Cellular control and Patterns of inheritance

Question 1

What is a mutation

Answer 1

-a mutation is a random change to the genetic material
-some mutations involve changes to the structure or number of chromosomes
-a gene mutation is a change to the DNA
-mutations may occur spontaneously during DNA replication before cell division
-certain chemicals such as tar in tobacco smoke and ionising radiation such as UV light, X rays and gamma rays may be mutagenic

Question 2

Why do mutations occur and compare mitotic and meiosis mutation

Answer 2

-the structure of DNA molecule makes it stable and fairly resistant to corruption of genetic information stored within it
-errors may occur, however, during replication of DNA molecule
-mutations associated with mitotic division are somatic mutations and are not passed to offspring; they may be associated with development of cancerous tumours
-mutations associated with meiosis and gamete formation may be inherited by offspring
-genes mutations may affect protein production and function

Question 3

What are the two main classes of DNA mutation

Answer 3

-point mutation= one base pair replaces (substituted for) another
-insertion or deletion (indel) mutation= one or more nucleotides are inserted or deleted from a length of DNA, these may cause a frameshift

Question 4

What are point mutations

Answer 4

-the genetic code consists of nucleotide base triplets within the DNA
-during transcription of a gene this code is copied to a length of mRNA as codons, complementary to the base triplets on template strand on length of DNA
-the sequence of codons on mRNA therefore a copy of the sequence of base triplets on the gene
-there are three types point mutation: silent, missense and nonsense

Question 5

POINT MUTATION: silent mutations

Answer 5

-all amino acids involved in protein synthesis apart from methionine have more than one base triplet code
-this reduces the effect of point mutations as they do not always a cause a change to sequence of amino acids in a protein
-this is often called redundancy or degeneracy of genetic code
-a point mutation involving a change to base triplet where that triplet still codes for same amino acid is a silent mutation
-the primary structure of protein and therefore secondary and tertiary structure not altered

Question 6

POINT MUTATION: missense mutation

Answer 6

-a change to base triplet sequence that leads to a change in the amino acid sequence in a protein is a missense mutation
-within a gene, such a point mutation may have significant effect on protein produced
-the alteration to the primary structure leads to a change in tertiary structure of the protein, altering its shape and preventing from carrying out its usual function

Question 7

What can occur as a result of missense mutations

Answer 7

-sickle cell anaemia results from a missense mutation on the sixth base triplet of the gene for the B polypeptide chains of haemoglobin: the amino acid valine, instead of glutamic acid is inserted at this position
-this result in deoxygenated haemoglobin, crystallising within erythrocytes, causing them to become sickle shaped, blocking capillaries and depriving tissues of oxygen

Question 8

POINT MUTATION: nonsense mutations

Answer 8

-a point mutation may alter a base triplet so that it becomes a termination (stop) triplet
-this particularly disruptive point mutation results in a truncated protein that will not function
-this abnormal protein will most likely be degraded within a cell
-the genetic disease Duchenne muscular dystrophy is a result of a nonsense mutation

Question 9

INDEL MUTATIONS: insertions and deletions

Answer 9

-both insertions and deletions cause a frameshift
-if nucleotide base pairs, not in multiples of three, are inserted in the gene or deleted from gene, because code is non overlapping and read in groups of three bases, all subsequent base triplets are altered
-this is a frameshift
-when the mRNA from such a mutated gene is translated the amino acid sequence after the frameshift is severely disrupted
-the primary sequence of the protein and subsequently the tertiary structure is much altered
-if the protein is very abnormal it will be rapidly degraded within a cell

Question 10

What can happen due to indel mutations

Answer 10

-some forms of thalassaemia, a haemoglobin disorder, result from frameshifts due to deletions of nucleotide bases
-insertions or deletions of a triplet base pairs results in the addition or loss of an amino acid and not in a frameshift

Question 11

What are triple nucleotide repeats

Answer 11

-some genes contain a repeating triplet such as CAG, CAG, CAG
-in an expanding triple nucleotide repeat, the number of CAG triplets increases at meiosis and again from generation to generation
-Huntington disease results from expanding triple nucleotide repeats
-if number of repeating CAG sequences goes above a certain critical number, then the person with the genotype will develop the symptoms of Huntington disease later in life

Question 12

Give examples of how not all mutations are harmful

Answer 12

-many mutations are beneficial and have helped drive evolution through natural selection
-different alleles of particular gene produced via mutation
-the mutation that gave rise to blue eyes arose in human population 6000-8000 years ago
-such mutation could be harmful in areas where sunlight intensity is high, as lack of iris pigmentation could lead to lens cataracts
-however, in more temperate zones it could enable people to see better in less bright light
-early humans in africa would have black skin, the high concentrations of melanin protecting them from sunburn and skin cancer
-when humans migrate to temperate regions, a paler skin would be advantage, enabling vitamin D to be made with lower intensity sunlight
-people with fairer skin would have an advantage and be selected as vitamin D not only protects us from rickets, it protects us from heart disease and cancer

Question 13

How are some mutations neutral

Answer 13

-they appear to be neither beneficial nor harmful
-inability to smell certain flowers, including freesias and honeysuckle
-differently shaped ear lobes

Question 14

List the DNA and polypeptide sequences of mutations

Answer 14

NORMAL
ATG CAG CAG CAG TTT TTA CGC AAT CCC
Met Gin Gin Gin Phe Leu Arg Asn Pro
SILENT
ATG CAG CAG CAG TTT TTG CGC AAT CCC
Met Gin Gin Gin Phe Leu Arg Asn Pro
NONSENSE
ATG CAG CAG CAG TTT TAA CGC AAT CCC
Met Gin Gin Gin Phe STOP
FRAMESHIFT
ATG CAG CAG CAG TTT TAC GCA ATC CC-
Met Gin Gin Gin Phe Tyr Ala Ile
MISSENSE
ATG CAG CAG CAG TTT TCA CGC AAT CCC
Met Gin Gin Gin Phe Ser Arg Asn Pro

Question 15

How is gene expression regulated in prokaryotic cells

Answer 15

-enzymes that catalyse the metabolic reactions in basic cellular functions are synthesised at a fairly constant rate
-enzymes that may only be needed under specific conditions are synthesised at varying rates according to the needs of the cell

Question 16

How does E.coli metabolise respiratory substrates if glucose absent and disaccharide lactose present

Answer 16

-lactose permease which allows lactose to enter the bacterial cell
-B galactosidase which hydrolyses lactose to glucose and galactose

Question 17

Structure of the lac operon

Answer 17

-the lac operon region consists of a length of DNA, about 6000 base pairs long, containing an operator region lac0 next to the structural genes lacZ and lacY that code for the enzymes B galactosidase and lactose permease respectively
-next to the operator region lac0 is the promoter region P to which the enzyme RNA polymerase binds to begin transcription of the structural genes lacZ and lacY
-the operator region and promoter region are the control sites

Question 18

What is the regulatory gene

Answer 18

-a small distance away from the operon is the regulatory gene, I, that codes for a repressor protein lac1
-when the regulatory gene is expressed the repressor protein produced binds to the operator preventing RNA polymerase from binding to promoter region
-the repressor protein therefore prevents the genes lacZ and lacY from being transcribed (expressed) so the enzymes for lactose metabolism are not made
-the genes are ‘off’

Question 19

What happens to the lac operon in the presence of oxygen

Answer 19

-when lactose is added to the culture medium, once all the glucose has been used, molecules of lactose bind to the lacI repressor protein, preventing it from binding to the operator
-the RNA polymerase enzyme can then bind to the promoter region and begin transcribing the structural genes into the mRNA that will then be translated to the two enzymes
-this lactose induces the enzymes needed to break it down

Question 20

How do eukaryotic cells differ in gene expression

Answer 20

-every cell in a eukaryotic organism has the dame genome but because different cells use it differently they function differently
-this is the basis of cell differentiation
-in neurones the genes being expressed differ to some extent from those being expressed in a liver or kidney cell although all cells express the basic housekeeping genes

Question 21

How is gene expression regulated in eukaryotic cells

Answer 21

-transcription factors are proteins, or short non coding pieces of RNA that act within the cells nucleus to control which genes in a cell are turned on or off
-transcription factors slide along a part of DNA molecule, seeking and binding to their specific promoter regions
-they may then aid or inhibit attachment of RNA polymerase to DNA and activate or supress transcription of gene
-they are essential for regulation or gene expression in eukaryotes making sure that different genes in different types of cells are activated or suppressed
-some transcription factors are involved in regulating cell cycle
-tumour suppressor genes and proto oncogenes help regulate cell division via transcription factors
-mutations to genes can lead to uncontrollable cell division or cancer

Question 22

What are transcription factors

Answer 22

-about 8% of genes in human genome encode transcription factors
-many genes have their promoter regions some distance away along the unwound length of DNA but because of how the DNA can bend, the promoter region may not be too far away spatially

Question 23

POST TRANSCRIPTIONAL GENE REGULATION: introns and exons

Answer 23

-within a gene, there are non coding regions of DNA called introns which are not expressed
-they separate the coding or expressed regions which are called exons
-all the DNA of a gene, both introns and exons is transcribed
-the resulting mRNA is called primary mRNA
-primary mRNA then edited and the RNA introns - length corresponding to the DNA introns are removed
-the remaining mRNA exon, corresponding to the DNA exons are joined together
-endonulcease enzyme may be involved in the editing and splicing proteins

Question 24

What are introns

Answer 24

-non coding region of DNA
-some introns may themselves encode proteins and some may become short non coding lengths of RNA involved in gene regulation
-some genes can be spliced in different ways
-a length of DNA with its introns and exons can according to how it is spliced, encode more than one protein

Question 25

Define exon

Answer 25

-the coding or expressed region of DNA

Question 26

Describe important factors to post translational level of gene regulation

Answer 26

-post translational regulation of gene expression involves the activation of proteins
-many enzymes are activated by being phosphorylated
-cyclic AMP is an important second messenger involved in this activation

Question 27

What are homeobox genes

Answer 27

-the large and ancient family of genes, the homeotic genes, are involved in controlling the anatomical development or morphogenesis of an organism so that all structures develop in the correct location, according to the body plan
-several of these genes contain homeobox sequences and they are sometimes called homeobox genes

Question 28

What are homeobox sequences

Answer 28

-each homeobox sequence is a stretch of 180 DNA base pairs (excluding introns) encoding a 60 amino acid sequence, called a homeodomain sequence within a protein
-the homeodomain sequence can fold into a particular shape and bind to DNA regulating the transcription of adjacent genes
-these proteins are transcription factors and act within the cell nucleus
-the shape that these homeodomain containing proteins fold into is called H-T-H
-it consists of two alpha helices (H) connected by one turn (T)
-part of the homeodomain amino acid sequence recognises the TAAT sequence of enhancer region (region that initiates or enhances transcription) of gene to be transcribed

Question 29

Describe homeobox genes

Answer 29

-homeobox genes are very conserved and very similar

Question 30

What are Hox genes

Answer 30

-the Hox genes regulate the development of embryos along the anterior-posterior axis
-they control which body parts grow where
-if Hox genes are mutated, abnormalities can occur such as the attennae on the head of Drosophila developing as legs or mammalian eyes developing on limbs

Question 31

How are Hox genes arranged

Answer 31

-Hox genes are arranged in clusters and each cluster may contain up to 10 genes
-in tetrapods, including mammals and therefore humans, there are four clusters
-some stage during evolution, Hox clusters have been duplicated
-in early embryonic development Hox genes are active and are expressed in order along anterior-posterior axis of developing embryo
-sequential and temportal order of the gene expressions corresponds to the sequential and temporal development of the various body parts, a phenomenon known as colinearity

Question 32

How do Hox genes contain body plan development in animals

Answer 32

-Hox genes encode homeodomain proteins that act in the nucleus as transcription factors and can switch on cascades of activation of other genes that promote mitotic cell division, apoptosis, cell migration and help regulate cell cycle
-Hox genes are similar across different classes of animals; a fly can function properly with a chicken Hox gene inserted in a place of its own

Question 33

How are regulators regulated

Answer 33

-Hox genes are regulated by other genes called gap genes and pair rule genes
-in turn these genes are regulated by maternally supplied mRNA from egg cytoplasm

Question 34

How do Hox genes help regulate mitosis

Answer 34

-from zygote to embryo to fully formed adult, there are many mitotic cell divisions
-mitosis is part of cell cycle that is regulated with help of homeobox and hox genes
-it ensures that each new daughter cell contains full genome and is clone of parent cell
-during cell differentiation some of genes ina particular type of cell are switched off and not expressed
-in 1962 Leonard Hayflick showed that normal body cells divide a limited number of times before dying (around 50 times, known as Hayflick constant)

Question 35

How was theory of apoptosis developed

Answer 35

-in 1965, John Foxton Ross Kerr re examined and researched the idea of programmed cell death, first described in 1842 by Carl Vogt
-in 1972, term apoptosis was used for programmed cell death
-apoptosis is different from cell death due to trauma which involves hydrolytic enzymes

Question 36

Sequence of events during apoptosis

Answer 36

1) enzymes break down cell cytoskeleton
2) cytoplasm becomes dense with tightly packed organelles
3) cell surface membrane changes and small protusions called blebs form
4) chromatin condenses, the nuclear envelope breaks and DNA breaks into fragments
5) cell breaks into vesicles that are ingested by phagocytic cells so that cell debris does not damage any other cells of tissues. The whole process happens quickly

Question 37

How is apoptosis controlled

Answer 37

-many cell signals help control apoptosis
-some of these signalling molecules may be released by cells when genes that are involved in regulating the cell cycle and apoptosis respond to internal cell stimuli and external stimuli such as stress
-these signalling molecules include cytokines from cells of the immune system, hormones, growth factors and nitric oxide
-nitric oxide can induce apoptosis by making inner mitochondrial membrane more permeable to hydrogen ions and dissipating the proton gradient
-proteins are released into the cytoplasm where they bind to apoptosis inhibitor proteins, allowing apoptosis to occur

Question 38

How does apoptosis affect development

Answer 38

-apoptosis is an integral part of plant and animal tissue development
-extensive proliferation of cell types is prevented by pruning through apoptosis without release of any hydrolytic enzymes that could damage surrounding tissues
-during limb development, apoptosis causes digits to separate from each other
-apoptosis removes ineffective or harmful T lymphocytes during development of immune system

Question 39

Compare apoptosis in children and adults

Answer 39

-in children aged between 8-14 years, 20-30 billion cells per day apoptose
-in adults about 50-70 million cells per day apoptose
-the rate of cells dying should equal the rate of cells produced by mitosis
-not enough apoptosis leads to formation of tumours
-too much apoptosis leads to cell loss and degeneration
-cell signalling plays a crucial role in maintaining right balance

Question 40

Compare definitions of genotype and phenotype

Answer 40

genotype = genetic makeup of an organism
phenotype = visible characteristics of an organism

Question 41

PHENOTYPIC VARIATION: genetic factors

Answer 41

-the appearance of a living organism is influenced by both its genotype and its environment
-mutations have contributed to the process of evolution
-a mutation is a change to genetic material
-this may involve changes to the structure of DNA or changes to the number of gross structure of chromosomes
-sexual reproduction may also lead to genetic variation

Question 42

Describe the different gene mutations

Answer 42

-certain physical and chemical agents described as mutagen can increase the rate of mutation
-physical agents= gamma rays, xrays, UV light
-chemical agents= benzopyrene (found in tobacco smoke), mustard gas, nitrous acid, aromatic amines, reactive oxygen species, colchicine
-biological agents= some viruses, transposons, food contaminants such as mycotoxins and chemicals in charred meat and alcohol
-these mutations may be harmful, advantageous or neutral

Question 43

Describe the mutations that occur during gamete formation

Answer 43

-persistent- they can be transmitted through many generations without change
-random- they are not directed by a need on the part of the organism in which they occur

Question 44

CHROMOSOME MUTATION: deletion

Answer 44

-chromosome mutations occur during meiosis
-part of a chromosome containing genes and regulatory sequences is lost

Question 45

CHROMOSOME MUTATION: inversion

Answer 45

-section of chromosome may break off, turn through 180 degrees and then join again although all the genes are still present, some may now be too far away from their regulatory nucleotide sequences to be properly expressed

Question 46

CHROMOSOME MUTATION: translocation

Answer 46

-a piece of one chromosome breaks off and then becomes attached to another chromosome
-this may also interfere with regulation of genes on translocated chromosome
-linked to many genetic disorders such as sarcoma, lymphoma, down syndrome, schizophrenia etc

Question 47

CHROMOSOME MUTATION: duplication

Answer 47

-piece of chromosome may be duplicated
-overexpression of genes can be harmful because too many of certain proteins or gene regulating nucleic acids may disrupt metabolism

Question 48

CHROMOSOME MUTATION: non-disjunction

Answer 48

-one pair of chromosomes or chromatids fails to separate leaving one gamete with an extra chromosome
-this is because a spindle fibre fails to be produced from the centriole in anaphase 1
-when fertilised by a normal haploid gamete, the resulting zygote has one extra chromosome
-down syndrome or trisomy 21 is caused by non disjunction
-bananas are an example of a non disjunction mutation
-they are infertile as they cannot produce pairs of chromosomes

Question 49

CHROMOSOME MUTATION: aneuploidy

Answer 49

-the chromosome number is not an exact multiple of haploid number of that organism
-sometimes chromosomes or chromatids fail to separate during meiosis - e.g. trisomy

Question 50

CHROMOSOME MUTATION: polyploidy

Answer 50

-if a diploid gamete is fertilised by a haploid gamete the resulting zygote will be triploid (has three sets of chromosomes)
-the fusion of two diploid gametes can make a tetraploid zygote
-many cultivated plants are polyploid- they have more than two sets of chromosomes
-only occurs in plants
-one centriole fails to produce any spindle fibres
-when fertilised with a normal gamete, the triploid gamete produced is infertile because it cannot produce pairs of chromosomes

Question 51

How does sexual reproduction (meiosis) lead to genetic variation

Answer 51

-allele shuffling (swapping of alleles between non sister chromatids) during crossing over in prophase 1
-independent assortment of chromosomes during metaphase/anaphase 1
-independent assortment of chromatids during metaphase/anaphase 2

Question 52

Describe the gametes produced by meiosis and how random fusion creates more genetic diversity

Answer 52

-gametes produced by meiosis are individual and genetically dissimilar
-they are also haploid, containing only one of each pair of homologous chromosomes and one allele for every gene
-the random fusion of gametes creates more genetic diversity
-any male gamete can potentially combine with any female gamete from an organism of the same species
-the random fertilisation of gametes that are already genetically unique produces extensive genetic diversity among resulting offspring

Question 53

How can variation occur solely by the environment

Answer 53

-some phenotypical variation is caused by the environment and not passed through genes
-speaking with a particular regional dialect: a persons offspring would not inherit dialect through their genes although they might learn to speak this way by listening to other people
-losing a digit or limb or having a scar following an injury

Question 54

How is variation cause by the environment interacting with genes

Answer 54

-if plants are kept in dim light after germination, or if soil in which they are grown contains insufficient magnesium then the leaves do not develop enough chlorophyll and are yellow/ yellow white
-the plant is described as chlorotic or suffering from chlorosis
-the plant cannot photosynthesise
-chlorotic plants have genotype making chlorophyll but environmental factors are preventing expression of these genes

Question 55

Compare heterozygous and homozygous definitions

Answer 55

HETEROZYGOUS= not true breeding; having different alleles at a particular gene locus on a pair of homologous chromosomes e.g. C^BC^b
HOMOZYGOUS= true breeding, having identical alleles at a particular gene locus on a pair of homologous chromosomes e.g. C^BC^B

Question 56

What did Mendel deduce from his dihybrid crosses

Answer 56

-the alleles of two genes are inherited independently of each other so each gamete has one allele for each gene locus
-during fertilisation any one of an allele pair can combine with any one of another allele pair

Question 57

Define codominance

Answer 57

-where both alleles present in genotypes of a heterozygous individual contribute to individuals phenotype

Question 58

Describe genes with multiple alleles

Answer 58

-so far we have considered characteristics with distinct traits where there are two alleles for each gene locus in question
-overtime a huge number of changes can occur anywhere within a gene, as a result many genes have more than two alleles
-when three or more alleles at a specific gene locus are known then the gene is said to have multiple alleles
-however any individual can only possess two alleles one on each gene locus in a pair of homologous chromosomes

Question 59

Describe the human blood groups

Answer 59

-the inheritance of human ABO blood groups is a good example of multiple alleles
-it also demonstrates both dominance and codominance of alleles involved
-the four blood groups (phenotypes) A, B, AB ,O are determined by three alleles of a single gene on chromosome 9
-the gene encodes an isoagglutinogen I on the surface of erythrocytes
-the alleles present in the human gene pool at IA, IB, and IO
-IA and IB are both dominant to IO which is recessive
-IA and IB are codominant
-if both IA and IB are present in the genotype they will both contribute to the phenotype
-any individual will have only two or three alleles within genotype
-a man of blood group A and woman of blood group B both heterozygous at this gene locus can produce children of any of these four blood groups

Question 60

What does it mean if the genetic code is degenerate/ redundant

Answer 60

-a single amino acid may be coded for by different codons

Question 61

Summarise transcription

Answer 61

-occurs within nucleus
-gyrase unwinds the DNA and helicase unzips the DNA
-reveals the template stand and coding strand
-in RNA, T is replaced with U
-RNA free nucleotides match with complementary base pair
-H bonds form temporarily
-A-U has 2H bonds
-G-C has 3H bonds
-RNA polymerase zips all mRNA free nucleotides together to form mRNA molecule
-leaves nucleus via nuclear pore

Question 62

Summarise translation

Answer 62

-tRNA is activated by adding corresponding amino acid, this is coded for by the anticodon
-the anticodon on the tRNA matches with the complementary codon on mRNA (start codon)
-the 2nd anticodon on tRNA matches with the mRNA codon
-a peptide bonds forms between the 2 adjacent amino acids
-this uses the enzyme peptidyl transferase
-this process occurs again repeatedly until a stop codon is met
-this forms the polypeptides primary structure (exact sequence of amino acids)
-tRNA molecules detach and are reactivated

Question 63

Why is the genetic code a 3 base code

Answer 63

-if it were a 4 base code (4^3) then it would be wasteful
-however if it were a 2 base code (4^2) it wouldn’t be enough to code for all amino acids

Question 64

What environmental factors may affect mutations

Answer 64

-some mutagens cause damage to the DNA molecule by altering its structure
-for example smoking tobacco, UV, ionising radiation etc

Question 65

What is the difference between mutations before mitosis and before meiosis

Answer 65

-prior to mitosis is a somatic mutation - it cannot be passed to offspring
-prior to meiosis mutations can be inherited

Question 66

Why is the structure of DNA fairly stable and resistant to mutations

Answer 66

-has check points in replication
-stable body temperature
-found in nucleus
-hydrogen bonds form between base pairs
-the base pairs are complementary to one another

Question 67

Give an example of a deletion mutation

Answer 67

-cystic fibrosis
-deletes the triplet code and causes chloride ions to build up as they cannot be pumped out
-results in the formation of mucus

Question 68

Give an example of a missense mutation

Answer 68

-may lead to oncogenes - genes that promote unregulated and uncontrollable cell division or growth (cancer)
-there is an inability to stop expressing these oncogenes

Question 69

Describe sex linkage in humans

Answer 69

-in humans sex is determined by one of the 23 pairs of chromosomes called the sex chromosomes
-the other 22 pairs are called autosomes
-each of the autosomal pairs is fully homologous-they match for length and contain the same genes at the same gene loci
-the sex chromosomes are XY in males and XX females
-the X and Y chromosomes are not fully homologous
-a small part of one matches a small part of the other so that these chromosomes can pair up before meiosis

Question 70

Describe the inheritance of female chromosomes

Answer 70

-the human X chromosome contains over 1000 genes that are involved in determining many characteristics or metabolic functions, not concerned with sex determination, and most of these have no partner alleles on the Y chromosome
-if a female has one abnormal allele on one of her X chromosomes she will probably have a functioning allele of same gene on her other X chromosome

Question 71

Describe the inheritance of male chromosomes

Answer 71

-if a male inherits, from his mother, an X chromosome with an abnormal allele for a particular gene, he will suffer from a genetic disease as he will not have a functioning allele for that gene
-males are functionally haploid or hemizygous for X linked genes
-they cannot be heterozygous or homozygous for X linked genes
-sex linked characteristics in humans include haemophilia A and colour blindness

Question 72

What is haemophilia A

Answer 72

-a person with haemophilia A is unable to clot blood fast enough
-injuries may cause bleeding or internal haemorrhage
-one of the genes on the non homologous region of the X chromosome codes for a blood clotting protein called factor 8
-a mutated form of allele codes for non functioning factor 8

Question 73

How How is haemophilia A inherited

Answer 73

-a female with one abnormal and one functioning allele produces enough factor 8 to enable her blood to clot normally when required
-however, this female is a carrier for the disease
-if such female passes the X chromosome containing the faulty allele to her son, he will have no functioning allele for factor 8 on his Y chromosome
-as a result he will suffer from haemophilia A

Question 74

Describe colour blindness

Answer 74

-one of the genes involved in coding for a protein involved in colour vision is on the X chromosome but not on Y chromosome
-a mutated allele of this gene may result in colour blindness- an inability to distinguish between red and green
-a female with one abnormal allele and one functioning allele will not suffer from colour blindness but a male with an abnormal allele on his X chromosome will not have a functioning allele on his Y chromosome and therefore will suffer from red-green colour blindness
-the inheritance patter is that of a recessive sex linked disorder

Question 75

Describe the sex linkage in cats

Answer 75

-one of the genes C for coat colour in cats is sex linked
-it is on the non homologous region of the X chromosome
-the allele C^O produces orange/ginger fur
-the allele C^B produces black fur
-these alleles are codominant as cats with the genotype XC^OXC^B are tortoiseshell and have patches of black fur and patches of orange fur
-both the orange and black alleles contribute to the phenotype but the orange allele is only expressed in cells where the X chromosome bearing the black coat colour is inactivated and vice versa
-male cats may either be black or ginger but no tortoiseshell as they only have one X chromosome

Question 76

Describe the inactivation of X chromosomes in female mammals

Answer 76

-it may appear that females have twice the number of X-linked genes being expressed as do males
-however a mechanism prevents this disparity
-in every female cell nucleus one X chromosome is inactivated
-determination of which member of the pair of X chromosomes becomes inactivated is random and happens during early embryonic development

Question 77

Describe codominant alleles

Answer 77

-when both alleles of a gene in the genotype of a heterozygous individual contribute to that individuals phenotype, the alleles are described as codominant
-the two alleles are responsible for two distinct and detectable gene products
-the phenotype of heterozygotes is different from phenotype of the homozygous

Question 78

CODOMINANT INHERITANCE HUMANS: ABO blood groups

Answer 78

-alleles I^A and I^B are codominant to each other
-an individual of genotype I^AI^B expresses both and has both types of isoagglutinogen proteins on their erythrocytes
-the inheritance of these blood groups also shows dominance as both I^A and I^B are dominant to allele I^O

Question 79

CODOMINANY INHERITANCE HUMANS: sickle cell anaemia

Answer 79

-sickle cell anaemia is caused by a mutation in the gene that codes for the B globin chain of haemoglobin
-the mutant allele is given the symbol Hb^5 and the normal allele is given the symbol Hb^N
-in heterozygous people at least half of the haemoglobin in their blood cells is normal and half is abnormal
-however heterozygous people do not suffer from sickle cell anaemia
-if we consider the type of haemoglobin as the phenotype then these alleles are considered as codominant
-however if we take sickle cell anaemia to be the phenotype the Hb^5 allele is considered to be recessive as this disorder has a recessive inheritance pattern

Question 80

Interaction of genes and environment

Answer 80

-environment has a greater effect on the expression of polygenes/ polygenic characteristics than it does on monogenic characteristics
-for example each person has a genetic potential for height and intelligence but without proper nutrition and also intelligence, mental stimulation these potentials will not be met

Question 81

Compare alloploidy and autoploidy

Answer 81

-when 2n zygote from nondisjunction fertilised forms tetraploid
AUTOPLOIDY= possessing more than 2 sets of chromosomes where all sets come from same species
ALLOPLOIDY= possessing 2 sets of chromosomes where sets come from 2 different species but have very similar DNA sequence
-more likely to be fertile because have enough differentiation between chromosomes

Question 82

Describe continuous variation

Answer 82

-no distinct categories- is a continuum
-lots of intermediates
-e.g. height, weight, eye colour, intelligence, bean length
-polygenic- controlled by more than one gene
-more affected by environment
-quantitative data
-use histogram to show data- bars touching

Question 83

Describe discontinuous variation

Answer 83

-distinct categories
-no/ few intermediates
-e.g. ear lobes (attached or unattached), sex, blood type, pea shape (wrinkled or smooth)
-monogenic- controlled by one gene with different alleles
-not very affected by environment
-qualitative data
-data shown with bar chart- has non touching bars

Question 84

Define monohybrid cross

Answer 84

-where one gene is crossed

Question 85

Describe linkage

Answer 85

• the genes assorted independently
  -there are many more genes in a genome than there are chromosomes
  -when two or more gene loci are on same chromosome, they are said to be linked
  -chromosome is the unit of transmission during sexual reproduction therefore linked genes are not free to undergo independent assortment; they are usually inherited together as a single unit
  -if linked genes ae not affected by crossing over of non sister chromatids during prophase 1 of meiosis then they are always inherited as one unit

Question 86

Define autosomal linkage

Answer 86

-where genes are linked by being on same autosome inheritance of autosomally linked genes with no crossing over

Question 87

What does inheritance of autosomally linked genes with crossing over form

Answer 87

-gamete formation where there is crossing over between two non sister chromatids during prophase 1 will form non crossover gametes and recombinant gametes
-the further apart the two gene loci are on a chromosome, greater is chance of recombinant gametes forming

Question 88

How do genes interact

Answer 88

-in some cases, different genes at different loci on different chromosomes, can interact to affect one phenotypic characteristic
-when one gene masks or suppresses the expression of another gene this is termed epistasis
-the genes in question may work together antagonistically or in complementary fashion
-because the gene loci are not linked they assort independently during gamete formation
-epistasis reduces the number of phenotypes produced in the F2 generation of dihybrid crosses and therefore it reduces genetic variation

Question 89

Recessive epistasis

Answer 89

-the homozygous presence of a recessive allele at the first locus prevents expression of another allele at a second locus
-the alleles at the first locus are epistatic to those at the second locus which are hypostatic to those at first locus

Question 90

Example of dominant epistasis

Answer 90

-inheritance of feather colour in chickens is an example of dominant epistasis
-there is an interaction between two gene loci I/i and C/c
-the hypostatic gene C/c codes for coloured feathers
-the I allele of the epistatic gene I/i prevents formation of colour even if one C allele is present
-individually carrying at leas one dominant allele I have white feather even if they also have one dominant allele for coloured feathers
-birds that are homozygous for recessive allele c are also white as this mutated allele does not cause pigment to be made

Question 91

Genes working in a complementary fashion

Answer 91

-as scientists are discovering more about how genes work and interact with each other, epistasis is more often explained in terms of genes working to code for two enzymes that work in succession, catalysing sequential steps of a metabolic pathway

Question 92

Directional selection

Answer 92

-type of natural selection that occurs when an environmental change favours a new phenotype and so results in a change in population mean

Question 93

Founder effect

Answer 93

-when small sample of an original population establishes in a new area; its gene pool is not as diverse as that of parent population

Question 94

Genetic bottleneck

Answer 94

-sharp reduction in size of a population due to environmental catastrophes such as earthquakes, floods, disease or human activities such as habitat destruction, overhunting or genocide which reduces genetic diversity
-as population expands it is less genetically diverse than before

Question 95

Stabilising selection

Answer 95

-natural selection leading to constancy within a population
-intermediate phenotypes are favoured and extreme phenotypes may be removed from the population
-stabilising selection reduces genetic variation within the population

Question 96

Describe natural selection

Answer 96

-mutations and migration introduce new alleles into populations
-some individuals within a population will be better adapted than others to the environment, due to differences in their genotypes and phenotypes
-these individuals are more likely to survive and reproduce, passing on advantageous alleles
-overtime allele frequencies within population will change
-this is natural selection
-natural selection may also maintain constancy of a species as well as leading to new species
-there are three main types of selection: stabilising, directional and disruptive selection

Question 97

Dewcribe stabilising selection

Answer 97

-stabilising selection normally occurs when the organisms environment remain unchanged
-it favours intermediate phenotypes
-in humans babies of birth mass close to 3.5kg are more likely to survive
-their offspring inherit alleles from them also leading to this mean birth mass

Question 98

Describe directional selection

Answer 98

-if environment changes for example by become colder, there may now be an advantage to being larger, so a new larger mass becomes ideal and will be selected for
-if more larger individuals survive and reproduce they will be more likely to pass genes and alleles for larger size to their offspring
-over several generations there is a gradual shift in optimum value for the trait
-directional selection is used by plant and animal breeders to produce desirable traits
-in nature within a population periods of directional selection may alternate with periods of stabilising selection

Question 99

Describe genetic drift

Answer 99

-if a population descends from a small number of parents, the gene pool will lack genetic variation
-some alleles resulting from mutation confer neither an advantage nor disadvantage on individual so will be no selection pressure acting upon them
-however chance events may drastically alter the allele frequency

Question 100

Example of genetic drift

Answer 100

-imagine a small population descended from one set of heterozygous parents
-there are only two alleles A and a in a population
-if a catastrophic event occurs such as an earthquake, flood, outbreak of disease or a severe shortage of food which leads to the death of many of already small population, one of alleles may disappear from this population
-when the population recovers and increases in size it will have less genetic diversity than before and may lack particular alleles
-the alleles in question did not disappear due to selection pressure but due to genetic drift
-genetic drift can arise after genetic bottleneck or as a result of founder effect

Question 101

Describe genetic bottleneck

Answer 101

-when a population size shrinks and then increases again it is said to have gone through a genetic bottleneck
-after this event the genetic diversity within that population will be reduced
-there may be loss of some advantageous alleles or disproportionate frequency of deleterious (harmful) allele putting that population chances of long term survival at risk
-sometimes after a genetic bottleneck a population shrinks to such a small size that its fertility is affected leading to the specie becoming endangered and then extinct
-however if the ones that survive are those that have a particular advantage, e.g. resistance to pathogen, then bottleneck could improve gene pool whilst shrinking genetic diversity
-species that have been selectively bred for certain traits have also been through a genetic bottleneck

Question 102

Describe the founder effect

Answer 102

-if a new population is established by a very small number of individuals originate from a larger parent population, the new population is likely to exhibit loss of genetic variation
-some groups of migrating humans, not fully genetically representative of parent population have set up populations in new areas
-if they have remained isolated from other human populations for example because of religious and cultural differences or due to geographic isolation then the new population will have a small gene pool
-this has happened in iceland, the faroe island, pitcairn island, easter island and among amish people of north america
-founder effect is special case of genetic drift

Question 103

Population genetics

Answer 103

-population genetics attempts to study the changes in allele frequencies within a population over time
-if a species is to succeed and not become extinct it needs genetic variation between the individuals in its populations
-individuals inherit their genomes from there parents and pass on some of their genetic material to their offspring
-population genetics studies the variation in alleles and genotypes within gene pool and how their frequencies vary over time
-factors affecting allele frequencies within populations and hence genetic diversity within gene pool include: population size, mutation rate, migration, genetic drift, gene flow, natural selection, changes to environment etc
-if two populations of same species become so genetically different that they can no longer interbreed and produce infertile offspring they have undergone speciation and formed two new species

Question 104

What does the Hardy Weinberg principle assume

Answer 104

-principle assumes that:
-population is large enough to make sampling error negligible
-mating within population occurs at random
-there is no selective advantage for any genotype and hence no selection
-there is no mutation, migration or genetic drift

Question 105

Hardy Weinberg equation

Answer 105

alleles: p +q = 1
genotypes: p^2 + 2pq + q^2
p is the dominant allele
q is the recessive allele
always start with the q^2

Question 106

What is speciation

Answer 106

-over time one species may evolve into another or it may evolve into two new species
-for a species to evolve into two species it must be split into two isolated populations
-if this happens then any mutations that occur in one population are not transmitted by interbreeding to the other population
-in each location there will be different selection pressures and each population will accumulate different allele frequencies
-hence each population can evolve along its own lines
-at times during the evolutionary process, the two populations will be different but still able to interbreed
-they are then called sub species
-when there have been sufficient genetic, behavioural and physiological changes in the two populations so that they can no longer interbreed they are then separate species
-the process by which species are formed is called speciation

Question 107

Isolating mechanisms

Answer 107

-there are two main types of isolating mechanisms, geographical and reproductive

Question 108

Geographical isolation

Answer 108

-if populations are separated and isolated from each other by geographical features such as lakes, rivers, oceans and mountains these also act as barriers to gene flow between the populations
-the isolated populations being subject to different selection pressures in two different environments then undergo independent changes to the allele frequencies and/ or chromosome arrangements within their gene pools
-these genetic changes may be result of a mutation selection and genetic drift
-as a result of natural selection, each population becomes adapted to its environment
-this type of speciation is called allopatric speciation (means in different countries)

Question 109

Describe reproductive isolation

Answer 109

-biological and behavioural changes within a species may lead to reproductive isolation of one population from another
-if a mutation leads to some organisms in a population changing their foraging behaviour and becoming active at dawn, dusk or at night rather than during the day, enabling them to exploit a new niche, the members of the diurnal population will be unlikely to mate with members of either crepuscular (active at dawn and dusk) or nocturnal populations

Question 110

How do genetic changes lead to reproductive isolation

Answer 110

-a change in chromosome number may:
-prevent gamete fusion
-make zygotes less viable so that they fail to develop
-lead to infertile hybrid offspring with an odd number of chromosomes so that chromosome pairing during meiosis cannot occur

Question 111

What does mating between members of reproductively isolated populations

Answer 111

-mating between members of the reproductively isolated populations may also be prevent by mutations leading to changes in:
-courtship behaviour e.g. time of year for mating or courtship rituals that precede mating
-animal genitalia or plant flower structure
-speciation resulting from reproductive isolation is called sympatric (meaning same country) speciation

Question 112

Principles and uses of artificial selection

Answer 112

-whereas natural selection is operating the environment is agent of selection, during artificial selection, humans are agents of selection
-breeders select individuals with desired traits and allow them to interbreed whilst at same time preventing those without desired characteristics from breeding
-humans have domesticated very few species
-desirable characteristics in plants include increased yield, and pest and disease resistance
-in livestock, desirable characteristics include docility, placidity and ability to be trained
-animals that normally live in social groups (herds) with a dominance hierarchy may be able to be trained to accept a human as pack leader and to tolerate being penned with other animals
-artificial selection produces new breeds of organisms

Question 113

Selective breeding programmes

Answer 113

-new breeds may be produced by selective breeding programmes
-breeders may grow many plants of a particular type under conditions they wish these plants to withstand- such as low temperatures or high salinity
-they will then select those individuals that grow best under these conditions and cross pollinate them; collect and sow seeds and repeat this process over many generations
-a selective breeding programme takes about 20 years

Question 114

Interbreeding depression

Answer 114

-at each stage of selective breeding the individuals with the desirable characteristics and no or few undesirable traits are selected
-inevitably the genetic diversity in the gene pool of selected breed is reduced
-if related individuals are crossed, interbreeding depression can occur
-the chances of an individual inheriting two copies of a recessive harmful allele is increased

Question 115

Hybrid vigour

Answer 115

-breeders sometimes outcross individuals belonging to two different varieties to obtain individuals that are heterozygous at many gene loci
-this property is termed hybrid vigour

Question 116

How increase hybrid vigour

Answer 116

-selective breeding whilst developing bigger and better varieties of crop plants and animal breeds has reduced organisms genetic diversity
-number of commercially grown varieties of crop has greatly reduced over last 100 years
-all commercial varieties are genetically similar; if a pathogen was introduced most plants would die from infection
-breeders may need to outcross cultivated varieties with varieties more like their wild ancestors to increase hybrid vigour
-samples of such wild ancestral types need to be conserved often in gene banks

Question 117

Gene bank examples

Answer 117

-gene banks store genomes but in their organisms
-examples include:
-rare breed farms
-wild populations of organisms
-crops in cultivation
-botanic gardens and zoos
-seed banks
-sperm banks
-cells in tissue culture
-frozen embryos

Question 118

Importance of gene banks

Answer 118

-provide genetic variation
-sources of alleles
-increase hybrid vigour
-prevent inbreeding depression

Question 119

Ethical considerations of artificial selection

Answer 119

-domesticated animals retain many juvenile characteristics making them friendly, docile, playful bur less able to defend themselves
-the loss of their nervous disposition can also make them easy prey
-livestock animals such as pigs, selected to have more lean more and less fat, might succumb to low environmental temperatures during winter if they are not housed

Question 120

Heterozygous x homozygous recessive

Answer 120

1:1:1:1

Question 121

Heterozygous x heterozygous

Answer 121

9:3:3:1

Question 122

Autosomal linkage ratio

Answer 122

genotype 1:2:1
phenotype 3:1

Question 123

Recessive epistasis

Answer 123

9:3:4

Question 124

Dominant epistasis

Answer 124

12:3:1 or
13:3

Question 125

Complementary action epistasis

Answer 125

9:7 or
9:3:4 or
9:3:3:1