Chapter 16: Inherited Change

Question 1

How many pairs of homologous chromosomes are there?

Answer 1

22 pairs

Question 2

How many chromosomes are there in total and what are the types and where do they come from?

Answer 2

There are 22 pairs of autosomal homologous chromosomes, wherein one chromosome of each pair is maternal, and the other is paternal.
The 23rd pair is a non-matching sex chromosome pair made of x and Y chromosomes which determine the sex of the individual.

Question 3

What are homologous chromosomes?

Answer 3

Homologous chromosomes are a pair of chromosomes
in a diploid cell that have the same structure as each
other, with the same genes (but not necessarily the same alleles of those genes) at the same loci, and that pair together to form a bivalent during the first division of meiosis.

Question 4

What is a gene?

Answer 4

A gene is a length of DNA that codes for a particular

protein or polypeptide

Question 5

What is a locus?

Answer 5

A locus is the position at which a particular gene is found on a particular chromosome; the same gene is always found at the same locus.

Question 6

What is an allele?

Answer 6

Different varieties of the same gene are called alleles.

Question 7

What is a diploid cell? Give an example.

Answer 7

A diploid cell is one that possesses two complete sets of chromosomes; the abbreviation for diploid is 2n. Normal body cells are diploid and have 46 chromosomes.

Question 8

What is a haploid cell? Give an example.

Answer 8

A haploid cell is one that possesses one complete set of chromosomes; the abbreviation for haploid is n. Gametes are haploid and contain only one set of chromosomes and therefore have 23 chromosomes.

Question 9

What happens in prophase 1 of meiosis 1?

Answer 9

o Homologous chromosomes pair up up. This process is called synapsis. Each pair is called a bivalent.
o Centrosomes begin moving to opposite ends of the nucleus.
o The nuclear envelope breaks up as in mitosis.
o Crossing over of chromatids may occur.
o Nucleolus disappears as in mitosis.

Question 10

What happens in metaphase 1 of meiosis 1?

Answer 10

Centrosomes have reached opposite ends of the pole and bivalents line up across the equator of the spindle formed attached by centromeres.

Question 11

What happens in anaphase 1 of meiosis 1?

Answer 11

Centromeres do not divide, unlike in mitosis.

Whole chromosomes move towards opposite ends of the spindle, centromeres first pulled by microtubules.

Question 12

What happens in telophase 1 in meiosis 1?

Answer 12

0 As in mitosis, the nuclear envelope and nucleolus begins to reform. Cytokinesis occurs and the remains of the spindle are broken down.
o Chromosomes have reached opposite ends of the spindle.
o Animal cells usually divide before entering meiosis 2.

Question 13

What is the difference between animal and plant cells when entering meiosis 2?

Answer 13

Many plant cells go straight into meiosis 2 with no reformation of nuclear envelops or nucleoli.

Question 14

What happens in meiosis 2?

Answer 14

The chromatids separate as in mitosis, but in meiosis telophase 2, four haploid daughter cells are formed.

Question 15

What is the formation of male and female gametes called?

Answer 15

Male- spermatogenesis

Female- oogenesis

Question 16

What happens in spermatogenesis?

Answer 16

Sperm production takes place inside tubules in the testes. Here, diploid cells divide by mitosis to produce numerous diploid spermatogonia, which grow to form diploid primary spermatocytes. The first division of meiosis then takes place, forming two haploid secondary spermatocytes. The second division of meiosis then produces haploid spermatids, which mature into spermatozoa.

Question 17

What happens in oogenesis?

Answer 17

It takes place inside the ovaries, where diploid cells divide by mitosis to produce many oogonia. These begin to divide by meiosis, but stop when they reach prophase I. At this stage, they are called primary oocytes, and they are still diploid. All of this happens before a baby girl is born, and at birth she has around 400 000 primary oocytes in her ovaries. When she reaches puberty, some of the primary oocytes get a little further with their division by meiosis. They proceed from prophase I to the end of the first meiotic division, forming two haploid cells. However, the division is uneven; one cell gets most of the cytoplasm, and becomes a secondary oocyte, while the other is little more than a nucleus, and is called a polar body. The polar body can be thought of as simply a way of getting rid of half of the chromosomes, and has no further role to play in reproduction.
Each month, one secondary oocyte is released into
the oviduct from one of the ovaries. If it is fertilised, it
continues its division by meiosis, and can now be called an ovum.

Question 18

What forms a zygote and how does it form a fetus?

Answer 18

The chromosomes of the spermatozoan and the
ovum join together to form a single diploid nucleus, and the cell that is made by this process is called a zygote. The zygote can now divide repeatedly by mitosis to form first an embryo, and then a fetus.

Question 19

How are the male gametes formed in flowering plants?

Answer 19

Inside the anthers, diploid pollen mother cells divide
by meiosis to form four haploid cells. The nuclei of each of these haploid cells then divide by mitosis, but the cell itself does not divide (cytokinesis does not take place), resulting in cells that each contain two haploid nuclei. These cells mature into pollen grains, each surrounded by a protective wall made up of a tough exine and thinner intine. One of the haploid nuclei is called the tube nucleus, and the other is the generative nucleus. These are the male gametes.

Question 20

How is the female gamete formed in plants?

Answer 20

Inside each ovule, a large, diploid, spore mother cell
develops. This cell divides by meiosis to produce four
haploid cells. All but one of these degenerates, and the one surviving haploid cell develops into an embryo sac The embryo sac grows larger, and its haploid nucleus divides by mitosis three times, forming eight haploid
nuclei. One of these becomes the female gamete.

Question 21

How does fertilisation occur in plants and what happens?

Answer 21

Fertilisation occurs when a male gamete from a pollen grain fuses with a female gamete inside an ovule. This
forms a diploid zygote, which grows into an embryo plant.

Question 22

What is a genotype?

Answer 22

A genotype is the alleles possessed by an organism.

Question 23

What is homozygous?

Answer 23

Homozygous means having two identical alleles of

a gene.

Question 24

What is heterozygous?

Answer 24

Heterozygous means having two different alleles of

a gene.

Question 25

What is the sickle cell trait?

Answer 25

A person with the genotype HbAHbS has one allele of the β-globin gene in each cell coding for the production of the normal β-globin, and one coding for the production of the sickle cell β-globin. Half of the person’s haemoglobin will be normal, and half will be sickle cell haemoglobin. Such people have sickle cell trait

Question 26

What is a phenotype?

Answer 26

An organismʼs phenotype is its observable characteristics, often resulting from an interaction between its genotype and its environment.

Question 27

What does a gamete contain?

Answer 27

It contains one of each pair of chromosomes and therefore they contain only one copy of each gene.

Question 28

What is a dominant allele?

Answer 28

A dominant allele is one whose effect on the phenotype of a heterozygote is identical to its effect in a homozygote.

Question 29

What is a recessive allele?

Answer 29

A recessive allele is one that is only expressed when no dominant allele is present.

Question 30

What are codominant alleles?

Answer 30

Codominant alleles both have an effect on the phenotype of a heterozygous organism.

Question 31

What is F1 generation?

Answer 31

The F1 generation is the offspring resulting from a cross between an organism with a homozygous dominant genotype, and one with a homozygous recessive genotype.

Question 32

What is F2 generation?

Answer 32

The F2 generation is the offspring resulting from a cross between two F1 (heterozygous) organisms

Question 33

What is a test cross?

Answer 33

A test cross is a genetic cross in which an organism
showing a characteristic caused by a dominant allele
is crossed with an organism that is homozygous
recessive; the phenotypes of the offspring can be a
guide to whether the first organism is homozygous or
heterozygous.

Question 34

What are the three alleles that determine blood groups?

Answer 34

IA, IB, IO

Question 35

Which of the blood group alleles are codominant and which are recessive?

Answer 35

IA and IB are codominant, whereas Io is recessive to both IA and IB.

Question 36

What two groups are the chromosomes in human organised into?

Answer 36

A pair of sex chromosomes and 22 pairs of autosomes.

Question 37

How can the two sex chromosomes be different?

Answer 37

They do not always have the same genes in the same position and so they are not always homologous.

Question 38

What is factor viii and where is it found?

Answer 38

It is a gene that is found on the X chromosome that codes for the production of a protein needed for blood clotting.

Question 39

Describe the recessive and dominant allele of the factor viii gene and how it is a sex linked gene.

Answer 39

The recessive allele Xh causes the disease haemophilia, in which the blood fails to clot properly.
The fact that the gene for haemophilia is on the X
chromosome, and not on an autosome, affects the way that it is inherited. Females, who have two X chromosomes, have two copies of the gene. Males have only one X chromosome, and so have only one copy of the gene. Therefore, the possible genotypes for men and women are different. The factor VIII gene is said to be sex linked.

Question 40

What is a sex linked gene?

Answer 40

A sex-linked gene is one that is found on a part of the X chromosome not matched by the Y, and therefore not found on the Y chromosome.

Question 41

What are monohybrid crosses?

Answer 41

Monohybrid crosses look at the inheritance of one gene.

Question 42

What are dihybrid crosses?

Answer 42

Dihybrid crosses look at the inheritance of two genes at once.

Question 43

What is independent assortment?

Answer 43

It states that different genes get sorted into gametes independently of one another.

Question 44

What is the typical ratio of a dihybrid cross between a heterozygous and homozygous species?

Answer 44

1:1:1:1

Question 45

What is the typical ratio of a dihybrid cross between two heterozygous organisms?

Answer 45

9:3:3:1

Question 46

Give a brief overview of interactions between loci.

Answer 46

The effect on phenotypic character by interaction

between different gene loci; one gene locus could mask the phenotypic expression of another.

Question 47

What is linkage?

Answer 47

Linkage is the presence of two genes on the same
chromosome, so that they tend to be inherited together
and do not assort independently.

Question 48

Why is total linkage rare?

Answer 48

Total linkage is very rare, almost always links are broken due to crossing over during meiosis

Question 49

What is a chiasmata and how is it formed?

Answer 49

The chromatids of a bivalent may break and reconnect to another, non-sister chromatid. This results in an exchange of gene loci between a maternal and paternal chromatid. The point where crossing over occurs is called a chiasma. p. chiasmata.

Question 50

What are recombinant offspring?

Answer 50

The exchanged genes through the crossing over result in recombinant offspring.

Question 51

How is the cross over value calculated?

Answer 51

Cross over value is calculated by adding the percentage of offspring that belong to recombinant classes

Question 52

What does the cross over value determine?

Answer 52

This value can be used to determine the distance between the two gene loci on their chromosomes. The smaller the cross over value, the closer the loci are together. The chance of a cross over taking place between two loci is directly related to their distance apart.

Question 53

What is the x^2 chi-squared test used for?

Answer 53

Th is test allows us to compare our observed results with the expected results, and decide whether or not there is a significant difference between them.

Question 54

What is a null hypothesis?

Answer 54

There is no significant difference between observed and expected results

Question 55

How are the degrees of freedom found?

Answer 55

number of different categories – 1

Question 56

Describe the relationship between expected x^2 and calculated x^2. CHECK

Answer 56

If calculated 𝑥^2 is lower than the expected 𝑥^2 in the table, we accept the null hypothesis, meaning the results are due to chance and so no significant difference
And if calculated 𝑥^2 greater or equal to expected 𝑥^2, we reject null hypothesis as difference must be
significant and is not due to chance.

Question 57

What is a mutation?

Answer 57

It is an unpredictable change of the nucleotide

sequence in DNA.

Question 58

What is a mutagen? What is an example?

Answer 58

A substance that increases the chances of a mutation occurring is said to be a mutagen. e.g. all types of ionising radiation

Question 59

What are two types of mutation?

Answer 59

Gene mutation and chromosome mutation

Question 60

What is a gene mutation?

Answer 60

It is a change in the structure of DNA molecule producing different allele of a gene

Question 61

What is a chromosome mutation?

Answer 61

It is when there are changes in structure or number of whole chromosomes in cell.

Question 62

What are the three ways in which a gene mutation can occur?

Answer 62

o Base substitution where simply one base takes
another base’s place
o Base addition where on/more extra bases are added
o Base deletion where bases are lost from sequence

Question 63

What is a silent mutation?

Answer 63

A mutation that has no apparent effect on an organism is said to be a silent mutation.

Question 64

What does base addition and deletion affect?

Answer 64

Base addition and deletion have significant effects on the structure and function of the polypeptide as it alters the following codons causing frame shifts in the code. Often, the effects are so large that the protein that is made is totally useless. It may also introduce a stop triplet part way through a gene so that a complete protein is never made at all.

Question 65

What do base substitutions affect and why?

Answer 65

Base substitution may not even have an effect at all(silent mutation), as there is more than one codon that codes for the same amino acid. However base substitutions can have a very large effect. It can result in a stop triplet so the synthesis of a protein stop at this point.

Question 66

What can all mutations cause?

Answer 66

All mutations can cause a STOP codon to form,

causing the polypeptide production to be halted.

Question 67

What are the effects of the Hb^S allele?

Answer 67

It makes the haemoglobin molecule much less soluble. The molecules tend to stick to each other, forming long fibres inside the red blood cells. The red cells are pulled out of shape, into a half-moon or sickle shape. When this happens, the distorted cells become useless at transporting oxygen. They also get stuck in small capillaries, stopping any unaffected cells from getting through. A person can suffer sever anaemia and may die.

Question 68

What is the difference in the base sequence of a Hb^S allele and a normal allele?

Answer 68

The base sequence CTT is replaced by CAT.

Question 69

What is albinism and what is a symptom?

Answer 69

Albinism is when melanin is deficient or completely
missing from irises, skin and hair. The condition also can be accompanied by poor vision causing jerky movements of the eye to avoid light

Question 70

What is the classic version of albinism?

Answer 70

It is an autosomal recessive mutation wherein individuals must be homozygous for the recessive allele to show albinism.

Question 71

What form of albinism is sex-linked?

Answer 71

Albinism which affects the eyes.

Question 72

What enzyme is responsible for melanin production?

Answer 72

Tyrosinase

Question 73

What is the mutation that occurs in albinism and how does it affect melanin production?

Answer 73

A mutation in the gene for the enzyme tyrosinase
results in either the absence of tyrosinase or the presence of inactive tyrosinase in the cells responsible for melanin production. In these melanocytes, the first two steps of the conversion of the amino acid, tyrosine into melanin cannot take place. Tyrosine cannot be converted into DOPA and dopaquinone

Question 74

Give a brief overview of tyrosinase.

Answer 74

Tyrosinase is an oxidase and has two copper atoms
in its active site which bind an oxygen molecule. It is a
transmembrane protein and is found in the membrane of large organelles in the melanocytes called melanosomes. Tyrosinases occur in plants as well as in animal tissues. The action of the enzyme can be seen in the blackening of a slice of potato left exposed to the air

Question 75

What is Huntington’s disease?

Answer 75

It is a mutation inherited as a dominant allele. It is a neurological disorder that causes involuntary
movements and progressive mental deterioration.

Question 76

What is the mutation that occurs in Huntington’s disease?

Answer 76

Mutation occurs on chromosome 4 on a gene that codes for the protein huntingtin
o Normally, people have small number of triplet CAG
repeats
o With the diseased, there are a larger number of CAG repeats(stutter).
There is an inverse correlation between number of triplet repeats and onset age of condition.

Question 77

What are transcription factors?

Answer 77

The transcription of a gene is controlled by transcription factors which are proteins that bind to a specific DNA sequence and control the flow of information from DNA to RNA by controlling mRNA formation.

Question 78

What are structural genes?

Answer 78

Structural genes are genes that code for proteins or

enzymes required by a cell.

Question 79

What are regulatory genes?

Answer 79

Regulatory genes are genes that code for proteins that regulate the expression of other genes.

Question 80

What are repressible enzymes?

Answer 80

Repressible enzymes are produced continuously unless production is repressed by binding a repressor protein to the operator.

Question 81

What are inducible enzymes?

Answer 81

Inducible enzymes are only produced when its substrate is present. Transcription of the gene occurs as a result of the inducer (the enzyme’s substrate) interacting with the protein produced by the regulatory gene

Question 82

What is an operon?

Answer 82

An operon is a length of DNA making up a unit of gene expression in a bacterium. It consists of one or more structural genes and also control regions of DNA that are recognised by the products of regulatory genes

Question 83

What is B-galactosidase?

Answer 83

It is an inducible enzyme that hydrolyses the disaccharide lactose to the monosaccharides glucose and galactose.

Question 84

What does the lac operon consist of?

Answer 84

The lac operon consists of promoter, operator and 3
structural genes which are:
o lacZ which codes for 𝛽-galactosidase.
o lacY coding for permease which allows lactose to enter cell
o lacA which codes for transacetylase

Question 85

What is the sequence of events that occurs when there is no lactose in the medium in which the bacterium is growing?

Answer 85

■ the regulatory gene codes for a protein called a repressor
■ the repressor binds to the operator region, close to the gene for β-galactosidase
■ in the presence of bound repressor at the operator, RNA polymerase cannot bind to DNA at the promoter region
■ no transcription of the three structural genes can
take place

Question 86

What is described as being allosteric and why?

Answer 86

The repressor protein is allosteric, meaning it has two
binding sites, one for DNA and another for lactose. When lactose binds to its site, the shape of the protein changes so that the DNA-binding site is closed.

Question 87

What happens when lactose is present in the medium?

Answer 87

■ lactose is taken up by the bacterium
■ lactose binds to the repressor protein, distorting its
shape and preventing it from binding to DNA at the
operator site
■ transcription is no longer inhibited and messenger
RNA is produced from the three structural genes.
The genes have been switched on and are transcribed
together.

Question 88

What are some advantages of the lac operon?

Answer 88

o This mechanism allows the bacterium to produce
β-galactosidase, permease and transacetylase only when lactose is available in the surrounding medium and to produce them in equal amounts.
o It avoids the waste of energy and materials in producing enzymes for taking up and hydrolysing a sugar that the bacterium may never meet. However, the sugar can be hydrolysed when it is available.

Question 89

What happens when both glucose and lactose are found in the medium?

Answer 89

The bacterium uses glucose in preference to other
sugars. When a bacterium finds both glucose and lactose in the medium in which it is growing, it represses the use of lactose by suppressing the lac operon by means of a different transcription control factor.

Question 90

What are some of the effects of transcription factors?

Answer 90

■ General transcription factors are necessary for
transcription to occur. They form part of the protein
complex that binds to the promoter region of the
gene concerned.
■ Other factors activate appropriate genes in sequence, allowing the correct pattern of development of body regions.
■ A transcription factor is responsible for the
determination of sex in mammals.
■ Transcription factors allow responses to environmental stimuli, such as switching on the correct genes to respond to high environmental temperatures.
■ Some transcription factors, including the products
of proto-oncogenes and tumour suppressor genes,
regulate the cell cycle, growth and apoptosis
(programmed cell death)
■ Hormones have their effect through transcription factors.

Question 91

How does the application of gibberellin cause an increase in the transcription of mRNA coding for amylase?

Answer 91

Gibberellin has this effect by causing the breakdown of DELLA proteins. A DELLA protein inhibits the binding of a transcription factor, such as phytochrome interacting protein (PIF), to a gene promoter. By causing the breakdown of the DELLA protein, gibberellin allows PIF to bind to its target promoter. Transcription of the gene can then take place, resulting in an increase in amylase production.

Question 92

What is the role of transcription factors in humans?

Answer 92

The factors may bind to the promoter region of a gene. They may increase or decrease the transcription of the gene. Whatever the mechanism, their role is to make sure that genes are expressed in the correct cell at the correct time and to the correct extent.