6.2 PATTERNS OF INHERITANCE AND VARIATION

Question 1

What is genetic variation?

Answer 1

Genetic variation is the variety of different combinations of alleles in a population.
e.g changes to the genetic code, hair and eye colour.

Question 2

What is environmental variation?

Answer 2

Environmental variation is the differences in individuals that are not inherited but are caused by the environment that the organism lives in.
e.g tattoos, scars.

Question 3

What is continuous variation?

Answer 3

Continuous variation is a characteristic that can take any value within a range. controlled by a number of genes (polygenes).
- can be graphed as a bell curve as individuals fall into a range and not into distinct categories
- often causes by a mixture of genes and environment. for example, genes that make you grow may be inherited but if you don’t eat nutritious food you may not grow as tall as potential

Question 4

What is discontinuous variation?

Answer 4

Discontinuous variation is a characteristic that can only result in certain discrete values.
e.g blood type.
- can be graphed as a bar chart as individuals fall into distinct categories
- usually caused by one or two genes and is not effected by the environment

Question 5

How are genes involved in the production of chlorophyll?

Answer 5

Genes are involved in the production of chlorophyll as most plants are genetically coded to produce large amounts of chlorophyll.

Question 6

What are the symptoms of chlorosis?

Answer 6

Symptoms of chlorosis:
- leaves pale or yellow
- reduced ability to photosynthesise

Question 7

Why does chlorosis in plants occur?

Answer 7

Chlorosis occurs because cells are not producing the normal amount of chlorophyll. Change in phenotype is due to environmental factors as most plants with chlorosis have normal genes coding for chlorophyll.

Question 8

Why is chlorosis not beneficial to the plant?

Answer 8

Chlorosis is not beneficial to the plant as it reduces the ability to photosynthesise due to the lack of chlorophyll.

Question 9

What environmental factors cause chlorosis?

Answer 9

Environmental factors that cause chlorosis:
1. lack of light - in the absence of light plants turn off chlorophyll production to conserve resources
2. mineral deficiencies - e.g lack of Fe or Mg. iron needed as a cofactor by some enzymes that make chlorophyll, magnesium found at the heart of the chlorophyll molecule. if either element is lacking in the soil, the plant cannot make chlorophyll and the leaves turn yellow
3. virus infections - when viruses infect plants they interfere with the metabolism of cells. common symptom is yellowing in the infected tissues as they can no longer support the synthesis of chlorophyll

Question 10

What determines body mass in organisms?

Answer 10

Both genetic and environmental factors determines body mass in organisms.

Question 11

What are the main environmental factors that affect body mass?

Answer 11

Environmental factors that affect body mass:
- amount and quality of food eaten
- quantity of exercise the organism gets
- presence of disease

Question 12

When can obesity be caused by genetics?

Answer 12

Obesity can be caused by genetics by a mutation of chromosome 7 which causes the pattern of fat deposition in the body to be altered. The mutated gene acts in conjunction with other genes that regulate the energy balance in the body.

Question 13

What is codominance?

Answer 13

Codominance occurs when two different alleles occur for a gene - both of which are equally dominant. as a result both alleles of the gene are expressed in the phenotype of the organism if present
. e.g the colour of snapdragon flowers = two equally dominant alleles exist, each of which codes for the colour of the flower
- an allele that codes for red flowers = codes for the production of an enzyme which catalyses the production of red pigment from a colourless precursor
- an allele that codes for white flowers = codes for an altered version of the enzyme which does not catalyse the production of the pigment, therefore the flowers are white
- three colours may be produced
1. red flowers = plant is homozygous for the allele coding for production of red pigment
2. white flowers = plant is homozygous for the allele coding for no pigment production
3. pink flowers = plant is heterozygous. single allele present which codes for red pigmentation produces enough pigment to produce pink flowers
. upper and lowercase letters are not used to represent the alleles. a letter is instead chosen to represent the gene e.g c^R for red flowers and c^W for white

Question 14

What are multiple alleles?

Answer 14

Multiple alleles:
- more than two alleles for a gene
- organism carries only two versions of the gene (one on each of the homologous chromosomes) only two alleles can be present in an individual
. e.g blood group determined by a gene with multiple alles
- immunoglobin gene (gene 1) codes for the production of different antigens present on the surface of red blood cells. 3 alleles for the gene
1. I^A = results in production of antigen A
2. I^B = results in production of antigen B
3. I^O = results in production of neither antigen
- I^A and I^B are codominant whereas I^O is recessive to both other alleles. different combinations of these alleles result in the four blood groups
1. blood group A = I^A/I^A or I^A/I^O
2. blood group B = I^B/I^B or I^B/I^O
3. blood group AB = I^A/I^B
4. blood group O = I^O/I^O

Question 15

How is sex determined in humans?

Answer 15

Determining sex:
- humans have 23 chromosomes of varying size and shape, in 22 of the pairs (autosomes) both members of the pair are the same, but in the 23rd pair (the sex chromosomes) are different
- human female have two x chromosomes, human males have an x and a y chromosome
- x chromosome is large and contains many genes not involved in sexual development
- y chromosome is very small, containing almost no genetic information, but does carry a gene that causes the embryo to develop as male
- sex of offspring determined by whether the sperm fertilising the egg will contain an x or a y chromosome

Question 16

What is sex linkage?

Answer 16

Sex linkage:
- some characteristics are determined by genes carried on the sex chromosomes = genes are sex linked
- as the y chromosome is much smaller than the x chromosome, there are a number of genes in the x chromosome that males have only one copy of
= this means that any characteristic caused by a recessive allele on the section of the x chromosome, which is missing the y, occurs more frequently in males. this is because many females will also have a dominant allele present in their cells

Question 17

What is haemophilia?

Answer 17

Haemophilia:
- sex linked genetic disorder
- blood which blood-clots extremely slowly due to the absence of a protein blood-clotting factor (in the majority of cases this is a factor VIII)
= as a result, injury can cause prolonged bleeding which is potentially fatal if left untreated
- if a male inherits the recessive allele that codes for haemophilia (on their x chromosome) they cannot have a corresponding dominant allele on their y chromosome and so develop the condition
= as a result the vast majority of haemophilia sufferers are male
- females who are heterozygous for the haemophilia coding gene are known as carriers so do not suffer from the disease but can still pass it on to their children
- when showing the inheritance of a sex linked condition the alleles are shown linked to the chromosomes they are found on
= x^H is used to represent the dominant ‘healthy’ allele
= x^h is used to represent the recessive allele coding for haemophilia
= y is used to represent the y chromosome
- if a carrier female and normal male have children, then in theory half the male offspring produced will have the disorder, half the female offspring will be carriers
- an affected male can pass on the faulty allele to daughters, resulting in them becoming carriers

Question 18

What is monohybrid inheritance?

Answer 18

Monohybrid inheritance:
- each character of a diploid organism is controlled by a pair of alleles
- e.g cystic fibrosis (cf)
= faulty allele causing cf is recessive
= normal allele is dominant

Question 19

What is a homeobox gene?

Answer 19

Homeobox genes determines how how an organisms body develops as it grows from a zygote to a complete organism. Determines polarity (head and tail ends of each body segment) and positioning of organs.

Question 20

What is a homeobox sequence?

Answer 20

A homeobox sequence is where each gene contains a homeobox (DNA sequence of about 180 nucleotides) which is highly conserved.

Question 21

What are hox genes?

Answer 21

Hox genes are a class of homeobox genes only found in animals. Every hox gene is a homeobox gene, but not every homeobox gene is a hox gene.
- found in clusters
- mammals have four clusters on different chromosomes
- genes within these clusters are expressed in certain body segments at certain stages of development
- homology between homeobox genes in mice and humans (similarities)
- linked (or lined up) in a sequence which relates to the pattern the genes are expressed in

Question 22

What are homeodomain proteins?

Answer 22

Homeodomain proteins are a product when a gene is transcribed. A transcription factor that switches particular genes on and off in certain body segments at particular stages of the development. The homeodomain is the part of the homeodomain protein that binds to DNA and is coded by the homeobox.

Question 23

What are the effects of homeodomain proteins?

Answer 23

Effects of homeodomains:
activate a developmental gene
- switches on so RNA polymerase can bind (transcription occurs)
repress a developmental gene
- switches off so RNA polymerase cannot bind (transcription does not occur)

Question 24

Why are fruit flies studied?

Answer 24

Fruit flies commonly studied by geneticists:
- genome quite small an well understood
- reproduces and develops rapidly
- each body segment is characterised by the structures it contains

Question 25

How have homeobox genes evolved?

Answer 25

Homeobox genes and evolution:
- homeobox genes are highly conserved (if mutation occurs to them organism wont survive)
- homeobox genes are so fundamental that any mutation will mean that the organism will not survive
- each kingdom has its own set of homeobox genes
- plant homeobox genes are similar to those in other plants but different to those in animals
- homeobox genes haven’t really changed through evolution = importance

Question 26

What is the evidence of homeobox genes?

Answer 26

Evidence for homeobox genes:
- if a mouse eye homeobox gene is inserted into the DNA of a fruit fly, a fruit fly eye will be produced even if it is in the wrong part of the body
- in a normal fly, a homeobox gene called Antp causes legs to develop in the necessary body segments
- in the head, this homeobox gene is turned off so that legs do not develop
- in some mutant flies, Antp is switched on and legs develop where antennae should be
- mice have a similar homeobox gene - if it is put into a fruit fly head it had the same effect as Antp

Question 27

What is thalidomide and how is it used now?

Answer 27

Thalidomide:
- a drug previously used for morning sickness that effects embryo development
- homeobox genes hoxA11 and hoxD11 switch on genes that cause forelimb development
- the drug affected the behaviour of these homeobox genes at a critical stage in development
- it is now used as a cancer drug

Question 28

What is apoptosis?

Answer 28

Apoptosis is programmed and controlled cell death important in controlling the body from and in the removal of damaged or diseased cells.

Question 29

What occurs during apoptosis?

Answer 29

During apoptosis:
1. DNA/nucleus condenses and becomes more tightly packed
2. chromatin condenses and cell envelope breaks down, causing cell shrinkage
3. blebs form and so apoptic bodies may also form
4. nucleus fragments
5. apoptic bodies are engulfed by phagocytes

Question 30

What role does apoptosis play in limb development?

Answer 30

During limb development, apoptosis destroys tissues that aren’t needed.

Question 31

What is morphogenetic apoptosis?

Answer 31

Morphogenetic apoptosis is when apoptosis is used to induce cellular reorganisation in surrounding tissues.

Question 32

What happens if the rate of mitosis does not equal the rate of apoptosis?

Answer 32

If the rate of mitosis does not equal the rate of apoptosis, there will be too many or too little cells so the structure will be different than it should be. can cause cancer.

Question 33

What signals control apoptosis?

Answer 33

Apoptosis is controlled by cell signalling. these signals can be:
- cytokines
- hormones
- growth factors
- nitric oxide; which can induce apoptosis

Question 34

What external factors can affect the expression of regulatory genes?

Answer 34

External factors that can affect the expression of regulatory genes:
- change in temp
- light intensity
- stress
- drugs like thalidomide

Question 35

What internal factors can affect the expression of regulatory genes?

Answer 35

Internal factors that can affect the expression of regulatory genes:
- release of hormones
- psychological stress

Question 36

Why does the process of apoptosis not damage nearby cells?

Answer 36

Apoptosis does not damage nearby cells as it does not release chemical signals.

Question 37

When a cell is underdoing apoptosis, it can release chemicals, what do these stimulate?

Answer 37

Chemicals released by apoptosis stimulate phagocytosis.

Question 38

What is a dihybrid cross?

Answer 38

A dihybrid cross considers the inheritance of 2 genes at the same time
- the 2 genes must be on different chromosomes to allow one pair of alleles to be able to combine freely with another pair of alleles
- each gene considered has 2 or more alleles
- pairs are separated by meiosis into gametes so you have to consider all of the possible gamete combinations
. for example
- plant genes for stem colour
A = purple, a = green
- plant genes for leaf shape
D = jagged, d = smooth
combinations = AaDd, AAdd, aaDD, AaDD, Aadd, AADd, aaDd

Question 39

What is chi squared?

Answer 39

Chi squared is a statistical test that can be used to determine if any difference between the observed and expected results is significant or due to chance.
chi squared = the sum of the observed minus the expected frequencies squared over the expected frequencies

Question 40

What are autosomal chromosomes?

Answer 40

Autosomal chromosomes are non-sex chromosomes (22 pairs)
- each chromosome has more than one gene on it
- genes on the same chromosome are linked together = autosomal linkage

Question 41

How are autosomal genes inherited?

Answer 41

Inheritance of autosomal genes:
- autosomal linked genes are inherited together
- however, crossing over may occur between genes and form recombinant offspring
= these are offspring that have different allele combinations to the gametes
- both of these scenarios will alter the expected ratio of phenotypes in the offspring

Question 42

What can crossing over during meiosis alter?

Answer 42

Crossing over during meiosis can alter the alleles for linked genes
- the closer the genes the less likely it is that they will be separated

Question 43

What are phenotypic ratios in relation to autosomal linkage?

Answer 43

Phenotypic ratios:
- if two genes are autosomally linked, you wont get the phenotypic ratio you expect
- for dihybrid (not linked) you would usually expect a 9:3:3:1 ratio
- if two genes are inherited together due to linkage, it is more like a monohybrid cross 3:1

Question 44

What is recombinant frequency?

Answer 44

Recombinant frequency:
- a measure of the amount of crossing over that has occurred
- 50% < = no linkage of genes
- 50% > = gene linkage
- reduced crossing over also reduces the % frequency
- recombination frequencies can then be used to map genes
recombination frequency = number of recombinant offspring / total number of offspring x 100

Question 45

What factors affect the frequency of alleles in a population and therefore the rate of evolution?

Answer 45

Factors affecting the frequency of alleles and the rate of evolution:
mutations - necessary for existence of alleles in the first place, and formation of new alleles leads to genetic variation
sexual selection - leads to increase in frequency of alleles which code for characteristics that improve mating success
gene flow - immigration and emigration result in changes of allele frequency within a population
genetic drift - occurs in small populations, change in allele frequency due to the random nature of a mutation, appearance of new allele will have greater impact in a smaller population
natural selection - leads to an increase in the number of individuals that have characteristics that improve their chance of survival

Question 46

Why is a larger population better with respect to the gene pool?

Answer 46

In respect to the gene pool, larger populations are better as they ensure lots of genetic diversity due to the presence of many different genes and alleles. This leads to variation within a population which is essential in the process of natural selection.

Question 47

What is a limiting factor in relation to population size?

Answer 47

Limiting factors in relation to population size:
- density dependent factors = competition, predation, parasitism, communicable disease
- density independent factors = climate change, natural disasters, human activities

Question 48

What is the difference between density-dependent and density-independent factors?

Answer 48

Density dependent factors are dependent on population size, density independent factors affect populations of all sizes in the same way.

Question 49

What is stabilising selection?

Answer 49

Stabilising selection is natural selection/survival of the fittest. Norm/average is selected (for positive selection) and extremes are selected (negative selection), therefore results in reduction of the frequency of alleles at the extremes, and an increase in the frequency of ‘average’ alleles
- e.g birth weight of babies

Question 50

What is directional selection?

Answer 50

Directional selection occurs when there is change in the environment and the most common phenotypes is no longer the most advantageous. Organisms that are less common and have more extreme phenotypes are positively selected. The allele frequency then shifts towards the extreme phenotypes and evolution occurs.
- e.g peppered moths changing frequency of lighter and darker colours when pollution is high and low respectively

Question 51

What is disruptive selection?

Answer 51

Disruptive selection is where the extremes are selected for and the norm selected against, opposite to stabilising selection
- e.g Darwin’s finches

Question 52

How do population bottlenecks affect the ability of a population of an endangered species to recover from its plight?

Answer 52

Population bottlenecks affect the ability of a population to recover as populations of endangered species have very low genetic diversity and there are no other groups (at all or in close enough radius) to breed with and attempt to increase genetic diversity, loss of alleles that were advantageous to selection pressures.

Question 53

Why does the lack of genetic diversity in cheetahs increase their susceptibility to disease.

Answer 53

Lack of genetic diversity in cheetahs increases their susceptibility to disease as they are endangered and they all carry the bottlenecked gene of sensitivity to disease so cannot breed it out. Rest of cheetahs are likely to be affected, unlikely to find individuals with advantageous genes.

Question 54

What is a genetic drift?

Answer 54

A genetic drift is random changes in allele frequency that occur in all populations, but are more pronounced by smaller populations.

Question 55

How does genetic drift affect the amount of genetic variation within very small populations?

Answer 55

Genetic drift affects the amount of genetic variation within very small populations as it decreases the amount of genetic variation as in small populations there are fewer individuals and alleles may become lost or fixed

Question 56

What is speciation?

Answer 56

Speciation is the formation of a new species through evolution
- members of a population can be isolated and can no longer breed with the rest
- random mutations mean different characteristics may be selected for under selection pressures
- accumulation of mutations and allele frequencies can eventually lead to organisms not being able to interbreed

Question 57

What is allopatric speciation?

Answer 57

Allopatric speciation is speciation that occurs as a result of a physical barrier between populations.

Question 58

What are geographical barriers involved in speciation?

Answer 58

Geographical barriers:
- separates two populations of a species and they become isolated from each other
- two groups evolve along different lines
= different selection pressures
= genetic drift
- if barrier breaks and two populations come together again, they may have changed so much that they can no longer interbreed
- they are now two different species

Question 59

What is sympatric speciation?

Answer 59

Sympatric speciation is speciation that occurs when there is no physical barrier between populations put the populations no longer interbreed
- e.g blind mole rats live in a small area in northern israel but exist as two separate populations above 2 different types of bedrock (chalk and basalt that support different plants). they only breed with mole rats living in the same type of soil (even though they still come into contact with the other population as there is no physical barrier)

Question 60

What are reproductive barriers involved in speciation?

Answer 60

Reproductive barriers:
- even if species share the same habitat and are reproductively active at the same time, they may not be able to interbreed
- reproductive barriers can prevent formation of a zygote (prezygotic) or reduce viability/fertility (postzygotic)
= different courtship behaviours - not recognising behaviour (pre)
= mechanical problems with mating (pre)
= zygote inviability - forms but ends in miscarriage/still birth (post)
= hybrid sterility - offspring not fertile (post)
= gamete incompatibility - sperm cannot enter egg (pre)

Question 61

What is the Hardy-Weinberg equilibrium?

Answer 61

The Hardy-Weinberg equilibrium states that dominant alleles do not replace recessive ones. This means that if 23% of the population has the genotype AaTTRR in a generation, 23% of that population will also have that genotype.

Question 62

What are the conditions for the Hardy-Weinberg equilibrium?

Answer 62

Conditions:
- a large isolated population, to ensure no statistical flukes
- random mating
- no mutations
- no migration between populations (ie the population remains static)
- no natural selection or selection pressures (ie no genotype is more likely to survive than another)

Question 63

What are the Hardy-Weinberg equations?

Answer 63

Equations:
p + q = 1
where p = frequency of dominant allele and q = frequency of recessive allele
p^2 + 2pq + q^2 = 1
where p^2 = the frequency of the homozygous dominant genotype, 2pq = the frequency of the heterozygous genotype and q^2 = the frequency of the homozygous recessive genotype

Question 64

What is epistasis?

Answer 64

Epistasis is the interaction of genes at different loci
- one gene (epistatic) masks or supresses the expression of another gene locus (hypostatic)
= often in relation to one characteristic
= just an interaction between the 2 loci
= reduces phenotypic variation
- 2 ways in which this can occur

Question 65

What is antagonistic epistatis?

Answer 65

Antagonistic epistasis:
two gene loci work against each other
- recessive epistasis
= 2 recessive alleles at one locus can affect another genes expression
= aa can prevent Bb, BB or bb from expressing their phenotype
- dominant epistasis
= a dominant allele for one gene affects the expression of another gene
= Aa or AA will prevent Bb, BB or bb from expressing their phenotype

Question 66

What is complementary epistasis?

Answer 66

Complementary epistasis:
- one gene may code for an intermediate compound
- the second gene then codes for an enzyme that converts it
gene 1 (A/a) = colourless compound = gene 2 (B/b) = orange pigment
- so gene 1 needs a dominant allele to get a compound made
- and gene 2 needs a dominant allele to make the compound orange
- if neither of the dominant alleles is missing, there will be no colour

Question 67

What is the process of apoptosis?

Answer 67

Apoptosis process:
Programmed cell death is known as apoptosis. Firstly, the fine network of protein filaments and microtubules known as the cytoskeleton , which gives structure to the cell, is broken down and digested by hydrolytic enzymes. The plasma (cell surface) membrane then changes, forming small bulges called ‘blebs’. The cell breaks into membrane-bound fragments that are removed by the process of phagocytosis so that harmful substances are not released into surrounding tissues. Programmed cell death is a controlled process. However, mutation in a gene called p53 can prevent programmed cell death. When this occurs, the rate at which somatic cells are produced by the process of mitosis becomes greater than the rate at which cells die, resulting in the formation of a mass of cells known as a tumour.