Meiosis and Variation

Question 1

Describe, with the aid of diagrams and photographs, the behaviour of chromosomes during meiosis, and the associated behaviour of the nuclear envelope, cell membrane and centrioles. (Names of the main stages are expected, but not the subdivisions of prophase)

Answer 1

Meiosis I

• *Prophase I**
  1. The chromatin condenses and supercoils so that chromosomes shorten and thicken.
  2. The chromosomes come together in their homologous pairs to form a bivalent. (Each member of the pair has the same genes at the same loci, but not necessarily the same alleles. Each pair consists of one maternal and one paternal chromosome)
  3. The non-sister chromatids wrap around each other and attach at chiasmata
  4. They may cross over and swap sections of chromatids with each other ‘crossing over’
  5. The nucleolus disappears and the nuclear envelope breaks down
  6. A spindle forms (made of protein microtubules)
• *Metaphase I**
  1. Bivalents line up along the equator of the spindle in a random order ‘random assortment’, attached to spindle fibres by centromeres
  2. The homologous pairs that make up each bivalent are still attached by chiasmata
• *Anaphase I**
  1. Spindle fibres contract; homologous chromosomes of each bivalent are pulled to opposite poles
  2. The centromeres do not divide/break
  3. The chiasmata separate; the crossed over lengths of chromatid remain with the new chromatid to which they’ve become newly attached
• *Telophase I**
  1. In (most) animal cells two new nuclear envelopes form- one around each set of chromosomes at each pole and the cell divides by cytokinesis. There is a brief interphase (replication) and the chromosomes uncoil.
  2. In most plant cells the cell goes straight from Anaphase I to Meiosis II

Meiosis II
This occurs in a plane at right angles to Meiosis I
Prophase II
1. If a nuclear envelope has reformed, it breaks down again
2. The nucleolus disappears, chromosomes condense and new spindle fibres form at right angles to the previous spindle axis

• *Metaphase II**
  1. The chromosomes line up along the equator of the spindle in a random order, attached to spindle fibres at centromeres.
• *Anaphase II**
  1. Spindle fibres contract, centromeres divide (as a result), randomly separating the chromatids by pulling them to opposite poles by the spindle fibres.
• *Telophase II**
  1. Nuclear envelopes reform around the haploid daughter nuclei
  2. Cytokinesis (division) occurs again:
• In animals, the two cells now divide to give four daughter cells
• In plants, a tetrad of four haploid cells is formed

Question 2

Explain the term ‘allele’

Answer 2

An alternative version of a gene; one of multiple versions of a gene, which codes for a slightly different polypeptide than a different allele of the same gene at the same locus. (e.g. the gene for eye colour has an allele for blue eye colour and an allele for brown.)

Question 3

Explain the term ‘locus’

Answer 3

Specific position of a specific gene on a chromosome.

Question 4

Explain the term ‘phenotype’

Answer 4

Observable characteristics of an organism which are determined by the genotype.

Question 5

Explain the term ‘genotype’

Answer 5

The genetic makeup of an organism in terms of the alleles it contains for specific genes (for a particular trait/characteristic) in the genome.

Question 6

Explain the term ‘dominant’

Answer 6

Referring to an allele that is always expressed (the characteristic) in the phenotype even if a different allele for the same gene is present (heterozygous genotypes)

Question 7

Explain the term ‘codominant’

Answer 7

A characteristic where both alleles contribute to the phenotype; where two, non-identical alleles are both expressed in the phenotype of a heterozygote.

Question 8

Explain the term ‘recessive’

Answer 8

Referring to an allele that is only expressed in the phenotype in the presence of another, identical allele for the same gene; when there is no dominant allele present.

Question 9

Explain the term ‘linkage’

Answer 9

Genes for different characteristics that are present at different loci on the same chromosome are linked; genes whose loci are closer together on the same chromosome to are more likely to be inherited together during meiosis since there is a smaller distance for a chiasma to form between them.

Question 10

Explain the term ‘crossing-over’

Answer 10

Where non-sister chromatids exchange alleles at chiasmata during prophase I of meiosis.

Question 11

Explain how meiosis and fertilisation can lead to variation through the independent assortment of alleles

Answer 11

Meiosis:
• Crossing over of chromatids ‘shuffles’ alleles; non-sister chromatids swap equivalent portions of chromatids, giving new combinations of alleles.
• Independent assortment:

 Random distribution and subsequent segregation of maternal and paternal chromosomes in the homologous pairs during meiosis I leads to genetic reassortment (50:50 chance which way round a pair of homologous chromosomes [a bivalent] will be placed on the equator of the cell)
 Random distribution and segregation of the chromatids at meiosis II leads to genetic reassortment

• Random mutations

Fertilisation:
• Randomly combining two sets of chromosomes, one from each of two genetically unrelated individuals; any egg can fuse with any sperm; produces mixture of alleles.

Question 12

Use genetic diagrams to solve problems involving sex linkage and codominance.

Answer 12

• *Sex linkage**; A characteristic or phenotype whose gene is found on one of the sex chromosomes; females need two copies of the recessive allele for the characteristic to be expressed; males only one; males only have one X chromosome, one copy, expressed always even if recessive.
• *Codominace**; two non-identical alleles that are both expressed in the phenotype of a heterozygous organism.

Recessive alleles are normally shown in lower case letters; dominant upper case.

Question 13

Describe the interactions between loci (epistasis). (Production of genetic diagrams is not required)

Answer 13

Epistasis is the interaction of different gene loci so that one gene locus masks/suppresses the expression of another gene locus. The epistatic gene at one locus alters or inhibits the expression of a second locus, the hypostatic gene. Protein product of former influences/controls the expression of the latter.

Recessive Epistasis
The homozygous presence of a recessive allele prevents the expression of another allele at a second locus. A 12:3:1 ratio is typical of this.
E.g. flower colour in Salvia:
The alleles for purple (B) and pink (b) can only be expressed in the presence of the allele A. When the genotype is aa—the phenotype is white

Dominant Epistasis
A dominant allele at one gene locus masks the expression of alleles at the second gene locus.
A 9:3:4 ratio is typical of this.
E.g. feather colour in poultry:
If the dominant allele A is present, the chickens will be white; even if the dominant allele of the second gene, B/b is present.
The genotype must be aaB- for any colour to be expressed

Question 14

Use the chi-squared (χ2) test to test the significance of the difference between observed and expected results. (The formula for the chi-squared test will be provided).

Answer 14

O is observed
E is expected
The smaller the value of χ2, the more certain we can be that that difference between observed and expected data is due to chance and is therefore not a significant difference.
To calculate how significant the χ2 value is, a χ2 table is used. Using n-1 (where n= number of classes) degrees of freedom, and a 5% critical value, we can see if the value is due to chance.
If the value is smaller than the value on the table, the null hypothesis can be accepted- any difference is due to chance and therefore not significant. If the value is larger than the value on the table then the null hypothesis is rejected- any difference is significant and not due to chance.

Question 15

Explain the basis of continuous and discontinuous variation by reference to the number of genes which influence the variation

Answer 15

Discontinuous variation; qualitative differences that fall into clearly distinguishable categories; there are no intermediates, each individual falls into only one of these categories. E.g. gender (sex), blood group.

• Different alleles at a single gene locus have large effects on the phenotype. (normally only one gene contributes to the phenotype; if multiple genes are involved, they are interacting in an epistatic way)
• Different gene loci have different effects on the trait

Continuous variation; quantitative differences where there are no distinguishable categories; instead there are a range of values (phenotypes) between two extremes. E.g. human height, plant mass.

• Different alleles at a single locus have a small effect on the phenotype because there are a large number of different genes contributing, with each providing just a small additive component to the phenotype/trait
• A large number of gene loci may have a combined effect on the trait

Question 16

Explain that both genotype and environment contribute to phenotypic variation. (No calculations of heritability will be expected)

Answer 16

While an organism may have the genetic potential to achieve a certain characteristic, e.g. length of corn cob, the environment also has an influence. The corn cob may have the genetic potential to be
12cm long, but the plant may be short of water, light or certain minerals (limiting factors) meaning that the cob is shorter, as the environmental factors have limited the expression of the genes, thus the phenotype.
Environmental factors may allow full genetic potential to be reached, or may restrict in some way.

• Inherited characteristics that show continuous variation are usually influenced by many genes; polygenic.
  E.g. human skin colour; many different shades of colour.
• Inherited characteristics that show discontinuous variation are usually influenced by only one gene (or a small number), e.g. violet flower colour (it is either coloured or white), is controlled by only one gene; monogenic.

Question 17

Explain why variation is essential in selection

Answer 17

So that when the environment changes, there will be individuals that are better adapted with advantageous characteristics (beneficial alleles), so they will survive and reproduce passing on the advantageous alleles to their offspring, allowing the species to continue and thrive, in turn surviving to pass on their genes.

Question 18

Use the Hardy–Weinberg principle to calculate allele frequencies in populations;

Answer 18

Allele frequency: how often an allele occurs in a population. (allele frequency changes over time; evolution BITCH)

• p - the frequency of the dominant allele A
• q - the frequency of the recessive allele a
• ∴ p + q = 1 as everyone in the population has the alleles (the sum of the allele frequencies always equals one) allele frequency
• q2 - the frequency of the homozygous recessive genotype aa
• p2 - the frequency of the homozygous dominant genotype AA
• 2pq - the frequency of the heterozygous genotype Aa
• ∴ p2 + 2pq + q2 = 1 as everyone in the population has one of the genotypes (the sum of genotype frequencies always equals one) genotype frequency

E.g.
Suppose that the incidence of the aa genotype is 1%.
• Then q2= 0.01 and q = √0.01 = 0.1
• P + 0.1=1
• ∴ p= 0.9
• So, p2 = 0.92 = 0.81 (81% of the population are homozygous AA)
• 0.81 + 2pq + 0.01 = 1
• So 2pq = 1 - (0.01 + 0.81)
• = 0.18
• So, 18% of the population are heterozygous Aa.

Question 19

Explain, with examples, how environmental factors can act as stabilising or evolutionary forces of natural selection

Answer 19

In unchanging conditions, stabilising selection maintains existing adaptations and so maintains existing allele frequencies; the allele and genotype frequency stays the same because the selection pressure is on the extreme phenotypes.

In changing conditions, directional selection alters allele frequencies.

A mutation may be disadvantageous in existing conditions, and so is removed in stabilising selection, but if the conditions change, the mutation might be advantageous and selected for, meaning that selection becomes an evolutionary force (a factor that brings about a change in allele and genotype frequency).

Question 20

Explain how genetic drift can cause large changes in small populations

Answer 20

• Genetic drift is a change in allele frequency that occurs by chance because only some of the organisms in each generation reproduce.
• It is particularly noticeable when a small number of individuals are separated from the rest of the large population.
• They form a small sample of the original population and so are unlikely to be representative of the large population’s gene pool.
• Genetic drift in the small population will alter the allele frequency still further.

‘A change in allele frequency in a population that can lead to an allele being eliminated from the population. It is normally caused by a small or decreasing population size, since in smaller populations; fluctuations in allele frequency are greater. Genetic drift reduces genetic variation and may reduce the ability of a population to survive in a new environment with new selection pressures.’

Question 21

Explain the role of isolating mechanisms in the evolution of new species, with reference to ecological
(geographic), seasonal (temporal) and reproductive mechanisms;

Answer 21

If two sub-populations are separated from each other, they will evolve differently as they have different selection pressures, so different alleles will be eliminated or increased within each sub population. Eventually the sub populations will not be able to interbreed and so will be different species; speciation (development of a new species).

The sub populations may be split by various isolating mechanisms:

Geographical isolation e.g. rivers or mountains
This happens when a physical barrier divides a population of a species; floods, volcanic eruptions, earthquakes; all cause barriers that isolate some individuals from the main population. Mutations take place independently in each population, also changing allele frequencies, as well as the above.

Seasonal barriers e.g. climate change throughout the year

Reproductive mechanisms e.g. their genitals, breeding seasons or courtship rituals may be different
Reproductive isolation occurs as the changes in alleles and phenotypes of the two populations prevent them from successfully breeding together. These changes include:

1. Seasonal changes; individuals from same population develop different flowering/mating seasons, or become sexually active at different times of the year.
2. Mechanical changes; changes in genitalia.
3. Behavioural changes; a group of individuals develop courtship rituals that aren’t attractive to the main population.

Question 22

Explain the significance of the various concepts of the species, with reference to the biological species
concept and the phylogenetic (cladistic/evolutionary) species concept.

Answer 22

The biological species concept
A species is ‘a group of similar organisms that can interbreed and produce fertile offspring and are reproductively isolated from such other groups’; a biospecies.
But:
• Excludes organisms that reproduce asexually
• Species may be extinct (only found in fossils; palaeospecies); can’t study their reproductive behaviour
• Members of the same species can look very different appearances/behavioural patterns
• Males can look different to females
• Fertilisation may fail, zygote may fail, hybrids may infertile
• Isolated populations may appear to be very different from each other
• Single specimens excluded
• Practical/ethical issues; humans + chimps are classed as same species, for example.
• Ring species; individuals from opposite extremes may not interbreed

The phylogenetic species concept
A species is ‘a group of organisms that have similar morphology, physiology, embryology and behaviour, and occupy the same ecological niche’. They show a close similarity in a number of characteristics. This classification shows the evolutionary relationships, or phylogeny. The phylogenetic linage is called a clade (A monophyletic group of organisms with haplotypes [a particular series of base sequences] which are more similar to each other than anyone else.)

Question 23

Compare and contrast natural selection and artificial selection.

Answer 23

Natural selection
The organisms best adapted for their environment are more likely to survive and reproduce to pass on the alleles for the favourable characteristics to their offspring

Artificial selection
A mechanism for evolution where organisms with useful characteristics are selected by humans and allowed to breed, while organisms without these characteristics aren’t allowed to breed.

Similarities:

• Both change the allele frequencies in the next generation; the alleles that code for the beneficial/desirable characteristic will become more common in the next generation.
• Both may make use of random mutation when they occur – if a random mutation produces an allele that gives a beneficial/desirable phenotype, it will be selected for in the next generation.

Differences:

• In natural selection, the organisms that reproduce are selected by the environment, but the selection in artificial selection is carried out by humans. (selective agent)
• Artificial selection aims for a predetermined result, e.g. a farmer aims for a higher yield of milk, but the result is unpredictable in natural selection.
• Natural selection makes the species better adapted to the environment; artificial selection makes the species more useful for humans.
• Only a single trait is selected for artificial selection, but many different traits contributing to the fitness of an organism are selected in natural selection.

Question 24

Describe how artificial selection has been used to produce the modern dairy cow and to produce bread wheat (Triticum aestivum)

Answer 24

Dairy cow

• Each cow’s milk yield is measured and recorded (performance testing)
• The progeny of bulls is tested to find out which bulls have produced daughters with high milk yields
• Only a few good-quality bulls need to be kept are the semen from one bull can be used to artificially inseminate many cows.
• Some elite cows are given hormones so they produce many eggs
• The eggs are fertilized in vitro and the embryos are implanted into surrogate mothers
• These embryos could also be clones and divided into many more identical embryos

Bread wheat

Wheat can undergo polyploidy (chance doubling of the chromosome number) - the nuclei can contain more than one diploid set of chromosomes. Modern bread wheat is hexaploid, having 42 chromosomes in the nucleus of each cell, meaning that the cells are bigger. Wheat is bred for high yield, resistance to fungal infection, high protein content, straw (stem) stiffness and resistance to lodging. (stems bending in wind/rain)