Control,Genomes and environment(F215)-Meiosis and Variation

1. The chromatin condenses and supercoils
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. Each pair consists
of one maternal and one paternal chromosome
3. The non sister chromatids wrap around each other and attach at points called
chiasmata
4. They may cross over and swap sections of chromatids with each other
5. The nucleolus disappears and the nuclear envelope breaks down
6. A spindle forms

Metaphase I
1. Bivalents line up across the equator of the spindle, attached to spindle fibres
at the centromeres
2. The bivalents are arranged randomly (random assortment) with each member
of the homologous pair facing opposite poles

Anaphase I
1. The homologous chromosomes in each bivalent are pulled by the spindle
fibres to opposite poles
2. The centromeres do not divide
3. The chiasmata separate and the lengths of chromatid that have been crossed
over remain with the chromatid to which they have 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 cytokenesis. There is a brief
interphase and the chromosomes uncoil
2. In most plant cells the cell goes straight from Anaphase I to Meiosis II

Meiosis II:
This occurs 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 spindles form
Metaphase II:
1. The chromosomes arrange themselves on the equator of the spindle. They are
attached to spindle fibres at the centromeres
2. The chromatids of each chromosome are randomly assorted
Anaphase II:
1. The centromeres divide and the chromatids are pulled to opposite poles by
the spindle fibres. The chromatids randomly segregate
Telophase II:
1. Nuclear envelopes reform around the haploid daughter nuclei
2. In animals, the two cells now divide to give four daughter cells
3. In plants, a tetrad of four haploid cells if formed

• Allele-An alternative version of a gene
• Locus-Specific position on a chromosome, occupied by a specific gene
• Phenotype-Observable characteristics of an organism
• Genotype-Alleles present within cells of an individual, for a particular trait/characteristic
• Dominant-Characteristic in which the allele responsible is expressed in the phenotype,even in those with heterozygous genotypes
• Codominant-A characteristic where both alleles contribute to the phenotype
• Recessive-Characteristic in which the allele responsible is only expressed in the phenotype is there is no dominant allele present

• Linkage-Genes for different characteristics that are present at different loci on the same chromosome are linked
•  
• Crossing-over-Where non-sister chromatids exchange alleles during prophase I of meiosis.
• Sex Linkage- A characteristic is sex-linked if the gene that codes for it is found on one of the sex (X and Y) chromosome

• Meiosis:
  Crossing over ‘shuffles’ alleles
• Random distribution and subsequent segregation of maternal and paternal chromosomes
  in the homologous pairs during meiosis I leads to genetic reassortment
• 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

Epistasis is the interaction of different gene loci so that one gene locus makes or suppresses the expression of another gene locus.
Recessive Epistasis:
The homozygous presence of a recessive allele prevents the expression of another
allele at a second locus
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
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

Recessive epistasis in Salvia
9:3:4 or 9:3:3:1
Dominant epistasis in Poultry
13:3

• Discontinuous variation describes qualitative differences between phenotypes- they fall into clearly
  distinguishable categories with no intermediates.
  E.g. blood type is either A, B, AB or O
• Continuous variation is quantitative differences between phenotypes- there is a wide range of
  variation within the population with no distinct catagories
  E.g. height

• Discontinuous variation
  Different alleles at a single gene locus have large effects on the phenotype.
  Different gene loci have different effects on the trait
• Continuous variation
  Different alleles at the same gene locus have small effects
  Different gene loci have the same, often additive effect on the trait
  A large number of gene loci may have a combined effect on the trait

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, meaning that the cob is
shorter, as the environmental factors have limited the expression of the genes.

So that when the environment changes, there will be individuals that are better adapted, so they will survive and reproduce passing on the advantageous alleles to their offspring and allowing the species to continue

p is the frequency of the dominant allele A
q is the frequency of the recessive allele a
∴ p+q = 1 as everyone in the population has the alleles
q2 is the frequency of the homozygous recessive genotype aa
p2 is the frequency of the homozygous dominiant genotype AA
2pq is the frequency of the heterozygous genotype Aa
∴ 1= p2+2pq+q2 as everyone in the population has one of the genotypes
The frequency of aa can be measured. Suppose that the incidence is 1%.
If q2= 0.01 then q = √0.01 = 0.1
p+0.1=1
∴ p= 0.9
So, p2 = 0.92= 0.81
0.81+2pq+0.01 = 1
So 2pq = 1- (0.01 + 0.81)
= 0.18
So, 18% of the population are heterozygous Aa

In unchanging conditions, stabilising selection maintains existing adaptations and so maintains existing allele frequencies.
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

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 ofindividuals 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 will alter the allele frequency still further.

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.
The sub populations may be split by various isolating mechanisms:
Geographical barriers e.g. rivers or mountains
Seasonal barriers e.g. climate change throughout the year
Reproductive mechanisms e.g. their genitals, breeding seasons or courtship rituals may be different

• The biological species concept:
  A species is ‘a group of similar organisms that can interbreed and produce fertile offspring and it reproductively isolated from such other groups’.

But not all organisms reproduce sexually
Members of the same species can look very different to each other
Males can look different to females
Isolated populations may appear to be very different from each other

• 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’. This classification shows the evolutionary relationships, or phylogeny. The phylogenetic linage is called a clade.
• 

• Natural selection
  The organisms best adapted for their environment are more likely to survive and pass on the favourable characteristics to their offspring
• Artificial selection
  Humans select the organisms with the useful characteristics
  Humans allow those with useful characteristics to breed and prevent the ones without the characteristics from breeding
  Thus, humans have a significant impact on the evolution of these populations or species

Dairy cow
Each cow’s milk yield is measured and recorded
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.

Wheat can undergo polyplody- 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.