Meiosis F215

Meiosis 2

Prophase 2- nucluer envelope breaks down and nucleolus disappears; chromosomes condense; spindle forms.

Metaphase 2- chromosomes randomly assorted line up on equator; attached to spindle fibres at the centromeres.

Anaphase 2- Centromeres divide; chromatids pulled to opposite poles by spindle fibres; chromatids randomly segregate.

Telophase 2- nucluer envelope reforms around haploid daughter nuclei, results in four haploid cells (gametes, half the number of chromosomes)

Allele= alternative form of a gene/different DNA base sequence expressed as a slightly different polypeptide.

Locus= position of a gene on a chromosome

Phenotype= characteristics expressed as a result of interaction between genotype and environment.

Genotype= the genetic constitution of an organism in terms of the alleles it contains

Homozygous= identical alleles at corresponding loci

Heterozygous= dissimilar alleles at corresponding loci

Dominant= same effect on the phenotype if the genotype is homozygous or heterozygous.

Recessive= only expressed if the genotype is homozygous.

Codominant= alleles are codominant if they both contribute to the phenotype, no linear relationship between alleles of a gene and resulting phenotype. 

Linkage= two or more genes located on the same chromosome, therefore more likely to be inherited together unless crossing-over occurs. Sex linkage is when the gene coding for a characteristic is found on a sex chromosome.

Crossing-over= occurs during prophase 1, non-sister chromatids wrap around at points called chiasmata similar sections of non-sister chromatid break at the chiasmata and swap around, producing new combination of alleles. Chiasmata stay in place until separation in anaphase 1. 

Fertilisation- genetic material from two unrelated individuals in combined; leads to a larger mix of alleles.

Meiosis- Reassortment of chromes on the equator in metaphase 1 is random, leads to a different combination of chromosomes in reduced cells; reassortment of chromatids in metaphase 2 on equator is random; crossing over in prophase 1 means genetic material is further reasserted. 

Epistasis- the interaction of different loci so that one masks or suppresses the expression of another gene loci; the genes may be antagonistic or complementary.

Recessive epistasis- the homozygous recessive genotype masks the expression of another gene.

Dominant epistasis- the presence of one or more dominant alleles in the genotype of one gene masks or suppresses the expression of another gene.

Chi-squared test- determines the siniliarity between the observed and expected frequencies; h0 and h1 hypothesis given; chi-squared calculated using given formular (easier to make a table of observed and expected values); the bigger the chi-squared value the more likely the O and E values are different for a reason other than chance; compare chi-squared value to critical value (found by looking at a table with significance level [e.g. 5%] and degrees of freedom [number of categories -1]); if chi-squared< accept H0, no significant difference. 

Discontinous variation= Qualitative differences, categories e.g. blood group.

Continuous variation= Quantitiative differences, numbers, e.g. height.

Discontinuous variation= most characteristics with discontinuous variation are expressed by one gene loci; epistasis is normally the only way two or more gene loci can affect the phenotype; dominant, codominant and recessive patterns of inheritance cause discontinuous variation

continuous variation= influenced by many genes; different alleles at each gene locus has a smaller effect on the monotype [polygenes]; each genes has a small but additive component on the phenotype.  

Environmental conditions affect the expression of phenotypes

e.g. intelligence is partly genetically inherited but a stimulating learning environment is needed for the characteristic to be fully expressed. 

Variation useful in developing characteristics to aid survival; natural selection couldn’t occur without variation in a population

Used to predict allele frequency in a population; for this to be valid there must be: large population, random mating, no immigration, no mutation or natural selection; for two alles:

P= frequency of A

Q= frequency of a

P+Q=1

P^2= frequency of AA genotype

2PQ= frequency of Aa genotype

Q^2= frequency of aa genotype

P^2+2PQ+Q^2=1   <- learn

Will be given one genotype frequency so we can work out others.

Evolution=change in allele frequencies in a population

If the environment changes selection pressures change; the frequencies of alleles in the gene pool begin to change; directional selection is occurring as the population changes in a direction to be better adapted to changing conditions.

Environmental factors limit population growth and allows stabilizing selection to take place; there are biotic factors e.g. pathogen or predator; there are abiotic factors e.g. tempreature or water availability. 

Evolution also occurs by genetic drift alongside natural selection; genetic drift is when chance dictates which alleles are passed on and by chance one allele is passed on more than another in a population; in a large population this is unlikely as chance tends to balance out; has a greater effect on small populations because chance has a greater influence.

Evolution leads to the development of new species [speciation] through several methods; must involve reproductive isolation/barrier between populations of the same species.

Allopatric speciation= Geographical isolation between populations of the same species; must have different environment/selection pressures; different mutations will occur within populations; eventually reproductively incompatible

Sympatric speciation= Disrupts the reproductive mechanisms of two groups within the population; mutations in genitalia may mean mating is no longer possible; this forms a reproductive barrier.

Temporal or habitat isolation may cause populations of the same species to become reproductively isolated; Seasonal mechanisms e.g. individuals in the same population developing different mating seasons. 

difficult to define what a species is, here are two concepts:

Biological species concept- ‘A species is a group of similar organisms that can interbreed and produce fertile offspring and is reproductively isolated from other such groups’ but unsuitable for organisms that produce asexually; can also be difficult to determine reproductive behavior of some organisms, e.g. rare or extinct species.

Phylogenetic species concept- ‘a species is a group of organisms that have similar morphology, physiology, embryology and behaviours and occupy the same ecological niche’; classification usually looks at DNA and how similar the organisms DNA is in respect to how closely related they are meaning they are more likely to be in the same species; however no definite cut-off as to ‘how similar’ DNA sequences need to be in order to be classed as the same species. 

Genetic process is the same; instead of selection pressures selecting individuals with selective advantage humans select the desired characteristics; over many generations this increases the allele frequency for the desired characteristic; breeding is often done artificially e.g. cross pollination in plants or IVF in animals.

Dairy cow- cows selected based on quantity and quality of milk; bulls selected on basis of temperament and health; artificial insemination/embryo transfer may take place; process repeated over many generations.

Bread wheat- the original species was taken and went through normal natural selection to breed individuals with best characteristics; the original species was bred with wild grass and then that offspring bred with goat grass; induced mutations were used to turn the infertile offspring fertile; genetically engineered to improve the following characteristics: resistance to infection, high protein content, stem stiffness, increased yield.