Genetics and Evolution

Mendel's law of independent assortment

Monohybrid crosses with pea plants showed that the two alleles of gene separate into different haploid gametes during meiosis. Called the law of segregation.

Mendel discovered the law of independent assortment by doing crosses in which parents differed in two characteristics that are controlled by two different genes. These are dihybrid crosses,

Ratios:

  • Genotypic ratio is the proportions of the various genotypes produced by the cross.
  • Phenotypic ratio is the proportions of the various phenotypes.

Dihybrid crosses

Predicting ratios of dihybrid crosses

The 9:3:3:1 ratio is often found when parents that are heterozygous for two genes are crossed together. The ratio is the product of two 3:1 ratios. Other ratios are also possible.

Genes - linked and unlinked

Unlinked genes

Genes that assort independently are unlinked genes.

Genes assort independently if they are located on different chromosomes.

Pairing of homologous chromosomes occurs during prophase 1 of meiosis and the alleles of unlinked genes are on different pairs of homologous chromosomes.

A pair of homologous chromosomes is called a bivalent.

Unlinked genes segregate independently as a result of meiosis

Linked genes

Combinations of genes tend to be inherited together. This is called gene linkage.

Linked genes have loci on the same chromosomes

New combinations of the alleles of linked genes can only be produced if DNA is swapped between chromatids. This is called recombination.

Involves crossing over.

Mendel and Morgan

Thomas Morgan developed the idea of linked genes to account for anomalies.

He did this while investigating Drosophila where the inheritance pattern is different in males and females - sex linkage.

He explains that for sex linkage genes were located on sex chromosomes.

Other anomalies where the pattern of inheritance was the same in both males and females but the ratios were non-mendelian could be explained by two genes being located together on the same non sex chromosome (autosome).

Crossing over

Prophase 1 of Meiosis

Each homologous chromosome consists of two sister chromatids because all DNA has been replicated in interphase before the start of meiosis.
Chromatids of the two different chromosomes in a pair are non sister chromatids.
While the chromosomes are paired, sections of chromatid exchange in a process called crossing over

Recombination of linked genes

There is always at least one cross over per bivalent so most of chromatids will have new combination of alleles.

The process of crossing over:

  • At one stage in prophase 1 all the chromatids of two homologous chromosomes become tightly paired up together. This is called synapsis.
  • The DNA molecule of one of the chromatids is cut. A second cut is made at the exactly the same point in the DNA of a non sister chromatid.
  • The DNA of each chromatid is joined up to the DNA of the non sister chromatid. This has an effect of swapping sections of DNA between chromatids.
  • In later stages of prophase 1 the tight pairing of the homologous chromosomes ends, but the sister chromatids remain tightly connected. Where each cross over has occurred there is an X shaped structure called a chiasma.

Continuous variation

Variation can be discrete or continuous.

With discrete variation every individual fits into one of a number of non-overlapping classes. E.g. humans are in a blood group, A,B,AB or O

With continuous variation any level of the characteristic is possible, between the two extremes. E.g. any height is possible in humans between the smallest and the largest height

Discrete variation is usually due to one gene. If continuous variation is genetically determined it is due to the combined effects of two or more genes. Known as polygenic inheritance. The environment can effect polygenic traits.

Speciation

Gene pools

Consists of all the genes and their different alleles in an interbreeding population.

Evolution always involves a change over time in allele frequency in a population's gene pool.

Types of natural selection

Directional

Stabilizing

Disruptive

One extreme in the range of variation is selected for; the other is selected against.

Intermediates are selected for and extremes are selected against

Extreme types are selected for and intermediates are selected against.

Polyploidy in allium:

  • Most allium species have a diploid number that is a multiple of 16.
  • The ancestral allium probably had this number.
  • Allium species with 32 chromosomes evolved by polyploidy.
  • In a species with a diploid number of 16 an individual with 32 chromosomes is tetraploid.
  • Polyploidy is an instant speciation.
  • Tetraploids are re-productively isolated from diploids.

Reproductive isolation

Formation of a new species

Usually involves one population not interbreeding with any other populations of its species becoming reproductively isolated.

Temporal:
When populations of a species breed at different times.

Can be gradual or abrupt. Gradualism or punctuated equilibrium

Behavioural:
When populations of a species have behaviour that prevents interbreeding.

Geographical:
When populations of a species live in different areas and therefore do no interbreed.

Examples: Great tit

Geographically isolated populations often have different allele frequencies from the rest of the species. e.g. cystic fibrosis 0.04 on faroe islands by only 0.03 in northern Europe.