Topic 10: HL Genetics and evolution Flashcards
Mendel’s law of independent assortment
Law of segregation - Mendel’s monohybrid crosses with pea plants showed that the two alleles of a gene separate into different haploid gametes during meiosis.
Law of independent assortment - dihybrid crosses with parents who differed in two characteristics controlled by two different genes - inheriting the allele of one gene does not affect the chances of inheriting the allele of another different gene (only if genes are unlinked).
Ratios in genetic crosses
Genotypic ratio - the proportions of the various genotypes produced by the cross.
Phenotypic ratio - the proportions of the various phenotypes.
Independent assortment in dihybrid cross
- Only in case of autosomal, unlinked genes.
- The 9:3:3:1 ratio shows that the four types of gametes are all equally common - the inheritance of the two genes is independent.
- The presence of an allele of one of the genes in a gamete has no influence over which allele of the other gene is present in the gamete - law of independent assortment.
Predicting ratios in 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 - each of the two genes would give a 3:1 ratio in a monohybrid cross between two heterozygous parents.
- In a dihybrid cross, they follow the law of independent assortment as unlinked.
- Dihybrid crosses give other ratios if: either of the genes has co-dominant alleles, either of the parents is homozygous, either of the genes is not autosomal (sex-linked), there is an interaction between genes.
Unlinked genes
- Genes that assort independently are unlinked genes (on different chromosomes).
- Independent assortment of unlinked genes can be explained in terms of chromosome movements during meiosis - pairing of homologous chromosomes during Prophase I - alleles of unlinked genes are on different pairs of homologous chromosomes (bivalents).
- Bivalents are orientated randomly during Metaphase I on the equator - orientation of one bivalent does not affect the orientation of the other bivalents - the pole to which one bivalent moves in Anaphase I does not affect the pole to which another bivalent moves.
Linked genes
- Some pairs of genes do not follow the law of independent assortment and expected ratios for unlinked genes are not found.
- Combinations of genes tend to be inherited together - gene linkage.
- Some pairs of genes are located on the same type of chromosome in the same loci.
- New combinations of the alleles of linked genes can only be produced if DNA is swapped between chromatids - recombination involves crossing over in Prophase I.
- Individuals that have different characteristics to their parents due to crossing over are called recombinants.
Mendel and Morgan
- Mendel performed careful dihybrid crosses and developed a theory that explained all his results -the law of independent assortment.
- In the 20th Century results were found that did not fit his theory as got different ratios.
- Morgan proposed the idea of linked genes after his experiments with fruit flies - inheritance pattern was different in males and females. Genes located on the sex chromosomes.
- Also, anomalies where the pattern was the same in females and in males could be explained by gene linkage on autosomal non-sex chromosomes.
Prophase I of meiosis
- Homologous chromosomes pair up in prophase I of meiosis.
- Each homologous chromosome consists of two sister chromatids as all DNA was replicated before meiosis in interphase.
- Chromatids in the pair are non-sister chromatids.
- While the chromosomes are paired in prophase I, sections of chromatid are exchanged in a process called crossing-over.
Recombination of linked genes
- Without crossing over it would be impossible to produce new combinations of linked genes.
- Homologous chromosomes separate in meiosis I and sister chromatids separate in meiosis II - each of the four haploid cells produced by meiosis receives one chromatid from each bivalent.
- Always at least one cross-over per bivalent.
- The point where crossing-over occurs is random - meiosis produces an almost indefinite amount of genetic variety.
Chiasmata and crossing over
- Crossing-over - the exchange of DNA material between non-sister homologous chromatids.
- At one stage in Prophase I all of the chromatids of two homologous chromosomes become tightly paired up - synapsis.
- The DNA molecule of one of the chromatids is cut and a second cut is made exactly at the same point in DNA of a non-sister chromatid.
- The DNA of each chromatid is joined up to the DNA of the non-sister chromatid. Swapping sections of DNA between the chromatids occurs.
- In the later stages of prophase I, 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.
Chi-squared tests in genetics
Significant difference between the expected and observed results.
Method:
1. Draw a table of observed frequencies.
2. Calculate the expected frequencies based on the Mendelian ratio and the total nr of offspring.
3. Determine the nr of df which is one less than the total nr of possible phenotypes. Dihybrid cross - df is 3.
4. Find the critical region using a table of chi-squared values and a statistical level of 5% (0.05). Critical region is any value larger than the value in the table.
5. Calculate the chi-squared using: (obs-exp)squared/exp
6. Compare the calculated value of chi-squared with the critical region. If the calc value is less than the critical region, the differences are statistically significant - the results do not fit the mendelian ratio (linked genes perhaps).
Continuous variation
Variation - can be discrete or continuous.
- Discrete variation: usually due to one gene, every individual fits into one of a number of non-overlapping classes (blood groups…).
- Continuous variation: due to the combined effects of two or more genes (polygenic inheritance), any level of the characteristic is possible between the two extremes (height…). Height is a polygenic trait as influenced by genes and environment like nutrition, just like skin colour is by the amount of light received.
Gene pools
- Gene pool - all of the genes and their different alleles in an interbreeding population.
Many genes have different alleles - in a typical interbreeding population, some alleles are more common than others.
Evolution always involves a change over time in allele frequency in a population’s gene pool.
Differences in allele frequency
- Allele frequency - the number of that allele there is in a population divided by the total number of alleles of the gene.
- Can range from 0.0 to 1.0 (1.0 is 100%).
- Geographically isolated populations often have different allele frequencies from the rest of the species - cystic fibrosis allele F508 has a frequency of 0.04 on the Faroe islands but only 0.03 in northern Europe and below 0.01 in most other parts of the world.
- Differences in allele frequency may be due to either differences in natural selection or to a random drift.
Types of natural selection
3 types of natural selection:
- Directional - one extreme is chosen for and the other extreme is selected against. For example - Parus major have a greater breeding success if they breed early.
- Stabilising - intermediates are selected for and extremes are selected against. For example - Parus major have a greater survival rate if intermediate clutch sizes.
- Disruptive - extremes are selected for and intermediates are selected against. For example - in Passerina amoena birds with the dullest and brightest plumage are more successful at finding a mate than intermediate plumage ones.
