- Dihybrid cross is used to show the inheritance of two different genes which may be located on different pairs of homologous chromosomes but control one phenotype --> each gene could have two or more alleles
- Homozygous parents produce all heterozygous offspring
- Heterozygous parents produce 9:3:3:1 offspring (usually)
- Expected ratio different to actual because of random fertilisation and crossing over doesn't always occur
Autosomal linkage
- Autosomal linkage = linked genes are found on pairs of chromosomes that aren't X or Y
- Linked genes are inherited as one unit, there is no independent assortment during meiosis (unless the genes are separated by chiasmata)
- The closer the genes are on the chromosome the less likely they are to be separated in crossing over and the fewer recombinant offspring (different combinations of alleles than either parent)
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Recombination frequency = measure of the amount of crossing over that has happened in meiosis
- Recombination frequency = number of recombinant offspring / total number of offspring
- Recombination frequency of 50% means there is no linkage and the genes are on separate chromosomes
- Less than 50% means that there is linkage and independent assortment had been hindered
- Degree of crossing over reduces = recombination frequency reduces <-- determined by how close the genes are on the chromosome
Chi-squared test
- Observed ratios will be different to expected --> due to chance
- Number of observations made determines how chance will effect the results
- Important to compare observed and expected to see if the differences are due to chance or if there's another reason
- Use chi-squared to measure the size of the difference between the expected and observed results and see if they're significant or not
- If chi-squared result is GREATER than critical value, ACCEPT null hypothesis
- If chi-sqaured result is FEWER than the critical value, REJECT null hypothesis
Epistasis
- Epistasis = interaction of genes at different loci
- Gene regulation is a form of epistasis with regulatory genes and TFs controlling the activity of structural genes
- One gene's expression can be "masked" by the lack of or expression of another gene
- Baldness masks the gene for widow's peak
Dominant and recessive epistasis
- Recessive epistasis = recessive alleles at one locus mask the phenotypic expression of another gene locus
- Dominant epistasis = dominant allele masks the effects of either allele on the second gene
