Genetics and evolution: Inheritance

In a dihybrid cross the expected ratio was 9 : 3 : 3 : 1. Of 160 offspring, the observed numbers were 85, 31, 28 and 16. Use the chi-squared test to decide whether the results differ significantly from the expected ratio. The critical value for 3 degrees of freedom at p = 0.05 is 7.815.

Haemophilia is caused by a recessive allele on the X chromosome. A carrier woman and an unaffected man have a child. Use a genetic diagram to find the probability that the child is a son with haemophilia.

ABO blood groups are controlled by multiple alleles: Iᴬ and Iᴮ are codominant and Iᴼ is recessive. A man of genotype IᴬIᴼ and a woman of genotype IᴮIᴼ have children. Give the possible genotypes and phenotypes of the children and their ratio.

In a dihybrid test cross the expected ratio was 1 : 1 : 1 : 1, but the offspring were 45 : 5 : 6 : 44 (data are illustrative, written for this paper). Explain these results.

In mice, gene A is needed for coat colour (A coloured, a albino), and gene B determines colour (B black, b brown). aa mice are albino whatever their B alleles. Two AaBb mice are crossed. Give the expected phenotypic ratio and explain it.