Dihybrid inheritance, multiple alleles and sex linkageAQA A-Level Biology: Mind map

Exam questions on Dihybrid inheritance, multiple alleles and sex linkage

1. In a species of garden pea, round seeds are produced by the dominant allele $R$ and wrinkled seeds by the recessive allele $r$. Yellow seeds are produced by the dominant allele $Y$ and green seeds by the recessive allele $y$. The two genes are on different chromosomes. A plant breeder crosses plants that are heterozygous for both genes ($RrYy$).
   An $RrYy$ plant is crossed with a plant that has wrinkled green seeds. State the expected phenotypic ratio in the offspring and explain how it arises.2 marks
2. The ABO blood group in humans is controlled by one gene, $I$, with three alleles. The alleles $I^A$ and $I^B$ are codominant, and the allele $I^O$ is recessive to both. A woman with blood group A (genotype $I^AI^O$) and a man with blood group B (genotype $I^BI^O$) have children. In a separate paternity enquiry, a woman of blood group A has a child of blood group O.
   A man with blood group AB is named as the father of the child of blood group O. Explain why he cannot be the father.2 marks
3. Haemophilia A is a blood-clotting disorder caused by a recessive allele, $h$, on the X chromosome. The dominant allele, $H$, codes for a working clotting factor. There is no corresponding gene on the Y chromosome. A woman who is a carrier and a man with normal clotting plan to have children.
   Explain why haemophilia is much more common in males than in females.3 marks