Genetic crosses and inheritanceEdexcel A-Level Biology B: Mind map

Exam questions on Genetic crosses and inheritance

1. A mother with blood group A and a father with blood group B have a first child with blood group O. The ABO blood group gene has three alleles: $I^{A}$ and $I^{B}$, which are codominant, and $I^{O}$, which is recessive to both.
   Explain how two parents with blood groups A and B can have a child with blood group O.2 marks
2. In pea plants, round seeds (R) are dominant to wrinkled seeds (r) and yellow seeds (Y) are dominant to green seeds (y). The two genes are on different chromosomes. A plant heterozygous for both genes has the genotype RrYy.
   State the genotypes of the four types of gamete made by a plant of genotype RrYy and explain why they are produced in equal numbers.2 marks
3. Haemophilia A is caused by a recessive allele, $X^{h}$, on the X chromosome. The dominant allele, $X^{H}$, gives normal blood clotting. A woman who is a carrier of haemophilia ($X^{H}X^{h}$) has a child with a man who does not have haemophilia.
   Construct a genetic cross to show the genotypes of the possible children and state the probability that the next child will have haemophilia.3 marks