Alkaptonuria is an extremely rare disease. The gene for Alkaptonuria (ALK) has r

Question

Alkaptonuria is an extremely rare disease. The gene for Alkaptonuria (ALK) has recently been shown to lie on human chromosome 9 and to be linked to the gene encoding the ABO blood group, with a recombination frequency of 11% between the loci. A pedigree of a family with the disease is shown below, with affected individuals indicated in black. In addition, the blood type of family members is given. The two alleles at the ALK locus will be denoted ALK+ and ALK-. The three alleles at the ABO blood group locus will be denoted IA, IB (which are co-dominant) and i (which is recessive to IA and IB). (a) What is the genotype of individual 1 at the ALK and ABO loci? (b) What is the genotype of individual 2 at the ALK and ABO loci? (c) What is the genotype of individual 3 at the ALK and ABO loci? Which alleles of each gene are carried on the chromosome he inherited from his father and which alleles are carried on the chromosome he inherited from his mother? (d) Individuals 3 and 4 are expecting their fifth child. A physician draws a prenatal blood sample and determines that the child has blood type B. What is the probability that the child will have alkaptonuria? Explain your answer.

Explanation / Answer

(a) ALK- is recessive because parents are normal . At ABO individual 1 is A type and with O type offspring . At ALK it is either ALK+ or ALK- because offspring is affected. Genotype of individula 1 at the ALK and ABO loci is

IA ALK-

i    ALK+

(b) At ABO locus the individual 2 has recessive phenotype O so it must be i/i. At ALK locus he must be ALK+ / ALK- because the offspring is not affected himself. Genotype is

i   ALK-

i   ALK+

(c) Similarly for individual C the genotype is

i   ALK-

IA ALK+

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