Question

the abo blood type locus has been mapped on chromosome 9. a father who has type ab blood and a mother type o blood have a child with trisomy 9 and type a blood. using this information, can you tell in which parent the nondisjunction occurred?

Explanation:

The father has type AB blood (genotype \(I^A I^B\)) and the mother has type O blood (genotype \(ii\)). The child has type A blood. Since the mother can only contribute an \(i\) allele, the child must have received an \(I^A\) allele from the father. Trisomy 9 means there is an extra chromosome 9. Nondisjunction in meiosis of the father could lead to a gamete with two chromosome 9s (one with \(I^A\) and the other with \(I^B\)) and a normal gamete from the mother (with \(i\)). If the gamete with two chromosome 9s (one \(I^A\) - carrying) fuses with the mother's gamete (\(i\)), the child can have trisomy 9 and type A blood. Nondisjunction in the mother would not explain the presence of \(I^A\) allele as she only has \(i\) alleles.

Answer:

The nondisjunction occurred in the father.