From this information, can we know in which parent the nondisjunction began?

Suppose that a pregnant woman shows up to a clinic to get her fetus evaluated for chromosomal disorders. The tests revealed trisomy 9, and blood type A. The father is type AB and mother is type O (ABO blood type has been mapped on chromosome 9).

1 Answer
Oct 19, 2016

Answer:

Nondisjunction occurs generally during female gamete formation.

Explanation:

Nondisjunction means nonseparation: of either homologous chromosome in meiosis I or of sister chromatids in meiosis II.
In both the cases abnormal gametes will be formed and after fertilisation, there will be trisomy or monosomy of chromosome.

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In the above case if non-disjunction of homologous chromosomes hapened in father then both alleles A and B would have entered single gamete (father with AB blood group means one chromosome 9 has A allele while the other homologous copy of chromosome 9 has B allele). That would have resulted in detection of AB blood type in the fetus; but it has A blood type.

From mother, the baby can receive only recessive alleles, even if there is nondisjunction because the mother is of blood type O.

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