A child with trisomy 18 has the genotype AAA for one of the

A child with trisomy 18 has the genotype AAA for one of the genes on chromosome 18. The parents\' genotypes are AA (mom) and Aa (dad). Consider the following statement: \"Nondisjuction must have occurred in mom.\" Is this statement correct or incorrect? Support your answer with an explanation.

Solution

During meiosis, the homologous chromosomes align during the prophase. These chromosomes crossover at some point and rejoin to form recombinant chromatids, and move towards the opposite poles during anaphase.

Sometimes, error occurs in meiosis during the meiosis – I or II, i.e. during the separation of homologous chromosomes. These errors result in excess or less chromosomes in the daughter cell, and this is called “nondisjunction.”

The likely cause of occurrence of primary nondisjunction is due to the defects that occur during the recombination process. In meiosis, recombination occurs during meiosis-I, which is characteristic of meiosis.

The likely cause of occurrence of secondary nondisjunction is due to failure in the separation of sister chromatids. In meiosis, sister chromatids are separated during meiosis-II.

Gametes produced due to the non-disjunction in meiosis-I of females: Two XX and two gametes without chromosomes.

Gametes produced due to the non-disjunction in meiosis-I of males: One XX gamete , one YY gamete and two gametes without chromosomes.

Gametes produced due to the non-disjunction in meiosis-II of females: One XX, two gametes with a single X chromosome (normal) and one gamete without chromosomes.

Gametes produced due to the non-disjunction in meiosis-II of males: Two XY and two gametes without chromosomes.

Thus, the non-disjunction might have occurred in either the mother or father. Thus, the statement \"the non-disjunction must have occurred in mother\" is incorrect.