Trisomy could occur as a result of nondisjunction during meiosis. in order for nondisjunction to lead to trisomy, how many chromosomes would be present that egg cell?

24 (n+1)

Explanation:

Trisomy refers to the presence of one extra chromosome in the genome of the organism. This means that the affected individuals have a total of 47 chromosomes, instead of 46.  

Nondisjunction during meiosis I or meiosis II produce the gametes with one extra chromosome. For example, nondisjunction in egg mother cell results in the production of some egg cells with "n+1; 24 chromosomes".  

Fertilization of the egg cell carrying 24 chromosomes with a sperm carrying 23 chromosomes forms a trisomic zygote with 47 chromosomes.  

The egg cell, to occur trisomy, has (n+1) = 24 chromosomes.

Explanation: Since an individual has 23 chromossomes in your gametes, to occur trisomy, one of the gametes has to have 24. This disorder in the chromosomes can be a result of nondisjunction.

Nondisjunction occurs when pair of chromosomes fail to separate. It can happen in Meiosis I: If the homologous chromosomes don't divide equally, it can produce an aneuploid, which is a cell with extra or missing chromosome, gamete; or in Meiosis II: if the chromatides fail to separate.

The trisomy will occur when the aneuploid gamete fertilized a normal one, creating a aneuploid zygote.

In this case, if an aneuploid egg cell, (n+1) =24 chromosome, combine with a normal sperm, (n) = 23 chromosome, results in an embryo with (2n+1) = 47 chromosomes.

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