Q1: If in meiosis I, chromosome number 21 underwent a NONDISJUNCTION, what could

Question

Q1: If in meiosis I, chromosome number 21 underwent a NONDISJUNCTION, what could be the chromosome constitution of the resulting egg?

Q2: If in meiosis I, chromosome number 21 underwent a NONDISJUNCTION, what could be the chromosome constitution of the resulting zygote? Assume that the egg is fertilized by a normal sperm.

Q3: If in meiosis I, chromosome number 21 underwent a NONDISJUNCTION, what could be the chromosome constitution of the resulting newborn? Assume that the egg is fertilized by a normal sperm.

Explanation / Answer

1)

If the chromosome 21 underwent nondisjunction during meiosis I, then the homologous chromosomes do not seperate (fail to seperate) and would be in pairs and during Meiosis II a sisterchromatid would be extra and in the resultant egg cell there would be an extra copy of chromosome 21.

2)

The nondisjunction of chromosome 21 during meiosis I results in extracopy of chromosome 21 in zygote though it is fertilized with normal sperm. It would result in XXX or down's syndrome.

3)

The resulting new born expresses a third copy of X chromosomes due to nondisjunction of chromosome 21 and results in the genetic disorder Down's syndrome.

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