QUESTION 15 Nondisjunction is the failure of homologous chromosomes or sister ch

Question

QUESTION 15 Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate in meiosis or mitosis. In the following figure, nondisjunction is occurring in one cell during anaphase ll of meiosis. correct n+1 Metaphase Anaphase I Metaphase Il nondisjunction Anaphase ll Gametes (a) What has gone wrong? (1 mark) (b) What potential problems do you foresee for the gametes that result from this nondisjunction event? (2.5 marks) Please note that we will revisit this nhenomenon again in Module 3. Chromosomal Variation

Explanation / Answer

a) In above mentioned condition there is Non disjunction in Anaphase II. It indicates till metaphase II all chromosomes were aligned properly on spindle axis, but during anaphase II while seperation of sister chromatids by degradation of cohesin proteins (which holds sister chromatids together). At one place both sister chromatids instead of seperating remain together. so in the resulting gamete one of gamete have one less chromatid and other have one extra chromatid. Seperation of sister chromatids in anaphase II went wrong.

usually meiosis results in 4 haploid gametes as n,n,n,n. But now due to non disjunction in anaphase II gametes formed are n,n,n+1,n-1.

b) When such gametes are formed mostly they are abnormal- n+1 or n-1. When these gametes fuse with normal gametes it will result in zygotes with genetic content 2n+1 or 2n-1.

This condition is called as Aneuploidy. 2n+1 is hyperaneuploidy and 2n-1 is hypoaneuploidy. In aneuploidy chromosomal number change occurs in particular pair. Mostly these zygotes do not develop and leads to miscarriage. some genetic abnormalities which can survive are as follows.

It is called as trisomy(2n+1) condition or monosomy (2n-1) condition among aneuploidy.

e.g of trisomy- for 21 number chromosome 2n+1 condition causes Down syndrome

for trisomy of 18 th chromosome- Edward syndrome, for 13 th chromosome patau syndrome.

This type of genetic abnormalities occur.

In case of monosomy condition as loss of chromosome is lethal to cell generally it dont survive. only monosomy which results in survival is in individuals suffering from Turner syndrome (2n-1)they lack one of X chromosome. But they only survive with genetic abnormalities, they cannot reproduce.

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