Question

A child with Down Syndrome is born to a normal couple who have no prior history of the disease. Illustrate with a diagram how it would occur during gametogenesis. What factors would influence the birth defects of an individual with Down Syndrome?

Answer 1

Down syndrome is a type of genetic disorder occur when all or a part of a third copy of chromosome is found in chromosome number 21. It is also called as trisomy 21. In most of the cases, down syndrome is not inherited and it happens because of chromosomal abnormality as a result of random event during the formation of reproductive cells called gametogenesis in a parent. This chromosomal abnormality usually happens in egg and it is rare in sperm cells.

During the cell division (mitosis and meiosis), at a particular stage the chromosome pair will be equally separated and as a result the new cells will have a copy of every chromosome present in the parent cell. Sometimes, the chromosome does not separate equally leading to new cells with an extra part of chromosome. This process of unequal separation of chromosome is called nondisjunction. In the case of down syndrome, disjunction process occurs in multiple ways in which trisomy is the most common form of down syndrome. Trisomy 21 caused by unequal separation of chromosome 21 during the process of gametogenesis. Due to the disjunction, the gamete cells are formed with an additional copy on chromosome 21. If a zygote is formed from this gamete leads to offspring with an extra or missing chromosomes. The disjunction can take place at meiosis stage I or stage II which is depicted in the figures below.

Figure 1. Chromosomal disjunction at meiosis stage I of gametogenesis

Figure 2. Chromosome disjunction at meiosis stge II of gametogenesis.

The main risk factors of down syndrome is the maternal age. The risk factor of down syndrome increases with increase in mother's age because the older eggs have a greater risk for abnormal chromosome division.