Life Sciences Term 1

Unit 2: What is disjunction and what are the consequences of this?

The homologous chromosomes do not separate due to failure of the centromere to divide during meiosis I & II. This usually happens during anaphase 1 or 2. The resulting gametes will have either an extra chromosome/copy on chromosome pair or another gamete will have one less chromosome.

The following diagram illustrates how non-disjunction takes place:

Down syndrome

The condition is named after Dr Down who described it the first time in 1866. People with Down syndrome have 47 chromosomes instead of 46 because the 21^st set of chromosomes did not divide properly during anaphase 1 in oogenesis. Both chromosomes of the homologous pair then end up in one ovum (which now has 24 chromosomes). If fertilisation takes place, the resulting zygote will have 47 instead of 46 chromosomes. This condition is called trisomy 21, because there are three (tri) chromosomes on the 21^st pair of chromosomes.

Karyotype is the number and visual appearance of the chromosomes in the cell nuclei of an organism or species. It is an individual's collection of chromosomes. This is sometimes used by examiners to ask questions about Down syndrome.  The following images illustrate the differences between normal karyotypes and the karyotype of a woman with Down syndrome.

The first 22 pairs of chromosomes are non-sex chromosomes and are called autosomes. The 23^rd pair of chromosomes is sex chromosomes, also known as gonosomes. They are composed of one large X and one small Y chromosome in the case of males or two large X chromosomes in the case of females.

In the Down syndrome karyotype one can clearly see that there are three chromosomes at position 21 instead of only two as the karyotypes of the normal male and female above indicate.