A karyotype is an organised picture of a personʼs chromosomes that helps scientists to identify certain genetic disorders.
The chromosomes are shown in pairs and numbered. The last two chromosomes are marked to show the personʼs gender: XY for male and XX for female.
Down Syndrome
We know that normal body cells have 46 chromosomes. If a zygote receives the wrong number of chromosomes from the sperm or egg cells (gametes) during fertilisation, it leads to a genetic disorder.
The most common example of this form of mutation is Down syndrome, where the offspring receives an extra copy of chromosome number 21.
Figure 4.3.4 shows the karyotype for a male with Down syndrome. Note the extra copy of chromosome 21.
Fig. 4.3.4 Chromosomes from a male with Down syndrome
