What happens when chromatids fail to segregate during cell division cycle ? Explain your answer with an example.

OR

ABO blood groups is a good example of co-dominance. Justify.

When chromatids fail to segregate during the cell division cycle, it causes the gain or loss of chromosomes, which leads to aneuploidy. A normal human has 46 chromosomes, but due to failure of segregation of chromatids, if a gain of chromosome (i.e., 47 chromosomes) occurs then, it can lead to Down syndrome while if there is a loss of chromosome (i.e., 45 chromosomes), it can lead to Turner syndrome.

OR

(i) The four possible phenotypes in the human blood groups are A, B, AB, and O. These blood groups are controlled by gene I.

(ii) There are three alleles of these genus I^A, I^B and i. They control the blood grouping.

(iii) I^A and I^B are dominant over i.

(iv) When both the dominant genes I^A and I^B are present, they are seen to express themselves together.

Thus, it is a good example of co-dominance as both of these genes are dominant, but they express themselves equally and independently.