(a) Explain briefly; (i) independent assortment of genes (ii) the reason blood group O in humans can only exist in homozygous form while blood groups A and ...

(a)(i) Independent assortment of genes

Independent assortment is the principle that during the formation of gametes (in meiosis), the two alleles of one gene separate into gametes independently of the alleles of another gene carried on a different pair of chromosomes. As a result, the members of one pair of contrasting characters are distributed to the gametes randomly and independently of the members of another pair, producing new combinations of characters.

(a)(ii) Why blood group O exists only in the homozygous form

Blood group O is produced by the allele i, which is recessive to both I^A and I^B. A recessive character can only be expressed when the allele is present in a double (homozygous) dose. Therefore group O can only appear as ii. Blood groups A and B, however, are produced by the dominant alleles I^A and I^B. A dominant allele shows its effect whether it is paired with an identical allele or with the recessive i. Hence group A can be I^AI^A (homozygous) or I^Ai (heterozygous), and group B can be I^BI^B (homozygous) or I^Bi (heterozygous).

(b) Classes of vertebrates in evolutionary order with examples