(a) Explain the following terms: (i) trait (ii) recessive gene (iii) test cross (iv) heterozygous (b) How do you determine whether a man with blood group '0...

(a) Explanation of terms

• (i) Trait: a distinct inherited characteristic or feature of an organism, e.g. eye colour, height or blood group.
• (ii) Recessive gene: an allele whose effect is masked in the presence of a dominant allele; it is only expressed when it is present in the homozygous condition (two recessive alleles together).
• (iii) Test cross: a cross between an individual of unknown genotype (showing the dominant trait) and a homozygous recessive individual, done to find out whether the unknown is homozygous or heterozygous.
• (iv) Heterozygous: having two different alleles of a gene for a particular trait, e.g. Bb.

(b) Determining fatherhood using blood groups

A person with blood group O has the genotype ii; he can only pass on an i allele. A person with blood group AB has the genotype I^AI^B. A child of blood group AB must have received I^A from one parent and I^B from the other. Since the group-O man can only give i, he cannot supply either I^A or I^B. Therefore a group-O man cannot be the biological father of an AB child; the AB child cannot be his.

(c)(i) Determining that a brown male rabbit is homozygous

Carry out a test cross: mate the brown male with several white (homozygous recessive, ww) females. If the male is homozygous WW, all the offspring will be brown. If any white offspring appear, the male is heterozygous Ww. So if all offspring are brown, the male is homozygous.

(c)(ii) Difference in chromosome number

A gamete has the haploid number of chromosomes (n), which is half the number in a body cell. A body (somatic) cell has the diploid number (2n), which is twice that of the gamete.