Theory question 107
Mendelian Inheritance(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 'O' and married to a woman with blood group 'AB' is the father of a child with blood group 'AB' born to them?
(c) (i) In rabbit brown hair is dominant over white hair colour. How could you determine that a brown coloured male rabbit is homozygous? (ii) What is the difference between the chromosome number of a gamete and that of the body cell of the same organism?
Show the model answer
(a)(i) Trait: an inherited characteristic, controlled by a pair of genes, e.g. height or eye colour.
(ii) Recessive gene: the allele whose effect is hidden when the dominant allele is present; it shows only when two copies are present.
(iii) Test cross: crossing an individual showing the dominant trait with a homozygous recessive one to find out its genotype.
(iv) Heterozygous: having two different alleles for a trait, e.g. Tt.
(b) Man OO gives only O gametes; woman AB gives A or B gametes.
Cross OO × AB gives AO and BO children: blood groups A or B only (half each).
No child can be AB, so the man cannot be the father of an AB child.
(c)(i) Mate the brown male with a white (homozygous recessive, bb) female.
If all the offspring are brown, he is homozygous (BB).
If about half are white, he is heterozygous (Bb).
(ii) A gamete has the haploid number (n), half the chromosomes; a body cell has the diploid number (2n).
Show the cross with parents, gametes and offspring.