Theory question 104
Mendelian Inheritance(a) (i) Define the law of independent assortment (ii) State the findings from Mendel's experiment involving white- flowered and red-flowered pea plants (iii) Illustrate these findings by means of simple diagrams
(b) Explain why blood groups A and B in man can exist both in heterozygous and homozygous conditions while blood group O can only exist in homozygous state.
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(a)(i) Law of independent assortment (Mendel's second law): when two or more pairs of characters are inherited together, each pair of alleles separates into the gametes independently of the other pairs.
(ii) Crossing pure red-flowered with pure white-flowered plants gave all red-flowered plants in the F1 (red is dominant). Selfing the F1 gave red and white plants in the F2 in a ratio of 3 red : 1 white; the factors for colour separate in gamete formation.
(iii) Let R = red, r = white. Parents RR × rr; gametes R and r; F1 all Rr (red). F1 Rr × Rr; gametes R, r × R, r; F2 RR, Rr, Rr, rr = 3 red : 1 white (genotypes 1 : 2 : 1). (Diagram or Punnett square showing these.)
(b) The alleles A and B are both dominant over O (O is recessive). Group A can be AA or AO, and group B can be BB or BO, because one dominant allele is enough to show. Group O shows only when no dominant allele is present, so the person must be OO (homozygous).
Show parents, gametes, F1 and F2 clearly in the genetic diagram.