Question

4(b). In a mango plant, the allele for bean-shaped seed is r and is recessive to round-shaped seed R. With the aid of a genetic diagram, determine the genotypes of the offspring if: (i) a homozygous bean-shaped parent (rr) is crossed with a homozygous round-shaped parent (RR); (ii) a heterozygous bean-shaped parent is crossed with a heterozygous round-shaped parent.

Accepted Answer

Let r = bean shape (recessive), R = round shape (dominant).
(i) Cross rr × RR: all gametes from the rr parent are r, and all gametes from the RR parent are R. Every offspring genotype is Rr (round-shaped, since round is dominant) - i.e. the F1 generation is 100% heterozygous round-shaped (Rr).
(ii) Since 'bean shape' is recessive to round shape, a bean-shaped seed can never be expressed in a heterozygous condition (bean shape can only appear as rr). Therefore, the phrase 'heterozygous bean-shaped' describing a parent is not biologically correct/possible, since the bean-shaped phenotype can only occur in the homozygous recessive form (rr).

WAEC Biology 2015 Theory — Question 13

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