3 Fig. 3.1 shows a maize cob with grains of two different colours. In maize, purple grain colour (P) is dominant over yellow grain colour (p). A farmer cros...

Labelled answer diagram:

(a) The genotype of the yellow-grained parent plant is pp. [1] Yellow grain colour is the recessive trait, so the plant must carry two copies of the recessive allele to express it. There is no dominant P allele present to mask the yellow phenotype.

(b) The phenotype of all F1 offspring is purple-grained. [1] Since one parent is homozygous dominant (PP) and the other is homozygous recessive (pp), every F1 individual inherits one P allele and one p allele, making them all Pp. The dominant P allele produces the purple colour.

(c) The genotype of all F1 plants is Pp (heterozygous). [1]

(d)(i) The gametes produced by an F1 plant (Pp) are P and p. [1] During meiosis, the two alleles separate into different gametes, so each gamete carries only one allele.

(d)(ii) Punnett square for the F1 cross (Pp x Pp):

The gametes P and p are shown across the top (from one parent) and down the side (from the other parent). [1] The four offspring genotypes are correctly placed: PP, Pp, Pp, and pp. [1]

(d)(iii) The expected phenotypic ratio in the F2 generation is 3 purple : 1 yellow (3:1). [1] Three of the four genotypes (PP, Pp, Pp) express the dominant purple phenotype; only pp expresses the recessive yellow phenotype.

(e) A farmer might carry out a test cross on a purple-grained plant because:

1. A purple-grained plant could be either homozygous (PP) or heterozygous (Pp), since the dominant allele masks the recessive one in both cases. The phenotype alone does not reveal the genotype. [1]
2. The farmer needs to determine the genotype to predict offspring from future crosses and to select reliable breeding stock that will breed true for purple grain. [1]

(f) How a test cross is carried out and interpreted:

1. Cross the purple-grained plant with a homozygous recessive (yellow-grained, pp) plant. [1] The recessive parent can only contribute p alleles, so the offspring's phenotype depends entirely on what allele the purple parent donates.
2. If all offspring are purple, the parent is likely homozygous (PP), because every offspring receives P from the tested parent and p from the yellow parent, giving Pp (purple) every time. [1]
3. If approximately half the offspring are purple and half are yellow (a 1:1 ratio), the parent is heterozygous (Pp), because half the gametes carry P (giving Pp, purple offspring) and half carry p (giving pp, yellow offspring). [1]