Starch synthesis in pea seeds is controlled by one gene that has two alleles, B and b. Choose the correct statements from the following:
A. In pea seeds, BB homozygotes synthesise starch effectively and produce large starch grains.
B. In pea seeds, the bb homozygotes produce round seeds after maturation.
C. In pea seeds, the Bb heterozygotes produce starch grains of intermediate size.
D. In pea seeds, when starch grain size is taken as the phenotype, the alleles B and b show co-dominance.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A and C only
Starch synthesis in pea seeds is controlled by one gene with two alleles, B and b. The same pair of alleles can look dominant or incompletely dominant. It depends on which phenotype we look at: the shape of the seed or the size of the starch grains.
A. In pea seeds, BB homozygotes synthesise starch effectively and produce large starch grains. — True. The BB homozygote makes starch with full efficiency, so its starch grains are large. The bb homozygote makes starch less efficiently and has smaller grains.
B. In pea seeds, the bb homozygotes produce round seeds after maturation. — False. After maturation the bb seeds are wrinkled. It is the BB seeds (and the Bb seeds) that are round, which is why B looks dominant for seed shape.
C. In pea seeds, the Bb heterozygotes produce starch grains of intermediate size. — True. The Bb seed has one allele of each kind, and its starch grains are of intermediate size, in between those of BB and bb seeds.
D. In pea seeds, when starch grain size is taken as the phenotype, the alleles B and b show co-dominance. — False. An intermediate phenotype in the heterozygote is incomplete dominance. In co-dominance the F1 resembles both parents and both alleles are fully expressed, as in the AB blood group. For grain size the Bb seed is in between, not like both.
NCERT Extract:
Co-dominance – “… Occasionally, a single gene product may produce more than one effect. For example, starch synthesis in pea seeds is controlled by one gene. It has two alleles (B and b). Starch is synthesised effectively by BB homozygotes and therefore, large starch grains are produced. In contrast, bb homozygotes have lesser efficiency in starch synthesis and produce smaller starch grains. After maturation of the seeds, BB seeds are round and the bb seeds are wrinkled. Heterozygotes produce round seeds, and so B seems to be the dominant allele. But, the starch grains produced are of intermediate size in Bb seeds. So if starch grain size is considered as the phenotype, then from this angle, the alleles show incomplete dominance.”