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  4. Starch synthesis in the pea seed is controlled by one gene. It has two alleles B and b. If starch grain size is considered as the phenotype, then from this angle allele show

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Q. Starch synthesis in the pea seed is controlled by one gene. It has two alleles $B$ and $b$. If starch grain size is considered as the phenotype, then from this angle allele show

Principles of Inheritance and Variation

Solution:

• 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.