Q.Explain the three kinds of plants that have recessive lethal gene in Antirrhinum sp.

Antirrhinum's chlorophyll gene shows incomplete dominance: CC is green, Cc is pale green, and cc is albino and dies -- making cc a recessive lethal genotype.

In the snapdragon Antirrhinum majus, a single gene (commonly denoted C/c) controls the formation of the green pigment (chlorophyll/plastid pigment) in the leaves, and this gene shows incomplete dominance together with a recessive lethal effect. Three kinds of plants result from the three possible genotypes:

1. CC (homozygous dominant) -- Green plants: these plants produce the full amount of chlorophyll, appear normal green, and are completely viable and fertile.

2. Cc (heterozygous) -- Pale green plants: because dominance is incomplete, the heterozygote produces an intermediate (reduced) amount of chlorophyll, giving the leaves a pale or light-green appearance rather than the full green of CC plants. These plants are viable, though somewhat less vigorous, and are capable of photosynthesising (at a reduced level) and reproducing.

3. cc (homozygous recessive) -- White (albino) plants -- lethal: these plants fail to produce chlorophyll at all and appear white/albino. Because they cannot photosynthesise, they cannot manufacture their own food, and the seedlings die shortly after germination once their stored seed reserves are exhausted. Thus the cc genotype behaves as a recessive lethal: it is only lethal in the homozygous condition, while a single copy of the recessive allele (as in Cc) is not lethal.

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