Question

Explain monohybrid cross taking seed coat colour as a trait in Pisum sativum. Work out the cross up to F2 - generation.

Answer 1

(b) Law of dominance: When two alternative forms of a trait or character (genes or alleles) are present in an organism, only one factor expresses itself in the F1 progeny and is called dominant and the other which remains masked is called recessive.  

Law of segregation: The factors or alleles of a pair segregate from each other during gamete formation such that a gamete receives only one of the two factors. They do not show any blending.

(c) Phenotypic ratio of the F2 generation in a monohybrid cross is 3:1, whereas in a dihybrid cross, it is 9:3:3:1.

Answer 2

monohybrid cross of colour of seed coat in pisum sativum

[parents]  yy [homozygous yellow] * YY [ homozygous green] [CROSS POLL.]

 [gametes]           y     y       Y   Y

[F1 progeny]        Yy      Yy     Yy    Yy

phenotypic ratio - all green seeds

genotypic - all are heterozygous

[parents] Yy [heterozygous green] *Yy [heterozygous green] [self poll.]

[gametes]           Y       y                  Y        y

[F2 progeny]       YY       Yy            Yy      yy

phenotypic ratio - 1:3  yellow : green

genotypic ratio-  1:2:1  homozygous green : heterozygous green : homozygous yellow