A cross was carried out between 2 pea plants homozygous dominant for yellow and round seeds with homozygous recessive for the same trait. The F2 progeny of such cross showed phenotypic ratio of 9:3:3:1
a) State the different laws of Mendel that could be derived from such a cross
b) Write the possible genotypes for the progeny for such a cross having:
(i) yellow and wrinkled seeds
(ii) green and round seeds

Dear student,
Please find below the answer:

​​​​​​Cross between a pea plant homozygous dominant for round and yellow seeds and pea plant homozygous recessive for the same trait is as follows:

Parent generation:  YYRR ( Yellow-Round seed)       X         yyrr (Green -Wrinkled seed)

Gametes:                                YR                                                             yr

F1 generation:                                    YyRr (Yellow-Round seed)

The F1 hybrid after self-fertilzation:
                                           YyRr                             X                         YyRr

​Gametes :             1/4 YR , 1/4 Yr,  1/4 yR, 1/4 yr                        1/4 YR, 1/4 Yr, 1/4 yR, 1/4 yr

F2 generation:

Gametes YR Yr yR yr
Yr YYRr YYrr YyRr Yyrr
yR YyRR YyRr yyRR yyRr
yr YyRr Yyrr yyRr yyrr

a. Different laws of Mendel that could be derived from such a cross​​​​​​​:

Law of Segregation

This law states that the two alleles of a pair segregate or separate during gamete formation such that a gamete receives only one of the two factors. In homozygous parents, all gametes produced are similar; while in heterozygous parents, two kinds of gametes are produced in equal proportions.

Law of Independent Assortment

When two pairs of traits are combined in a hybrid, one pair of character segregates independently of the other pair of character. In a dihybrid cross between two plants having round yellow (RRYY) and wrinkled green seeds (rryy), four types of gametes (RY, Ry, rY, ry) are produced. Each of these segregates independent of each other, each having a frequency of 25% of the total gametes produced.

Both laws are interlinked. The law of segregation states that the alleles will separate and the law of independent assortment tells us that this separation will be independent of each other.  

b. Genotypes of the progeny with yellow and wrinkled seeds would be YYrr, Yyrr and with green and round seeds would be yyRR, yyRr.
Hope this clears your doubt.

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