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  • (a) How would you find out whether a given tall garden pea plant is homozygous or heterozygous? Sustantiate your answer with the help of Punnett squares. (b) Given below are F2 Phenotypic ratios of two independently carried mo

(a) How would you find out whether a given tall garden pea plant is homozygous or heterozygous? Sustantiate your answer with the help of Punnett squares.

(b) Given below are F2 Phenotypic ratios of two independently carried monohybrid crosses:

(i) 1 : 2 : 1

(ii) 3 : 1

Mention what does each ratio suggest.

 

 

 

 
 
 
 
 

Answers (1)

a) To determine the genotype of a tall plant i.e whether a tall plant from F1 or F2 has TT or Tt composition, we can cross the tall plant from F2 with a dwarf plant which is known as a test cross. In a typical test cross, a pea plant showing a dominant phenotype (and whose genotype is to be determined) is crossed with the recessive parent instead of self-crossing. The progenies of such a cross can easily be analyzed to predict the genotype of the test organism.

b)

i) 1:2:1 This ratio suggests that the phenomenon is Incomplete Dominance. The F1 had a phenotype that did not resemble either of the two parents and was in between the two. In a cross between true-breeding homozygous dominant and true-breeding homozygous recessive, the F1 hybrid was intermediate between the two parents. When the F1 was self-pollinated the F2 resulted in the following ratio 1 : 2 : 1 . The genotype ratios were exactly as we would expect in any mendelian monohybrid cross, but the phenotype ratios had changed from the 3:1 dominant: recessive ratio. This is because neither of the alleles is completely dominant over the other. Example- Inheritance of flower colour in the dog flower (snapdragon or Antirrhinum sp.)

ii) 3:1 This suggest the typical monohybrid cross between the true-breeding tall plants and true-breeding dwarf plants. In the F1 generation a cross between homozygous TT (tall) and tt (short) , there was the formation of a hybrid Tt (heterozygote) ie all the resulting plants were tall. The F1 plants of genotype Tt were self-pollinated which resulted in the genotypes TT, Tt, tt. 1/4th of the random fertilisations lead to TT, 1/2 lead to Tt and 1/4th to tt.

At F2, 3/4th of the plants are tall, where some of them are TT while others are Tt. Hence the character T or ‘tall’ is said to dominate over the other allele t or ‘dwarf’ character. It is thus due to this dominance of one character over the other that all the F1 are tall (though the genotype is Tt) and in the F2 3/4th of the plants are tall (though genotypically 1/2 are Tt and only 1/4th are TT). This leads to a phenotypic ratio of 3/4th tall : (1/4 TT + 1/2 Tt) and 1/4th tt, i.e., a 3:1 ratio, but a genotypic ratio of 1:2:1. Example- Monohybrid cross between the true-breeding tall plants and true-breeding dwarf plants.

Figure - Punnett square to understand a monohybrid cross between true-breeding tall plants and true-breeding dwarf plants.

Posted by

Priyanka Kumari

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