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Question

In a plant species, yellow flower colour is dominant over white. Round fruit shape is dominant over elongated. Crossing was performed between two pure lines, i.e., one having yellow flower and round fruit and another with white flower and elongated fruit. About 20 plants survived in F1 progeny. Plants of F1 were allowed to self-fertilize and about 960 plants survived in
F2. If the traits follow Mendelian inheritance, the number of plants that would have yellow flowers and round fruit in F1 and F2 are respectively

A
20, 960
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B
20, 540
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C
10, 180
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D
10, 60
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Solution

The correct option is B 20, 540
Let, Y = yellow flower, y = white flower, R = round fruit, r = elongated fruit.

The dihybrid cross between RRYY and rryy is shown below:


In a cross between two pure lines, all the progenies in F1 generation will show only the dominant traits. Therefore, the number of plants that would have yellow flowers and round fruit in F1 = 20.

In F2 generation,

the progenies showing yellow round = 1 (RRYY) + 2 (RRYy) + 2 (RrYY) + 4 (RrYy) = 9

the progeny showing white elongated = 1 (rryy)

the progenies showing round white = 1 (RRyy) + 2 (Rryy) = 3

the progenies showing elongated yellow = 1 (rrYY) + 2 (rrYy) = 3

9 out of 16 progenies will show both the dominant characters (yellow flowers and round fruit). Therefore, the number of plants that would have yellow flowers and round fruit in F2 out of 960 plants = 9/16×960 = 540.

Hence, the number of plants that would have yellow flowers and round fruit in
F1 and F2 are 40 and 540 respectively.

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