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 $F_1$ progeny. Plants of $F_1$ were allowed to self-fertilise and about 960 plants survived in $F_2$. If the traits follow Mendelian inheritance, the number of plants that would have yellow flowers and round fruit in $F_1$ and $F_2$ are respectively

• A. 20, 960
• B. 20, 540
• C. 10, 180
• D. 10, 60

Answer 1

The correct option is B 20, 540

Let Y = yellow flower, y = white flower, R = round fruit and r = elongated fruit.

The dihybrid cross between RRYY and rryy is shown below:


In a cross between two pure lines, all the progenies in $F_1$ generation will show only the dominant traits. Therefore, the number of plants that would have yellow flowers and round fruit in $F_1$ generation = 20.

In $F_2$ 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 $F_2$ generation out of 960 plants = $\frac{9}{16}\times 960$ = 540.

Hence, the number of plants that would have yellow flowers and round fruit in $F_1$ and $F_2$ are 20 and 540, respectively.