1
Question
In a plant, gene 'A' is responsible for tallness and its recessive allele 'a' for dwarfness. 'B' is responsible for red flower colour and it's recessive allele 'b' for white flower colour. A tall and red flowered plant with genotype (AaBb) is crossed with dwarf and red flowered plant (aaBb). What is the percentage of dwarf-white flowered plant offsprings in the cross?
In a plant, gene 'A' is responsible for tallness and its recessive allele 'a' for dwarfness. 'B' is responsible for red flower colour and it's recessive allele 'b' for white flower colour. A tall and red flowered plant with genotype (AaBb) is crossed with dwarf and red flowered plant (aaBb). What is the percentage of dwarf-white flowered plant offsprings in the cross?
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Solution
The correct option is C 12.5%
It is an example of dihybrid cross as two characters are under study at a time. According to the Mendel's rule of independent assortment, alleles of one gene segregate independently of alleles of other gene. Hence the alleles of plant height [A and a] will segregate independently of alleles of flower colour [B and b] resulting in formation of four types of gametes by tall and red flowered plant with genotype AaBb and two types of gametes by the other parent plant. Combination of these gametes will give the phenotypic ratio of 3 tall and red flowered plant: 3 dwarf and red flowered plant : 1 tall and white flowered plant: 1 dwarf and white flowered plant. Hence, the percentage of dwarf and white flowered plant is 1/8*100=12.5%. The correct option is C.
Gamete :
AaBb -->
aaBb AB Ab aB ab aB AaBB
(tall and red flower) AaBb
(tall and red flower)
aaBB
(dwarf and red flower) aaBb
(dwarf and red flower)
ab AaBb
(tall and red flower)
Aabb
(tall and white flower)
aaBb
(dwarf and red flower)
aabb
(dwarf and white flower)
It is an example of dihybrid cross as two characters are under study at a time. According to the Mendel's rule of independent assortment, alleles of one gene segregate independently of alleles of other gene. Hence the alleles of plant height [A and a] will segregate independently of alleles of flower colour [B and b] resulting in formation of four types of gametes by tall and red flowered plant with genotype AaBb and two types of gametes by the other parent plant. Combination of these gametes will give the phenotypic ratio of 3 tall and red flowered plant: 3 dwarf and red flowered plant : 1 tall and white flowered plant: 1 dwarf and white flowered plant. Hence, the percentage of dwarf and white flowered plant is 1/8*100=12.5%. The correct option is C.
| Gamete : AaBb --> aaBb | AB | Ab | aB | ab |
| aB | AaBB (tall and red flower) | AaBb (tall and red flower) | aaBB (dwarf and red flower) | aaBb (dwarf and red flower) |
| ab | AaBb (tall and red flower) | Aabb (tall and white flower) | aaBb (dwarf and red flower) | aabb (dwarf and white flower) |
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