Question Details

Generally it is observed that human males suffer from hemophilia more than human females, who rarely suffer from it. Explain giving reasons. or F₁ progeny of pea plant bearing violet flowers and snapdragon plant bearing red flowers were selfed to produce their respective F₂ progeny. Compare the phenotypes, the genotypes and the pattern of inheritance of their respective F₂ progeny.

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Correct Answer :

This is a sex linked / X chromosomes linked recessive disease ,the heterozygous female / carrier for hemophilia may transmit the disease to sons (male progeny), The possibility of hemophilic female/daughter is rare because mother of such a female has to be at least carrier and the father should be hemophilic.

The red colour in the snapdragon flower shows incomplete dominance if crossed with white colour flower the genotype Rr is pink colour in it. So they will have RR as genotype.

When self-pollination:

R R

R RR RR Genotype 4:0, phenotype all red

R RR RR

Violet genotype is Rr, violet is a dominant trait. When selfed in F2 generation

R r

R RR rR Genotype ratio 1:2:1, phenotype 3:1 (3 violet, 1 white)

r Rr rr

The pattern of inheritance is complete dominance. The dominant allele is expressed and the recessive allele is suppressed in both the case. The phenotype is of the dominant allele.

Solution :

Correct Answer / Explanation:

The question provides two options to explain. Both explanations are detailed below based on the provided correct answer guidelines.

Part 1: Why human males suffer from hemophilia more than human females
Hemophilia is a sex-linked recessive disorder, where the gene responsible for the disease is located on the X chromosome.
Since females possess two X chromosomes (XX), a female will only suffer from hemophilia if she inherits two defective alleles (one from each parent, representing a homozygous recessive state). If she is heterozygous (having only one defective allele on one of her X chromosomes), she will act as a carrier but will not exhibit the disease herself.
On the other hand, males possess only one X chromosome and one Y chromosome (XY). Because males are hemizygous for the X chromosome, inheriting a single defective allele is sufficient to cause the disease. Therefore, a carrier mother can easily transmit the defective X chromosome to her sons, who will develop hemophilia.
For a female to be hemophilic, her mother must be at least a carrier and her father must be hemophilic, which is a rare genetic combination.

Part 2: Comparison of F₂ progeny of pea plant and snapdragon plant
1. Snapdragon Plant (Red Flowers):
The red flower colour in snapdragon exhibits incomplete dominance when crossed with white flowers (producing pink Rr heterozygotes). However, for a pure red-flowered snapdragon, the genotype is homozygous RR.
When these red-flowered plants are self-pollinated (RR × RR):
Genotype ratio: 4:0 (all RR)
Phenotype: All red flowers (100% red)
The Punnett square for this selfing is:
R R
R RR RR
R RR RR

2. Pea Plant (Violet Flowers):
Violet flower colour is a dominant trait in pea plants, and the heterozygous F₁ progeny has the genotype Rr. When self-pollinated (Rr × Rr) to produce the F₂ generation:
Genotype ratio: 1:2:1 (1 RR : 2 Rr : 1 rr)
Phenotype ratio: 3:1 (3 violet flowers, 1 white flower)
The Punnett square for this selfing is:
R r
R RR Rr
r rR rr

Pattern of Inheritance:
The pattern of inheritance demonstrated here is complete dominance. In this system, the dominant allele (R) is expressed and masks the recessive allele (r) in heterozygous individuals (Rr), resulting in the dominant phenotype.