If a colour-blind man marries a woman who is homozygous for normal colour vision, the probability of their son being colour-blind is [2016]
0
0.5
0.75
1
(1)
Genotype of colour blind man –
Genotype of woman homozygous – XX
for normal woman
[IMAGE 28]
Hence, there is zero (0) probability of their son to be colour-blind.
Pick out the correct statements. [2016]
(1) Haemophilia is a sex-linked recessive disease.
(2) Down’s syndrome is due to aneuploidy.
(3) Phenylketonuria is an autosomal recessive gene disorder.
(4) Sickle cell anaemia is an X-linked recessive gene disorder.
(1), (3) and (4) are correct
(1), (2) and (3) are correct
(1) and (4) are correct
(2) and (4) are correct
(2)
Sickle-cell anaemia is an autosomal recessive genetic disorder. It can be transmitted from parents to the offspring when both the partners are carriers of the gene (or heterozygous).
Which of the following most appropriately describes haemophilia? [2016]
Chromosomal disorder
Dominant gene disorder
Recessive gene disorder
X-linked recessive gene disorder
(4)
Haemophilia is a sex-linked disease. It occurs due to the presence of a recessive sex linked gene h, which is carried by X-chromosome.
A colour blind man marries a woman with normal sight who has no history of colour blindness in her family. What is the probability of their grandson being colour blind? [2015]
Nil
0.25
0.5
1
(2)
When a colour blind man () marries a normal woman (XX), all of their daughters are carriers and all of their sons are normal, as shown in following figure:
[IMAGE 29]
When the carrier daughter () is married to a normal man, the probability of their son being colour blind is 0.25, as shown in following figure:
[IMAGE 30]
From above crosses, it is clear that the probability of occurrence of colour blindness in the grandson of a colour blind man and a normal woman is 0.25.
In the following human pedigree, the filled symbols represent the affected individuals. Identify the type of given pedigree. [2015]
[IMAGE 31]
Autosomal recessive
X-linked dominant
Autosomal dominant
X-linked recessive
(1)
Autosomal recessive traits are the traits which are caused by recessive autosomal genes when present in homozygous condition. The given pedigree can be explained as:
[IMAGE 32]
As the trait appears only in homozygous recessive individuals (aa), therefore it is an autosomal recessive trait.
An abnormal human baby with ‘XXX’ sex chromosomes was born due to [2015]
fusion of two ova and one sperm
fusion of two sperms and one ovum
formation of abnormal sperms in the father
formation of abnormal ova in the mother
(4)
The abnormal baby has an extra X chromosome, thus it must have been produced by fusion of abnormal XX ovum with a normal X sperm. Abnormal XX sperm is not possible because, males have XY genotype, and if produce abnormal sperms, then XY sperms and O sperms will be produced. If fusion of multiple gametes have occurred (either two ova with one sperm or two sperms with one ovum), then the human baby will have triploid genotype not the trisomy of sex chromosomes.
A man whose father was colour blind marries a woman who had a colour blind mother and normal father. What percentage of male children of this couple will be colour blind? [2014]
25%
0%
50%
75%
(3)
It is given that the man had colour blind father, i.e., man's genotype would be XY.
Now, the woman had a colourblind mother and normal father, thus her genotype would be . A cross between them can be represented as below.
[IMAGE 33]
Therefore, 50% of male children of this couple will be colour blind.
A human female with Turner’s syndrome [2014]
has 45 chromosomes with XO
has one additional X chromosome
exhibits male characters
is able to produce children with normal husband.
(1)
A human female with Turner’s syndrome has single sex chromosome i.e., 44 + X0 (45). Such females are called sterile females with rudimentary ovaries. Other associated phenotypes of this condition are short stature, webbed-neck, broad chest, lack of secondary sexual characteristics and sterility. Thus, any imbalance in the copies of the sex chromosomes may disrupt the genetic information necessary for normal sexual development.
