Select the incorrect statement from the following:

Galactosemia is an inborn error of metabolism
Small population size results in random genetic drift in a population
Baldness is a sex -limited trait
Linkage is an exception to the principle of independent assortment in heredity

Solution

Baldness is a sex influenced trait. The dominance of alleles may differ in heterozygotes of the two sexes.

The genetic defect – Adenosine deaminase (ADA) deficiency may be cured permanently by:

administering adenosine deaminase activators.
introducing bone marrow cells producing ADA into cells at early embryonic stages.
enzyme replacement therapy.
periodic infusion of genetically engineered lymphocytes having functional ADA cDNA.

Solution

By introducing bone marrow cells producing ADA into cells at early embryonic stages, the genetic defect adenosine deaminase(ADA) deficiency may be cured permanently. It is due to mutation.

A human male produces sperms with the genotypes AB, Ab, aB, and ab pertaining to two diallelic characters in equal proportions. What is the corresponding genotype of this person ?

AaBB
AABb
AABB
AaBb

Solution

Two genes R and Y are located very close on the chromosomal linkage map of maize plant. When RRYY and rryy genotypes are hybridized, the F₂ segregation will show

segregation in the expected 9 : 3 : 3 : 1 ratio
segregation in 3 : 1 ratio
higher number of the parental types
higher number of the recombinant types.

Solution

When the linked genes are situated quite close, the chances of crossing over are highly reduced. Due to this, large number of parental gametes are formed and only few recombinant gametes are formed. This results in higher number of parental types in F₂ generation as compared to recombinants.

Phenotype of an organism is the result of

cytoplasmic effects and nutrition
environmental changes and sexual dimorphism
genotype and environment interactions
mutations and linkages

Solution

Phenotype is the appearance one organism shows while genotype is the gene complement it has from its ancestors. These genes only show their effect in phenotype but environment also plays an important role in this hence phenotype is a result of genotype and environmental interaction.

In Mendel’s experiment with garden pea, round seed shape(RR) was dominant over wrinkled seeds (rr), yellow cotyledon(YY) was dominant over green cotyledon (yy). What are the expected phenotypes in the F₂ generation of the cross RRYY × rryy?

only wrinkled seeds with yellow cotyledons
only wrinkled seeds with green cotyledons
round seeds with yellow cotyledons, and wrinkled seeds with yellow cotyledons
only round seeds with green cotyledons

Solution

Cri-du-chat syndrome in humans is caused by the

loss of half of the short arm of chromosome 5
loss of half of the long arm of chromosome 5
trisomy of 21st chromosome
fertilization of an XX egg by a normal Y-bearing sperm

Solution

Cri-du-chat syndrome in humans caused by the partial deletion of the short arm of chromosome no. 5. In this child has abnormalities in his/her facial expressions, mental retardation, speak like and they cry like that of cat, heart does not work properly.

Duchenne muscular dystrophy (DMD) is caused by a sex-linked recessive. Its victims are invariably boys, who usually die before the age of 20. Why is this disorder almost never seen in girls?

Females need two copies of an X-linked recessive before they express it.
The allele is carried on the Y chromosome.
Non-disjunction occurs in males but not in females.
Males carrying the alleles don't live long enough to be fathers.

Solution

Males express all information on that portion of their X chromosome that is not homologous to their Y chromosome.

Red-green colour blindness is a human recessive sex-linked trait. A man and a woman with normal vision have a colour-blind son. What is the probability that their next child will also be a color-blind son?

Zero
¹⁄₈
¹⁄₄
¹⁄₂

Solution

The probability of being male is ¹⁄₂. Since they have one color-blind son,the woman must be heterozygous.

Haemophilia is mentioned as a trait carried by the mother and passed to her sons. What is the pattern of inheritance for this trait ?

Haemophilia is an allele carried on one of the mother’s autosomal chromosomes.
Haemophilia is an allele carried on the Y chromosome because more males have this genetic disorder than females.
Haemophilia is an allele carried on the X chromosome and can be directly inherited by the son from the father or the mother.
Haemophilia is carried on the X chromosome and can only be inherited by the son if the mother is a carrier.

Solution

Haemophilia is an X-linked trait, and can only be inherited by the son from his mother’s X chromosome. The father contributes the Y chromosome to his son (not his X chromosome) and thus cannot pass any of his X-linked alleles to his son.

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