A single gene exhibiting multiple phenotypic expressions is known as a:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• Polygenic gene
• Pleiotropic gene
• Linked gene
• Mutant gene
Correct Option: B  [ Pleiotropic gene ]

Remark: Instances where a single gene can exhibit multiple phenotypic expression are known as pleiotropy, and the gene is called a pleiotropic gene.

The human disease phenylketonuria, caused by a mutation in a single gene, results in mental retardation and changes in hair and skin pigmentation. This is an example of:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• Polygenic inheritance
• Incomplete dominance
• Pleiotropy
• Co-dominance
Correct Option: C  [ Pleiotropy ]

Remark: Phenylketonuria is an example of pleiotropy. The disease is caused by a single gene mutation that affects the enzyme phenylalanine hydroxylase, leading to multiple phenotypic expressions like mental retardation and reduced pigmentation.

Who first traced a specific nuclear structure through spermatogenesis in insects, which was later identified as the X chromosome?

[Principles-of-Inheritance-and-Variation] [class-xii ]

• T.H. Morgan
• Gregor Mendel
• Henking
• Sutton and Boveri
Correct Option: C  [ Henking ]

Remark: Henking (1891) traced a specific nuclear structure in a few insects and observed that 50% of sperm received it. He named it the X body, which was later identified as a chromosome.

In the XO type of sex determination, such as in grasshoppers, males have:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• One X chromosome and one Y chromosome
• Two X chromosomes
• Only one X chromosome and no Y chromosome
• Two Y chromosomes
Correct Option: C  [ Only one X chromosome and no Y chromosome ]

Remark: Grasshopper is an example of XO type of sex determination in which the males have only one X-chromosome besides the autosomes, whereas females have a pair of X-chromosomes.

In humans and Drosophila, the XY type of sex determination means that males are:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• Homogametic (XX)
• Heterogametic (XY)
• Homogametic (ZZ)
• Heterogametic (ZW)
Correct Option: B  [ Heterogametic (XY) ]

Remark: In humans and Drosophila, males have one X and one Y chromosome (XY) and are heterogametic, producing two different types of gametes. Females have two X chromosomes (XX) and are homogametic.

In birds, the mechanism of sex determination involves:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• Male heterogamety (XY)
• Female heterogamety (ZW)
• Male heterogamety (XO)
• Haplodiploidy
Correct Option: B  [ Female heterogamety (ZW) ]

Remark: In birds, females produce two different types of gametes in terms of sex chromosomes, which is called female heterogamety. The female has one Z and one W chromosome, while the male has a pair of Z-chromosomes.

In humans, the sex of a child is determined by:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• The genetic makeup of the ovum
• The genetic makeup of the sperm
• The autosomes of the parents
• The environment
Correct Option: B  [ The genetic makeup of the sperm ]

Remark: Females produce only one type of ovum with an X-chromosome. Males produce two types of sperm, 50% with X and 50% with Y. Therefore, it is the genetic makeup of the sperm that determines the sex of the child.

The sex determination system in honey bees is called:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• XY system
• ZW system
• XO system
• Haplodiploid system
Correct Option: D  [ Haplodiploid system ]

Remark: The sex determination in honey bees is based on the number of sets of chromosomes an individual receives. This is called the haplodiploid sex-determination system.

In honey bees, a male (drone) develops from:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• A fertilised egg and is diploid
• An unfertilised egg and is haploid
• A fertilised egg and is haploid
• An unfertilised egg and is diploid
Correct Option: B  [ An unfertilised egg and is haploid ]

Remark: An unfertilised egg develops as a male (drone) by means of parthenogenesis. This means that males have half the number of chromosomes (haploid, 16) than that of a female (diploid, 32).

A phenomenon which results in the alteration of DNA sequences and consequently changes the genotype and phenotype is:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• Recombination
• Linkage
• Mutation
• Segregation
Correct Option: C  [ Mutation ]

Remark: Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism. It is a source of variation in DNA, in addition to recombination.