CBSE Class 12 Biology Principles of Inheritance and Variation Question Bank

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Biology: Chapter 5 – Principles of Inheritance and Variation (Question Bank)

State the difference between a gene and an allele.

Answer: A gene is a unit of inheritance that controls a particular character. Alleles are the different forms of the same gene (for example, T for tall and t for dwarf).

Write the possible genotypes Mendel got when he crossed F₁ tall pea plants with a dwarf pea plant.

Answer: The F₁ tall plants are Tt and the dwarf plant is tt. The cross gives Tt (tall) and tt (dwarf) in the ratio 1 : 1.

Who proposed the chromosomal theory of inheritance?

Answer: Sutton and Boveri.

Write the scientific name of the organism which T.H. Morgan used in his genetic experiments.

Answer: Drosophila melanogaster (the fruit fly).

List the advantages of selecting the pea plant for his experiments by Mendel.

Answer:

1. It is bisexual, with flowers that can be self-pollinated or cross-pollinated.
2. It has a short life cycle, so several generations can be grown quickly (one season).
3. It has many contrasting traits, such as tall and dwarf.

Briefly mention the contributions of T.H. Morgan in genetics.

Answer: Morgan worked on Drosophila. He discovered linkage and recombination, and showed that genes are located on chromosomes.

What is a point mutation? Give an example.

Answer: A point mutation is a mutation that changes a single nitrogen base in a gene. Example: sickle cell anaemia.

What is pedigree analysis? How is it useful?

Answer: Pedigree analysis is the study of the inheritance of a trait over several generations of a family, drawn as a family tree (pedigree chart). It is useful for studying how a disease is inherited and for genetic counselling.

The male fruit fly and the female fowl are heterogametic, while the female fruit fly and the male fowl are homogametic. Why are they termed so?

Answer: An organism that produces two different kinds of gametes (with respect to sex chromosomes) is heterogametic. An organism that produces only one kind of gamete is homogametic. The male fruit fly (XY) and the female fowl (ZW) make two kinds of gametes, so they are heterogametic. The female fruit fly (XX) and the male fowl (ZZ) make only one kind of gamete, so they are homogametic.

Define and explain test cross.

Answer: A test cross is a cross between an organism showing the dominant phenotype (genotype unknown) and a homozygous recessive organism. If all the offspring show the dominant character, the unknown parent is homozygous (TT). If the offspring show the dominant and recessive characters in the ratio 1 : 1, the unknown parent is heterozygous (Tt).

Explain the law of dominance with an example.

Answer: When two contrasting alleles of a character are present together in a hybrid, only one of them is expressed. That allele is the dominant allele, and the other is the recessive allele. Example: in a cross between a tall (TT) and a dwarf (tt) pea plant, all the F₁ plants are Tt and are tall, because T is dominant over t.

With the help of an example, differentiate between incomplete dominance and co-dominance.

Answer:

Explain the mechanism of sex determination in honeybees.

Answer: Honeybees show the haplo-diploid system. The unfertilised eggs develop into males (drones), which are haploid. The fertilised eggs develop into females (queen and workers), which are diploid.

How does the Mendelian disorder phenylketonuria occur?

Answer: Phenylketonuria is an inborn error of metabolism. It is a recessive autosomal disorder. The person lacks the enzyme that converts the amino acid phenylalanine into tyrosine. Phenylalanine builds up and is changed into phenylpyruvic acid and other derivatives, which are excreted in the urine. They also affect the brain and cause mental retardation.

Why did Mendel select the garden pea plant for his experiments?

Answer: The pea plant has a short life span, it is easy to grow, and it has many clear contrasting characters. It is bisexual, so self-pollination and cross-pollination can both be controlled. It gives a large number of seeds in one generation.

Tabulate the different types of sex determination in organisms.

Answer:

Give the characteristic features of Turner's syndrome.

Answer: Turner's syndrome is caused by the absence of one X chromosome, so the karyotype is 44 + XO (45 chromosomes). The person is a female who is sterile, with rudimentary ovaries. She lacks the other secondary sexual characters, is short in height and has a webbed neck.

What will be the genotype and phenotype of the offspring if a colour-blind man marries a carrier woman?

Answer: The man is X^cY and the carrier woman is X^CX^c.

Genotypes: X^CX^c, X^cX^c, X^CY and X^cY in the ratio 1 : 1 : 1 : 1. Phenotypes: a normal (carrier) daughter, a colour-blind daughter, a normal son and a colour-blind son. So 50% of the children are colour-blind.

Anita was happy when she gave birth to her first child. Her in-laws were dissatisfied at her not giving birth to a male child and blamed Anita. Anita tried to convince her in-laws that she had no role in the child's gender. They understood the biological reason but were yet to be satisfied. Anita's husband took up the matter and convinced the parents.
(a) What values did Anita's husband show in the above situation?
(b) What governs sex determination in humans? How is it different from birds?
(c) Why can't Anita be blamed for not giving birth to a male child?

Answer:

1. He showed a strong and determined personality, responsibility towards his family, and respect for his elders.
2. The sex chromosomes govern sex determination. In humans, the male is XY and the female is XX. In birds, the male is ZZ and the female is ZW, so the female is the heterogametic sex in birds.
3. A woman produces only one kind of egg (22 + X). A man produces two kinds of sperm, 22 + X and 22 + Y. If an X-bearing sperm fertilises the egg, the child is a girl. If a Y-bearing sperm fertilises it, the child is a boy. So the sex of the child depends on the father's sperm, not on the mother.