Chapter 4: Principles of Inheritance and Variation Short Questions | biology Class 12 CBSE

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Chapter 4: Principles of Inheritance and Variation Short Questions | biology Class 12 CBSE | ByteNotes.in

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Question 1

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What is a true-breeding line in pea plants? Why did Mendel use such lines in his experiments?

A true-breeding line is one that, having undergone continuous self-pollination, shows stable trait inheritance and expression for several generations.
Mendel used true-breeding lines to ensure that the parents in his crosses were homozygous for the character being studied, eliminating pre-existing genetic variation.
This allowed him to clearly observe which trait was dominant and which was recessive in the F1 generation, and accurately interpret the 3:1 ratio at F2.

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Question 2

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Distinguish between genotype and phenotype with one example each.

Genotype refers to the genetic constitution or allelic composition of an organism (e.g., TT, Tt, or tt for height in pea plants).
Phenotype refers to the observable physical or functional expression of the genotype (e.g., 'tall' or 'dwarf' plant).
The same phenotype (tall) can correspond to different genotypes (TT or Tt), demonstrating that genotype determines but is not always directly readable from the phenotype.

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Question 3

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State Mendel's Law of Segregation. On what observation is it based?

The Law of Segregation states that alleles of a gene pair segregate from each other during gamete formation, so each gamete receives only one of the two alleles.
It is based on the observation that a recessive character not seen in F1 reappears in F2 without any blending, indicating alleles remain discrete and unmodified.
This segregation is a random process, giving a 50 per cent chance of a gamete containing either allele, resulting in a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio at F2.

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Question 4

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What is a test cross? What information does it provide about an organism?

A test cross is a cross between an organism showing a dominant phenotype (whose genotype is to be determined) and a homozygous recessive parent.
If all progeny of the test cross show the dominant phenotype, the organism being tested is homozygous dominant (e.g., TT).
If the progeny show a 1:1 ratio of dominant to recessive phenotype, the organism is heterozygous (e.g., Tt), thereby revealing its genotype.

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Question 5

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What is incomplete dominance? How does it differ from complete dominance?

Incomplete dominance is a condition where neither allele in a heterozygote is completely dominant, resulting in an intermediate phenotype (e.g., pink flowers in Antirrhinum when red RR is crossed with white rr).
In complete dominance, one allele fully masks the other, so F1 phenotype resembles one parent (e.g., all tall in a TT x tt cross in peas).
In incomplete dominance, the F2 phenotypic ratio changes from 3:1 to 1:2:1 (red:pink:white), whereas the genotypic ratio (1:2:1) remains the same as in complete dominance.

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Question 6

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Explain co-dominance using ABO blood grouping as an example.

Co-dominance is a condition where both alleles of a heterozygote are fully expressed simultaneously, producing a phenotype that shows both parental traits.
In ABO blood groups, alleles IA and IB are co-dominant — when both are present (genotype IAIB), both A and B sugar polymers are expressed on red blood cell surfaces, resulting in blood group AB.
This differs from dominance where only one allele's effect is expressed; co-dominance is possible because IA and IB each direct production of their own distinct sugar type independently.

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Question 7

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What are multiple alleles? Give one example from the chapter.

Multiple alleles refer to the existence of more than two alleles controlling the same trait at a single gene locus in a population, though any diploid individual can carry only two alleles.
ABO blood grouping in humans is an example — the gene I has three alleles: IA, IB, and i, giving rise to six possible genotypes (IAIA, IAi, IBIB, IBi, IAIB, ii) and four phenotypes (A, B, AB, O).
Multiple alleles can only be detected through population-level studies, as each individual can only possess two of the possible alleles.

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Question 8

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What is pleiotropy? Give one example from the chapter.

Pleiotropy is the ability of a single gene to influence more than one phenotypic trait in an organism.
Phenylketonuria (PKU) in humans is an example — a mutation in the gene coding for phenylalanine hydroxylase causes both mental retardation and a reduction in hair and skin pigmentation.
Pleiotropy usually arises because the gene product participates in multiple metabolic pathways, each contributing to a different phenotype.

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Question 9

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State Mendel's Law of Independent Assortment. In which type of cross was it discovered?

The Law of Independent Assortment states that when two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair of characters.
It was discovered through dihybrid crosses — for example, between round yellow (RRYY) and wrinkled green (rryy) pea plants.
The F2 phenotypic ratio of 9:3:3:1 observed in dihybrid crosses provides experimental support for the independent assortment of alleles of different gene pairs during meiosis.

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Question 10

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Who proposed the Chromosomal Theory of Inheritance? What was the basis of their proposal?

Walter Sutton and Theodore Boveri independently proposed the Chromosomal Theory of Inheritance in 1902.
They noted that the behaviour of chromosomes during meiosis was parallel to the behaviour of Mendel's genes — both occur in pairs, segregate during gamete formation, and different pairs assort independently.
They argued that the pairing and separation of chromosomes during meiosis would mechanistically explain the segregation of alleles and the law of independent assortment as described by Mendel.

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Question 11

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What is linkage? How does it affect the results of a dihybrid cross?

Linkage is the physical association of two or more genes located on the same chromosome that tend to be transmitted together to offspring.
When two genes in a dihybrid cross are linked (on the same chromosome), they do not segregate independently and the F2 ratio deviates significantly from the expected 9:3:3:1 ratio.
Linked genes show a higher proportion of parental gene combinations in offspring; genes that are closely linked show very low recombination frequency, while loosely linked genes show higher recombination.

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Question 12

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Distinguish between autosomes and sex chromosomes.

Autosomes are the 22 pairs of chromosomes in humans that are identical in both males and females and do not determine the sex of an individual.
Sex chromosomes (the 23rd pair in humans) differ between males (XY) and females (XX) and are responsible for determining the sex of an individual.
Genes located on autosomes follow regular Mendelian inheritance, while genes on sex chromosomes (especially the X chromosome) show sex-linked inheritance patterns.

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Question 13

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How is sex determined in human beings? Which parent determines the sex of the child?

Human sex determination is of the XY type. Females are XX (produce only X-bearing eggs) and males are XY (produce 50% X-bearing and 50% Y-bearing sperm).
If an X-bearing sperm fertilises the egg, the zygote is XX and develops into a female; if a Y-bearing sperm fertilises the egg, the zygote is XY and develops into a male.
Thus, it is the genetic makeup of the sperm (contributed by the father) that determines the sex of the child — not the mother, despite socially prevalent misconceptions.

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Chapter 4: Principles of Inheritance and Variation | Short Questions

What is a true-breeding line in pea plants? Why did Mendel use such lines in his experiments?

Distinguish between genotype and phenotype with one example each.

State Mendel's Law of Segregation. On what observation is it based?

What is a test cross? What information does it provide about an organism?

What is incomplete dominance? How does it differ from complete dominance?

Explain co-dominance using ABO blood grouping as an example.

What are multiple alleles? Give one example from the chapter.

What is pleiotropy? Give one example from the chapter.

State Mendel's Law of Independent Assortment. In which type of cross was it discovered?

Who proposed the Chromosomal Theory of Inheritance? What was the basis of their proposal?

What is linkage? How does it affect the results of a dihybrid cross?

Distinguish between autosomes and sex chromosomes.

How is sex determined in human beings? Which parent determines the sex of the child?

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