Principles of Inheritance and Variation
12 solved NEET Biology previous year questions on Principles of Inheritance and Variation, each with the correct answer and a full explanation.
Q1
An allele is dominant if it is expressed in:
✓ Correct answer: a) both homozygous and heterozygous states.
ExplanationAn allele is dominant if it is expressed itself in the homozygous as well as heterozygous condition.(NCERT 12th Edition (2025-26) Page No. 56)
Q2
A polygenic inheritance in human beings is
✓ Correct answer: a) skin colour
ExplanationThe correct answer is A. skin colour.Explanation: Skin color in humans is determined by the interaction of multiple genes, not just a single gene pair.This is known as polygenic inheritance, where several genes contribute to a single trait.
Q3
Sex is determined in human beings
✓ Correct answer: b) At time of fertilisation
ExplanationThe sex of a human baby is genetically determined at the exact moment of fertilization (conception), when the sperm and the egg fuse.The egg (ovum) always carries an X sex chromosome.The sperm carries either an X or a Y sex chromosome.The combination at fertilization dictates the genetic sex:If an X-carrying sperm fertilizes the egg, the zygote is XX (female).If a Y-carrying sperm fertilizes the egg, the zygote is XY (male).While the physical differentiation of genitals occurs much later in fetal development (around the 7th to 8th week), the underlying genetic blueprint for sex is set immediately at fertilization.
Q4
The most striking example of point mutation is found in a disease called;
✓ Correct answer: b) Sickle cell anaemia.
ExplanationA) Down's syndrome Not due to point mutation.It is caused by trisomy of chromosome 21 (a chromosomal abnormality).B) Sickle cell anaemia Correct — It is the most striking example of point mutation.A single base substitution (A → T) in the gene coding for the β-globin chain of haemoglobin.This changes glutamic acid to valine at the 6th position.C) Thalassaemia Caused by mutations or deletions in the genes controlling haemoglobin production, but not typically a simple point mutation.
Q5
Genetic identity of a human male is determined by
✓ Correct answer: a) sex-chromosome
ExplanationA. Sex-chromosome ✅ Correct.The presence of a Y-chromosome determines the male genetic identity in humans.Males: XY, Females: XX.B. Cell organelles ❌ Organelles like mitochondria, ER, etc., are present in all cells and do not determine sex.C. Autosome ❌ Autosomes are chromosomes that do not determine sex (humans have 22 pairs of them).D. Nucleolus ❌ It is a structure inside the nucleus involved in ribosome production, not related to genetic sex identity. ✅ Correct Answer: A. sex-chromosome
Q6
Which one of the following conditions of the zygotic cell would lead to the birth of a normal human female child?
✓ Correct answer: a) Two X chromosomes
ExplanationThe correct answer is Option A: Two X chromosomes.In humans, sex determination depends on the combination of sex chromosomes present in the zygote.A normal human female has two X chromosomes (XX).This condition arises when an X-bearing sperm fertilizes an X-bearing ovum.The presence of two X chromosomes ensures normal female development, including proper formation of reproductive organs and secondary sexual characters.Option B (only one Y chromosome) is not viable, as at least one X chromosome is essential for survival.Option C (only one X chromosome) results in Turner syndrome (XO), which is not a normal female condition.Option D (one X and one Y chromosome) produces a normal male child (XY).Therefore, a zygotic cell with two X chromosomes leads to the birth of a normal human female child.
Q7
A gene showing co-dominance has:
✓ Correct answer: c) both alleles independently expressed in the heterozygote.
ExplanationThe alleles which do not show dominance - recessive relationship and are able to express themselves independently and equally when present together.(NCERT 12th Edition (2025-26) Page No. 67)
Q8
Which one from those given below is the period for Mendel's hybridisation experiments?
✓ Correct answer: d) 1856-1863
ExplanationGregor Mendel conducted his hybridisation experiments on garden peas (Pisum sativum) during the period 1856 to 1863. Hence, the correct answer is option D.
Q9
The genotypes of a husband and wife are and i respectively. Among the blood types of their children, how many different genotypes and phenotypes are possible?
✓ Correct answer: b) 4 genotypes; 3 phenotypes
Explanation(2) If the genotypes of a husband and wife are and i, then the possible offsprings will be; Gametes IA i IA IAIA IAi IB IAIB IBi Therefore, the offspring will have 4 genotypes (IAIA , IAi, IBi, IAIB) ; 3 phenotypes (Blood group A, B and AB). [New NCERT Class 12th Page No. 61]
Q10
Which of the following occurs due to the presence of autosome linked dominant trait? [NEET 2022]
✓ Correct answer: c) Myotonic dystrophy
ExplanationAn autosomal dominant trait appears when a person inherits one defective allele from either parent — the presence of a single mutant gene is enough to express the disorder. eg.Myotonic dystrophy: An autosomal dominant disorder; even one abnormal gene copy causes the condition.Thalassemia: Caused by mutations in haemoglobin genes and is autosomal recessive, meaning both alleles must be defective for the disease to appear.Sickle cell anaemia: Also an autosomal recessive condition; the disease is expressed only in individuals who are homozygous recessive.Haemophilia: This is X-linked recessive, primarily affecting males since they have only one X chromosome.
Q11
Given below are two statements: Statement I: Mendel studied seven pairs of contrasting traits in pea plants and proposed the Laws of Inheritance. Statement II: Seven characters examined by Mendel in his experiment on pea plants were seed shape and colour, flower colour, pod shape and colour, flower position, and stem height. In light of the above statements, choose the most appropriate answer from the options given below: [NEET 2022]
✓ Correct answer: b) Both Statement I and Statement II are correct.
ExplanationStatement I is correct- Mendel conducted hybridisation experiments on garden peas for seven years (1856-1863) and proposed the laws of inheritance in living organisms- the law of dominance, the law of segregation, and the law of independent assortment.Statement II is also correct- 7 pairs of contrasting traits examined by Mendel were seed shape, seed colour, flower colour, pod shape, pod colour, flower position and stem height.
Q12
Given below are two statements: Statement I : Failure of segregation of chromatids during cell cycle resulting in the gain or loss of whole set of chromosome in an organism is known as aneuploidy. Statement II : Failure of cytokinesis after anaphase stage of cell division results in the gain or loss of a chromosome is called polyploidy. In the light of the above statements, choose the most appropriate answer from the options given below: [Re-NEET 2024]
✓ Correct answer: d) Both Statement I and Statement II are false.
ExplanationFailure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy.For example, Down’s syndrome results in the gain of extra copy of chromosome21. Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and, this phenomenon is known as polyploidy.This condition is often seen in plants.
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