Principles of Inheritance and Variation — Class 12 MCQs with Answers

Principles of Inheritance and Variation — important questions & MCQs with answers (Class 12 Biology)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   The father of modern genetics is:

   ACharles DarwinBWatson and CrickCGregor MendelDThomas Morgan
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   Answer: Gregor Mendel

   Explanation: Gregor Mendel is called the father of genetics for discovering the laws of inheritance using garden pea (Pisum sativum) between 1856 and 1863, decades before chromosomes or DNA were known; Darwin studied evolution, not inheritance mechanics.

2. Q2 of 6Easy

   In a heterozygous organism (Tt), the trait that appears in the phenotype is:

   ADominant (T)BRecessive (t)CBoth equallyDNeither
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   Answer: Dominant (T)

   Explanation: In a heterozygous Tt individual, the dominant allele T is expressed and masks the recessive allele t, so the phenotype matches the homozygous dominant TT — students often think the traits blend instead of one being masked.

3. Q3 of 6Medium

   Haemophilia is an example of:

   AAutosomal dominant disorderBX-linked recessive disorderCAutosomal recessive disorderDY-linked disorder
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   Answer: X-linked recessive disorder

   Explanation: Haemophilia is X-linked recessive: the faulty allele sits on the X chromosome, so a hemizygous male (one X) shows the disease from a single copy, while a female needs two copies — far rarer than in males.

4. Q4 of 6Medium

   A test cross of a heterozygous tall plant (Tt) gives offspring in the ratio:

   

   AAll tallB3 tall : 1 dwarfC1 tall : 1 dwarfD1:2:1
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   Answer: 1 tall : 1 dwarf

   Explanation: A test cross of a heterozygous Tt tall plant with a homozygous recessive tt dwarf plant gives gametes T and t from the tall parent, so offspring split 1 Tt (tall) : 1 tt (dwarf) — a 1:1 ratio, not the 3:1 seen when selfing Tt.

5. Q5 of 6Hard

   ABO blood-group inheritance is an example of:

   AComplete dominance onlyBPolygenic inheritanceCPleiotropyDCodominance and multiple alleles
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   Answer: Codominance and multiple alleles

   Explanation: ABO blood groups involve three alleles — Iᴬ, Iᴮ and i — at one locus (multiple alleles); Iᴬ and Iᴮ are both fully expressed together in the AB phenotype (codominance), while i is recessive to both.

6. Q6 of 6Hard

   In the pedigree, a haemophilic son is born to phenotypically normal parents. The genotypes of the parents must be:

   

   ABoth affectedBCarrier mother and normal fatherCNormal homozygous mother and affected fatherDAffected mother and normal father
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   Answer: Carrier mother and normal father

   Explanation: A haemophilic son born to phenotypically normal parents means the affected X came from his mother, who must be a carrier while appearing normal because her second X is unaffected; the father, being normal, contributes only a normal X and Y.

Principles of Inheritance and Variation notes: key concepts (Class 12 Biology)

Principles of inheritance: Mendel's laws of dominance, segregation and independent assortment from pea crosses; deviations like incomplete dominance, co-dominance and multiple alleles (ABO); the chromosomal theory, linkage and recombination; sex determination; mutation; and human genetic disorders.

Law of dominance

Of a pair of contrasting alleles, the dominant one expresses in the heterozygote and masks the recessive; alleles do not blend.

Law of segregation

The two alleles of a gene separate during gamete formation, so each gamete carries only one; monohybrid F2 is 3:1 phenotype, 1:2:1 genotype.

Law of independent assortment

Alleles of different genes assort independently in gamete formation (genes on different chromosomes); a dihybrid F2 gives the 9:3:3:1 ratio.

Test cross

Crossing an individual with a homozygous recessive to reveal its genotype; a heterozygote (Tt × tt) gives a 1:1 phenotypic ratio.

Incomplete dominance

The heterozygote shows an intermediate phenotype (e.g., pink Antirrhinum); the F2 phenotype ratio equals the genotype ratio, 1:2:1.

Co-dominance & multiple alleles

Co-dominance: both alleles express fully (AB blood group). ABO is governed by three alleles Iᴬ, Iᴮ, i — an example of multiple allelism.

Pleiotropy

A single gene affecting several traits; e.g., phenylketonuria affects both mental ability and pigmentation from one gene defect.

Polygenic inheritance

A trait controlled by several genes with additive effects, giving continuous variation; e.g., human skin colour and height.

Linkage and recombination

Genes on the same chromosome tend to be inherited together (linkage, shown by Morgan); crossing over makes recombinants, whose frequency gives map distance.

Sex determination

In humans XX = female, XY = male (male heterogametic); birds use ZZ/ZW, grasshoppers XO. The sperm decides the sex of the child.

Mendelian disorders

Single-gene traits: haemophilia and colour blindness (X-linked recessive), sickle-cell anaemia and PKU (autosomal recessive), Huntington's (dominant).

Chromosomal disorders

From aneuploidy/polyploidy due to non-disjunction: Down's (trisomy 21), Turner's (XO), Klinefelter's (XXY).

Point mutation

A single base-pair change in DNA; substitution of glutamic acid by valine at position 6 of β-globin causes sickle-cell anaemia.

Pedigree analysis

Study of inheritance of a trait through a family chart; used to trace dominant/recessive and autosomal/sex-linked patterns of human disorders.