Principles of Inheritance and Variation MCQs

Mendel needed a plant with easily observable, distinct traits and controllable pollination.

Pea plants produce many seeds per pod, allowing large sample sizes for statistical analysis.

Their flowers are self‑fertilizing but can be cross‑pollinated manually, making hybridisation straightforward.

Therefore Mendel chose Pisum sativum.

Hybrid shows intermediate phenotype (pink) between red and white parents, not a blend of both distinct traits.

In incomplete dominance the heterozygote expresses a phenotype midway between the two homozygotes.

Thus the red × white cross giving pink F1 exemplifies incomplete dominance.

Phenylketonuria is caused by a single gene defect that affects the enzyme phenylalanine hydroxylase, leading to multiple unrelated symptoms such as mental retardation, skin lesions and eczema.

Since one gene influences several distinct phenotypic traits, it exemplifies pleiotropy.

This classic case has been studied both in Drosophila mutants and in human patients.

In birds sex chromosomes are Z and W; males have two Z chromosomes (ZZ) and females have one Z and one W.

The heterogametic sex possesses two different sex chromosomes, so the female is ZW.

Thus the correct representation for the female bird is ZW.

HbA (normal β‑globin) and HbS (mutant β‑globin) are co‑dominant; the heterozygote produces both normal and sickle hemoglobin.

Presence of sufficient normal HbA prevents severe disease, but some sickling occurs under low‑oxygen stress.

Thus the individual shows the sickle‑cell trait, acting as a carrier without full disease.

A carrier with sickle‑cell trait.

Two identical alleles mean both copies of the gene are the same.

When the alleles are alike, the genotype is not mixed (not heterozygous).

Such a genotype is termed homozygous.

Hence the organism is Homozygous.

A single gene can produce products that influence different biochemical pathways.

When mutations in that gene cause changes in several distinct characteristics, the gene shows pleiotropy.

Thus the phenomenon described matches pleiotropy.

Grasshoppers have males with one X chromosome and females with two X chromosomes.

Males are hemizygous (XO) and produce sperm carrying either no sex chromosome or an X.

Females are homogametic (XX) and produce eggs all bearing an X chromosome.

Thus sex is determined by the presence or absence of a second X → XX‑XO.

The normal adult hemoglobin is HbA (α2β2).

In sickle‑cell disease a point mutation replaces glutamic acid by valine in the β‑chain, producing a variant called hemoglobin S.

This mutant hemoglobin polymerizes under low oxygen, causing sickling of red cells.

Hence the mutant hemoglobin is designated HbS.

Thalassaemia involves reduced production of one or more globin chains, so the amount of globin synthesized is decreased.

Sickle‑cell disease results from a point mutation that alters the structure of the β‑globin chain, a qualitative change.

Thus thalassaemia is a quantitative defect in globin synthesis. Quantitative defect in globin synthesis.

During meiosis each gene exists as a pair of homologous alleles in the diploid parent.

At anaphase I the paired alleles separate so that each gamete receives only one allele.

Thus the two alleles are split apart during gamete formation, which is the basis of the law of segregation.

In incomplete dominance the heterozygote shows an intermediate phenotype, so the three genotypes (AA, Aa, aa) each produce distinct phenotypes.

The F2 generation from a heterozygote cross (Aa × Aa) yields genotypic ratios 1 AA : 2 Aa : 1 aa.

Since each genotype corresponds to a different phenotype, the phenotypic ratio is also 1:2:1.

Thus the correct answer is 1:2:1.

Starch provides turgor to the seed; without starch the seed collapses and contracts.

The gene controlling starch synthesis is pleiotropic, also determining seed shape.

Lack of starch leads to a shrunken, wrinkled surface rather than a smooth round one.

Hence starchless seeds appear wrinkled.

Unfertilised egg contains only the maternal set of chromosomes.

During development the egg does not undergo fertilisation, so no paternal chromosomes are added.

Thus the resulting male (drone) has a single set of chromosomes, i.e., it is haploid.

Haemophilia is a genetic disorder of the clotting cascade, usually due to deficiency of factor VIII or IX.

Without these clotting factors the intrinsic pathway cannot generate fibrin, so bleeding persists.

The clinical hallmark is prolonged bleeding and inability of blood to form a stable clot.

Therefore the correct answer is Failure of blood to clot.

Independent assortment refers to the random segregation of two different gene pairs during gamete formation.

A dihybrid cross involves individuals heterozygous for two traits (AaBb × AaBb), allowing observation of all allele combinations.

The resulting 9:3:3:1 phenotypic ratio shows that the alleles of one gene sort independently of the other.

Thus the law is best demonstrated by a dihybrid cross.

Both A and B alleles are expressed simultaneously in the phenotype of an AB individual.

Since neither allele masks the other and each produces its own antigen, the traits are visible together.

This pattern of two alleles being fully expressed in the heterozygote is called codominance.

Genes on the same chromosome are physically close, so during meiosis they usually do not separate by crossing‑over.

Because they travel together to gametes, they are inherited as a unit.

Such genes are described as linked, matching the given answer. Linked

In honey bees, fertilized eggs develop into diploid females and unfertilized eggs develop into haploid males.

Males arise from a single set of chromosomes (haploid) while females are diploid, a hallmark of haplodiploidy.

Thus the sex‑determination system in bees is called haplodiploidy. Haplodiploidy.

Haemophilia gene is located on the X chromosome, so males (XY) express the disease when they inherit the defective X.

Females (XX) need two copies of the mutant allele to be affected, otherwise they are carriers.

Thus the pattern fits an X‑linked recessive inheritance.

Correct answer: X-linked recessive trait