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Principles of Inheritance and Variation

Q1. What is linkage and recombination?


Linkage- The physical Association of two genes on a chromosome is termed as linkage.

Recombination- The generation of non-parental gene combinations is termed as recombination.

Q2. Mention the karyotype of chromosomal disorders.


Down’s syndrome: 45A+XX, 45A+XY (Trisomy of 21)

Klinefelter’s syndrome: 44+XXY (47 Chromosomes)

Turner’s syndrome: 44+X0 (45 Chromosomes)

Q3. Explain the sex determination mechanism in human beings.


  • Humans show XY type of sex determination mechanism.

  • Out of 23 pair of chromosomes, 22 are autosomes (same in both males and females).

  • Females have a pair of X chromosomes.

  • Males have an X and a Y chromosome.

  • During spermatogenesis Males produce two types of gametes with equal probability - sperm carrying either X or Y chromosome.

  • During oogenesis females produce only one type of gamete having X chromosome.

  • An ovum fertilized by the sperm carrying X-chromosome develops into a female (XX) and an ovum fertilized by the sperm carrying Y-chromosome develops into a male (XY).

Q4. Describe Haplodiploid sex determination system Honey Bees.


  1. Honeybee show haploid sex determination system.

  2. Offsprings formed from union of a sperm and an egg develops a female (queen or workers), which are diploid, having 32 chromosomes.

  3. Unfertilized eggs developed by parthenogenesis form male (drone), which are haploid having 16 chromosomes.

  4. Male produce sperms by Mitosis, so they, neither have fathers nor sons but have grandfathers and grandsons.

Q5. Write a note on Down's syndrome.


Down’s syndrome: Down’s syndrome is caused by an extra copy of chromosome number 21 (trisomy of 21).


  1. Short statured with small round head.

  2. Partially open mouth with protruding furrowed tongue.

  3. Palm is broad with characteristic palm crease.

  4. Physical, psychomotor and mental development is retarded.

Q6. Write a note on Klinefelter's syndrome.


Klinefelter’s Syndrome: Klinefelter’s syndrome is caused by the presence of an additional copy of X-chromosomes, resulting in the karyotype of 44+XXY.


  1. Sex of the individual is masculine but possess feminine characters.

  2. Gynaecomastia i.e., development of breasts.

  3. Poor beard growth and often sterile.

  4. Feminine pitched voice

Q7. What is co-dominance? Give an example.

Ans: The Alleles which are able to express themselves independently, even when present together are called co-dominant alleles and this biological phenomenon is called co-dominance.

E.g., ABO type of blood group in human.

Q8. Give an account of Haemophilia.


  1. It is a sex-linked recessive disorder. Which shows its transmission from unaffected carrier female to some of the male progeny.

  2. Patient continues to bleed even with a minor cut because of a defect in blood coagulation

  3. The gene for haemophilia is located on X-chromosome.

  4. More males suffer from haemophilia than females because in males single gene for the defect is able to express as male have only one X-chromosome.

  5. The defective alleles produce non-functional proteins which later form a non-functional cascade of proteins involved in blood clotting.

Q9. Explain in complete dominance with a suitable example.

Ans: It’s a phenomenon in which the F1 hybrid exhibits intermediate characters of the parental gene.

  1. It is seen in flower colours of Mirabilis jalapa (4 O'clock plant and Antirrhinus majus (Snapdragon).

  2. In a cross between true breeding red flowered (RR) and true breeding white flowered plants (rr), the F1 (Rr) was pink. When the F1 was self-pollinated the F2 resulted in the following ratio 1 Red (RR) : 2 Pink (Rr) : 1 White (rr). Here the phenotypic ratio deviates from the Mendel's monohybrid cross.

  3. Both phenotypic and genotypic ratio will be the same 1:2:1.

Q10. Explain Mendel’s dihybrid cross by giving suitable example.

Ans: The cross between two homozygous individuals differing in two characters is called dihybrid cross.

  1. Mendel crossed true breeding round yellow-coloured seeds (RRYY) with wrinkle green coloured seeds (rryy).

  2. The gametes RY and ry unite on fertilisation to produce the F1 hybrid RrYy.

  3. Mendel found that the seeds resulting from the crossing of the parents, had round shaped yellow coloured seeds (RrYy) in F1 generation

  4. In the next step, F1 hybrids were self-pollinated.

  5. In the F2 generation 9 Round, yellow: 3 Wrinkled, yellow: 3 Round, green: 1 Wrinkle, green seeds are produced.

  6. The phenotypic ratio of dihybrid cross is 9:3:3:1.

Q11. Explain the inheritance of one gene / monohybrid cross / law of segregation with suitable example.

Ans: The crossing between two homozygous plants, with respect to a single contrasting character is called monohybrid cross.

  1. Mendel conducted hybridization experiment by crossing true-breed tall pea plant (TT) with true-breed dwarf pea plant (tt).

  2. In F1 (First filial) generation only one of the parental traits (tall) was seen and trait of other plant (dwarf) was not seen.

  3. Then Mendel self-pollinated the tall F1 plants.

  4. He found that in the F2 generation some of the offspring were ‘dwarf’; the character that was not seen in the F1 generation was now expressed.

  5. In the F2 generation both tall and dwarf plants were appeared in the ratio of 3:1 respectively.

  6. F2 plants were identical to their parental type and did not show any blending.

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