Exercises

1. Mention the advantages of selecting pea plant for experiment by Mendel.

• Pea plants have many distinct contrasting characters (e.g., Tall/Dwarf, Round/Wrinkled seeds).
• They are normally self-pollinating but can be easily cross-pollinated.
• They are easy to grow and have a short life cycle.
• A large number of seeds are produced in a single generation, providing a good sample size for statistical analysis.

2. Differentiate between the following –

• (a) Dominance and Recessive: Dominance refers to the trait that is expressed in the F1 generation and masks the other trait. Recessive refers to the trait that is suppressed in the F1 and only expressed in the homozygous condition.
• (b) Homozygous and Heterozygous: Homozygous means having two identical alleles for a particular gene (e.g., TT or tt). Heterozygous means having two different alleles for a particular gene (e.g., Tt).
• (c) Monohybrid and Dihybrid: A monohybrid cross involves one pair of contrasting traits. A dihybrid cross involves two pairs of contrasting traits.

3. A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?

• Number of gamete types = 2^n, where n is the number of heterozygous loci.
• For n = 4, types of gametes = 2^4 = 16.

4. Explain the Law of Dominance using a monohybrid cross.

• In a monohybrid cross between a homozygous tall plant (TT) and a homozygous dwarf plant (tt), the F1 generation consists of all tall plants (Tt).
• Even though the F1 plants have both 'T' and 't' alleles, only the 'T' trait (tallness) is expressed.
• This proves the Law of Dominance: in a pair of dissimilar factors, one dominates (T) and the other is recessive (t).

5. Define and design a test-cross.

• Definition: A test cross is a cross between an individual with a dominant phenotype (whose genotype is unknown) and a homozygous recessive individual.
• Design: If we have a tall pea plant of unknown genotype (TT or Tt), we cross it with a dwarf plant (tt).
  • If all offspring are tall, the unknown parent is TT.
  • If 50% are tall and 50% are dwarf, the unknown parent is Tt.

6. Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.

• Assume the trait is height (T/t).
• Female (Homozygous recessive): tt
• Male (Heterozygous): Tt
• Punnett Square:

  	T	t
  t	Tt	tt
  t	Tt	tt

• Phenotypic distribution: 50% Tall (Tt) and 50% Dwarf (tt).

7. When a cross in made between tall plant with yellow seeds (TtYy) and tall plant with green seed (Tt yy), what proportions of phenotype in the offspring could be expected to be (a) tall and green, (b) dwarf and green.

• Cross: TtYy x Ttyy
• Gametes from TtYy: TY, Ty, tY, ty
• Gametes from Ttyy: Ty, ty
• Punnett Square:

  	Ty	ty
  TY	TTYy	TtYy
  Ty	TTyy	Ttyy
  tY	TtYy	ttYy
  ty	Ttyy	ttyy

• (a) Tall and green (TTyy, Ttyy, Ttyy): 3/8
• (b) Dwarf and green (ttyy): 1/8

8. Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in F1 generation for a dibybrid cross?

• If two loci are completely linked, they will segregate together as a single unit.
• The phenotypic ratio will resemble a monohybrid cross (3:1) rather than the standard dihybrid ratio (9:3:3:1), because no independent assortment occurs between the linked genes.

9. Briefly mention the contribution of T.H. Morgan in genetics.

• Experimental verification of the chromosomal theory of inheritance.
• Discovery of sex-linked inheritance (using white-eyed Drosophila).
• Coined the terms linkage and recombination.
• Initiated the mapping of chromosomes based on recombination frequencies.

10. What is pedigree analysis? Suggest how such an analysis, can be useful.

• Pedigree analysis is the study of the inheritance of a particular trait across several generations of a family, represented as a family tree.
• Usefulness:
  • It helps in tracing the inheritance of genetic disorders (like haemophilia, colour blindness).
  • It helps genetic counselors to predict the probability of a disorder occurring in future offspring.
  • It can identify whether a trait is dominant, recessive, or sex-linked.

11. How is sex determined in human beings?

• Human sex determination is of the XY type.
• Females have two X chromosomes (XX) and are homogametic (all ova carry an X).
• Males have one X and one Y chromosome (XY) and are heterogametic (50% sperms carry X, 50% carry Y).
• Fusion of an X-ovum with an X-sperm results in a female (XX).
• Fusion of an X-ovum with a Y-sperm results in a male (XY).
• The sex of the child is determined by the father's sperm.

12. A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.

• Child's blood group O = genotype ii.
• This means both parents must carry the 'i' allele.
• Father (Group A) must be IAi.
• Mother (Group B) must be IBi.
• Possible genotypes of other offspring:
  • IAIB (Group AB)
  • IAi (Group A)
  • IBi (Group B)

13. Explain the following terms with example:

• (a) Co-dominance: Both alleles express themselves simultaneously in a heterozygote. Example: AB blood group in humans, where both IA and IB alleles are expressed.
• (b) Incomplete dominance: The F1 phenotype is an intermediate between the two parents. Example: Pink flowers in Snapdragon, resulting from a cross between red (RR) and white (rr) flowered plants.

14. What is point mutation? Give one example.

• A mutation resulting from a change in a single base pair of DNA.
• Example: Sickle-cell anaemia, where GAG is changed to GUG in the beta-globin gene.

15. Who had proposed the chromosomal theory of the inheritance?

• Walter Sutton and Theodore Boveri in 1902.

16. Mention any two autosomal genetic disorders with their symptoms.

1. Sickle-cell anaemia: Symptoms include anaemia, jaundice, and organ damage due to sickle-shaped RBCs clogging capillaries.
2. Down’s syndrome: Symptoms include short stature, small round head, furrowed tongue, partially open mouth, and mental retardation.