NCERT Solutions

1. Distinct Characters: Pea plants have many visible and contrasting characters (e.g., tall/dwarf, violet/white flowers).
2. Short Life Span: They complete their life cycle in one season, allowing multiple generations to be studied quickly.
3. Self-Pollination: Flowers are bisexual and naturally self-pollinating, making it easy to obtain pure lines.
4. Cross-Pollination: They can be easily cross-pollinated artificially (emasculation and bagging).
5. Large Progeny: A single cross produces a large number of seeds, providing statistical reliability.

Formula: The number of types of gametes = \(2^n\), where \(n\) is the number of heterozygous loci.

Here, \(n = 4\).

Number of gametes = \(2^4 = 2 \times 2 \times 2 \times 2 = 16\).

The Law of Dominance states that:

• Characters are controlled by discrete units called factors (genes).
• Factors occur in pairs.
• In a dissimilar pair of factors (heterozygous), one member of the pair dominates (dominant) the other (recessive).

Cross: Pure Tall (TT) x Pure Dwarf (tt).

F1 Generation: All offspring are Hybrid Tall (Tt). Here, the allele ‘T’ (Tallness) expresses itself, while ‘t’ (Dwarfness) is suppressed. This proves dominance.

F2 Generation: On selfing (Tt x Tt), 3 Tall : 1 Dwarf plants are obtained.

Definition: A cross between an organism with a dominant phenotype (whose genotype is unknown) and a homozygous recessive parent is called a Test Cross. It is used to determine the genotype of the dominant parent.

Let’s test a Tall Plant (T?). It could be TT or Tt. Cross it with Dwarf (tt).

Let the single locus be for Height (T).
Female (Homozygous): TT (Tall) or tt (Dwarf). Let’s assume Dominant TT.
Male (Heterozygous): Tt (Tall).

Result: All offspring (100%) in F1 generation will be Tall Phenotypically.

Parent 1: TtYy (Tall, Yellow)
Parent 2: Ttyy (Tall, Green)

Gametes:
P1: TY, Ty, tY, ty
P2: Ty, ty

Total Combinations: \(4 \times 2 = 8\).

• (a) Tall and Green: Genotypes TTyy, Ttyy. Frequency: 3/8.
• (b) Dwarf and Green: Genotype ttyy. Frequency: 1/8.

If the two loci are completely linked (on the same chromosome and close together), they do not sort independently. The parental gene combinations are inherited together.

• If parents are Heterozygous (e.g., AaBb x AaBb) and linkage is complete (AB/ab arrangement).
• Gametes formed will be only parental types (AB and ab). No recombinants (Ab, aB).
• Result:
  • 1 AABB (Dominant)
  • 2 AaBb (Dominant)
  • 1 aabb (Recessive)
• Phenotypic Ratio: 3 Dominant : 1 Recessive (Similar to Monohybrid).
• Note: This deviates from the expected 9:3:3:1 ratio of independent assortment.

Thomas Hunt Morgan is known as the “Father of Experimental Genetics”. His contributions using fruit flies (Drosophila melanogaster) include:

1. Chromosomal Theory Verification: He experimentally verified the chromosomal theory of inheritance.
2. Linkage and Recombination: He discovered that genes located on the same chromosome are linked and inherited together, while recombination (crossing over) breaks this linkage.
3. Sex-Linked Inheritance: He discovered sex-linked traits (like white eye colour in Drosophila) located on the X-chromosome.
4. Gene Mapping: His student Sturtevant used recombination frequencies to map gene positions on chromosomes.

Definition: Pedigree analysis is the study of the inheritance of genetic traits in several generations of a family, represented in the form of a family tree diagram.

• It helps in tracing the inheritance of a specific trait, abnormality, or disease (e.g., Haemophilia, Colour blindness).
• It helps to determine if a trait is dominant or recessive, and autosomal or sex-linked.
• It is useful for Genetic Counselling to predict the probability of genetic disorders in future offspring.

Sex determination in humans is of the XX-XY type.

• Females: Homogametic. They have two X chromosomes (XX). They produce only one type of ovum (22 + X).
• Males: Heterogametic. They have one X and one Y chromosome (XY). They produce two types of sperms: (22 + X) and (22 + Y).
• Determination:
  • If an X-sperm fertilizes the egg \(\to\) XX (Female).
  • If a Y-sperm fertilizes the egg \(\to\) XY (Male).

Thus, the sex of the child is determined by the father (sperm).

• Child Blood Group O: Genotype must be ii.
• This implies the child received one ‘i’ allele from each parent.
• Father (Group A) must have genotype \(I^A i\).
• Mother (Group B) must have genotype \(I^B i\).

Cross: \(I^A i \times I^B i\)

Other possible genotypes: \(I^A I^B\) (AB), \(I^A i\) (A), \(I^B i\) (B).

A condition where both alleles of a gene pair in a heterozygote are fully expressed, neither being dominant or recessive.

Example: ABO blood grouping in humans. In blood group AB (\(I^A I^B\)), both A and B antigens are expressed on RBCs.

A condition where the F1 hybrid exhibits a phenotype that is intermediate between the two homozygous parents.

Example: Flower colour in Snapdragon (Antirrhinum majus). Cross between Red (RR) and White (rr) gives Pink (Rr) flowers.

Point Mutation: A mutation arising due to a change in a single base pair of DNA.

Example: Sickle Cell Anaemia. It is caused by the substitution of Glutamic acid (GAG) by Valine (GUG) at the 6th position of the beta-globin chain of haemoglobin due to a single base change (A to U in mRNA, T to A in DNA).

The Chromosomal Theory of Inheritance was proposed by Sutton and Boveri (Walter Sutton and Theodore Boveri) in 1902.

1. Sickle Cell Anaemia:
   • Defect: Substitution of Glutamic acid by Valine in Haemoglobin.
   • Symptoms: RBCs become sickle-shaped under low oxygen tension, causing blockage of blood vessels, anaemia, and pain.
2. Down’s Syndrome:
   • Cause: Trisomy of Chromosome 21 (Autosomal aneuploidy).
   • Symptoms: Short stature, round head, furrowed tongue, partially open mouth, broad palm with palm crease, physical and mental retardation.