Explain how color blindness is inherited as a sex linked traits in humans. Illustrate the inheritance of colour blindness using suitable genetic crosses and analyze the resulting phenotypic…

Inheritance of Color Blindness as a Sex-Linked Trait in Humans

1. Explanation of Sex-Linked Inheritance of Color Blindness (2 marks)

Color blindness is a sex-linked recessive trait primarily inherited through the X-chromosome. The gene responsible for this condition is located on the X-chromosome (Xⁱ), while the normal allele is Xᴺ. Since males have only one X-chromosome (XY), they express the trait if they inherit the defective allele (XⁱY). Females (XX) must inherit two recessive alleles (XⁱXⁱ) to exhibit color blindness, making them carriers (XⁱXᴺ) if heterozygous.

2. Genetic Crosses and Phenotypic Outcomes (4 marks)

We analyze inheritance using Punnett squares for different crosses:

Cross 1: Carrier Mother (XⁱXᴺ) × Normal Father (XᴺY)

       Xᴺ   |   Xⁱ
-------|-------|-------
  Xᴺ   | XᴺXᴺ | XᴺXⁱ
-------|-------|-------
  Y    | XᴺY   | XⁱY

Phenotypic Ratio:

• 25% Normal daughters (XᴺXᴺ)
• 25% Carrier daughters (XᴺXⁱ)
• 25% Normal sons (XᴺY)
• 25% Color-blind sons (XⁱY)

Cross 2: Color-Blind Mother (XⁱXⁱ) × Normal Father (XᴺY)

       Xᴺ   |   Xⁱ
-------|-------|-------
  Xⁱ   | XⁱXᴺ | XⁱXⁱ
-------|-------|-------
  Y    | XⁱY   | XⁱY

Phenotypic Ratio:

• 100% Carrier daughters (XⁱXᴺ)
• 100% Color-blind sons (XⁱY)

Cross 3: Carrier Mother (XⁱXᴺ) × Color-Blind Father (XⁱY)

       Xᴺ   |   Xⁱ
-------|-------|-------
  Xⁱ   | XⁱXᴺ | XⁱXⁱ
-------|-------|-------
  Y    | XⁱY   | XⁱY

Phenotypic Ratio:

• 25% Normal daughters (XᴺXᴺ)
• 25% Carrier daughters (XⁱXᴺ)
• 50% Color-blind sons (XⁱY)

3. Justification for Criss-Cross Inheritance (2 marks)

The term "criss-cross inheritance" arises because:

• The defective allele (Xⁱ) moves from mother to son (vertical transmission).
• The carrier status (XⁱXᴺ) is passed from affected father to daughters (horizontal transmission).
• The trait skips generations and appears more frequently in males due to their single X-chromosome.

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OR

Biologically Feasible Model of DNA Replication: Semi-Conservative Model

1. Identification of the Correct Model (3 marks)

The semi-conservative model (proposed by Watson and Crick) is the biologically feasible mechanism of DNA replication. It states that:

• Each parental DNA strand acts as a template for synthesizing a new complementary strand.
• After replication, each new DNA molecule consists of one old strand and one newly synthesized strand.

2. Mechanism of Semi-Conservative Replication (5 marks)

DNA replication occurs during the S-phase of interphase and involves the following steps:

Step 1: Initiation

• Helicase unwinds the double helix, breaking hydrogen bonds between base pairs.
• Single-Strand Binding Proteins (SSBPs) stabilize the unwound strands.
• Topoisomerase (Gyrase) relieves supercoiling ahead of the replication fork.

Step 2: Primer Synthesis

• Primase synthesizes a short RNA primer to provide a 3’-OH group for DNA polymerase.

Step 3: Elongation

• DNA Polymerase III adds deoxyribonucleotides in the 5’ → 3’ direction.
  • Leading strand: Synthesized continuously.
  • Lagging strand: Synthesized discontinuously as Okazaki fragments.
• DNA Polymerase I removes RNA primers and fills gaps with DNA.

Step 4: Termination & Ligation

• DNA Ligase seals nicks between Okazaki fragments.
• Two identical semi-conservative DNA molecules are formed.

3. Role of Enzymes in Replication

4. Illustration of Semi-Conservative Replication