Heredity

Detailed Solutions Q1 – Q4

Question 1

A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as:

(a) TTWW (b) TTww
(c) TtWW (d) TtWw

Correct Answer: (c) TtWW

Reasoning:
1. Flower Colour: Since all the progeny bore violet flowers, the parent must carry the dominant allele for violet in a homozygous state ($WW$).
2. Height: Since the progeny contained both tall and short plants, the tall parent cannot be pure tall ($TT$). It must be heterozygous ($Tt$) to pass on the recessive ‘short’ trait to half the offspring.
3. The Cross: $$TtWW \times ttww \rightarrow TtWw \text{ (Tall)} + ttWw \text{ (Short)}$$

[Image of Mendelian dihybrid cross]

Question 2

A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?

Answer: No, it cannot be concluded.

Reasoning:
To decide if a trait is dominant or recessive, we need to know the frequency of the trait in the population or see a cross where a trait “skips” a generation.
• If Light eyes ($L$) are dominant and parents are $LL$, children are $LL$ (Light).
• If Light eyes ($l$) are recessive and parents are $ll$, children are $ll$ (Light).
Since both scenarios fit the observation, the data is insufficient.

Question 3

Outline a project which aims to find the dominant coat colour in dogs.

Project Plan:

1. Select a purebred Homozygous Black ($BB$) male dog and a purebred Homozygous Brown ($bb$) female dog.

2. Breed them to obtain the F1 generation.

3. Observation: If all puppies in the F1 generation are black, then Black is the dominant trait.

F1 Generation Cross
	B (Male)	B (Male)
b (Female)	Bb (Black)	Bb (Black)
b (Female)	Bb (Black)	Bb (Black)

F2 Generation: If we cross the F1 heterozygous dogs ($Bb \times Bb$):
$$Bb \times Bb \rightarrow 25\% BB : 50\% Bb : 25\% bb$$
This would result in a ratio of 3 Black : 1 Brown.

Question 4

How is the equal genetic contribution of male and female parents ensured in the progeny?

Answer: This is ensured by the processes of Gamete Formation (Meiosis) and Fertilization.

1. Halving the Chromosomes (Meiosis):
Human body cells have 46 chromosomes (23 pairs). During the formation of gametes (sperm and egg), a special cell division called meiosis occurs. This reduces the chromosome count by half.
• Sperm: 23 Chromosomes
• Egg: 23 Chromosomes

2. Restoring the Number (Fertilization):
When the sperm fuses with the egg, their genetic material combines.
$$23 \text{ (Father)} + 23 \text{ (Mother)} = 46 \text{ (Offspring)}$$
This ensures that the offspring receives exactly half of its genetic material from each parent.

[Image of sex determination in humans]