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This chapter explores how traits are passed from parents to offspring. Here are the key takeaways:

  • Heredity: The transmission of features from parents to offspring.
  • Variation: The differences seen among individuals of a species.
  • Mendel’s Experiments: Pioneering work by Gregor Mendel on pea plants that established the basic rules of inheritance.
  • Genes: The units of heredity found on chromosomes, carrying instructions for traits.
  • Alleles: Different versions of a gene that can determine the expression of a trait.
  • Dominant and Recessive Alleles: Dominant alleles mask the effect of recessive alleles for a particular trait.
  • Punnett Squares: A tool used to predict the probability of inheriting specific traits in offspring.
  • Sex Determination: The process by which the sex of an offspring is determined (e.g., XX for female, XY for male in humans).

Key points to remember:

  • Offspring inherit half their genes from each parent.
  • Sexual reproduction allows for genetic variation, which is essential for adaptation and evolution.
  • Understanding heredity has applications in agriculture, medicine, and selective breeding.

Questions (Page 129)

1. If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier ?

Ans : In asexual reproduction, more common traits are older. Here, Trait B (60%) is more widespread than Trait A (10%), so Trait B likely appeared earlier and had more time to spread through the population.

2. How does the creation of variations in a species promote survival ?

Ans : Variations in a species act like a kind of survival insurance in a changing world. Here’s how:

  • Variety is Key: Variations create a pool of different traits within a population. These traits might include things like size, color, resistance to disease, or tolerance for different environments.
  • Environmental Challenges: When the environment changes (e.g., new predator, climate shift), some variations might be more beneficial for survival than others. For example, thicker fur might be an advantage in colder climates.
  • Natural Selection: Individuals with beneficial traits are more likely to survive and reproduce, passing those traits on to their offspring. Over generations, these traits become more common in the population.
  • Adapting as a Whole: Through this process of natural selection, the population as a whole becomes better adapted to the new environment, increasing its chances of survival.

Questions (Page 133)

1. How do Mendel’s experiments show that traits may be dominant or recessive ?

Ans : Mendel’s pea plant experiments showed dominant and recessive traits. Here’s how:

  • Crossbred plants with contrasting traits (e.g., tall vs short).
  • Surprise: F1 generation all showed one trait (e.g., all tall).
  • Explanation: Dominant trait hides recessive trait in F1.
  • F2 generation showed 3:1 ratio (e.g., 3 tall: 1 short).
  • This 3:1 ratio proves recessive traits weren’t lost, just hidden by dominant ones in F1.

2. How do Mendel’s experiments show that traits are inherited independently ? 

Ans : Mendel’s pea plant experiments showed traits are inherited independently. Here’s the key:

  • Crossed plants with contrasting traits in TWO features (e.g., seed color and shape).
  • F1 generation ALL showed ONE form of EACH trait (e.g., all yellow & round).
  • F2 generation showed a 9:3:3:1 ratio for ALL combinations (e.g., yellow round, yellow wrinkled, etc.).
  • This ratio suggests traits like seed color and shape are inherited separately, not blended.

3. A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O – is dominant ? Why or why not ?

Ans : No, the info isn’t enough to say if A or O is dominant in blood types.

Here’s why:

  • Blood types A and B are co-dominant, O is recessive.
  • Dad could be AA (pure A) or AO (A and O). Mom is OO (pure O).
  • Daughter is O, so she got an O gene from each parent.
  • This confirms Dad has at least one O gene, but we don’t know if he’s AA or AO (pure A or A and O).

Without knowing if Dad is AA or AO, we can’t say for sure if A is dominant over O based on their daughter’s blood type.

4. How is the sex of the child determined in human beings ?

Ans : The sex of a child in humans is determined by a chromosomal system involving the sex chromosomes. Here’s the breakdown:

  • Sex Chromosomes: Humans have 23 pairs of chromosomes, with one pair determining sex: X and Y.
  • Females: Females have two X chromosomes (XX).
  • Males: Males have one X chromosome and one Y chromosome (XY).

The Key Player: The Y Chromosome

  • The Y chromosome carries a gene called SRY (sex-determining region of the Y chromosome) that triggers the development of male sex organs. In the absence of SRY, the default pathway leads to female development.

How It Works:

  • Sperm Determine Sex: During fertilization, the sperm determines the sex of the child. Sperm cells can carry either an X or a Y chromosome.
  • Egg Contributes an X: The egg cell from the mother always carries an X chromosome.
  • X + X = Female: If a sperm carrying an X chromosome fertilizes the egg (XX), the resulting zygote will develop into a female.
  • X + Y = Male: If a sperm carrying a Y chromosome fertilizes the egg (XY), the resulting zygote will develop into a male.


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

Ans : (c) TtWW

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 ?

Ans : No, based solely on the observation that children with light-colored eyes tend to have parents with light-colored eyes, we cannot definitively say whether light eye color is dominant or recessive.

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

Ans : This project aims to identify the dominant coat color in dogs through controlled breeding and observation. It’s important to note that coat color in dogs is a complex trait often influenced by multiple genes. This project is a simplified model and may not capture the full picture.

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

Ans : Baby’s genetic mix is 50/50 from mom and dad!

  1. Halving the Chromosomes: Special cell division (meiosis) in mom and dad cuts their genetic info in half (from 46 chromosomes to 23) for sperm and egg cells.
  2. Random Shuffle: During meiosis, chromosomes are randomly mixed, creating unique combinations in each sperm and egg cell.
  3. Egg and Sperm Meet: Fertilization combines the 23 chromosomes from sperm and egg, giving the baby 23 from each parent (46 total).

This combo of halving and shuffling ensures an equal and varied genetic contribution from mom and dad!

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