PSEB 10th Science – Chapter 8 Heredity and Evolution

Heredity and Evolution – In-text Questions

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?
Trait B is likely to have arisen earlier. In asexual reproduction, traits are passed down from parent to offspring with little change. For a trait to be present in a large part of the population (60%), it must have been around for a longer time, allowing it to be copied and passed on through many generations.

2. How does the creation of variations in a species promote survival?
Variations are small differences among individuals in a species. This is very important for survival because if the environment changes (for example, a new disease appears or the climate gets hotter), some individuals may have variations that help them survive these new conditions. These survivors can then reproduce and pass on their useful traits, ensuring the entire species doesn’t get wiped out.

3. How do Mendel’s experiments show that traits may be dominant or recessive?
When Mendel crossed a pure-bred tall pea plant with a pure-bred short pea plant, all the offspring in the first generation were tall. The short trait seemed to have disappeared. This showed that the tall trait was dominant. However, when he then crossed these tall offspring with each other, the short trait reappeared in the second generation. This showed that the short trait was recessive—it was hidden in the first generation but not lost.

4. How do Mendel’s experiments show that traits are inherited independently?
Mendel studied two traits at the same time, such as seed shape (round or wrinkled) and seed color (yellow or green). He found that the inheritance of seed shape did not affect the inheritance of seed color. A seed could be round and yellow, round and green, wrinkled and yellow, or wrinkled and green. This showed that the genes for different traits are passed down to offspring independently of each other.

5. 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 whether the trait – blood group A or O – is dominant or recessive? Why or why not?
Yes, this information is enough. The gene for blood group O must be recessive. For the daughter to have blood group O, she must have inherited the O gene from both parents. We know she got one from her mother (who has blood group O). This means she must have also received an O gene from her father. Since the father has blood group A but still carries the O gene, his genetic makeup must be AO. This proves that the A gene is dominant because it shows up even when the O gene is present.

6. How is the sex of the child determined in human beings?
In humans, sex is determined by the sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).

• A mother always passes an X chromosome to her child.

• A father can pass on either an X or a Y chromosome through his sperm.
  If a sperm carrying an X chromosome fertilizes the egg, the child will be a girl (XX). If a sperm carrying a Y chromosome fertilizes the egg, the child will be a boy (XY). Therefore, it is the father’s genetic contribution that determines the sex of the child.

Textbook Exercise Questions

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:
(c) TtWW

• Since all the offspring had violet flowers, the violet trait (W) must be dominant, and the tall parent must be pure-bred for this trait (WW) to ensure no white flowers appeared.

• Since half the offspring were short (tt), they must have received a recessive ‘t’ gene from both parents. This means the tall parent must have been heterozygous for height (Tt) to be able to pass on the ‘t’ gene.

2. An example of homologous organs is:
(d) all of the above.
Homologous organs are those that have a similar basic structure from a common ancestor but are used for different functions. All the examples fit this description: our arm and a dog’s leg have similar bones; our teeth and an elephant’s tusks are structurally related; and a potato and grass runners are both modified plant stems.

3. In evolutionary terms, we have more in common with:
(a) a Chinese school-boy.
In evolutionary terms, all modern humans (Homo sapiens) are members of the same species. We are more closely related to other humans, regardless of where they are from, than to any other species, including our closest animal relative, the chimpanzee.

4. 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?
No, we cannot say for sure based only on this information.

• If light eye color is a recessive trait, then two parents with light eyes can only have children with light eyes. This fits the observation.

• But if light eye color is a dominant trait, two parents with light eyes could both be heterozygous and could possibly have a child with dark (recessive) eyes.
  Since the study only says it’s “likely,” it doesn’t rule out any possibilities. We would need more information, such as whether two light-eyed parents can ever have a dark-eyed child.

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

1. Select Parents: Find a pure-bred dog with one coat color (e.g., black) and another pure-bred dog with a different coat color (e.g., brown).

2. First Cross (F1 Generation): Breed these two dogs together.

3. Observe Offspring: Observe the coat color of all the puppies in the litter. The color that appears in all the puppies is the dominant trait. For example, if all the puppies are black, then black is the dominant coat color.

4. Confirmation (Optional): To confirm, breed two of the puppies from the F1 generation together. The recessive color (brown) should reappear in about 25% of their offspring (the F2 generation).

6. Explain the terms analogous and homologous organs with examples.

• Homologous Organs are organs that have a similar basic structure because they are inherited from a common ancestor, but they are used for different functions in different species.

  • Example: The forelimb of a human, the wing of a bat, and the flipper of a whale all have the same pattern of bones, but one is used for grasping, one for flying, and one for swimming.

• Analogous Organs are organs that have different structures and evolutionary origins but are used for the same function.

  • Example: The wing of a bird and the wing of an insect. Both are used for flying, but a bird’s wing is made of bone and feathers, while an insect’s wing is made of a thin membrane.

7. How is the equal genetic contribution of male and female parents ensured in the progeny?
This is ensured by the process of sexual reproduction:

1. Gamete Formation: Organisms create special reproductive cells called gametes (sperm in males, eggs in females). These gametes contain exactly half the genetic information of a normal body cell.

2. Fertilization: During fertilization, one male gamete fuses with one female gamete.
   Because each gamete carries half the required genetic information, the resulting offspring receives exactly half of its genetic material from the male parent and half from the female parent.

READ – How Do Organisms Reproduce