Heredity - Q&A
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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?
Answer: Trait B is likely to have arisen earlier.
Explanation: In asexual reproduction, DNA is copied almost exactly, with very few changes (variations) occurring over time. When a change does happen, it is passed on to all future generations. Since Trait B is present in a much larger portion of the population (60%) compared to Trait A (10%), it suggests that Trait B has been around for a longer time, allowing it to spread to more individuals through reproduction. Trait A is likely a more recent variation.
2. How does the creation of variations in a species promote survival?
Answer: Variations help a species adapt to changing environmental conditions.
Explanation: Imagine a population of bacteria living in water. If the water temperature suddenly rises due to a heatwave, most bacteria might die. However, if there were a few bacteria with a "heat-resistant" variation, they would survive and reproduce. This variation prevents the entire species from being wiped out. Thus, variations act like an insurance policy for the survival of a species.
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1. How do Mendel's experiments show that traits may be dominant or recessive?
Answer: Mendel crossed a pure tall pea plant (TT) with a pure short pea plant (tt).
1. In the first generation (F1), he observed that all the plants were tall. The short trait seemed to disappear. This showed that the tall trait was dominant because it expressed itself even when the short gene was present.
2. When he crossed these F1 tall plants (Tt) with each other, the second generation (F2) had both tall and short plants in a ratio of 3:1. The reappearance of the short plants showed that the short trait was still there but was hidden in the F1 generation. This hidden trait is called recessive.
2. How do Mendel's experiments show that traits are inherited independently?
Answer: Mendel performed a "dihybrid cross" using two traits: seed shape (Round/Wrinkled) and seed colour (Yellow/Green).
1. He crossed Round-Yellow seeds (RRYY) with Wrinkled-Green seeds (rryy).
2. In the F1 generation, all plants had Round-Yellow seeds.
3. In the F2 generation, he found new combinations of traits: Round-Green and Wrinkled-Yellow, in addition to the original parental types. The ratio was 9:3:3:1.
Conclusion: The fact that "Round" didn't always stick with "Yellow" and "Wrinkled" didn't always stick with "Green" showed that the traits for shape and colour are passed down (inherited) independently of each other.
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?
Answer: No, this information is not enough to determine which trait is dominant.
Explanation:
Case 1: If A is dominant and O is recessive, the father could be genotype AO and the mother OO. The daughter would inherit O from both, becoming OO (Blood group O). This works.
Case 2: If O were dominant (hypothetically) and A recessive, the mother could be OO and the father AA. The daughter would be OA, which would be Blood group O. This also works legally in this logic problem.
Since both scenarios could theoretically produce an O daughter depending on which is dominant, we cannot be 100% sure without more information (like the blood groups of the grandparents).
4. How is the sex of the child determined in human beings?
Answer: The sex is determined by the sex chromosomes inherited from the parents.
1. Females have two X chromosomes (XX) and produce only X-type eggs.
2. Males have one X and one Y chromosome (XY) and produce two types of sperm: half with X and half with Y.
3. If a sperm with an X chromosome fertilizes the egg, the child will be a girl (XX).
4. If a sperm with a Y chromosome fertilizes the egg, the child will be a boy (XY).
Therefore, the father's sperm determines the sex of the child.
EXERCISES
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
Answer: (c) TtWW
Explanation:
1. Flower Colour: All progeny were violet. This means the violet parent must be homozygous dominant (WW) because if it were Ww, some white flowers (ww) would likely appear.
2. Height: The progeny were half tall and half short. This is a 1:1 ratio, which happens when a heterozygous parent (Tt) crosses with a homozygous recessive parent (tt).
Therefore, the tall violet parent must be 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?
Answer: No, we cannot say for sure whether the trait is dominant or recessive based solely on this statement.
Explanation:
- If light eye colour is recessive: Two parents with light eyes (recessive) will produce only light-eyed children. This fits the statement.
- If light eye colour is dominant: Two parents with light eyes (if they are pure dominant) will also produce light-eyed children. This also fits.
Since both possibilities explain the observation, we need more data (like seeing if light-eyed parents can have a dark-eyed child) to decide.
3. Outline a project which aims to find the dominant coat colour in dogs.
Answer:
Objective: To identify whether Black coat colour is dominant over White coat colour in dogs.
Steps:
1. Select Purebred Parents: Find a male dog that comes from a long line of pure Black dogs (Homozygous BB) and a female dog from a long line of pure White dogs (Homozygous bb).
2. Breed Them: Cross these two dogs.
3. Observe the F1 Generation: Look at the coat colour of the puppies born.
Conclusion:
- If all puppies are Black, then Black is the dominant colour.
- If all puppies are White, then White is the dominant colour.
- (Note: If they are grey or spotted, it might be a case of incomplete dominance or codominance).
4. How is the equal genetic contribution of male and female parents ensured in the progeny?
Answer: It is ensured through the formation of gametes (germ cells) and fertilization.
1. Gamete Formation (Meiosis): Both parents produce specialized cells called gametes (sperm and egg). During this process, the number of chromosomes is reduced to half. For example, human cells have 46 chromosomes, but sperm and eggs have only 23 each.
2. Fertilization: When the sperm (from the father) fuses with the egg (from the mother) to form a zygote, the two halves combine (23 + 23 = 46).
This restores the full number of chromosomes, ensuring that the child receives exactly one set of genes from the mother and one set from the father.