CBSE Class 10 Science HOTs Heredity Set 05

This page has 20 higher order thinking (HOTs) questions with answers for CBSE Class 10 Science Chapter 8, Heredity, collected from question banks and board-style practice. The questions ask you to reason about dominant and recessive traits, test crosses, blood group and eye colour inheritance, the role of the father in deciding the sex of a child, and the 9 : 3 : 3 : 1 ratio, and each answer is explained step by step.

Very Short Answer HOTS Questions

Question 1. Do genetic combination of mothers play a significant role in determining the sex of a new born?
Answer:
No. Every egg of a woman (XX) carries one X chromosome, so the mother always passes an X.
The sex of the baby is decided by the sperm: an X sperm gives a girl (XX) and a Y sperm gives a boy (XY). Since the father produces both kinds of sperms, the father's chromosome decides the sex.
In simple words: All eggs are X, so only the father's X or Y decides the sex.

Teacher's Note:
1. State that the mother contributes only X.
2. Name the father's chromosomes X and Y.

Question 2. 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, the information is not enough. Let us test both possibilities.
If A is dominant: the father can be heterozygous (A with O), the mother is OO, and the daughter can receive an O allele from each, so she has blood group O.
If O is dominant: the mother (O) can pass an O allele and the father (A, recessive) passes his allele, and the daughter can still show O.
Because the daughter can have blood group O in both cases, the cross alone cannot tell which trait is dominant. We need data on the blood groups of more children or more generations.
In simple words: The daughter's blood group O is possible whichever trait is dominant, so we cannot decide.

Teacher's Note:
1. Check both possibilities one by one.
2. Say that data of more generations is required.

Question 3. Why is recessive trait found in fewer individuals?
Answer:
A recessive trait is expressed only when an individual has two copies of the recessive allele (for example tt).
Individuals with even one dominant allele (TT or Tt) show the dominant trait. So only a smaller fraction of individuals, for example one-fourth in an F2 generation, show the recessive trait.
In simple words: Both parents must pass the recessive allele, so it shows less often.

Teacher's Note:
1. Link the answer to homozygous recessive (tt).
2. Give the one-fourth example from the F2 generation.

Question 4. A plant has two varieties, one with red petals and the other with white petals. When these two varieties are cross-pollinated, all the offsprings have red petals?
(a) Which gene is dominant?
(b) Choose suitable letters to represent the two genes.
Answer:
(a) The gene for red petals is dominant, because only red petals appear in the offspring (the first generation).
(b) Use R for the dominant red gene and r for the recessive white gene. Pure red plant RR × pure white plant rr gives all Rr (red).
In simple words: Red shows in all the offspring, so red is dominant (R) and white is recessive (r).

Teacher's Note:
1. Use a capital letter for the dominant gene.
2. Show the genotype Rr of the offspring.

Teacher's Note:
1. Use the Punnett square to see all combinations.
2. Only ee gives blue eyes.

Question 6. A bacteria is haploid. Can we use the term homozygous and heterozygous for it? Explain why or why not.
Answer:
No. The terms homozygous and heterozygous describe the two alleles of a gene that a diploid organism has, one from each parent.
A haploid bacterium has only one copy of each gene, so it cannot have two similar or two different alleles for the same trait.
In simple words: One copy of each gene means no pair of alleles, so these terms do not apply.

Teacher's Note:
1. Link the terms to a pair of alleles.
2. Mention haploid means a single set.

Question 7. Why are asexually reproducing individuals capable of showing hereditary features?
Answer:
In asexual reproduction only one parent is involved, no gametes are formed and there is no fertilisation.
The cell copies its DNA and the copy passes directly to the offspring. Therefore the offspring inherit the parent's traits and show the hereditary features, with only small changes due to errors in DNA copying.
In simple words: The offspring get a copy of the parent's DNA, so they inherit its features.

Teacher's Note:
1. Mention DNA copying.
2. Note that variations are very small.

Short Answer HOTS Questions

Question 8. A child questioned his teacher that why do organisms resemble their parents more as compared to grandparents. In which way, will the teacher explain to the child?
Answer:
The teacher will explain that a child receives half of its genes (one set of chromosomes) from each parent.
A grandparent gives only about one-fourth of the genes, as the genes are passed through a parent, and in every generation the mixing of genes (and new changes in DNA) adds more differences. So the child resembles the parents more closely than the grandparents.
In simple words: The child gets 50% of its genes from each parent but only about 25% from each grandparent.

Teacher's Note:
1. Use the fractions one-half and one-fourth.
2. Mention mixing of genes in each generation.

Teacher's Note:
1. Name the dominant and the recessive trait first.
2. Show both crosses with Punnett squares.

Question 10. Pure-breed pea plants A are crossed with pure-breed pea plants B. It is found that the plants which look like A do not appear in F1 generation but re-emerge in F2 generation. Which of the plants A and B are: (i) tall and (ii) dwarf? Give reason for your answer.
Answer:
(i) Plants B are tall.
(ii) Plants A are dwarf.
Reason: The trait of A is absent in F1 and reappears in F2, so it is the recessive trait (dwarfness). The trait of B appears in F1, so it is the dominant trait (tallness).
In simple words: The trait that disappears in F1 and returns in F2 is recessive, so A is dwarf.

Teacher's Note:
1. Recessive traits hide in F1.
2. Give the reason with the word recessive.

Question 11. 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. The finding fits both possibilities.
If light eye colour is recessive (ee), two light-eyed parents can only have light-eyed children. If light eye colour is dominant, light-eyed parents (EE or Ee) can also have light-eyed children.
To decide, we need more information, such as the eye colour of children when both parents have dark eyes, and the data of at least three generations.
In simple words: Light-eyed children from light-eyed parents is possible either way, so the data is not enough.

Teacher's Note:
1. Test both cases, dominant and recessive.
2. Say what extra data is needed.

Question 12. A woman has only daughters. Analyse the situation genetically and provide a suitable explanation.
Answer:
A woman has two X chromosomes, so all her eggs carry X. Her husband (XY) makes two kinds of sperms, half with X and half with Y.
A daughter is born when an X sperm fertilises the egg. For each child the chance of an X sperm is 50%, so it is quite possible by chance that all the children are girls.
The sex of the children depends on the father's sperm, so the woman is not responsible for having only daughters.
In simple words: It is a matter of chance which sperm fertilises the egg; the mother cannot decide.

Teacher's Note:
1. State that eggs always carry X.
2. Remove blame from the mother; the father's gamete decides.

Teacher's Note:
1. The recessive parent is called the test parent.
2. Show both possible crosses.

Question 14. Sahil has black hair, his mother also has black hair but his father has brown hair. How can you explain on the basis of transmission of genes for hair colour from the mother and father?
Note: It is given that black hair colour (B) is dominant over brown hair colour (b).
Answer:
The father has brown hair, so his genotype is bb and he can pass only b.
Sahil has black hair, so he must have the dominant allele B. He got B from his mother, so her genotype is BB or Bb.
Sahil's genotype is therefore Bb: B from the mother and b from the father. The b allele is hidden by the dominant B, so he has black hair.
In simple words: Sahil gets B from his mother and b from his father, and B hides b.

Teacher's Note:
1. Start from the recessive parent, the father (bb).
2. Write the genotype of Sahil as Bb.

Teacher's Note:
1. A short plant needs the genotype tt.
2. Link the asexual variation to errors in DNA copying.

Long Answer HOTS Questions

Teacher's Note:
1. List all three parent pairs in part (A).
2. Draw the Punnett square for one pair.

Teacher's Note:
1. Mention selection of dwarf plants and repeating the cycle.
2. Check the gametes: Tt gives T and t; tt gives only t.

Teacher's Note:
1. Use the dominant allele V and the recessive allele v.
2. Convert the ratio to numbers out of 100.

Question 19. Mendel used a number of contrasting visible characters of garden peas. He also explained that each trait is determined by a pair of factors.
(A) (i) Which term has replaced factors for the representation in a trait?
(ii) Where are these factors found in gametes?
(B) What is the number of chromosomes present in an ovum and fertilised egg?
(C) What do the following stand for?
(i) XX
(ii) XY
Which one of them produces heterogametes and which contributes towards the sex determination in the human progeny?
Answer:
(A)(i) The term genes (the different forms of a gene are called alleles) has replaced Mendel's factors.
(ii) The factors (genes) are located on the chromosomes. A gamete carries only one of the two factors of each pair, since the two separate when gametes are formed.
(B) An ovum has 23 chromosomes and a fertilised egg (zygote) has 46 chromosomes, as 23 + 23 = 46.
(C)(i) XX is the sex chromosome pair of a human female. (ii) XY is the sex chromosome pair of a human male.
XY (the male) produces two kinds of gametes (heterogametes), sperms with X and sperms with Y, and the male gamete contributes towards determining the sex of the child. A female (XX) produces only one kind of egg.
In simple words: Factors are genes on chromosomes; the father (XY) makes two kinds of sperms and decides the child's sex.

Teacher's Note:
1. Give 23 and 46 for the chromosome numbers.
2. Use the word heterogametic for the male.

Question 20. Mendel's dihybrid cross resulted in 4 types of individuals. What was the rationale behind Mendel deducing independent assortment from the observation?
Answer:
In the dihybrid cross of round yellow (RRYY) and wrinkled green (rryy) plants, the F1 plants were all round yellow (RrYy). On selfing, the F2 plants were of four types in the ratio 9 : 3 : 3 : 1: round yellow, round green, wrinkled yellow and wrinkled green.
If the two traits were inherited together, only the parental types (round yellow and wrinkled green) would appear, in the ratio 3 : 1. The two new combinations, round green and wrinkled yellow, appeared, which means that seed shape and seed colour separate and recombine freely.
Also, each trait taken separately gives the 3 : 1 ratio (12 round : 4 wrinkled and 12 yellow : 4 green out of 16), and 3 : 1 × 3 : 1 gives 9 : 3 : 3 : 1. Therefore the factors of the two traits are inherited independently of each other.
In simple words: The appearance of new combinations, with a 9 : 3 : 3 : 1 ratio, shows that the two traits are inherited independently.

Teacher's Note:
1. Mention the two new combinations.
2. Use the 3 : 1 ratio of each trait to explain 9 : 3 : 3 : 1.