CBSE Class 10 Science Heredity Sure Shot Questions Set 03

This page gives 22 sure shot questions with answers for CBSE Class 10 Science Chapter 8, Heredity, chosen because these board-style questions are asked again and again. They cover Mendel's monohybrid and dihybrid crosses, dominant and recessive traits, acquired and inherited traits, genes and DNA, and how the sex of a child is determined, with step-by-step answers and Punnett squares.

Very Short Answer Questions

Question 1. All the variations in a species do not have equal chances of survival. Why?
Answer:
Variations differ in how useful they are to the organism in its surroundings.
A variation that helps an individual to cope with its environment (for example, bacteria that can withstand heat in a heat wave) improves its chance of survival; a variation that does not help gives no such advantage, and a harmful one may reduce survival.
So the chance of survival depends on the nature of the variation and on the environment.
In simple words: Only useful variations help an organism to survive.

Teacher's Note:
1. Mention the heat-wave bacteria example to earn full marks.
2. Link survival to the environment, not to the variation alone.

Question 2. Why is the progeny always tall when a tall pea plant is crossed with a short pea plant?
Answer:
A pure tall plant has TT and a short plant has tt, so every F1 plant is Tt.
Tallness (T) is the dominant trait and shortness (t) is recessive. A single copy of T is enough to make the plant tall, so the t allele is not expressed.
In simple words: Tall is dominant, so the hidden short allele cannot show in Tt plants.

Teacher's Note:
1. Write the genotype Tt to show the hidden allele.
2. Use the word dominant and explain it.

Question 3. A Mendelian experiment consisted of breeding pea plants bearing violet flowers with pea plants bearing white flowers. What will be the result in F1 progeny?
Answer:
All the F1 plants will bear violet flowers.
Violet is the dominant trait and white is recessive, so the white-flower trait is hidden in the F1 generation.
In simple words: Only the dominant colour, violet, appears in F1.

Teacher's Note:
1. Assume pure-breeding parents, as in Mendel's experiments.
2. Say clearly which trait is dominant.

Question 4. Name the information source for making proteins in the cells.
Answer:
DNA (deoxyribonucleic acid) in the cell nucleus. A section of DNA that provides information for one protein is called a gene.
In simple words: DNA carries the instructions for making proteins.

Teacher's Note:
1. The answer is DNA, not RNA or the nucleus.
2. Add that a gene is the part of DNA for one protein.

Question 5. Why do mice whose tails were surgically removed just after birth for generations, continue to produce mice with tails?
Answer:
Removal of the tail is an acquired trait that develops during the lifetime of the mouse.
It does not change the genes (DNA) in the germ cells, so it is not passed on to the next generation, and the young mice are born with tails.
In simple words: Cutting the tail changes the body, not the genes in the eggs and sperms.

Teacher's Note:
1. Name the term acquired trait.
2. Say that germ-cell DNA is unchanged.

Short Answer Questions

Teacher's Note:
1. Give one example for each column.
2. The key point is whether the germ-cell DNA changes.

Teacher's Note:
1. Draw the Punnett square for F2.
2. Ratio GG : Gg excludes gg, so it is 1 : 2.

Question 8. (a) Why is the F1 progeny always of tall plants when a tall plant is crossed with a short pea plant?
(b) How is F2 progeny obtained by self-pollination of F1 progeny different from F1 progeny? Give reason for this observation.
(c) State a conclusion that can be drawn on the basis of this observation.
Answer:
(a) The F1 plants are Tt. Tallness is dominant over shortness, so all of them are tall.
(b) The F2 progeny has both tall and short plants in the ratio 3 : 1, while F1 had only tall plants. The Tt plants form two kinds of gametes (T and t), and when they fuse at random, tt plants appear in one quarter of the offspring and show the recessive trait.
(c) Traits can be dominant or recessive. A recessive trait is not lost; it is only hidden when the dominant allele is present.
In simple words: The short trait was hidden in F1 and returns in F2 when two t alleles meet.

Teacher's Note:
1. Use the ratio 3 : 1 in part (b).
2. In part (c) name dominant and recessive traits.

Question 9. In one of his experiments with pea plants Mendel observed that when a pure tall pea plant is crossed with a pure dwarf pea plant, in the first generation, F1 only tall plants appear.
(a) What happens to the traits of the dwarf plants in this case?
(b) When the F1 generation plants were self-fertilised, he observed that in the plants of second generation, F2 both tall plants and dwarf plants were present. Why it happened? Explain briefly.
Answer:
(a) The dwarf trait is not lost. It is present in the Tt plants but is suppressed by the dominant tall trait, so it does not express in F1.
(b) The F1 plants (Tt) produce two kinds of gametes, T and t, in equal numbers. On random fusion they give TT, Tt, Tt and tt. The tt plants (one quarter) are dwarf, so tall and dwarf plants appear in the ratio 3 : 1 in F2.
In simple words: The dwarf trait stayed hidden in Tt and showed up again in tt plants.

Teacher's Note:
1. Say hidden, not lost.
2. Show the gametes T and t in part (b).

Question 10. In a monohybrid cross between tall pea plants (TT) and short pea plants (tt) a scientist obtained only tall pea plants (Tt) in the F1 generation. However, on selfing the F1 generation pea plants, he obtained both tall and short plants in F2 generation. On the basis of above observations with other angiosperms also, can the scientist arrive at a law? If yes, explain the law. If not, give justification for your answer.
Answer:
Yes. The scientist can arrive at the law of dominance.
When two pure parents with contrasting traits are crossed, only one trait (the dominant trait) appears in F1, although the F1 plants carry both factors. In F2 the recessive trait appears again along with the dominant trait, because it was only masked, not lost.
In simple words: One trait masks the other in F1, but the masked trait comes back in F2.

Teacher's Note:
1. The answer is yes; name the law of dominance.
2. State what happens in F1 and in F2.

Question 11. With the help of an example justify the following statement: "A trait may be inherited, but may not be expressed."
Answer:
Example: Mendel crossed a pure tall pea plant (TT) with a pure dwarf plant (tt).
All the F1 plants were tall (Tt). The dwarf trait was inherited, because each F1 plant carries a t allele, but it was not expressed, since T is dominant.
When the F1 plants were self-pollinated, dwarf plants (tt) reappeared in F2 (3 tall : 1 dwarf). This shows that a trait can be inherited without being expressed.
In simple words: Dwarfness is carried in Tt plants but cannot show because tallness is dominant.

Teacher's Note:
1. Use the tall and dwarf example with genotypes.
2. Mention that dwarf plants reappear in F2.

Teacher's Note:
1. Give the two differences in a table.
2. Round is dominant, so RR and Rr both look round.

Question 13. "It is a matter of chance whether a couple will have a male or a female child." Justify this statement by drawing a flow chart.

Answer:
A woman has two X chromosomes (XX), so all her eggs carry X. A man has X and Y (XY), so half of his sperms carry X and half carry Y.
The sex of the child depends on which sperm fertilises the egg: an X sperm gives a girl (XX) and a Y sperm gives a boy (XY). Both events are equally likely, so the chance of a boy or a girl is 50% each and it is only a matter of chance.
In simple words: Which sperm wins the race, X or Y, decides the baby's sex.

Teacher's Note:
1. Draw the flow chart with the chromosomes labelled.
2. End with the 50 : 50 chance.

Question 14. A cross was made between pure breeding pea plants, one with round and green seeds and the other with wrinkled and yellow seeds.
(a) Write the phenotype of F1 progeny. Give reason for your answer.
(b) Write the different types of F2 progeny obtained along with their ratio when F1 progeny was selfed.
Answer:
Parents: RRyy (round, green) × rrYY (wrinkled, yellow). Gametes: Ry and rY. F1: RrYy.
(a) All F1 plants have round, yellow seeds, because round and yellow are dominant traits.
(b) On selfing RrYy, the F2 plants are: round yellow : round green : wrinkled yellow : wrinkled green = 9 : 3 : 3 : 1. Round green and wrinkled yellow are new combinations.
In simple words: F1 shows only the dominant traits; F2 gives the 9 : 3 : 3 : 1 ratio.

Teacher's Note:
1. Give the genotype of F1 (RrYy).
2. Mention the two new combinations.

Teacher's Note:
1. Work out the F2 Punnett square.
2. The ratio of BB to Bb ignores bb.

Question 16. If we cross pure-breed tall (dominant) pea plant with pure-breed dwarf (recessive) pea plant we will get pea plants of F1 generation. If we now self-cross the pea plant of F1 generation, then we obtain pea plants of F2 generation.
(a) What do the plants of F1 generation look like?
(b) State the ratio of tall plants to dwarf plants in F2 generation.
(c) State the type of plants not found in F1 generation but appeared in F2 generation, mentioning the reason for the same.
Answer:
(a) All the F1 plants are tall (Tt).
(b) Tall : dwarf = 3 : 1 in F2.
(c) Dwarf plants are not found in F1 but appear in F2. In F1 the dominant tall trait masks the recessive dwarf trait; in F2 some plants get two t alleles (tt) and therefore show dwarfness.
In simple words: Dwarf plants hide in F1 and show in F2 when they are tt.

Teacher's Note:
1. Give the three answers in order.
2. Reason: both alleles must be t for dwarfness.

Question 17. With the help of a flow chart explain in brief how the sex of a newborn is genetically determined in human beings. Which of the two parents, the mother or the father, is responsible for determination of sex of a child?
Answer:
Human cells have 23 pairs of chromosomes; 22 pairs are autosomes and one pair is the sex chromosomes. A woman is XX and a man is XY.
Flow chart: Father XY → sperms X or Y; Mother XX → eggs X only; X sperm + X egg → girl (XX); Y sperm + X egg → boy (XY).
Every child gets an X from the mother, so the mother cannot decide the sex. The father decides it, by passing either an X or a Y chromosome.
In simple words: The father's sperm carries X or Y, so he decides whether the child is a girl or a boy.

Teacher's Note:
1. Write the chromosome pairs in the flow chart.
2. State clearly that the father is responsible.

Long Answer Questions

Teacher's Note:
1. Draw both crosses neatly.
2. Part (b) needs the new combinations.

Question 19. (a) Why did Mendel choose garden pea for his experiments? Write two reasons.
(b) List two contrasting visible characters of garden pea Mendel used for his experiment.
(c) Explain in brief how Mendel interpreted his results to show that the traits may be dominant or recessive.
Answer:
(a) (i) The garden pea is easy to grow and has a short life cycle, so many generations can be studied quickly. (ii) It has clear contrasting traits (such as tall/dwarf) and self-pollinating bisexual flowers, so pure lines are easy to get and cross-pollination is possible.
(b) Tall / dwarf plants, and round / wrinkled seeds (also yellow / green seeds or violet / white flowers).
(c) In the tall × dwarf cross, only tall plants appeared in F1. On selfing, F2 had tall and dwarf plants in the ratio 3 : 1. The trait that appears in F1 and in three-fourths of F2 (tall) is dominant; the hidden trait that reappears in one-fourth of F2 (dwarf) is recessive.
In simple words: Peas are easy to grow and have clear traits; the F1 and F2 results show which trait is dominant.

Teacher's Note:
1. Give exactly two reasons in part (a).
2. Part (c) needs both F1 and F2 results.

Question 20. (a) What is the law of dominance of traits? Explain with an example.
(b) Why are the traits acquired during the life time of an individual not inherited? Explain.
Answer:
(a) The law of dominance states that when two pure parents with contrasting traits are crossed, only one trait (the dominant trait) appears in the F1 generation, and the other (recessive) trait is hidden, although both factors are present. Example: TT × tt gives all Tt plants, which are tall because T is dominant over t.
(b) Acquired traits are changes in body cells (somatic changes) caused by the environment, use or effort. They do not change the DNA of the germ cells (eggs and sperms). Only changes in the DNA of germ cells are passed on to the next generation, so acquired traits are not inherited.
In simple words: Only changes in germ-cell DNA reach the children; changes in the body do not.

Teacher's Note:
1. Quote the tall and dwarf example.
2. Contrast somatic cells and germ cells in part (b).

Teacher's Note:
1. Give the phenotype and genotype ratios.
2. Show the gametes in the Punnett square.

Question 22. (a) Why did Mendel carry out an experiment to study inheritance of two traits in garden pea?
(b) What were his findings with respect to inheritance of traits in F1 and F2 generation?
(c) State the ratio obtained in the F2 generation in the above mentioned experiment.
Answer:
(a) A monohybrid cross showed how one trait is inherited. Mendel wanted to find out whether two different traits (for example seed shape and seed colour) are inherited together or independently of each other.
(b) In F1, all the plants had the dominant traits, round and yellow seeds (RrYy). In F2, four kinds of plants appeared: round yellow, round green, wrinkled yellow and wrinkled green. Two of them (round green and wrinkled yellow) were new combinations not seen in the parents.
(c) The F2 ratio was 9 : 3 : 3 : 1.
In simple words: The new combinations in F2 show that seed shape and seed colour are inherited independently.

Teacher's Note:
1. Name the two traits used in the dihybrid cross.
2. State the ratio with the four seed types.