Heredity: Class 10 Important Questions with Answers

Case-based questions (4 marks each)

(i) Which colour is dominant? (1) (ii) Write the genotypes of the parents and F1 (use R/r). (1) (iii) Explain the F2 ratio with a Punnett square. How can he find out whether a red F2 plant is RR or Rr? (2)

(i) Write the gametes of F1. (1) (ii) Name the four F2 phenotypes and their ratio. (1) (iii) Which are the new combinations, and what principle does this prove? (2)

(i) Who decides the sex of the child? (1) (ii) What are the chances that the fourth child will be a boy? (1) (iii) Explain with a cross why the relatives are wrong on both points. (2)

(i) Where did the heat-resistant trait come from? (1) (ii) Why is variation useful for the species but not always for the individual? (1) (iii) Will the bacteria population show more variation than a sexually reproducing species? Explain. (2)

20 MCQs (1 mark each)

1. Mendel worked on
   (a) Drosophila (b) garden pea (c) maize (d) mice
2. A section of DNA that codes for one protein is a
   (a) chromosome (b) gene (c) nucleus (d) zygote
3. F1 of TT × tt is
   (a) all TT (b) all Tt (c) all tt (d) 3:1
4. Phenotypic ratio of a monohybrid F2:
   (a) 1:2:1 (b) 3:1 (c) 9:3:3:1 (d) 1:1
5. Genotypic ratio of a monohybrid F2:
   (a) 1:2:1 (b) 3:1 (c) 2:1 (d) 1:1
6. Dihybrid F2 ratio:
   (a) 3:1 (b) 1:2:1 (c) 9:3:3:1 (d) 1:1:1:1
7. Number of gamete types from RrYy:
   (a) 1 (b) 2 (c) 3 (d) 4
8. A heterozygous plant is
   (a) TT (b) tt (c) Tt (d) none
9. Recessive trait shows only in
   (a) TT (b) Tt (c) tt (d) all
10. In F2 of RrYy × RrYy, wrinkled green are
    (a) 9/16 (b) 3/16 (c) 1/16 (d) 4/16
11. Tt × tt gives tall : short as
    (a) 3:1 (b) 1:1 (c) all tall (d) 1:2:1
12. Number of chromosomes in a human body cell:
    (a) 23 (b) 44 (c) 46 (d) 48
13. Number of pairs of autosomes in humans:
    (a) 22 (b) 23 (c) 44 (d) 1
14. Human male sex chromosomes:
    (a) XX (b) XY (c) YY (d) XO
15. Sex of a child is decided by
    (a) mother's X (b) father's chromosome (c) both equally (d) environment
16. In some reptiles sex is decided by
    (a) X/Y (b) temperature (c) diet (d) light
17. Animals that can change sex:
    (a) snails (b) humans (c) dogs (d) frogs
18. Gametes are made by
    (a) mitosis (b) meiosis (c) fission (d) budding
19. Which earlobe is generally dominant?
    (a) attached (b) free (c) both (d) neither
20. Mendel's “factors” are now called
    (a) alleles (b) genes (c) traits (d) characters

Answer key

Assertion–Reason (10)

(a) Both A and R true, R explains A · (b) Both true, R does not explain A · (c) A true, R false · (d) A false, R true

1. A: All F1 plants of TT × tt are tall. R: Tallness is dominant over shortness. → (a)
2. A: Short plants reappear in F2. R: The short trait was not inherited by F1. → (c)
3. A: New combinations appear in a dihybrid F2. R: Traits are inherited independently. → (a)
4. A: Asexual reproduction produces great variation. R: DNA copying is not perfectly accurate. → (d)
5. A: Each gamete carries one chromosome of each pair. R: This keeps the chromosome number constant over generations. → (a)
6. A: Women decide the sex of the child. R: Women give only X eggs. → (d)
7. A: Genes control traits. R: Genes make proteins such as enzymes that control body processes. → (a)
8. A: Tt and TT plants look the same. R: They have the same genotype. → (c)
9. A: A child is not an exact copy of either parent. R: The child receives half its DNA from each parent. → (a)
10. A: Mendel was successful in finding the laws. R: Mendel counted the individuals with each trait in every generation. → (a)

Give reasons (2 marks each)

1. Variation is more in sexual reproduction.

DNA from two different parents is combined, creating new combinations, in addition to copying errors. 2

2. F1 shows only one of the two parental traits.

F1 is Tt; the dominant allele T masks the recessive t, so only the dominant trait is expressed. 2

3. Sex chromosomes in males are called a mismatched pair.

One is a normal-sized X chromosome and the other a short Y chromosome. 2

4. Germ cells have half the number of chromosomes.

So that when two germ cells fuse, the zygote gets the normal number, keeping DNA content stable. 2

5. One family's data cannot tell whether a trait is dominant.

The same result can arise either way; we need many crosses and offspring ratios to decide. 2