CBSE Class 10 Science Heredity and Evolution MCQs Set 12

Answer: (c) 9 : 3 : 3 : 1

Explanation:
1. Two pairs of contrasting characters (shape and colour) are involved, so this is a dihybrid cross.
2. The F₂ phenotypic ratio of a dihybrid cross is 9 : 3 : 3 : 1.

Teacher's Note:
1. Monohybrid cross gives 3 : 1 and dihybrid gives 9 : 3 : 3 : 1.
2. The ratio 1 : 2 : 1 is a genotypic ratio.

Answer: (b) A: Yellow, B: Green, C: Wrinkled, D: Green

Explanation:
1. The numbers 315 : 108 : 101 : 32 are in the ratio 9 : 3 : 3 : 1.
2. The biggest class has both dominant traits: round and yellow, so A is yellow.
3. Round with the 3-part must be round green, so B is green.
4. The 101 class is wrinkled yellow, so C is wrinkled, and the smallest class is wrinkled green, so D is green.

Teacher's Note:
1. The four classes are round yellow, round green, wrinkled yellow and wrinkled green.
2. Match the numbers with the 9 : 3 : 3 : 1 ratio.

Answer: (d) All of the above

Explanation:
1. Selfing AaRr gives four phenotypic classes: axial round, axial wrinkled, terminal round and terminal wrinkled.
2. So the F₂ progeny include all of the above types.

Teacher's Note:
1. Four phenotypes appear in the ratio 9 : 3 : 3 : 1.
2. New combinations appear because the two traits assort independently.

Answer: (c) Law of independent assortment

Explanation:
1. The law of independent assortment states that the inheritance of one pair of characters is independent of the inheritance of another pair.
2. It is shown by a dihybrid cross.

Teacher's Note:
1. Segregation deals with the separation of alleles of one gene.
2. Independent assortment deals with two or more genes.

Answer: (d) 5643

Explanation:
1. F₂ shows round : wrinkled in the ratio 3 : 1.
2. Round seeds are 3/4 of 7524 = 5643.

Teacher's Note:
1. Round is dominant, so it forms 75% of F₂.
2. Wrinkled seeds would be 1881.

Answer: (c) A = 315, B = 105, C = 105, D = 35

Explanation:
1. This is a dihybrid cross, so the F₂ phenotypes appear in the ratio 9 : 3 : 3 : 1 (a total of 16 parts).
2. One part = 560 ÷ 16 = 35.
3. Yellow round = 9 × 35 = 315, yellow wrinkled = 3 × 35 = 105, green round = 3 × 35 = 105 and green wrinkled = 1 × 35 = 35.

Teacher's Note:
1. The largest class is always the one with both dominant traits.
2. The smallest class is the double recessive.

Answer: (c) A and C only

Explanation:
1. All offspring have round seeds, so round is dominant (A is true).
2. If the round parent were heterozygous, about half of the seeds would be wrinkled, so it is homozygous (B is false).
3. A wrinkled plant must be homozygous recessive (C is true).
4. So A and C only.

Teacher's Note:
1. A recessive trait is expressed only in the homozygous condition.
2. All-dominant offspring suggest one parent is homozygous dominant.

Answer: (b) Rr × rr

Explanation:
1. Rr × rr gives Rr (round) and rr (wrinkled) in the ratio 1 : 1.
2. RR × rr gives all round, RR × Rr gives all round and rr × rr gives all wrinkled.

Teacher's Note:
1. A 1 : 1 ratio is the sign of a test cross.
2. Draw a Punnett square to check.

Answer: (a) 750, 250

Explanation:
1. Rr × Rr gives round and wrinkled in the ratio 3 : 1.
2. Out of 1000 plants, round = 3/4 × 1000 = 750 and wrinkled = 1/4 × 1000 = 250.

Teacher's Note:
1. A monohybrid cross between two hybrids gives 3 : 1.
2. Divide the total in four equal parts.

Answer: (a) 75% plants will be tall and have wrinkled seeds and other 25% will be dwarf with wrinkled seeds

Explanation:
1. The wrinkled plants are rr, so all their seeds are wrinkled.
2. Heterozygous tall plants (Tt) on selfing give 3 tall : 1 dwarf.
3. So 75% are tall with wrinkled seeds and 25% are dwarf with wrinkled seeds.

Teacher's Note:
1. Consider each character separately.
2. Wrinkled is recessive, so rr × rr gives only wrinkled seeds.

Answer: (d) Statement-I is incorrect and Statement-II is correct.

Explanation:
1. The law of independent assortment is based on a dihybrid cross, so Statement-I is incorrect.
2. The phenotypic ratio of a monohybrid cross is 3 : 1, so Statement-II is correct.

Teacher's Note:
1. A monohybrid cross is the basis of the law of segregation.
2. A dihybrid cross is the basis of independent assortment.

Answer: (d) Statement-I is incorrect and Statement-II is correct.

Explanation:
1. Round and yellow are dominant, so all F₁ plants bear round yellow seeds and Statement-I is incorrect.
2. Statement-II is correct.

Teacher's Note:
1. Only dominant traits show in F₁.
2. Recessive traits reappear in F₂.

Answer: (a) Autosomes

Explanation:
1. Human cells have 23 pairs of chromosomes.
2. One pair is the sex chromosomes and the remaining 22 pairs, which are alike in males and females, are called autosomes.

Teacher's Note:
1. Meiosis is a type of cell division, not a chromosome.
2. Remember: 22 pairs of autosomes + 1 pair of sex chromosomes.

Answer: (a) 1

Explanation:
1. A human zygote has 23 pairs of chromosomes.
2. Of these, 22 pairs are autosomes and only 1 pair is the sex chromosomes (XX or XY).

Teacher's Note:
1. The zygote has the same chromosome number as an adult body cell.
2. Sex chromosomes are always one pair.

Answer: (c) (iii), (iv)

Explanation:
1. In males the sex chromosome pair is XY, which is not a perfectly matched pair.
2. The X chromosome is large and the Y chromosome is short.
3. So the unpaired chromosomes are the Y and X chromosomes, i.e. (iii) and (iv).

Teacher's Note:
1. In females both sex chromosomes are X, so they form a perfect pair.
2. The unequal XY pair is typical of human males.

Answer: (a) Copies of the same chromosome

Explanation:
1. The two alleles of a trait are carried on the two members of a homologous pair of chromosomes.
2. One copy comes from the father and the other from the mother, so the versions lie on copies of the same chromosome.

Teacher's Note:
1. Homologous chromosomes carry genes for the same traits at the same positions.
2. One chromosome of each pair comes from each parent.

Answer: (d) Combination of X and Y chromosome from either of its parents

Explanation:
1. A male has the sex chromosomes XY.
2. The X comes from the mother and the Y comes from the father.
3. So a male inherits the X and Y chromosome from either of its parents.

Teacher's Note:
1. A son always gets his Y from the father.
2. The X of a son comes from the mother.

Answer: (b) the Y-chromosome in zygote

Explanation:
1. A child is male when the zygote has the sex chromosomes XY.
2. The Y chromosome comes from the father's sperm, so maleness is decided by the Y-chromosome in the zygote.

Teacher's Note:
1. A zygote with XX develops into a girl.
2. The presence of Y makes the child a boy.

Answer: (a) girl

Explanation:
1. The egg always gives an X chromosome.
2. If the sperm also gives an X, the zygote is XX and develops into a girl.

Teacher's Note:
1. A zygote with Y from the father is XY and becomes a boy.
2. The father decides the sex of the child.

Answer: (b) (ii) and (iv)

Explanation:
1. Every egg carries an X chromosome, so the mother does not decide the sex; the sperm carrying X or Y does, so (ii) is true and (i) is false.
2. X-sperms and Y-sperms are equally likely, so a boy and a girl are equally probable and (iii) is false.
3. Sex is fixed at fertilisation when the zygote is formed, so (iv) is true.
4. Hence the correct statements are (ii) and (iv).

Teacher's Note:
1. A child who inherits X from the father is a girl and one who inherits Y is a boy.
2. The chance of a boy or a girl is 50% each.

Answer: (b) 23

Explanation:
1. A body cell has 23 pairs, i.e. 46 chromosomes.
2. A gamete (sex cell) has half the number, so it has 23 chromosomes.

Teacher's Note:
1. The number is halved during gamete formation.
2. Fusion of two gametes restores 46 chromosomes in the zygote.

Answer: (d) The presence of a Y chromosome in a sperm

Explanation:
1. Every ovum has an X chromosome, so the ovum does not decide the sex.
2. A sperm with a Y chromosome gives an XY zygote, which is a male, so sex is determined by the presence of a Y chromosome in a sperm.

Teacher's Note:
1. Pregnancy length and timing have no effect on sex.
2. The father's sperm decides the sex.

Answer: (b) 23

Explanation:
1. Human body cells have 46 chromosomes, which form 23 pairs.
2. Of these, 22 pairs are autosomes and 1 pair is the sex chromosomes.

Teacher's Note:
1. Do not confuse 46 chromosomes with 46 pairs.
2. Gametes have 23 chromosomes.

Answer: (b) 50%

Explanation:
1. Each pregnancy is an independent event.
2. The sperm that fertilises the egg carries X or Y with equal chance, so the probability of a girl is 50% every time, whatever the earlier children are.

Teacher's Note:
1. Past births do not change the chance of the next one.
2. Sex is decided by the father's sperm at fertilisation.

Answer: (b) Allosomes

Explanation:
1. The sex chromosomes (X and Y) are also called allosomes.
2. The other chromosomes are autosomes.

Teacher's Note:
1. Autosomes control body (somatic) traits.
2. Allosomes determine the sex.

Answer: (c) A is true, but R is false.

Explanation:
1. Mendel allowed the F₁ plants to self-pollinate to obtain F₂, so A is true.
2. The F₁ progeny of tall round and dwarf wrinkled plants are all tall with round seeds, because these are dominant, so R is false.

Teacher's Note:
1. Dominant traits appear in F₁.
2. Dwarf and wrinkled are recessive.

Answer: (c) A is true, but R is false.

Explanation:
1. A dihybrid cross gives the phenotypic ratio 9 : 3 : 3 : 1 in F₂, so A is true.
2. A monohybrid cross gives 3 : 1 in F₂, not 1 : 1, so R is false.

Teacher's Note:
1. 3 : 1 is the monohybrid phenotypic ratio.
2. 1 : 1 is the test-cross ratio.

Answer: (d) A is false, but R is true.

Explanation:
1. In asexual reproduction one parent is involved and the offspring are almost identical copies, so significant variations are not seen. The assertion is false.
2. Copying of DNA is not perfectly accurate, so the reason is true.

Teacher's Note:
1. Small copying errors do cause minor variations even in asexual reproduction, but they are rare.
2. Major variation arises from sexual reproduction.

Answer: (c) A is true, but R is false.

Explanation:
1. Mendel chose the pea plant because it has clear contrasting traits, is easy to grow and can be both self- and cross-pollinated, so A is true.
2. Pea flowers are bisexual and normally self-pollinating, so R is false.

Teacher's Note:
1. The source key marks both statements correct, but pea flowers are bisexual, so the key has been corrected.
2. Pea is naturally self-pollinating.

Answer: (c) A is true, but R is false.

Explanation:
1. The father passes either X or Y, so he decides the sex of the child, and A is true.
2. Human males have one X and one Y chromosome, not two X chromosomes, so R is false.

Teacher's Note:
1. Females are XX and males are XY.
2. The mother always gives an X, so the father's gamete decides.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Human males have the sex chromosomes X and Y.
2. During gamete formation these separate, so half of the sperms carry X and half carry Y.
3. Hence R explains A.

Teacher's Note:
1. Females have XX and produce only one type of egg.
2. The father's sperm therefore decides the sex of the baby.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. The two alleles of a gene separate so that each gamete has only one allele, so gametes are pure for a character.
2. This explains why segregation is called the principle of purity of gametes.

Teacher's Note:
1. Alleles remain distinct and never blend.
2. Both statements are true and linked.

Answer: (d) A is false, but R is true.

Explanation:
1. In grasshoppers females are XX, which is homogametic, and males are XO, which is heterogametic, so A is false.
2. R correctly describes the sex chromosomes, so R is true.

Teacher's Note:
1. Heterogametic means producing two kinds of gametes.
2. Grasshopper males produce gametes with and without X.

Answer: (c) A is true, but R is false.

Explanation:
1. A child with X from the father gets XX and is a girl, so A is true.
2. The mother has no Y chromosome, so R is false.

Teacher's Note:
1. A girl gets one X from each parent.
2. Only the father can pass on a Y chromosome.

Answer: (d) A is false, but R is true.

Explanation:
1. In human females the sex chromosomes are XX, which also form a perfect pair, so all chromosomes are perfectly paired and A is false.
2. Males have the sex chromosomes X and Y, so R is true.

Teacher's Note:
1. It is in males that all chromosomes are paired perfectly except the XY pair.
2. Check whether the assertion fits females or males.