CBSE Class 10 Science Heredity MCQs Set 13

Answer: (b) Gregor Mendel. Gregor Mendel worked out the basic laws of inheritance using garden pea plants.

Teacher's Note:
1. He studied seven pairs of contrasting characters.
2. His work was done in the 1860s.

Answer: (a) it has contrasting characters, short life cycle and can be self- or cross-pollinated. Pea plants have easily visible contrasting traits, mature quickly and are easy to cross or self-pollinate.

Teacher's Note:
1. These features gave him clear, repeatable results.
2. They also let him work with pure lines.

Answer: (a) monohybrid cross. A monohybrid cross follows one pair of contrasting traits.

Teacher's Note:
1. A dihybrid cross follows two pairs of traits.
2. Mendel started with monohybrid crosses.

Answer: (a) tallness is a dominant trait. The tall allele T is dominant over t, so Tt plants are tall.

Teacher's Note:
1. Dominance means the trait expresses in the heterozygote.
2. The dwarf trait is hidden but not lost.

Answer: (b) 3 : 1. Tt × Tt gives TT, Tt, Tt and tt, so 3 tall : 1 dwarf.

Teacher's Note:
1. The genotypic ratio of the same cross is 1 : 2 : 1.
2. Do not confuse the two ratios.

Answer: (b) 1 : 2 : 1. The offspring are 1 TT : 2 Tt : 1 tt.

Teacher's Note:
1. Genotype refers to the allele combination.
2. Phenotype refers to the visible trait.

Answer: (a) 9 : 3 : 3 : 1. F2 shows 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.

Teacher's Note:
1. This shows that the two characters are inherited independently.
2. New combinations appear in F2.

Answer: (c) 1/16. Only the rryy class shows both recessive traits, which is 1 out of 16.

Teacher's Note:
1. The parental combinations are round-yellow and wrinkled-green.
2. Round-green and wrinkled-yellow are the new combinations.

Answer: (a) 10. 160 × 1/16 = 10 plants.

Teacher's Note:
1. Use the 9 : 3 : 3 : 1 ratio with a total of 16 parts.
2. Each part here equals 10 plants.

Answer: (b) new combinations (recombinants). They carry trait combinations not seen in either parent.

Teacher's Note:
1. These appear because traits are inherited independently.
2. They provide variation.

Answer: (b) recessive. A recessive trait appears only in the homozygous recessive condition.

Teacher's Note:
1. It is hidden in the presence of a dominant allele.
2. In Tt, the t allele is not expressed.

Answer: (a) different forms of a gene. Alleles are alternative forms of the same gene, such as T and t.

Teacher's Note:
1. An individual carries two alleles for each gene, one from each parent.
2. Together they form the genotype.

Answer: (a) inaccuracies in DNA copying and the shuffling of genes during gamete formation. Small errors in DNA copying and the mixing of parental genes produce differences in offspring.

Teacher's Note:
1. Variations give some individuals a better chance of survival.
2. Asexual reproduction gives much less variation.

Answer: (a) genes. Genes on chromosomes carry the information for traits.

Teacher's Note:
1. A gene is a section of DNA that codes for a protein.
2. This protein controls a trait.

Answer: (b) 46. A human cell has 23 pairs of chromosomes, i.e. 46.

Teacher's Note:
1. Gametes have only 23 chromosomes.
2. Fertilisation restores the number to 46.

Answer: (a) 22 pairs and 1 pair. There are 22 pairs of autosomes and 1 pair of sex chromosomes.

Teacher's Note:
1. The sex chromosomes are XX in females and XY in males.
2. Autosomes are all the other chromosomes.

Answer: (b) XX. A female has two X chromosomes.

Teacher's Note:
1. All her eggs therefore carry an X chromosome.
2. A male has one X and one Y.

Answer: (b) the sperm of the father. The father's sperm may carry X or Y; the egg always carries X.

Teacher's Note:
1. If an X sperm fertilises the egg the child is a girl, and if a Y sperm does, a boy.
2. The chance is 50% each time.

Answer: (b) a boy. XY gives a male child.

Teacher's Note:
1. The egg always provides an X.
2. So the sperm decides the sex.

Answer: (c) 1 tall : 1 dwarf. Tt × tt gives Tt and tt in equal numbers.

Teacher's Note:
1. This is a test cross, used to find whether a tall plant is TT or Tt.
2. A 1 : 1 ratio shows Tt.

Answer: (b) TT. A pure (homozygous) tall plant has two dominant alleles, TT.

Teacher's Note:
1. Tt is a hybrid tall plant.
2. Both TT and Tt look tall.

Answer: (a) they do not change the DNA in germ cells. Changes in body cells do not alter the DNA of the germ cells.

Teacher's Note:
1. Only changes in the DNA of the gametes pass to offspring.
2. A trained skill is not inherited.

Answer: (b) as discrete units, with each parent contributing one copy. Each parent contributes one copy of each factor and traits do not blend.

Teacher's Note:
1. Tall and dwarf parents give tall, not medium, offspring in F1.
2. The dwarf trait reappears in F2.

Answer: (b) recessive trait. The recessive trait is masked in F1 by the dominant one but returns in F2 when two copies come together.

Teacher's Note:
1. In Mendel's tall × dwarf cross the dwarf trait returns in 1/4 of F2 plants.
2. This showed that the trait was not lost.

Answer: (b) 50%. Half the sperms carry X and half carry Y, so an X sperm fertilises the egg with a 50% chance.

Teacher's Note:
1. Each birth is an independent event.
2. A previous child does not change the chance.