Class 10 Science, Chapter 8 (NCERT). 15 questions by concept. Open “Show answer” for the answer and why.
How to teach this chapter · Common misconceptions & worked visual for this chapter
How do offspring of sexual reproduction compare with their parents?
(b) Similar, with some differences
Offspring share the basic body design of their parents but show some differences, because genes from two parents combine.
Sugarcane is grown from cuttings, without gametes. What is the source of the small differences among its descendants?
(d) Errors in DNA copying
Even without gametes, DNA copying is not perfectly exact. Its small errors cause small differences among descendants.
A heat wave begins. What does the environment do to the heat-resistant individuals already present in the population?
(c) Selects them
The environment selects among variants that already exist; it does not create the variation an organism needs. Heat-resistant individuals already present survive and reproduce more.
A moth colour pattern gives no camouflage advantage in a habitat with no sight-hunting predators. What is the survival value of that variation there?
(a) Neutral
The colour pattern does not help or hurt survival here, since no predator hunts by sight. Its survival value is neutral, not beneficial or harmful.
Two tall pea plants look alike but can have different genotypes. What does a dominant phenotype alone tell you about genotype?
(a) It cannot tell you if the genotype is TT or Tt
A dominant phenotype alone cannot distinguish a homozygous genotype, TT, from a heterozygous genotype, Tt. That is why two tall pea plants can look alike yet have different genotypes.
A trait appears in individuals carrying just one copy of its gene, but that gene copy is rare in the population. Is this trait dominant or recessive?
(b) Dominant
A trait that appears when only one copy of a gene is present is dominant. Dominance depends on how the trait is expressed, not on how common the gene copy is in the population.
How many different genotypes appear in the F2 of a Tt × Tt cross?
(d) Three
The F2 has TT, Tt and tt: three genotypes in a 1:2:1 ratio. The 3:1 ratio is for tall to short plants.
A true-breeding round-seeded pea plant is crossed with a true-breeding wrinkled-seeded pea plant. Round is dominant. What shape are all the F1 seeds?
(c) Round
Crossing a true-breeding dominant parent with a true-breeding recessive parent gives F1 offspring that all show the dominant trait. All F1 seeds are round.
Seed shape and seed colour are inherited independently in a dihybrid cross. Which phenotype combinations appear among the F2 plants?
(b) Both the parental combinations and new combinations
Seed shape and seed colour separate into gametes independently of each other. So the F2 plants show both the parental combinations, such as round-yellow and wrinkled-green, and new combinations, such as round-green and wrinkled-yellow.
A tall, round-seeded pea plant is crossed with a short, wrinkled-seeded plant. In the F2 generation, a short, round-seeded plant appears. What is this combination called?
(d) Recombinant combination
The parents combine tall stature with round seeds and short stature with wrinkled seeds. Short stature with round seeds is a new combination of these parental traits, so it is a recombinant combination. The traits in this cross are plant height and seed shape, not seed colour.
A pea plant carries the gene pair Tt. What is such a pair called?
(b) Heterozygous
Tt has two different alleles of the gene, so it is heterozygous. TT and tt are homozygous.
A gene for plant height affects a protein that takes part in a growth reaction. How does the gene control the height of the plant?
(c) By coding a protein that takes part in the growth process
The gene does not stretch the stem by itself. It codes for a protein that takes part in the growth reaction, and that reaction changes the height of the plant.
How many chromosomes of each pair does a gamete carry?
(d) One
A body cell has paired chromosomes, but a gamete has only one chromosome from each pair.
In snails, an individual can change sex during its life. What does this show about sex determination in snails?
(d) It is not genetically fixed
A snail that changes sex during its life is not locked into one sex by its chromosomes at birth, unlike the human XX-XY system. So sex determination in snails is not genetically fixed.
The mother's ovum always carries an X chromosome. Which parent's gamete decides whether a child is XX or XY?
(c) Father
Every ovum carries X. A sperm carries X or Y, so the father's sperm decides the child's sex.
Wrong
A dominant allele must be found in most people.
Why it feels right. The word dominant sounds like it means widespread.
Right
Dominance describes expression in a heterozygote.
Worked example
