CBSE Class 10 Science Heredity and Evolution MCQs Set 11

Answer: (a) 1 : 3

Explanation:
1. F₁ plants are all Rr and purple.
2. Rr × Rr gives RR, Rr, Rr, rr, so 3 purple and 1 white.
3. White : purple = 1 : 3.

Teacher's Note:
1. The ratio asked is white to purple, not purple to white.
2. Read the order carefully.

Answer: (b) 25%

Explanation:
1. GG × gg gives all Gg (green) in F₁.
2. Gg × Gg gives GG, Gg, Gg and gg.
3. Purple stems need gg, which is 1/4, i.e. 25%.

Teacher's Note:
1. Green is dominant here and purple is recessive.
2. A recessive phenotype is always 25% in F₂ of a monohybrid cross.

Answer: (a) meiosis

Explanation:
1. Meiosis is the division in which gametes are formed and the chromosome number is halved.
2. The two alleles of a gene separate during meiosis.

Teacher's Note:
1. Fertilisation brings the alleles together again.
2. Segregation is Mendel's first law.

Answer: (a) 1 : 2 : 1

Explanation:
1. A monohybrid F₂ consists of TT, Tt, Tt and tt.
2. The genotypic ratio is 1 : 2 : 1, whereas the phenotypic ratio is 3 : 1.

Teacher's Note:
1. Pure dominant : hybrid : pure recessive = 1 : 2 : 1.
2. Do not mix it with the phenotypic ratio.

Answer: (c) recessive

Explanation:
1. In a hybrid only one allele is expressed.
2. The allele that cannot express itself in the presence of the other is the recessive allele.

Teacher's Note:
1. The allele that expresses itself is dominant.
2. Recessive alleles show only in homozygous condition.

Answer: (c) violet flower colour, green pod colour and round seed shape

Explanation:
1. Dominant traits in pea are round seed, yellow seed, violet flower, axial flower, inflated pod, green pod and tall plant.
2. Option (c) lists three of them: violet flower, green pod and round seed.

Teacher's Note:
1. Green seed colour, terminal flower, wrinkled seed and yellow pod are recessive.
2. Each wrong option contains a recessive trait.

Answer: (b) heterozygous

Explanation:
1. A genotype with two different alleles, such as Tt, is called heterozygous.

Teacher's Note:
1. Homozygous means both alleles are the same.
2. The other two terms are not used in this sense.

Answer: (b) (i) dominant, (ii) recessive

Explanation:
1. Free earlobes are the common form and are expressed even when one dominant allele is present, so they are dominant.
2. Attached earlobes appear only when both alleles are recessive, so they are recessive.

Teacher's Note:
1. Earlobe type is an inherited trait of humans.
2. Use F for free and f for attached earlobes.

Answer: (b) There are atleast two different alleles for the gene for height

Explanation:
1. A gene exists in alternative forms called alleles.
2. Tt shows that the organism has two different alleles, T and t, for the gene for height.
3. So there are at least two different alleles for the gene.

Teacher's Note:
1. Alleles are forms of one gene, not different genes.
2. Tt is a heterozygous genotype.

Answer: (b) A are tall and B are dwarf

Explanation:
1. The trait A appears in all F₁ plants and in 3 of the 4 F₂ plants, so A is dominant.
2. The trait B is hidden in F₁ and appears in one-fourth of F₂, so B is recessive.
3. A and B must be contrasting traits of the same character, so A are tall and B are dwarf.

Teacher's Note:
1. Tall and round are traits of different characters, so they cannot be contrasting.
2. 3 : 1 in F₂ means dominant : recessive.

Answer: (c) 50% dominant, 50% recessive

Explanation:
1. The cross is tt × Tt.
2. The offspring are Tt and tt in equal numbers, so 50% show the dominant phenotype and 50% the recessive.

Teacher's Note:
1. This is a test cross.
2. Option (d) mixes genotypes with phenotypes and cannot be a phenotypic result.

Answer: (b) asexual reproduction

Explanation:
1. Sugarcane is propagated by vegetative (asexual) reproduction from a single parent.
2. Offspring are almost exact copies of the parent, so there is little variation.
3. Hence the cause is asexual reproduction.

Teacher's Note:
1. Sexual reproduction produces more variation because two parents contribute genes.
2. Asexual reproduction gives nearly identical offspring.

Answer: (a) F₁ generation

Explanation:
1. The progeny produced by crossing two parents is called the first filial generation, or F₁.
2. The progeny obtained by selfing F₁ is the F₂ generation.

Teacher's Note:
1. F stands for filial, meaning offspring.
2. The parents form the P generation.

Answer: (a) Law of purity of gametes

Explanation:
1. The recessive character is hidden in F₁ and reappears in F₂.
2. This shows that the alleles stay unchanged and separate in the gametes, which is the law of purity of gametes (segregation).

Teacher's Note:
1. The law of dominance explains why only one trait appears in F₁.
2. Reappearance in F₂ shows segregation.

Answer: (b) a gamete receiving only one of the two homologous chromosomes during meiosis.

Explanation:
1. The two alleles of a gene lie on a pair of homologous chromosomes.
2. During meiosis these chromosomes separate, so each gamete receives only one of them and hence only one allele.

Teacher's Note:
1. This is the physical basis of Mendel's first law.
2. Gametes carry half the number of chromosomes.

Answer: (c) An alternative form of a gene that is located at a specific position on a specific chromosome.

Explanation:
1. Alleles are alternative forms of the same gene.
2. They occupy the same position (locus) on the homologous chromosomes.
3. So the correct option is (c).

Teacher's Note:
1. T and t are two alleles of the height gene.
2. Alleles of one gene always lie at the same position.

Answer: (b) No, the plants of F₁ generation are not the same as parents. They are heterozygous tall.

Explanation:
1. F₁ plants are Tt, which look tall but carry one allele for dwarfness.
2. The tall parent was TT (homozygous).
3. So F₁ plants look the same but differ in genotype; they are heterozygous tall.

Teacher's Note:
1. Same phenotype does not mean same genotype.
2. F₁ tall plants give dwarf plants in F₂ but TT plants do not.

Answer: (d) 1 : 1

Explanation:
1. F₂ has TT : Tt : tt in the ratio 1 : 2 : 1.
2. Homozygous dominant (TT) and homozygous recessive (tt) are 1 part each, so the ratio is 1 : 1.

Teacher's Note:
1. Do not include the heterozygous Tt plants.
2. Homozygous means both alleles are the same.

Answer: (a) 50 : 50

Explanation:
1. The cross is Yy × yy, which gives Yy (yellow) and yy (green) in equal numbers.
2. So the ratio of yellow to green is 50 : 50.

Teacher's Note:
1. A heterozygote crossed with a recessive parent gives 1 : 1.
2. 3 : 1 comes from Yy × Yy.

Answer: (b) round and yellow

Explanation:
1. The F₁ plants are RrYy.
2. Round (R) is dominant over wrinkled and yellow (Y) is dominant over green.
3. So all F₁ seeds are round and yellow.

Teacher's Note:
1. In F₁ of a dihybrid cross only dominant traits are seen.
2. The recessive traits reappear in F₂.

Answer: (b) Dihybrid cross

Explanation:
1. A 9 : 3 : 3 : 1 phenotypic ratio is obtained in the F₂ generation when two pairs of contrasting characters are studied together.
2. Such a cross is a dihybrid cross.

Teacher's Note:
1. Monohybrid cross gives 3 : 1.
2. Dihybrid cross gives 9 : 3 : 3 : 1.

Answer: (d) wrinkle and green

Explanation:
1. The F₂ phenotypic ratio is 9 : 3 : 3 : 1, so the rarest class forms 1/16 of the progeny.
2. 1/16 of 240 = 15.
3. The class with 1 part is the double recessive, wrinkled green.

Teacher's Note:
1. 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
2. Total parts are 16, so 240 ÷ 16 = 15 per part.

Answer: (c) 9 and 4

Explanation:
1. A cross AaBb × AaBb gives 3 × 3 = 9 different genotypes.
2. The phenotypes are 2 × 2 = 4 in the ratio 9 : 3 : 3 : 1.
3. So the answer is 9 and 4.

Teacher's Note:
1. Each gene pair gives 3 genotypes and 2 phenotypes.
2. Multiply the numbers for the two gene pairs.

Answer: (d) 100%, 75%

Explanation:
1. RR × rr gives all Rr in F₁, so 100% of F₁ have round seeds.
2. Rr × Rr gives RR, Rr, Rr and rr in F₂, so 3 out of 4, i.e. 75%, have round seeds.

Teacher's Note:
1. Round is dominant over wrinkled.
2. F₂ always has 75% dominant phenotype in a monohybrid cross.

Answer: (d) that tallness was dominant over dwarfness.

Explanation:
1. The parents were true breeding, so they were homozygous.
2. All F₁ plants were tall, which shows that tallness is dominant over dwarfness.

Teacher's Note:
1. Recombination and assortment can be seen only in F₂.
2. F₁ shows only dominant traits.

Answer: (a) 25%

Explanation:
1. A YyRr plant produces four types of gametes, YR, Yr, yR and yr, in equal numbers.
2. So the yr gametes are 1/4, i.e. 25%.

Teacher's Note:
1. Each pair of alleles separates independently.
2. 2 × 2 = 4 types of gametes with equal frequency.

Answer: (a) Plants with round and yellow coloured seeds.

Explanation:
1. The numbers 315 : 108 : 101 : 32 are close to the ratio 9 : 3 : 3 : 1.
2. The largest class, round yellow, is the class produced by both dominant traits, so round and yellow are dominant.
3. All F₁ plants therefore have round and yellow seeds.

Teacher's Note:
1. F₁ shows only the dominant traits.
2. The largest F₂ class always shows both dominant traits.

Answer: (a) 9 : 3 : 3 : 1 ratio of phenotypes only

Explanation:
1. Selfing a dihybrid YyRr plant gives 16 combinations of gametes.
2. The phenotypes appear in the ratio 9 : 3 : 3 : 1, while the genotypes are nine kinds in the ratio 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1.
3. So the answer is 9 : 3 : 3 : 1 ratio of phenotypes only.

Teacher's Note:
1. The 9 : 3 : 3 : 1 ratio refers to phenotypes.
2. Genotypes are nine in number.

Answer: (b) 50%

Explanation:
1. YyRR produces only two kinds of gametes, YR and yR, because RR gives only R.
2. Each is 50%, so the yR gametes are 50%.

Teacher's Note:
1. Homozygous pairs give only one type of allele in gametes.
2. Count only the heterozygous gene pairs for the number of gamete types.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A back cross is a cross of a hybrid with one of its parents.
2. A test cross crosses an individual with the recessive parent, which is a type of back cross, so A is true and R is true.
3. But R does not explain why a test cross is a back cross, because a back cross can also be with the dominant parent.

Teacher's Note:
1. A test cross is used to find out whether a dominant individual is homozygous or heterozygous.
2. Not every back cross is a test cross.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A pure line consists of homozygous individuals that produce identical offspring, so it is a true breed and A is true.
2. True breeds are used for cross breeding to get improved varieties, so R is true, but it does not explain why they are called true breeds.

Teacher's Note:
1. True breeding plants gave Mendel clear results.
2. The name 'true' comes from breeding true to type.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Group O parents are I^OI^O and give only I^O, so all children are group O and A is true.
2. Blood group is determined by three alleles I^A, I^B and I^O, so R is true, but it does not by itself explain the result.

Teacher's Note:
1. Group O is the recessive homozygous genotype.
2. Two recessive parents can have only recessive children.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A dominant trait appears whether the other allele is dominant or recessive, so A is true.
2. A recessive trait appears only when the dominant allele is absent, so R is true.
3. R is the other side of the same rule and is not the reason why the dominant trait expresses itself.

Teacher's Note:
1. Tt and TT both show the dominant trait.
2. Only tt shows the recessive trait.

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

Explanation:
1. Violet is dominant over white, so all F₁ plants have violet flowers, and A is true.
2. The gene for white colour is passed on to F₁ and it is only masked; it reappears in F₂, so R is false.

Teacher's Note:
1. The source key marked R as the correct explanation, but R is false, so the key has been corrected.
2. A recessive allele is hidden, not lost.

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

Explanation:
1. According to the law of segregation the two alleles of a gene separate at gamete formation, so each gamete carries only one allele and is pure.
2. The alleles in the hybrid stay distinct and do not mix, which explains the name.

Teacher's Note:
1. Alleles never blend in a hybrid.
2. The recessive allele comes out unchanged in F₂.

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

Explanation:
1. Mendel self-pollinated F₁ round yellow plants to get F₂, so A is true.
2. The F₁ progeny of round yellow and wrinkled green parents are all round yellow, not green and wrinkled, so R is false.

Teacher's Note:
1. Round and yellow are dominant over wrinkled and green.
2. Only dominant characters appear in F₁.