NCERT Exemplar Class 12 Biology Solutions Chapter 5 Principles of Inheritance and Variation

Question:2

Conditions of a karyotype 2n +1, 2n –1 and 2n + 2, 2n – 2 are called:
a. Aneuploidy
b. Polyploidy
c. Allopolyploidy
d. Monosomy

Answer:

The answer is the option (a) Aneuploidy
Explanation: Aneuploidy is the condition in which loss or gain of chromosomes is observed due to the failure of the separation of chromatids during the cycle of cell division. Hence, (a) is the correct option

Question:3

Distance between the genes and the percentage of recombination shows:
a. a direct relationship
b. an inverse relationship
c. a parallel relationship
d. no relationship

Answer:

The answer is the option (b) An inverse relationship
Explanation: Reduced distance between genes increases the probability of recombination which shows an inverse relationship. Hence, (b) is the correct option.

Question:4

If a genetic disease is transferred from a phenotypically normal but carrier female to only some of the male progeny, the disease is:
a. Autosomal dominant
b. Autosomal recessive
c. Sex-linked dominant
d. Sex-linked recessive

Answer:

The answer is the option (d) Sex-linked recessive
Explanation: Here, the X-chromosome has the flawed gene, but the carriers of the disease are females and it is manifested in males. Thus, it is referred to as a sex-linked recessive disease. One example of such a disease is Haemophilia which is known as X-linked recessive disease.

Question:5

In sickle cell anaemia glutamic acid is replaced by valine. Which one of the following triplets codes for valine?
a. G G G
b. A A G
c. G A A
d. G U G

Answer:

The answer is the option (d) G U G
Explanation: At the sixth codon of the beta-globin gene, single base substitution takes place from GAG to GUG. Due to this, in the globin protein, replacement of glutamic acid with valine is seen. Hence, GUG is the code for valine.

Question:6

The person having genotype |A|B would show the blood group as AB. This is because of:
a. Pleiotropy
b. Co-dominance
c. Segregation
d. Incomplete dominance

Answer:

The answer is the option (b) Co-dominance
Explanation: Most commonly found in ABO blood grouping, co-dominance is when the F1 generation resembles both parents. Here, the result is AB group because both A and B sugars are dominant.

Question:7

zzzwype of sex determination is seen in:
a. Platypus
b. Snails
c. Cockroach
d. Peacock

Answer:

The answer is the option (d) Peacock
Explanation: Sex determination of the type ZZZW is observed in birds. Males have ZZ combination and females have ZW combination.

Question:8

A cross between two tall plants resulted in offspring having few dwarf plants. What would be the genotypes of both the parents?
a. TT and Tt
b. Tt and Tt
c. TT and TT
d. Tt and tt

Answer:

The answer is the option (b) Tt and Tt
Explanation: In the case of Tt and Tt (option (b)), a few of the offspring will be dwarf (TT, Tt, tt) and most of them are going to be tall (TT, Tt). In case of option (a) and (c), the offspring will be all tall and in option (d), one of the parents is dwarf. Hence, (b) is the correct option.

Question:9

In a dihybrid cross, if you get 9:3:3:1 ratio it denotes that:
a. The alleles of two genes are interacting with each other
b. It is a multigenic inheritance
c. It is a case of multiple allelism
d. The alleles of two genes are segregating independently

Answer:

The answer is the option (d) The alleles of two genes are segregating independently.
Explanation: A 9:3:3:1 ratio is the result of a dihybrid cross when two genes assort independently during gamete formation, as explained by Mendel’s law of independent assortment.

Question:10

Which of the following will not result in variations among siblings?
a. Independent assortment of genes
b. Crossing over
c. Linkage
d. Mutation

Answer:

The answer is the option (c) Linkage
Explanation: Whenever two genes are in close proximity and located on same chromosome, linkage takes place irrespective of the variation.

Question:11

Mendel's Law of independent assortment holds good for genes situated on the:
a. non-homologous chromosomes
b. homologous chromosomes
c. extranuclear genetic element
d. same chromosome

Answer:

The answer is the option (a) Non-homologous chromosomes
Explanation: Mendel's law of independent assortment applies when genes are located on different (non-homologous) chromosomes, allowing them to assort independently during gamete formation.

Question:12

Occasionally, a single gene may express more than one effect. The the phenomenon is called:
a. multiple allelism
b. mosaicism
c. pleiotropy
d. polygeny

Answer:

The answer is the option (c) Pleiotropy
Explanation: Pleiotropy is the phenomenon in which a single gene expresses more than one effect occasionally.

Question:13

In a certain taxon of insects, some have 17 chromosomes and the others have 18 chromosomes. The 17 and 18 chromosome-bearing organisms are:
a. males and females, respectively
b. females and males, respectively
c. all males
d. all females

Answer:

The answer is the option (a) Males and females, respectively
Explanation: Males have single X-chromosomes while females have pairs of X-chromosomes. This is XO sex determination which is seen in such insects. Hence, (a) is the answer.

Question:14

The inheritance pattern of a gene over generations among humans is studied by the pedigree analysis. Character studied in the pedigree analysis is equivalent to:
b. Mendelian trait
c. polygenic trait
d. maternal trait

Answer:

The answer is the option (b) Mendelian trait
Explanation: Pedigree analysis can be used to trace the Mendelian disorders and the pattern of their inheritance.

Question:15

It is said that Mendel proposed that the factor controlling any character is discrete and independent. His proposition was based on the
a. results of the F3 generation of a cross.
b. observations that the offspring of a cross made between the plants having two contrasting characters shows only one character without any blending.
c. self-pollination of F1 offsprings
d. cross-pollination of F1 generation with recessive parent

Answer:

The answer is the option (b) observations that the offspring of a cross, made between the plants having two contrasting characters shows only one character without any blending.
Explanation: It was observed by Mendel that when two pairs of varying characters are analyzed during dihybrid cross, the behavior of a particular character is independent of another character. Hence, the correct answer is (b).

Question:16

Two genes ‘A’ and ‘B’ are linked. In a dihybrid cross involving these two genes, the F1 heterozygote is crossed with the homozygous recessive parental type (aa bb). What would be the ratio of offspring in the next generation?
a. 1: 1 : 1: 1
b. 9 : 3 : 3: 1
c. 3: 1
d. 1: 1

Answer:

The answer is the option (a) 1:1:1:1
Explanation: Since genes A and B are linked, but a test cross is performed with a double recessive parent, the four types of gametes (parental and recombinant) are produced in equal proportion, giving a 1:1:1:1 ratio.

Question:17

In the F2 generation of a Mendelian dihybrid cross the number of phenotypes and genotypes are:
a. phenotypes – 4; genotypes – 16
b. phenotypes – 9; genotypes – 4
c. phenotypes – 4; genotypes – 8
d. phenotypes – 4; genotypes – 9

Answer:

The answer is the option (d) Phenotypes – 4; genotypes - 9
Explanation: For example, consider a dihybrid cross between wrinkled green (rryy) and round yellow (RRYY). The phenotypes were as follows: round green, round yellow, wrinkled green and wrinkled yellow and the genotypes were: rryy, rrYy, rrYY, Rryy, RrYy, RrYY, RRyy, RRYy, and RRYY. Hence, the correct option is (d).

Question:18

Mother and father of a person with ‘O’ blood group have ‘A’ and ‘B’ blood group, respectively. What would be the genotype of both mother and father?
a. Mother is homozygous for ‘A’ blood group and father is heterozygous for ‘B’
b. Mother is heterozygous for ‘A’ blood group and father is homozygous for ‘B’
c. Both mother and father are heterozygous for ‘A’ and ‘B’ blood groups, respectively
d. Both mother and father are homozygous for the ‘A’ and ‘B’ blood groups, respectively

Answer:

The answer is option (c) Both mother and father are heterozygous for the ‘A’ and ‘B’ blood groups, respectively
Explanation: Offspring ii (O blood group) is the feasible phenotype of parents and I^Ai (A blood group) and I^Bi (B blood group) are the feasible genotype of the parents.

Question:2

Do you think Mendel's laws of inheritance would have been different if the characters that he chose were located on the same chromosome?

Answer:

Yes, the results would have been completely different if the characters chosen were located on the same chromosome. In his experiments on Drosophila, Morgan found that the ratio of phenotype in the F1 generation was different than that in Mendel’s observation because the characters were on the same chromosome.

Question:3

Enlist the steps of controlled cross-pollination. Would emasculation be needed in a cucurbit plant? Give reasons for your answer.

Answer:

Controlled cross-pollination involves the following steps:

1. Emasculation

2. Transfer of pollen from a different flower

3. Pollination

4. Fertilization

Emasculation is necessary in the case of a monoecious plant, but it is not needed in the plants of Cucurbitaceae as they have unisexual flowers.

Question:4

A person has to perform crosses to study the inheritance of a few traits/characters. What should be the criteria for selecting the organisms?

Answer:

The following criteria should be followed for selecting the organisms:

• Easy hybridization should be possible.

• The sets of contrasting characters chosen should be easily recognizable.

• The organism should have a short life cycle for convenient and early completion of the study.
  

Question:5

The pedigree chart given below shows a particular trait which is absent in parents but present in the next generation irrespective of sexes. Draw your conclusion based on the pedigree.

Answer:

The breeding between parents is shown in the first row, and the number of progenies is depicted by the second row. The figure shows two female and three male children with one girl and one boy affected by some kind of genetic disease. From the figure, it can be concluded that the nature of the trait is recessive, and autosome linked in nature.

Question:6

To obtain the F1 generation Mendel pollinated a pure-breeding tall plant with a pure breeding dwarf plant. But forgetting the F2 generation, he simply self-pollinated the tall F1 plants. Why?

Answer:

To prevent the masking of the effect of inheritance to F2 generation by character set, Mendel self-pollinated the tall F1 plants. This is because all plants of the F1 generation turned out to be tall and he could not understand the fate of the recessive character.

Question:7

Genes contain the information that is required to express a particular trait.” Explain.

Answer:

The fact that genes are responsible for inheritance was discovered after the discovery of chromosomes. Before that, genes were not in the knowledge of the scientists. Only after Mendel’s period, it was known that the information on the expression of a particular trait is contained in the genes.

Question:8

How do the alleles of a particular gene differ from each other? Explain its significance.

Answer:

Alleles are genes which code for a pair of contrasting characters, and these are different from each other within a pair. However, the slight difference is not noticeable as characters or phenotype, but as the absence or presence of a molecule of a substance like sugar polymer. The substance is present on the gene I which regulates the ABO blood grouping.
Alleles are vital because they help in identifying whether a particular trait is recessive or dominant or even co-dominant (in some cases).

Question:9

In a monohybrid cross of plants with red and white-flowered plants, Mendel got only red-flowered plants. On self-pollinating these F1 plants got both red and white-flowered plants in 3:1 ratio. Explain the basis of using RR and rr symbols to represent the genotype of plants of the parental generation.

Answer:

The usage of particular alphabets is only meant for convenience. Generally, the character is shown by the first letter of a particular trait, and a different case of the same letter is used to show the contrasting character. Conventionally, a capital letter depicts the dominant trait, and a lowercase letter depicts the recessive trait.

Question:10

For the expression of traits, genes provide only the potentiality, and the environment provides the opportunity. Comment on the veracity of the statement.

Answer:

Genes carry the potentiality for the expression of a trait, i.e. phenotype and are responsible for inheritance. Phenotype depends on many factors. If a trait gets combined with a dominant one, it may get recessive, and the survival depends on the natural selection passage. Therefore, the opportunity for expression of a particular trait is provided by and depends on the environment.

Question:11

A, B, D are three independently assorting genes with their recessive alleles a, b, d, respectively. A cross was made between individuals of Aa bb DD genotype with aa bb dd. Find out the type of genotypes of the offspring produced.

Answer:

The genotype of the F1 generation is depicted by the following Punnett Square:

So, the offspring produced will have two types of genotypes

• AabbDd
• aabbDd

Question:12

In our society, a woman is often blamed for not bearing a male child. Do you think it is right? Justify.

Answer:

The Y chromosome, which is present in males, determines the sex of the human child. Hence, it is evident that the father is responsible for having a male or a female child. Therefore, a mother should not at all be blamed for the same. However, the probability of X or Y chromosome, making its way to the zygote is equal, i.e. 50-50. Hence, no one is to be blamed or rewarded for that. A female child is as important as a male child.

Question:13

Discuss the genetic basis of wrinkled phenotype of pea seed.

Answer:

Generally, seeds dry before dispersal and germination. The amount of starch in the seed decides whether the seed will be round or wrinkled, and the relative amount of starch is looked after by a gene. If there is less than the required starch in the seed, it will be wrinkled. In pea plants, a mutant gene’s enzymes are responsible for reducing the amount of starch in seeds and hence, the seeds become wrinkled.

Question:14

Even if a character shows multiple allelism, an individual will only have two alleles for that character. Why?

Answer:

Alleles are present only in pairs because maximum organisms are diploid. Thus, an individual will have only two alleles for the character even after showing multiple alleles. For example, the ABO blood grouping is governed by alleles I, I^A and I^B.

Question:15

How does a mutagen induce mutation? Explain with example.

Answer:

The mutation is induced by physical and chemical factors called mutagens. During replication, the base-pair sequence in the DNA is altered for inducing mutation.

Question:2

Can a child have blood group O if his parents have blood group ‘A’ and ‘B’? Explain.

Answer:

Yes, it is possible for a child to have blood group O if the parents have blood group A and B. If genotypes of the parents are I^Ai and I^Bi, then gametes can have one of the genotype from i, I^A or I^B. The child’s genotype will be ii if fertilization of gametes occurs with i and hence the child will have blood group O.

Question:3

What is Down’s syndrome? Give its symptoms and cause. Why is it that the chance of having a child with Down’s syndrome increases if the age of the mother exceeds forty years?

Answer:

First discovered and described by Langdon Down in 1866, Down’s syndrome refers to the presence of an additional copy of 21st chromosome. It is a chromosomal disorder. A person with this condition has:

• distinct facial appearance with partially opened mouth and furrowed tongue

• small round head

• delayed physical and mental development

• short stature

• broad palm

According to studies, with advancing maternal age, the chances of Down’s syndrome increase in the child. This is because cells get older with the age of the mother and the ova are present since birth in females. Various physio-chemical exposures also lead to increased chances.

Question:4

How was it concluded that genes are located on chromosomes?

Answer:

The behaviour of chromosome and genes was studied during meiosis by Walter Sutton and Theodor Boveri, and they observed similar movement in chromosome and gene. Hence, it was proposed by them that genes are located on chromosomes.

Question:5

A plant with red flowers was crossed with another plant with yellow flowers. If F1 showed all flowers orange in colour, explain the inheritance.

Answer:

Incomplete dominance of a particular trait over another is seen sometimes which results in the manifestation of both characters in some way. For example, consider genotype of red flowers to be RR and that of yellow flowers to be rr. In F1 generation, all progenies have genotype as Rr. Hence, orange flowers are produced by all plants in F1 generation because red colour does not completely dominate the yellow colour.

Question:6

What are the characteristic features of a true-breeding line?

Answer:

The characteristic features of a true-breeding line are:

• Throughout several generations, stable trait inheritance and expression of characters is seen.

• Self-pollination
  

Question:7

In peas, tallness is dominant over dwarfness, and red colour of flowers is dominant over the white colour. When a tall plant bearing red flowers was pollinated with a dwarf plant bearing white flowers, the different phenotypic groups were obtained in the progeny in numbers mentioned against them:

Answer:

TtRr and ttrr are the genotypes of the two parents.
The four offspring types are:

1. TtRr: Tall, Red

2. Ttrr: Tall, White

3. ttRr: Dwarf, Red

4. ttrr: Dwarf, White
   

Question:8

Why is the frequency of red-green colour blindness is many times higher in males than that in the females?

Answer:

The genes with photo-pigments are located on X-chromosome, and if the gene is destroyed or missing, it results in colour blindness. Chances of this disease are much higher in males than in females because males have only one X-chromosome; hence there is no second X-chromosome to compensate for the defect, like in females. Females have colour blindness if the defect is in both her X-chromosomes.

Question:9

If a father and son are both defective in red-green colour vision, is it likely that the son inherited the trait from his father? Comment.

Answer:

X-chromosome contains the genes for colour blindness. However, in the male child, the X-chromosome is contributed by the mother. Therefore, the son inherits the trait from the mother even if his father is suffering from colour blindness.

Question:10

Discuss why Drosophila has been used extensively for genetical studies.

Answer:

Drosophila is ideal for genetical studies because:

• Its life cycle is completed in two weeks.

• Many offspring are produced through a single mating.

• It can be in a laboratory.

• It exhibits sexual dimorphism and many hereditary variations.

Question:11

How do genes and chromosomes share similarity from genetical studies?

Answer:

From the point of view of genetical studies, the similarities in genes and chromosomes are:

• Both genes and chromosomes are found in pairs.

• Independent segregation is observed in independent pairs.

• Segregation takes place when the gamete is formed, and only one pair is transmitted to the gamete.

Question:12

What is recombination? Discuss the applications of recombination from genetic engineering.

Answer:

Recombination is a combination of genes which is different from parental gene combination. It can occur either naturally during meiosis or artificially through genetic engineering, which is used for recombining different species for producing beneficial products for humans. For example, production of BT cotton, BT brinjal, and vaccines like hepatitis B was done with genetic engineering.

Question:13

What is artificial selection? Do you think it affects the process of natural selection? How?

Answer:

The selective breeding of animals and plants is called artificial selection. It is done to include advantageous traits in plants and animals, and it can be natural or artificial. However, ethically, artificial selection is wrong. But according to the law of survival of the fittest, only the varieties which are fit to survive will survive in artificial selection. Therefore, it will not affect the process of natural selection.

Question:14

With the help of an example, differentiate between incomplete dominance and co-dominance.

Answer:

Question:15

It is said that the harmful alleles get eliminated from the population over some time, yet sickle cell anaemia is persisting in the human population. Why?

Answer:

A single pair of alleles, i.e. Hb^A and Hb^S, controls the sickle cell anaemia. The offspring may suffer from this disease if both parents are heterozygous (Hb^AHb^S). But the offspring should be homozygous (Hb^SHb^S) because heterozygous carry this disease. However, it persists in humans because, in adaptation, heterozygous individuals are advantageous.

Question:2

a. In humans, males are heterogametic, and females are homogametic. Explain. Are there any examples where males are homogametic and females heterogametic?

b. Also describe as to, who determines the sex of an unborn child? Mention whether the temperature has a role in sex determination.

Answer:

1. In humans, males are called heterogametic because they have X and Y chromosomes in their 23rd pair of chromosomes. On the other hand, females are called homogametic as they have two chromosomes, i.e. XX chromosomes in their 23rd pair of chromosomes. However, in birds, the scenario is quite the opposite. Male birds have ZZ chromosomes and female birds have ZW chromosomes. Hence, in cases like these, males are homogametic, and females are heterogametic.
2. The sex of an unborn child is determined by the 23rd pair of chromosomes in humans. The males have X and Y chromosomes and females have X and X chromosomes in that pair. So, when the ovum is fertilized with the sperm containing the X chromosome, the result is a female child, and when it is fertilized with the Y chromosome, the result is a male child. In some animals like crocodiles, the sex is determined by temperature. Male crocodiles are born if eggs are brooded at high temperatures.

Question:3

A normal visioned woman, whose father is colour blind, marries a normal visioned man. What would be the probability of her sons and daughters to be colour blind? Explain with the help of a pedigree chart.

Answer:

The pedigree analysis shown below depicts the colour blindness in offspring:

• The F1 generation depicts two female children and one male child – One of the female children might be a carrier, and the male child will suffer from colour blindness.

• The P generation depicts a normal husband and a carrier wife.

About 8% of the males are colour blind, while only 0.5% of the females are colour blind. This is because only one X-chromosome is present in males. If that one X chromosome carries the defect, the boy will suffer from colour blindness. But in females, there are two X chromosomes which help in the compensation of deficiency in one of the chromosomes. Thus, females are generally carriers of the disease and rarely the sufferer.

Question:4

Discuss in detail the contributions of Morgan and Sturvant in the area of genetics.

Answer:

Sturtevant was one of the students of Morgan. Morgan has carried out several experiments related to genetics. Some of their contributions are:

• In the various dihybrid crosses of Drosophila carried out by Morgan, it was observed by him that the phenotypic ratio did not resemble the standard ratio, which was observed by Mendel. Morgan and his group noticed that the genes did not show independent segregation when they were located on the same chromosome as they were aware that genes were located on the X chromosome. Morgan termed the interaction of genes on the same chromosome as linkage and the generation of non-parental combination as recombination. On the same chromosome, the probability of parental combination is much higher than that of non-parental combination.

• Sturtevant discovered that on the same chromosome, recombination or lack of recombination was largely dependent on the relative distance between the two genes. There was no recombination when genes were tightly linked, and chances of recombination were higher when genes had a large distance between them.

Contributions made by Morgan and his group, including Sturtevant, contributed hugely to the development of the genetic mapping.

Question:5

Define aneuploidy. How is it different from polyploidy? Describe the individuals having followed chromosomal abnormalities.
a. Trisomy of 21st Chromosome
b. XXY
c. XO

Answer:

Aneuploidy is a condition in which gain or loss of chromosome is seen during the cell division due to the failure of chromatid segregation. On the other hand, an increase in the number of chromosome set caused due to the failure of cytokinesis after telophase is called polyploidy. This condition is mostly seen in plants and rarely seen in animals.

1. Trisomy of 21st chromosome: First discovered and described by Langdon Down in 1866, it refers to the presence of an additional copy of 21st chromosome. It is known as Down’s syndrome. A person with this condition has a distinct facial appearance with partially opened mouth and small round head. Physical and mental development is delayed in such individuals.

2. XXY: In this condition, a karyotype of 47 is caused due to an additional copy of the X chromosome. It is a genetic disorder and is called Klinefelter’s syndrome. In this case, three chromosomes (XXY) are present in the 23rd set of chromosomes. A person suffering from this disorder is sterile and has enlarged breasts (gynecomastia) with overall masculine development.

3. XO: Lack of an X chromosome causes this genetic disorder of ploidy of 45 (XO). It is known as Turner’s syndrome. Such females have rudimentary ovaries and are sterile. The absence of secondary sexual characters is also seen.