CBSE Class 12 Biology Principles of Inheritance and Variation VBQs

Question. Non-disjunction in meiosis results in
(a) trisomy
(b) normal diploid
(c) gene mutation
(d) None of these
Answer : A

Question. A disease caused by an autosomal primary non-disjunction is 
(a) Down’s syndrome
(b) Klinefelter’s syndrome
(c) Turner’s syndrome
(d) Sickle-cell anaemia
Answer : A

Question. Karyotype of Down’s syndrome has how many chromosomes? 
(a) 43
(b) 46
(c) 47
(d) 45
Answer : C

Question. A normal-visioned man whose father was colourblind, marries a woman whose father was also colourblind.
They have their first child as a daughter. What are the chances that this child would be colourblind?
(a) 100%
(b) 0% 
(c) 25%
(d) 50%
Answer : B

Question. A man whose father was colourblind marries a woman, who had a colourblind mother and normal father. What percentage of male children of this couple will be colourblind ?
(a) 25%
(b) 0%
(c) 50%
(d) 75%
Answer : A

Question. I. Short statured body with small round head.
II. Furrowed tongue and partially opened mouth.
III. Palm is broad with characteristic palm crease.
IV. Slow physical, psycomotor and mental development.
These are the characters of
(a) Down’s syndrome
(b) Turner’s syndrome
(c) Klinefelter’s syndrome
(d) Edward syndrome
Answer : A

Question. Choose the correct pair.
(a) Gynacoemastia – Development of breasts
(b) Turner’s syndrome – Loss of an X-chromosome in females
(c) Polyploidy – Seen in plants
(d) All of the above
Answer : D

Question. What is the genetic disorder in which an individual has an overall masculine development gynaecomastia and is sterile?
(a) Klinefelter’s syndrome
(b) Edward syndrome
(c) Down’s syndrome
(d) Turner’s syndrome
Answer : A

Question. Klinefelter’s syndrome results from
(a) XX egg and Y from sperm
(b) XX egg and XY sperm
(c) X egg and XY sperm
(d) Both (a) and (c)
Answer : D

Question. In which genetic condition, each cell in the affected person, has three sex chromosomes XXY?
(a) Thalassemia
(b) Klinefelter’s syndrome
(c) Phenylketonuria
(d) Turner’s syndrome
Answer : B

Question. Monosomy and trisomy are represented respectively as
(a) n − 1, n + 2
(b) 2n + 2, 2n + 1
(c) 2n − 1, 2n + 1
(d) 2n − 2, 2n + 1
Answer : C

Question. Female suffering from Turner’s syndrome possess
(a) 45 + XO
(b) rudimentary ovaries
(c) lack of secondary sexual characters
(d) All of the above
Answer : D

Question. Which of the following are chromosomal disorders.
I. Colour blindness II. Down’s syndrome
III. Phenylketonuria IV. Turner’s syndrome
V. Thalassaemia
(a) I, II and III
(b) II, IV and V
(c) III, IV and V
(d) II and IV
Answer : B

Question. Thalassemia in humans
(a) is an autosome linked recessive blood disorder
(b) can transmit from parents to offspring when both parents are unaffected carriers (heterozygous)
(c) caused due to the mutation or deletion of one of the a or b-globin chain
(d) All of the above
Answer : D

Question. a-thalassemia in humans is controlled by
(a) HBA1 and HBA2 genes on chromosome 16
(b) HBA1 gene on chromosome 12
(c) HBA2 gene on chromosome 11
(d) HBA1 and HBA2 genes on chromosome 9
Answer : A

I. Assertion and Reason  
n Direction (Q. No. 149-163) In each of the following questions, a statement of Assertion (A) is given followed by corresponding statement of Reason (R). Of the
statements, mark the correct answer as
(a) If both A and R are true and R is the correct explanation of A
(b) If both A and R are true, but R is not the correct explanation of A
(c) If A is true, but R is false
(d) If A is false, but R is true

Question. Assertion (A) Offspring have characteristics of both the parents.
Reason (R) Characters pass from the parents to their progeny.
Answer : A

Question. Assertion (A) True breeding lines have stable trait inheritance for several generations.
Reason (R) Mendel conducted cross-pollination experiments on true breeding lines.
Answer : B

Question. Assertion (A) In F2-generation, the traits seen in the progeny were identical to their parents.
Reason (R) The progeny of the F2-generation show no blending of traits.
Answer : A

Question. Assertion (A) Genes are not passed on from one generation to the next.
Reason (R) Genes serves as the units of inheritance.
Answer : D

Question. Assertion (A) Gametes receives only one allele of a gene.
Reason (R) Mitosis occurs during gamete formation leading to the formation of haploid gametes.
Answer : C

Question. Assertion (A) In codominance, the F1-generation resembles both the parents.
Reason (R) An example is different type of red blood cells that determine ABO blood grouping in humans.
Answer : A

Question. Assertion (A) Behaviour of chromosome is parallel to gene.
Reason (R) Genes are located on the chromosome.
Answer : B

Question. Assertion (A) Some genes tend to pass together from one generation to another.
Reason (R) Such genes are located for away from each other on a chromosome.
Answer : C

Question. Assertion (A) Insects show female heterogamety.
Reason (R) In insects, males have XO sex chromosome and females have XX sex chromosome.
Answer : D

Question. Assertion (A) The mechanism of sex-determination in honeybee is called haplodiploidy.
Reason (R) Female honeybees are haploid while male honeybees are diploid.
Answer : C

Question. Assertion (A) The non-allelic genes for red hair and prickles are usually inherited together. 
Reason (R) The genes for red hair and prickles are located on the same chromosome in close association.
Answer : A

Question. Assertion (A) Down’s syndrome, Klinefelter’s syndrome and Turner’s syndrome are chromosomal disorders.
Reason (R) In Klinefelter’s syndrome females are sterile. 
Answer : B

Question. Assertion (A) Phenylketonuria is recessive hereditary disease caused by body’s failure to oxidise an amino acid phenylalanine to tyrosine, because of defective enzyme. 
Reason (R) It is characterised by in the presence of phenylalanine acid in urine.
Answer : B

Question. Assertion (A) Sickle-cell anaemia is an autosome linked recessive trait.
Reason (R) It is controlled by a single pair of allele.
Answer : B

Question. Assertion (A) Down’s syndrome is a Mendelian disorder.
Reason (R) It is caused due to the presence of an additional copy of chromosome 21.
II. Statement Based Questions
Answer : D

Question. Which of the following statement is not true for two genes that show 50% recombination frequency?
(a) The genes may be on different chromosomes
(b) The genes are tightly linked
(c) The genes show independent assortment
(d) If the genes are present on the same chromosome, they undergo more than one cross overs in every meiosis
Answer : B

Question. In Antirrhinum (Snapdragon), a red flower was crossed with a white flower and in F1-generation, pink flowers were obtained.
When pink flowers were selfed, the F2-generation showed white, red and pink flowers. Choose the incorrect statement from the following.
(a) Pink colour in F1 is due to incomplete dominance
(b) Ratio of F2 is 1/4 (Red) : 2/4 (Pink) : 1/4 (White)
(c) Law of segregation does not apply in this experiment
(d) This experiment does not follow the principle of dominance
Answer : C

Question. Select the incorrect statement. 
(a) In male grasshoppers, 50% of sperms have no sex-chromosome
(b) In domesticated fowls, sex of progeny depends on the type of sperm rather than egg
(c) Human males have one of their sex-chromosomes much shorter than the other
(d) Male fruitfly is heterogametic
Answer : B

Question. Which one of the following conditions correctly describes the manner of determining the sex?
(a) Homozygous sex chromosomes (ZZ) determine female sex in birds
(b) XO type of sex chromosomes determine male sex in grasshopper
(c) XO condition in humans as found in Turner’s syndrome determines female sex
(d) Homozygous sex-chromosomes (XX) produce males in Drosophila
Answer : B

Question. The following statements are regarding sexdetermination.
Choose the incorrect one.
(a) There are two types of sex-determining mechanism
(b) In male heterogamety, male has autosomes and sex chromosome XY
(c) In female heterogamety, it has autosomes and one Z and one W chromosomes
(d) Female heterogamety is found in mammals
Answer : D

Question. Which of the following statement is incorrect?
(a) X-body of Henking was given the name X-chromosomes
(b) In many insects, all eggs bear an additional X-chromosomes besides autosomes
(c) X-chromosomes is a sex chromosomes as it is involved in sex-determination
(d) None of the above
Answer : D

Question. Identify the incorrect statement for sex-determination in humans.
(a) Humans contain 23 pairs of autosomes
(b) Females produce only one type of ovum
(c) Genetic makeup of sperm determine the sex of the child
(d) In males, two types of gametes are produced
Answer : A

Question. Which among the following statement is not true for haemophilia?
(a) It is a sex-linked dominant disease
(b) It is transmitted to unaffected carrier female to male progeny
(c) The possibility of a female becoming a haemophilic is extremely rare
(d) The family pedigree of Queen Victoria shows a number of haemophilic descendents
Answer : A

Question. Thalassemia and sickle-cell anaemia are caused due to a problem in globin molecule synthesis. Select the correct statement.
(a) Both are due to a qualitative defect in global chain synthesis
(b) Both are due to a quantitative defect in globin chain synthesis
(c) Thalassemia is due to the synthesis of abnormal haemoglobin molecules
(d) None of the above
Answer : C

Question. Select the incorrect statement.
(a) RBCs become sickle-shaped under low oxygen tension in sickle-cell anaemia
(b) Phenylpyruvic acid gets accumulated in brain and excreted in urine due to poor absorption by kidney
(c) In thalassemia, a and b-globin chains are altogether absent in body
(d) Thalassemia is an autosome linked recesive disease
Answer : C

Question. Which of the following statement is correct with respect to monohybrid and dihybrid cross?
(a) A monohybrid cross is performed for one generation whereas dihybrid cross is performed for two generations
(b) A monohybrid cross involves a single parent, whereas a dihybrid cross involved two parents
(c) A monohybrid cross produces a single progeny whereas a dihybrid cross produce two progenies
(d) A monohybrid cross involves individuals with one heterozygous character, whereas a dihybrid corss involves individuals with two heterozygous characters
Answer : D

Question. Which of the following statement(s) is/are correct with respect to the law of segregation?
(a) Alleles do not show blending
(b) The paired factors or alleles segregate from each other such that a gamete receives the two factors as in the original paired form
(c) Homozygous parent produce similar types of gametes whereas heterozygous ones produce two types of gametes each having one allele with unequal proportion
(d) All of the above
Answer : A

Question. Which of the following statement(s) is/are true withrespect to sickle-cell anaemia?
(a) The mutant haemoglobin of sickle-cell anaemic individual undergo polymerisation under low oxygen tension causing sicking of RBCs
(b) Sickle-cell anaemia occur due to the single base substitution (GAG® GUG) at the sixth codon of b-globin gene
(c) Individuals heterozygous for sicke-cell anaemia (Hb Hb ) S A are resistant towards malaria
(d) All of the above
Answer : D

Question. Which of the following statement is incorrect?
(a) Mutations provide variations on which natural selection acts
(b) The vast majority of mutations produce dominant alleles
(c) Mutations arise spontaneously, infrequently and randomly
(d) Rate of mutation can be increased by artificial means
Answer : B

Question. On what basis is the skin colour in humans considered polygenic? 
Answer : The skin colour of human is controlled by three genes where the dominant alleles have cumulative effect. Each dominant allele expresses a part or unit of the trait (skin colour). Such type of genes are called polygenes and their inheritance is called as polygenic inheritance. So, the skin colour of human is a polygenic trait.

Question. How many kinds of phenotypes would you expect in F2 generation in a monohybrid cross exhibiting co-dominance?
Answer : Three types of phenotypes are obtained in a monohybrid cross exhibiting co-dominance. E.g., coat colour in cattle. In F2 generation three types of phenotypes were obtained – red, roan and white coat colour.

Question. When does a geneticist need to carry a test cross? How is it carried?
Answer : To determine the genotype of a plant i.e., whether the individual is exhibiting dominant character is homozygous or heterozygous, a test cross is carried out by a geneticist. The individual having dominant phenotype is crossed with its homozygous recessive parent. If heterozygous tall is crossed with homozygous recessive parent, tall and dwarf will be produced, in equal proportion while if homozygous tall is crossed with homozygous recessive, the upcoming progenies will contain all tall plant.

Question. State and explain the law of segregation as proposed by Mendel in a monohybrid cross? 
Answer : Principle of segregation states that, “when a pair of contrasting factor or gene are brought together in a hybrid; these factors do not blend or mix up but simply associate themselves and remain together and separate at the time of gamete formation”, i.e, allele pairs segregate during gamete formation and the paired condition is restored by random fusion of gametes during fertilisation. The above law is also known as “law of purity of gametes” because each gamete is pure in itself.

Question. Give an example of a gene responsible for multiple phenotypic expressions. What are such genes called ? State the cause that is responsible for such an effect?
Answer : Multiple alleles is the presence of more than two alleles of a gene. They are produced due to repeated mutation of the same gene but in different directions and show meristic type of germinal variations, e.g., eye colour in Drosophila. Multiple alleles occur on the same gene locus of the same chromosome or its homologue and are responsible for multiple phenotypic expression.
For example, the wild type of allele for red eye colour (w+ or W) in Drosophila melanogaster mutated to form allele for white eye (w). Further mutations in both have produced as much as 15 alleles which are recessive to wild type and dominant over white eye (w) but have incomplete intermediate dominance over one another.

Question. How does the gene ‘I’ control ABO blood groups in humans? Write the effect the gene has on the structure of red blood cells.
Answer : ABO blood groups are controlled by the gene I.
The plasma membrane of the red blood cells has sugar polymers that protrude from its surface. The kind of sugar is controlled by the gene I. The gene I has three alleles IA, IB and i. The alleles IA and IB produce a slightly different form of the sugar while allele i does not produce any sugar. Each person possesses any two of the three I gene alleles. IA and IB are completely dominant over i. When IA and IB are present together they both express their own types of sugar because of codominance. Hence red blood cells have both A and B types of sugars. Since there are three different alleles, there are six different combinations of these three alleles that are possible, and therefore, a total of six different genotypes are there in human ABO blood group.

Question. In snapdragon a cross between true-breeding red flowered (RR) plants and true-breeding white flowered (rr) plants showed a progeny of plants with all pink flowers.
(a) The appearance of pink flowers is not known as blending. Why?
(b) What is this phenomenon known as?
Answer : (a) When a cross is made between a red flowered plant with a white flowered plant of snapdragon, the F1 hybrid has pink flowers. When the F1 individual was self pollinated F2 individuals were obtained bearing red, pink and white flowers in the ratio 1 : 2 : 1. It is not a case of blending inheritance because the parental characters appear in the F2 generation without any change. It is due to law of seggregation which states that the members of the allelic pair that remained together in the parent, segregate during gamete formation and only one factor enters a gamete.
(b) In this neither of the two alleles of a gene is completely dominant over the other, hence the phenomenon is known as incomplete dominance.

Question. With the help of one example, explain the phenomena of co-dominance and multiple allelism in human population?
Answer : In the ABO system, there are four blood groups A, B, AB and O. ABO blood groups are controlled by gene I. The gene I has three alleles IA, IB and i. This phenomenon is known as multiple allelism. IA and IB are completely dominant over i. When IA and IB are present together they both express themselves and produce blood group AB. This phenomenon is known as codominance.

Question. Identify a, b and c in the table given below:

Answer : (a) Both the forms of a trait are equally expressed in F1 generation.
(b) Dominance.
(c) Phenotypic expression of F1 generation is somewhat intermediate between the two parental forms of a trait

Question. “Multiple alleles can be found only when population studies are made.” Explain with the help of an example in humans.
Answer : Human population is characterised by the presence of different blood groups although this trait is controlled by a single gene which indicates that more than two alleles of the gene are involved in the inheritance of this trait i.e., multiple allelism. Since, an individual contains only two alleles of a gene, therefore to study multiple allelism, population survey is required.
The gene involved in the inheritance of human blood group possesses four alleles which are present in different combinations that results in six genotypes and four phenotypes.

Question. How are dominance, co dominance and incomplete dominance patterns of inheritance different from each other? 
Answer : In dominance, F1 is similar to the dominant parent, phenotypic ratio is different from genotypic ratio. In incomplete dominance, F1 is different from either of the two parents. Phenotypic and genotypic ratios are the same. In codominance, the effect of both the alleles is equally conspicuous. Both the alleles produce their effect independently.