CBSE Class 12 Biology Principles of Inheritance And Variation Worksheet Set A

MULTIPLE CHOICE QUESTIONS

Question. XO-chromosomal abnormality in human beings causes
(a) Turner’s syndrome
(b) Down’s syndrome
(c) Klinefelter’s syndrome
(d) none of the above
Answer. A

Question. A normal woman whose father was colour blind, is married to a normal man. The sons would be 
(a) 75% colour blind
(b) 50% colour blind
(c) all normal
(d) all colour blind
Answer. B

Question. If a homozygous tall plant is crossed with homozygous dwarf plant, the offsprings will be
(a) all tall plants
(b) all dwarf plants
(c) half tall plants
(d) half dwarf plants
Answer. A

Question. Given below is a representation of a kind of chromosomal mutation. What is the kind of mutation represented?

(a) deletion
(b) duplication
(c) inversion
(d) reciprocal translocation
Answer. C

Question. How many different types of gametes can be formed by F1 progeny, resulting from the following cross Tt × Rr? 
(a) 4
(b) 8
(c) 27
(d) 64
Answer. A

Question. Grain colour in wheat is determined by three pairs of polygene. Following the cross AABBCC (dark colour) × aabbcc (light colour), in F2 generation what proportion of the progeny is likely to resemble either parent? 
(a) Half
(b) Less than 5 percent
(c) One third
(d) None of these
Answer. B

Question. Primary source of allelic variation is 
(a) independent assortment
(b) recombination
(c) mutation
(d) polyploidy
Answer. C

Question. Given below is a pedigree chart showing the inheritance of a certain sex-linked trait in humans.

The trait traced in the above pedigree chart is
(a) dominant X-linked
(b) recessive X-linked
(c) dominant Y-linked
(d) recessive Y-linked
Answer. B

Question. Frame shift mutation occurs when
(a) base is added
(b) base is deleted
(c) base is added or deleted
(d) none of the above
Answer. C

Question. The "Cri-du-Chat" syndrome is caused by change in chromosome structure involving
(a) deletion
(b) duplication
(c) inversion
(d) translocation
Answer. A

Question. Mating of an organism to a double recessive in order to determine whether it is homozygous or heterozygous for a character under consideration is called 
(a) reciprocal cross
(b) test cross
(c) dihybrid cross
(d) back cross
Answer. B

Question. Mother and father both have blood group ‘A’.They have two children one with blood group ‘O’ and second one with blood group ‘A’. They have 
(a) mother has homozygotic gene father has heterozygote IA IA.
(b) both are homozygotic (IAIA).
(c) mother is heterozygotic (IAi) and father is homozygotic (IAIA).
(d) both are heterozygotic (IAi).
Answer. D

Question. The experiment shown in the given figure has been carried out by Morgan to show the phenomenon of linkage and recombination. If in cross I, genes are tightly linked and in cross II,genes are loosely linked then what will be the percentage of recombinants produced in cross I and cross II respectively?

(a) 98.7% and 62.8%
(b) 1.3% and 37.2%
(c) 37.2 and 1.3%
(d) 62.8% and 98.7%
Answer. B

Question. When one gene controls two or more different characters simultaneously, the phenomenon is called 
(a) apomixis
(b) pleiotropy
(c) polyploidy
(d) polyteny
Answer. B

Question. Three children in a family have blood types O, AB and B respectively. What are the genotypes of their parents? [2013]
(a) IA i and IBi
(b) IAIB and i i
(c) IBIB and IAIA
(d) IAIA and IBi
Answer. A

Question. If both parents are carriers for thalassemia, which is an autosomal recessive disorder, what are the chances of pregnancy resulting in an affected child?
(a) 50%
(b) 25%
(c) 100%
(d) no chance
Answer. B

ASSERTION REASON QUESTIONS

Directions : Each of these questions contains an Assertion followed by Reason. Read them carefully and answer the question on the basis of following options. You have to select the one that best describes the two statements.
(a) If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
(c) If Assertion is correct but Reason is incorrect.
(d) If both the Assertion and Reason are incorrect.

Question. Assertion : Persons suffering from haemophilia fail to produce blood clotting factor VIII.
Reason : Prothrombin producing platelets in such persons are found in very low concentration.
Answer. C

Question. Assertion : In case of incomplete linkage, linked gene show new combination along with parental combination.
Reason : In case of incomplete linkage, linked genes are separated by crossing over.
Answer. A

Question. Assertion: Aneuploidy may be of hypoploidy or hyperploidy type.
Reason: Monosomy lacks one pair of chromosomes.
Answer. C

Question. Assertion : Cross of F1 individual with recessive homozygous parent is test cross.
Reason : No recessive individual are obtained in the monohybrid test cross. 
Answer. C

Question. Assertion : In Mirabilis, selfing of F1 pink flower plants produces same phenotypic & genotypic ratio.
Reason : Flower colour gene shows incomplete dominance. 
Answer. A

Question. Assertion : In humans, the gamete contributed by the male determines whether the child produced will be male or female.
Reason : Sex in humans is a polygenic trait depending upon a cumulative effect of some genes on X-chromosome and some on Y-chromosome.
Answer. C

Very Short Answer Questions

Question. The egg of an animal contains 10 chromosomes, of which one is X-chromosome. How many autosomes would there be in the karyotype of this animal?
Answer. There will be 9 pairs of autosomes in the karyotype of this animal.

Question. What is point mutation? Give one example.
Answer. Point mutation is a gene mutation that arises due to change in a single base pair of DNA.
Example: Sickle-cell anaemia.

A substitution of a single nitrogen base (GAG → GUG) at the sixth codon of the b-globin chain of haemoglobin molecule causes substitution of Glutamic acid by Valine at 6th position & thus the change in the shape of the RBC from biconcave disc to elongated spindle shaped, structure which results in sickle-cell anaemia.

Question. State a difference between a gene and an allele. 
Answer. Gene contains information that is required to express a particular trait whereas alleles are alternating forms of a gene and are the code for a pair of contrasting traits for e.g., for plant height has two alleles – for tallness and dwarfness.

Question. Observe the pedigree chart and answer the following questions: 

(a) Identify whether the trait is sex-linked or autosomal.
(b) Give an example of a disease in human beings which shows such a pattern of inheritance.
Answer. (a) The trait is sex-linked.
(b) Haemophilia, Colour blindness (Any one)

Question. A haemophilic man marries a normal homozygous woman. What is the probability that their daughter will be haemophilic? 
Answer. 0% because only one X chromosome will carry the haemophilia gene. So, she will be a carrier.

Question. A haemophilic son was born to normal parents. Give the genotypes of the parents and son.
Answer. Father : 44 + XY
Mother : 44 + XXh
Son : 44 + XhY.
(Xh= X chromosome with gene for haemophilia)

Question. State the chromosomal defect in individuals with Turner’s syndrome.
Answer. Monosomy of sex chromosome in females (XO condition).

Question. Name the event, during cell division cycle that results in the gain or loss of chromosome.
Answer. Failure of segregation of chromosomes.

Question. Name one autosomal dominant and one autosomal recessive Mendelian disorder in humans.
Answer. Huntington’s disease is an autosomal dominant disorder and sickle-cell anaemia is an autosomal recessive disorder.

Question. Why is it that the father never passes on the gene for haemophilia to his sons? Explain.
Answer. Haemophilia is a sex-linked recessive disease and the defective gene is present on X chromosome only and not on Y chromosome. Father never passes X chromosome to the son as father only contributes Y chromosome to the son.

Question. Who had proposed the chromosomal theory of inheritance?
Answer. In 1902, Walter Sutton and Theodore Boveri proposed the chromosomal theory of inheritance.

Short Answer Questions

Question. Mention any two autosomal genetic disorders with their symptoms.
Answer. Two autosomal genetic disorders are:
(a) Down’s Syndrome: It was first described by Langdon Down (1866). It is caused due to the presence of an additional copy of the chromosome number 21, i.e., trisomy (2n + 1).
Symptoms:
(i) Short stature with small round head
(ii) Partially open mouth
(iii) Protruding furrowed tongue
(iv) Short neck
(v) Retarded mental development

(b) Phenylketonuria: It is an inborn error of metabolism. The affected individual lack an enzyme called phenylalanine hydroxylase that converts the amino acid phenylalanine into tyrosine.
As a result, phenylalanine gets accumulated and converted into phenylpyruvic acid and other derivatives in brain, causing mental retardation. These are also excreted through urine
due to their poor absorption by kidney.

Question. In order to obtain the F1 generation, Mendel pollinated a true-breeding, say, tall plant with a true-breeding dwarf plant. But for getting the F2 generation, he simply self-pollinated the tall F1 plants. Why?
Answer. All the F1 offsprings of the cross are heterozygous so allowing self-pollination is sufficient to raise F2 offspring. Also he intended to understand the inheritance of the selected trait over generations.

Question. (a) Why is human ABO blood group gene considered a good example of multiple alleles?
(b) Work out a cross up to F1 generation only, between a mother with blood group A (Homozygous) and the father with blood group B (Homozygous). Explain the pattern of inheritance exhibited.
Answer. (a) This is because more than two alleles govern the human ABO blood group gene. (Image 166)

The cross exhibits co-dominance. When the two alleles IA and IB are present together, both the alleles express each other equally forming the blood group AB.

Question. Two independent monohybrid crosses were carried out involving a tall pea plant with a dwarf pea plant. In the first cross, the offspring population had equal number of tall and dwarf plants, whereas in the second cross it was different. Work out the crosses, and explain giving reasons for the difference in the offspring populations.
OR
Work out a cross to find the genotype of a tall pea plant. Name the type of cross.
Answer. This type of cross called a test cross. 

Question. A, B and D are three independently assorting genes with their recessive alleles a, b and d, respectively. A cross was made between individuals of AabbDD genotype and aabbdd. Explain the type of genotypes of the offspring produced.
Answer. 

Question. Why is pedigree analysis done in the study of human genetics? State the conclusions that can be drawn from it.
Ans. Pedigree analysis is done because control crosses are not possible in case of humans beings.
This can be useful for analysis of traits, in several generations of a family, to trace pattern of inheritance to check whether the trait is dominant or recessive or sex-linked or not.

Question. A man with blood group A married a woman with B group. They have a son with AB blood group and a daughter with blood group O. Work out the cross and show the possibility of such inheritance. (Image 166)
Answer.

Thus, the F1 progeny can have all the four possible blood groups, i.e., A, B, AB and O.

Question. The pedigree chart given below, present a particular generation which shows a trait irrespective of sexes (i.e., present in both male and female). Neither of the parents of the particular generation shows the trait. Draw your conclusion on the basis of the pedigree. 

Answer. The trait is autosome linked and recessive in nature. Both the parents are carriers (i.e., heterozygous). Hence, among the offsprings only few show the trait irrespective of sex. The other offsprings are either normal or carrier.

Question. Differentiate between male and female heterogamety. 
Answer. Differences between male heterogamety and female heterogamety 
Male heterogamety
(i) Male produces two types of gametes (while female produces only one type of gamete)
(ii) XY and XO type are two types of male heterogamety
(iii) Example, male grasshopper produce gametes of two types––X and O.
Female heterogamety
(i) Female produces two types of gametes (while male produces only one type of gamete)
(ii) ZW type is a type of heterogamety
(iii) Example, female birds produce gametes of two types––Z and W.

Question. Explain mechanism of sex determination in birds. 
Answer. In birds, females are heterogemetic and males are homogametic. Females have one Z sex chromosome and one W sex chromosome. Males have a pair of Z sex chromosome. If Z sperm fertilises Z ovum, a male offspring is produced, and if Z sperm fertilises W ovum a female offspring is produced.

Question. Both haemophilia and thalassemia are blood related disorders in humans. Write their causes and the difference between the two. Name the category of genetic disorder they both come under. 
Answer. Differences between haemophilia and thalassemia 
Haemophilia
(i) Cause
Single protein is involved in the clotting of blood is affected.
(ii) Genetic disorder
Sex-linked recessive disorder.
(iii) Difference
Blood does not clot due to lack of clotting factors.
Thalassemia
(i) Cause
Defects in the synthesis of globin leading to formation of abnormal haemoglobin.
(ii) Genetic disorder
Autosomal recessive disorder.
(iii) Difference
Results in anaemia (abnormal or lack of haemoglobin).

Question. Name the phenomenon that leads to situations like ‘XO’ abnormality in humans. How do humans with ‘XO’ abnormality suffer? Explain. 
Answer. Absence of one X chromosome due to non segregation of chromatids during cell division leads to XO abnormality. These are sterile female with rudimentary ovaries. They have shield-shaped thorax, webbed neck, poor development of breasts, short stature, small uterus and puffy fingers.

Question. Differentiate between “ZZ” and “XY” type of sex-determination mechanisms.
Answer. ZZ type is seen in birds. The males are homogametic (ZZ) and females are heterogametic (ZY).
Sex is determined by the type of egg getting fertilised.
XY type is seen in human beings The males are heterogametic (XY) and females homogametic (XX). Sex is determined by the type of sperm fertilising the ovum.

Question. Which chromosome carries the mutated gene causing b-thalassemia? What are the problems caused by the mutation?
Answer. Chromosome number 11 carries the mutant gene causing b-thalassemia. It causes formation of abnormal haemoglobin molecules, resulting into anaemia.

Question. Haemophilia is a sex-linked recessive disorder of humans. The pedigree chart given below shows the inheritance of haemophilia in one family. Study the pattern of inheritance and answer the questions given. 

(a) Give all the possible genotypes of the members 4, 5 and 6 in the pedigree chart.
(b) A blood test shows that the individual 14 is a carrier of haemophilia. The member numbered 15 has recently married the member numbered 14. What is the probability that
their first child will be a haemophilic male?
Answer. (a) Genotypes of member 4—XX or XXh
Genotype of membe

r 5—XhY and Genotype of member 6—XY
(b) The probability of first child to be a haemophilic male is 25%.

50% sons will be colour-blind and rest 50% will be normal.
50% daughters will be colour-blind and rest 50% will be carriers.

Question. Give the chromosomal constitution and the resulting sex in each of the following syndromes:
(i) Turner’s syndrome
(ii) Klinefelter’s syndrome
Answer. (i) XO, female
(ii) XXY, male with female characters

Question. How is sex determined in human beings?
Answer. • Humans show XY type of sex determining mechanism.
• Out of 23 pair of chromosomes, 22 are autosomes (same in both males and females).
• Females have a pair of X-chromosomes.
• Males have an X and a Y chromosome.
• During spermatogenesis males produce two types of gametes with e•al probability – sperm carrying either X or Y chromosome.
• During oogenesis, females produce only one types of gamete – having X chromosome.
• An ovum fertilised by the sperm carrying X-chromosome develops into a female (XX) and an ovum fertilised by the sperm carrying Y-chromosome develops into a male (XY).

Long Answer Questions

Question. What is Down’s syndrome? Give its symptoms and cause. Why is it that the chances of having a child with Down’s syndrome increases if the age of the mother exceeds forty years?
Answer. Down’s syndrome is a human genetic disorder caused due to trisomy of chromosome 21. Such individuals are aneuploid and have 47 chromosomes (2n + 1). The symptoms include mental retardation, growth abnormalities, constantly open mouth, dwarfness, etc. The reason for the disorder is the non-disjunction (failure to separate) of homologous chromosome of pair 21 during meiotic division in the ovum.
The chance of having a child with Down’s syndrome increase with the age of the mother (40+) because ova are present in females since their birth and therefore older cells are more prone to chromosomal non-disjunction because of various physicochemical exposures during the mother’s life-time.

Question. Explain how does trisomy of 21st chromosome occur in humans. List any four characteristic features in an individual suffering from it. 
Answer. Cause: Additional copy of chromosome number 21 or trisomy of chromosome 21.
Symptoms:
(i) Short statured with small round head.
(ii) Partially open mouth with protruding furrowed tongue.
(iii) Palm is broad with characteristic palm crease.
(iv) Physical, psychomotor and mental development retarded.

Question. Explain the mechanism of ‘sex determination’ in birds. How does it differ from that of human beings?  
Answer. In birds, female heterogamety is observed. They exhibit ZW type of sex determination. 

Question. List any four symptoms shown by Klinefelter’s syndrome sufferer. Explain the cause of this disease.
Answer. (ii) Klinefelter’s syndrome
Cause: Presence of an additional copy of X chromosome resulting in the karyotype 44+XXY. i.e., 47 chromosomes.
Symptoms:
(i) Sex of the individual is masculine but possess feminine characters.
(ii) Gynaecomastia, i.e., development of breasts.
(iii) Poor beard growth and often sterile.
(iv) Feminine pitched voice.
(v) They are sterile.
(vi) Tall stature.

Question. (a) How does mutation occur?
(b) Differentiate between point mutation and frameshift mutation.
Answer. (a) Mutation occurs due to loss by deletion or gain by insertion/duplication/addition or change in position of DNA segments or chromosomes.
(b) Mutation due to change in a single base pair of DNA is point mutation.
Insertion or deletion of one or two bases change the reading frame from the point of insertion or deletion. This is called as frameshift mutation.

Please click on below link to download CBSE Class 12 Biology Principles of Inheritance And Variation Worksheet Set A