Principles of Inheritance and Variation- Competency-Based Questions

Class 12 Biology Competency-Based Questions Chapter 5

1 Mark Questions

1. In mice, brown fur is dominant and white fur is recessive. A biologist crossed a brown fur male with a white fur female and obtained two brown fur and two white fur offspring. Which of the following statements is MOST likely to be true?

A. Both parents are heterozygous.

B. Only the male parent is homozygous.

C. Only the male parent is heterozygous.

D. Only the female parent is heterozygous.

Ans: C. Only the male parent is heterozygous.

2. In which of the following processes would genetic variation occur?

P) mutation in liver cells

Q) development of a zygote into an embryo

R) gene flow from one population to another

A. only P

B. only Q

C. only P and Q

D. only P and R

Ans: D. only P and R

3. Which of the following was a reason that Mendel’s experiments showed reproducible results?

Use of mathematical logic
Use of large sample sizes
Use of true-breeding parental lines
Use of pea plants that had a quick growth time

Ans: C. use of true breeding parental lines

Kernel colour in wheat ranges from dark red to white depending on the number of dominant alleles in the genotype of the plant, as shown below.

Kernel colour	No. of dominant alleles
dark red	4
medium red	3
intermediate red	2
light red	1
white	0

Assuming that a cross is performed between a homozygous dominant male and a homozygous recessive female, answer the questions that follow. Answer the Questions from Q72-Q75.

4. What will be the kernel colour of offspring in the F1 generation?

A. white

B. dark red

C. medium red

D. intermediate red

Ans: D. intermediate red

5. How many genes code for kernel colour in wheat?

A. 1

B. 2

C. 4

D. (Cannot say from the information provided)

Ans: B 2

6. What would be the ratio of kernel colours in the F2 generation?

A. 1:2:1

B. 9:3:3:1

C. 1:4:6:4:1

D. 1:6:15:20:15:6:1

Ans: C. 1:4:6:4:1

7. Which of the following phenomena is/are exhibited by alleles coding for kernel colour in wheat?

P) co-dominance

Q) incomplete dominance

R) polygenic inheritance

A. only P

B. only R

C. only P and R

D. only Q and R

Ans: B. only R

8. In a cross between pea plants having round green seeds (RRGG) and wrinkled yellow seeds(rrgg), what percentage of eggs will carry both the round and green alleles for the F2 generation?

A. 9%

B. 93%

C. 64%

D. D. 25%

Ans: D. 25%

9. Which of the following processes helped scientists corroborate Mendel’s findings long after he had discovered them?

A. recombinant DNA technology

B. DNA transcription

C. DNA replication

D. cell division

Ans: D. cell division

10. Marfan syndrome is a connective tissue disorder in humans caused due to the mutation of the FBN1 gene coding for fibrillin-1. Some symptoms of this disease are abnormally long limbs, heart murmurs, and extreme nearsightedness. Marfan syndrome is an example of.

A. a chromosomal disorder

B. polygenic inheritance

C. codominance

D. pleiotropy

Ans: D. pleiotropy

2 Marks Questions

11. (a) State ONE point of difference between a monohybrid cross and a test cross.

(b) What is/are the possible genotypic ratio/s in a test cross?

Ans: (a)

A monohybrid cross takes place between two parents of any genotype, whereas in a test cross, one parent is necessarily homozygous recessive for a/multiple trait/s.
A monohybrid cross between known parents is done to determine the pattern of inheritance of one single gene, whereas a test cross is done to determine the unknown genotype of one individual/parent of the cross.

(b)

All heterozygous dominant, if the unknown genotype is homozygous dominant
1:1 (heterozygous dominant: homozygous recessive) if the unknown genotype is heterozygous dominant.

12. State two ways in which sex determination in humans is different from that in honeybees.

Ans:

Sex determination in humans is of the XY type, i.e, the male has 2 different sex chromosomes (X and Y), and females have 2 copies of the X chromosome, whereas it is of the haplodiploid type in bees, i.e, females are diploid and males are haploid (only one set of chromosomes)
Males in bees are produced by parthenogenesis and females by fertilization, whereas both male and female offspring in humans are produced by fertilization. Sperm production by mitosis in bees and by meiosis in humans.

13. The pea plant is a self-pollinating plant.

(a) Why was this fact about the pea plant a concern in Mendel’s experiments?

(b) How did Mendel ensure that self-pollination was avoided in his experiments?

Ans:

(a) In Mendel’s experiment, self-pollination would have led to unknown and undesirable results as the experiment would not have been controlled.
(b) by the removal of the stamen or anther from the flower bud

14. In the process of sperm formation, a cell called the spermatogonium undergoes mitosis first to form the primary spermatocyte, which then undergoes meiosis to form the spermatozoa or the sperm cell. What percentages of the primary spermatocyte and the spermatozoa would have the X chromosome? Justify your answer.

Ans: 100% of primary spermatocytes will have the X chromosome, whereas 50% of the sperm cells will have the X chromosome.

Since the spermatogonium undergoes mitosis, the number of chromosomes remains the same in the daughter cell, i.e, all cells get all the chromosomes, whereas when the primary spermatocyte undergoes meiosis, where the number of chromosomes is divided, only half the cells get the X chromosome.

15. A colour-blind father will not have a colour-blind son. State if the above statement is true and justify your answer if the mother is homozygous for the:

(a) colour blind trait

(b) normal vision trait

Ans:

(a) Since colour blindness is an X-linked recessive disorder, a colour blind male will share the normal Y chromosome and the colour blind mother will share a chromosome with the colour blindness trait to the child, making him a carrier but having normal vision.
(b) A colour blind male will transfer the normal Y chromosome, and the mother with normal vision will transfer a chromosome with the normal vision trait to the child, so the child will not be a carrier too.

16. Rett syndrome is a rare X-linked dominant genetic neurological and developmental disorder that affects the way the brain develops. What would be the occurrence of the disease in their daughter and son, if:

(a) A normal male marries a female with Rett syndrome

(b) A normal female marries a male with Rett syndrome

Ans:

(a)
- Son: will have Rett syndrome
- Daughter: will have Rett syndrome
(b)
- Son: will not have Rett syndrome
- Daughter: will have Rett syndrome

17. State TWO points of difference and TWO points of similarity between Klinefelter’s syndrome and Turner’s syndrome.

Ans:

Differences
- Klinefelter’s syndrome is caused due to the presence of an additional copy of the X chromosome, whereas Turner’s syndrome is caused due to the absence of an X chromosome.
- Klinefelter’s syndrome occurs in both genders, whereas Turner’s syndrome is found in females.
Similarities
- Both are chromosomal disorders related to the sex chromosomes
- In both disorders, the individuals are sterile.

18. Using a Punnett square, determine the genotype of the parents if their child can have any of the four blood groups.

Ans:

A child can have any of the four blood groups – IAi, IBi, IAIB, ii

	IB	i
IA	IAIB	IAi
i	IBi	ii

The parent genotypes would have to be IAi, IBi

19. Duchenne muscular dystrophy is an X-linked recessive genetic disease that is also caused by mutations in the DMD gene. Predict the genotypic and phenotypic ratios of the possible offspring of a male with muscular dystrophy and a female who is a carrier using suitable symbols for the alleles in a Punnett grid.

Ans:

	XH	Xh
Xh	XHXh	XhXh
Y	XHY	XhY

Genotypic ratio – 1 XHXh: 1XhXh: 1XHY: 1XhY

Phenotypic ratio –

1 female with muscular dystrophy: 1 carrier female: 1 male with muscular dystrophy: 1 normal male

50% offspring with muscular dystrophy: 50% without muscular dystrophy

50% offspring with muscular dystrophy: 25% normal: 25% carriers

20. Sickle cell anemia (allele a) is a disorder where the RBCs stretch out and become thin as opposed to the round normal (allele A) RBCs. A person who is homozygous for normal RBCs marries a person who is heterozygous for sickle cell RBCs.

What is the probability of the offspring being homozygous for the sickle cell trait?

Blood from one of the offspring was checked, and the microscopic image of its RBCs is shown below.

What is the genotype of this individual?

(c) Based on the image shown, explain the type of dominance exhibited by the sickle cell allele?

Ans:

Probability of offspring being homozygous for sickle cell anemia – 0%
Heterozygous (Aa) for sickle cell anemia.
co-dominance – as both parental alleles are expressed in the offspring

21. Given below is a pedigree chart for an autosomal dominant disorder characterised by trait ‘A’.

What evidence in the pedigree chart helps establish that the condition is caused by a dominant allele?

What are the genotypes of the affected and unaffected individuals?

Ans:

In every generation, there is an affected individual.

An affected offspring has an affected parent.

Genotypes

Affected individual – heterozygous dominant OR Aa

Unaffected individual – homozygous recessive OR aa

22. Given below is the karyotype of an individual.

What are the characteristic reproductive and physical features of such an individual?

What is the category of such disorders called? How is it caused?

Ans:

(a)
- May or may not have an impact on reproductive features
- Short stature with a small, round head
(b)
- Category: chromosomal disorders/aneuploidy
- Cause: failure of segregation of chromatids during cell division

3 Marks Questions

23. Sex determination in cockroaches is the same as that seen in some other insects, like grasshoppers.

(a) Illustrate a cross between a female (XX) cockroach and a male (XO) cockroach.

(b) What will be the chromosome number of the offspring formed if the number of autosomes is 22?

Ans: (a)

(b)

XO – 23 chromosomes
XX – 24 chromosomes

24. ACHOO syndrome is characterized by uncontrollable sneezing in response to the sudden exposure to bright light, typically intense sunlight. It is inherited as an autosomal dominant condition.

(a) Draw a Punnett grid to determine the probability of producing an unaffected child by a heterozygous father and an unaffected mother.

(b) Depict the inheritance using a pedigree.

Ans: (a)

	A	A
a	Aa	Aa
a	Aa	Aa

The possibility of producing an unaffected child is 50%.

(b)

25. (a) What is the mode of inheritance of traits causing haemophilia?

(b) What would be the possibility of a female and male child having haemophilia in the following scenarios?

(i) the mother is unaffected and the father is affected

(ii) the mother is a carrier and the father is unaffected

Ans:

(a) X-linked recessive transmission
(b)
- (i)
  - Male child: All will be unaffected
  - Female child: All will be carriers
- (ii)
  - Male child: Half will be affected and half will be unaffected
  - Female child: Half will be carriers and half will be unaffected

26. (a) State two points of difference between aneuploidy and polyploidy.

(b) Failure in which phase of cell division can cause the conditions mentioned in (a)?

Ans:

(a)
- Aneuploidy is the loss or gain of an extra chromosome, whereas polyploidy is the gain of an entire chromosome set(s).
- Aneuploidy is common in humans, whereas polyploidy is common in plants.
(b)
- Aneuploidy – anaphase
- Polyploidy – anaphase/cytokinesis

27. A roan cattle is an animal that has both red (R) and white (R’) hair on its coat. Consider that two cows – one pure line for white hair and one pure line for red hair are crossed.

What would be the genotype and phenotype of the F1 and F2 generations?

What kind of dominance does the gene for hair colour exhibit? Give a reason to support your answer.

Ans:

(a)
- F1 generation genotype: all RR’
- F1 generation phenotype: all roan cows
- F2 generation genotype: 1RR:2RR’:1R’R’
- F2 generation phenotype: 1 red hair cow, 2 roan hair cows, 1 white hair cow.
(b)
- Codominance
- Both alleles are equally expressed in the offspring

28. State THREE advantages of using Drosophila as the model organism for studying genetics as compared to Mendel’s pea plant.

Ans:

Drosophila can be grown on a simple synthetic medium, making growth faster and more controlled, whereas pea plants were grown naturally, making growth slower and unpredictable.
Drosophila complete their life cycle in about two weeks, whereas pea plants take 10-12 weeks.
In Drosophila, there is a clear differentiation of the sexes, whereas pea plants are bisexual.

29. Given below is the pedigree of a family for a trait.

Identify the mode of inheritance of the trait. Give a reason(s) to support your answer.

Choose the correct answer to complete the following sentence:

Colour blindness is a sex-linked condition. However, a (heterozygous/homozygous) (male/female) will always have normal vision.

Ans:

(a) X-linked recessive disorder -The affected mother has all affected male children, but female children are not affected.
(b) Heterozygous female

30. Cystic fibrosis is a condition in which a defective CF gene (consider allele c) produces faulty transport channels that cause mucus to build up in the ducts of different organs, leading to vitamin deficiency diseases, respiratory infections, and excessive loss of salt through sweat.

Given below is a pedigree chart for a family. The great-grandmother, represented as I- 1, suffers from cystic fibrosis.

Identify the type of expression of the cystic fibrosis gene. Give a reason to support your answer.

From the pedigree, deduce the pattern of inheritance exhibited by the gene. Justify your answer.

Identify the genotypes of II-2 and III-2.

Ans:

(a)
- Pleiotropy
- One gene is affecting multiple organs and therefore contributing to multiple phenotypes.
(b)
- Recessive disorder
- Expression of the trait is skipping generations, so it cannot be dominant.
(c) II-2 – Cc III-2 – Cc

5 Marks Questions

31. Mendel crossed a pea plant having inflated pods with another pea plant having constricted pods. Both parents were pure lines. Next, he crossed the progeny obtained in the F1 generation.

(a) Depict the cross and observations using a Punnett square.

(b) How did his findings lead to the inference of the law of segregation?

Ans:

(a) Draws correct Punnett squares for F1 and F2 offspring with the correct signs and ratios
(b) Since the recessive parental trait is expressed without any blending in the F2 generation, the alleles from the parental pair separate or segregate from each other, and only one allele is transmitted to the offspring.
(c)
- He would have arrived at a 1:1 ratio in the F1 generation
- A trait would not disappear in F1 and reappear in F2.

32. The stem colour of a plant is characterised by a green (G) dominant allele and a yellow (g) recessive allele.

(a) What would be the genotype/s of a plant with a green phenotype of the stem colour?

(b) Using an illustration, explain how we can ascertain the correct genotype of the plant mentioned in (a).

(c) Name and define the process followed in (b).

Ans:

(a)
- GG (homozygous)
- Gg (heterozygous)
(b)
- Punnett square for GG x gg with the correct result and interpretation
- Punnett square for Gg x gg with the correct result and interpretation
(c) Test cross: A genetic cross between a homozygous recessive parent and a parent with an unknown genotype to determine the genotype of the latter.

33. In pea plants, inflated (A) and green (B) pods with round (C) seeds, and the male with constricted (a) and yellow (b) pods with wrinkled (c) seeds. Mendel performed a cross to study the inheritance of these three traits. He performed a cross between two parents that were heterozygous for all the traits.

Identify the:

(a) Genotype of the gametes.

(b) Number of offspring in the F1 generation. Give a reason to support your answer.

(c) Number of offspring who are homozygous for all traits. State their phenotype.

Ans:

(a) Genotype of gametes: ABC, ABc, AcB, aBC, Abc, abC, aBc, abc
(b) Number of offspring: 64
- Reason: Since 8 gametes will be produced by each parent, the cross will result in 8×8 = 64 offspring
(c) 2 offspring will be homozygous for all traits
- Inflated, green pods with round seeds
- Constricted, yellow pods with wrinkled seeds

34. A gardener has been planting white flowers that grow on the terminal ends of the plant in a garden for several months. One day, a purple flower grows along the terminal end of the plant. The gardener wants to know if these flowers would give rise only to purple terminal flowers if pollinated. She knows that purple axial alleles are recessive over white terminal ones.

(a) What are the possible genotypes of the new purple flower that sprouted?

(b) If she wants to use only one generation to determine its genotype, what should she do?

(c) With the help of a Punnett grid/s, help her make inferences to arrive to the conclusion she desires.

Ans:

(a)
- wwTT
- wwTt
(b) She will need to perform a test cross with a purple axial flower that are homozygous for both alleles.
(c)

(i) Possibility 1: wwTt x wwtt

	wT	wt	wT	wt
wt	wwTt	wwtt	wwTt	wwtt
wt	wwTt	wwtt	wwTt	wwtt
wt	wwTt	wwtt	wwTt	wwtt
wt	wwTt	wwtt	wwTt	wwtt

Inference: If the genotype of the unknown flower is wwTt, then only 50% progeny will have purple terminal flowers.

(ii) Possibility 2: wwTT x wwtt

	wT	wT	wT	wT
wt	wwTt	wwTt	wwTt	wwTt
wt	wwTt	wwTt	wwTt	wwTt
wt	wwTt	wwTt	wwTt	wwTt
wt	wwTt	wwTt	wwTt	wwTt

Inference: If the genotype of the unknown flower is wwTT, then all progeny will have purple terminal flowers.

35. Mia’s family consists of her grandparents, brother, sister, and parents. Her parents are the only offspring of her paternal and maternal grandparents. She is the only one in her family who has thalassemia. Of her 4 grandparents, her maternal grandmother had thalassemia.

Describe the method she can use to analyse the inheritance of thalassemia in her family.

Draw an illustration to show the pattern of inheritance.

What is the mode of inheritance? Justify your answer.

Ans:

(a)

Pedigree analysis can be done to analyse the inheritance of thalassemia in her family.
It is the study and representation of the family history with respect to the inheritance of a particular trait.

Autosome-linked recessive trait
Justification: This is an autosomal recessive blood disease transmitted from parents to the offspring when both the partners are unaffected carriers for the gene.