Grade 12 : Biology | Principles of Inheritance and Variation

Question 1

A researcher crosses a pure-breeding pea plant bearing violet flowers with a pure-breeding pea plant bearing white flowers. The resulting $F_1$ progeny are self-pollinated to produce an $F_2$ generation consisting of 600 plants.

How many plants in the $F_2$ generation are expected to be heterozygous violet?

Accepted Answer

C) 300

Answer

A) 150

Answer

B) 450

Answer

D) 200

Question 2

In Antirrhinum majus (snapdragon), flower color exhibits incomplete dominance. A cross between a true-breeding red-flowered plant (RR) and a true-breeding white-flowered plant (rr) yields pink flowers (Rr) in the $F_1$ generation.

If the pink-flowered $F_1$ plants are self-pollinated, what is the phenotypic ratio observed in the $F_2$ generation?

Accepted Answer

B) 1 Red : 2 Pink : 1 White

Answer

A) 3 Red : 1 White

Answer

C) 9 Red : 3 Pink : 3 White : 1 Yellow

Answer

D) All Pink

Question 3

Assertion (A): A cross between a homozygous recessive parent and an individual with a dominant phenotype is called a test cross.
Reason (R): A test cross is utilized to determine the unknown genotype of an organism exhibiting a dominant phenotype.

Select the correct option:

Accepted Answer

A) Both (A) and (R) are true and (R) is the correct explanation of (A).

Answer

B) Both (A) and (R) are true but (R) is not the correct explanation of (A).

Answer

C) (A) is true, but (R) is false.

Answer

D) (A) is false, but (R) is true.

Question 4

In human ABO blood groups, alleles $I^A$ and $I^B$ show codominance, while allele $i$ is recessive to both. A man with blood group A and a woman with blood group B have a child with blood group O.

What is the probability that their next child will possess blood group AB?

Accepted Answer

C) 0.25

Answer

A) 1.00

Answer

B) 0.50

Answer

D) 0.75

Question 5

In a dihybrid test cross involving two completely independently assorting genes ($AaBb imes aabb$), what is the expected phenotypic ratio among the offspring?

Accepted Answer

B) $1 : 1 : 1 : 1$

Answer

A) $9 : 3 : 3 : 1$

Answer

C) $1 : 2 : 1$

Answer

D) $3 : 1$

Question 6

Thomas Hunt Morgan chose the fruit fly, Drosophila melanogaster, as an ideal experimental organism for chromosomal studies and linkage analysis.

Which of the following is NOT a reason for Morgan's choice of Drosophila melanogaster?

Accepted Answer

D) They lack clear sexual dimorphism under low power magnification.

Answer

A) They can be grown on simple synthetic medium in the laboratory.

Answer

B) A single mating could produce a large number of progeny flies.

Answer

C) They complete their life cycle in about two weeks.

Question 7

Morgan conducted a dihybrid cross in Drosophila between yellow-bodied, white-eyed females and wild type brown-bodied, red-eyed males ($y^+ w^+$). The $F_1$ females were then test-crossed.

What was the percentage of non-parental recombinant offspring obtained for the body color and eye color genes?

Accepted Answer

A) 1.3%

Answer

B) 37.2%

Answer

C) 50.0%

Answer

D) 98.7%

Question 8

Assertion (A): The physical distance between two genes on a chromosome is directly proportional to their recombination frequency.
Reason (R): Alfred Sturtevant utilized the frequency of recombination between gene pairs as a measure of the distance between genes to construct genetic maps.

Select the correct option:

Accepted Answer

D) Both (A) and (R) are true and (R) is the correct explanation of (A).

Answer

A) (A) is false, but (R) is true.

Answer

B) (A) is true, but (R) is false.

Answer

C) Both (A) and (R) are true but (R) is not the correct explanation of (A).

Question 9

In insects such as grasshoppers, sex determination differs fundamentally from that of mammals.

Which type of sex determination mechanism is observed in grasshoppers?

Accepted Answer

B) XO male / XX female

Answer

A) ZW female / ZZ male

Answer

C) XY male / XX female

Answer

D) Haplo-diploidy

Question 10

Which of the following organisms exhibits female heterogamety where the female produces two distinct types of gametes ($Z$ and $W$)?

Accepted Answer

A) Birds

Answer

B) Humans

Answer

C) Grasshoppers

Answer

D) Drosophila

Question 11

A normal man whose father was color-blind marries a woman whose father was also color-blind. The woman herself has normal color vision, and both of their mothers had homozygous normal vision.

What proportion of their male children is expected to be color-blind?

Accepted Answer

C) 50%

Answer

A) 100%

Answer

B) 0%

Answer

D) 25%

Question 12

In human sickle-cell anemia, a single nucleotide substitution in the $\beta$-globin gene leads to amino acid replacement in the polypeptide chain.

Which specific amino acid substitution occurs at the sixth position of the $\beta$-globin chain?

Accepted Answer

A) Glutamic acid is replaced by Valine

Answer

B) Valine is replaced by Glutamic acid

Answer

C) Leucine is replaced by Valine

Answer

D) Glycine is replaced by Serine

Question 13

Assertion (A): Phenylketonuria is an inborn error of metabolism inherited as an autosomal recessive trait.
Reason (R): The affected individual lacks an enzyme that converts the amino acid tyrosine into phenylalanine.

Choose the correct alternative:

Accepted Answer

D) (A) is true, but (R) is false.

Answer

A) Both (A) and (R) are true and (R) is the correct explanation of (A).

Answer

B) Both (A) and (R) are true but (R) is not the correct explanation of (A).

Answer

C) (A) is false, but (R) is true.

Question 14

A clinical cytogenetics lab analyzes the karyotype of a patient exhibiting short stature, sterile ovaries, webbed neck, and absence of secondary sexual characteristics.

What is the karyotypic constitution characteristic of this condition (Turner's syndrome)?

Accepted Answer

B) $45, XO$

Answer

A) $47, XXY$

Answer

C) $47, XXX$

Answer

D) $47, +21$

Question 15

Down's syndrome is caused by chromosomal non-disjunction resulting in which of the following cytogenetic anomalies?

Accepted Answer

C) Trisomy of chromosome 21

Answer

A) Monosomy of chromosome 21

Answer

B) Nullisomy of chromosome 18

Answer

D) Trisomy of sex chromosomes

Question 16

In human pedigree charts, standardized symbols are employed to trace the inheritance of specific traits across multiple generations.

Which symbol correctly represents mating between relatives (consanguineous mating)?

Accepted Answer

A) Double horizontal line between a square and a circle

Answer

B) A shaded square connected to an open circle

Answer

C) A diamond symbol containing a number

Answer

D) A single horizontal line with a diagonal slash

Question 17

Thalassemia and sickle-cell anemia are both hereditary disorders affecting hemoglobin synthesis, but their pathogenic mechanisms differ fundamentally.

Which statement accurately distinguishes thalassemia from sickle-cell anemia?

Accepted Answer

D) Thalassemia is a quantitative problem of synthesizing too few globin molecules, while sickle-cell anemia is a qualitative problem of synthesizing aberrant globin chains.

Answer

A) Thalassemia is sex-linked recessive, whereas sickle-cell anemia is autosomal dominant.

Answer

B) Thalassemia is a qualitative defect, while sickle-cell anemia is a quantitative defect.

Answer

C) Thalassemia is due to an extra chromosome, while sickle-cell is a point mutation.

Question 18

Assertion (A): Queen and worker honeybees develop from fertilized diploid eggs, whereas drones develop parthenogenetically from unfertilized haploid eggs.
Reason (R): In honeybees, males produce sperms by mitosis rather than meiosis.

Choose the correct option:

Accepted Answer

B) Both (A) and (R) are true but (R) is not the correct explanation of (A).

Answer

A) Both (A) and (R) are true and (R) is the correct explanation of (A).

Answer

C) (A) is true, but (R) is false.

Answer

D) (A) is false, but (R) is true.

Question 19

Consider a cross between two individuals with genotypes $AaBbCc imes AaBbCc$. Assume that all three gene loci assort independently and display complete dominance.

What fraction of the offspring is expected to exhibit the completely homozygous recessive genotype aabbcc?

Accepted Answer

C) $\frac{1}{64}$

Answer

A) $\frac{1}{16}$

Answer

B) $\frac{3}{64}$

Answer

D) $\frac{1}{8}$

Question 20

Who independently rediscovered Mendel’s principles of inheritance in the year 1900?

Accepted Answer

A) Hugo de Vries, Carl Correns, and Erich von Tschermak

Answer

B) Walter Sutton and Theodor Boveri

Answer

C) Thomas Hunt Morgan and Alfred Sturtevant

Answer

D) George Beadle and Edward Tatum

Set 3Grade 12 : Biology | Principles of Inheritance and Variation