Worksheet with Solutions: Principles of Inheritance and Variation | Biology Class 12 - NEET PDF Download

Ans: Genes

Ans: Monohybrid

Ans: Pleiotropy

Ans: Down’s

Ans: Haplodiploid

Ans: 1 - B, 2 - C, 3 - A, 4 - D, 5 - E

Ans: (b) 3:1

Ans: (b) ABO blood groups

Ans: (b) XO type

Ans: (b) Sickle-cell anaemia

Ans: (b) 47, XXY

Ans: (a) Both assertion and reason are true, and the reason is the correct explanation of the assertion.
In a monohybrid cross (a cross between two individuals with contrasting traits for a single character), the F1 generation is heterozygous. For example, crossing a pure tall (TT) and a pure dwarf (tt) pea plant results in all F1 plants being tall (Tt).
This happens because the dominant allele (T) expresses itself even in the presence of the recessive allele (t). The recessive allele remains unexpressed in the heterozygous condition due to being masked by the dominant one.
Therefore, both the assertion and the reason are true, and the reason correctly explains why the dominant trait appears in the F1 generation.

Ans: (d) Assertion is false, but the reason is true.
Linked genes are genes located on the same chromosome, often close to each other. Because of their proximity, they tend to be inherited together and do not follow the principle of independent assortment unless crossing over separates them during meiosis.
The assertion is false because linked genes do not always assort independently—their inheritance is often dependent on their physical proximity.
The reason is true, as it correctly states the definition and behavior of linked genes.

Solution:

(i) In the F1 generation, all plants were tall (Tt), showing the dominant trait. In the F2 generation, self-pollination of F1 plants resulted in a phenotypic ratio of 3 tall (1 TT, 2 Tt) to 1 dwarf (tt), and a genotypic ratio of 1:2:1.

(ii) The Law of Segregation states that alleles of a gene pair segregate during gamete formation, so each gamete carries only one allele. This explains why both tall and dwarf traits reappeared in the F2 generation without blending.

Solution:

(i) In humans, females have XX chromosomes, and males have XY. During spermatogenesis, 50% of sperms carry X and 50% carry Y. Fertilisation of an ovum (X) with an X-sperm results in a female (XX), while a Y-sperm results in a male (XY).

(ii) In honey bees, females (queens/workers) are diploid (32 chromosomes) from fertilised eggs, while males (drones) are haploid (16 chromosomes) from unfertilised eggs via parthenogenesis. Males produce sperm by mitosis, unlike the XY system where both sexes are diploid.

Ans: Mendel chose pea plants due to their large sampling size, true-breeding varieties, contrasting traits (e.g., tall/dwarf), and ability to perform controlled cross-pollination, ensuring reliable inheritance data.

Ans: A test cross involves crossing an organism with a dominant phenotype (unknown genotype) with a homozygous recessive parent to determine its genotype. It reveals whether the organism is homozygous (all dominant offspring) or heterozygous (1:1 ratio).

Ans: Incomplete dominance occurs when the F1 heterozygote shows an intermediate phenotype between the two homozygous parents. Example: In snapdragons, red (RR) and white (rr) flowers produce pink (Rr) flowers in the F1 generation.

Ans: Linkage is the physical association of genes on the same chromosome, causing them to be inherited together more often than expected. It reduces recombination, as seen in Morgan’s Drosophila crosses.

Ans: A point mutation is a change in a single base pair of DNA. Example: Sickle-cell anaemia, caused by a single base substitution (GAG to GUG) in the beta-globin gene, altering haemoglobin structure.

Ans: Down’s syndrome individuals have short stature, a small round head, furrowed tongue, partially open mouth, broad palm with palm crease, and retarded physical, psychomotor, and mental development.

Ans: Pedigree analysis traces the inheritance of traits across generations in a family tree. It helps identify whether a trait is dominant, recessive, or sex-linked and predicts the risk of genetic disorders in offspring.

Ans: Male heterogamety (e.g., XY in humans) involves males producing two types of gametes (X or Y), determining offspring sex. Female heterogamety (e.g., ZW in birds) involves females producing two types of gametes (Z or W), determining sex.

Ans:

• Law of Dominance: In a heterozygous condition, one allele (dominant) expresses its trait, masking the recessive allele. In a monohybrid cross between true-breeding tall (TT) and dwarf (tt) pea plants, the F1 generation (Tt) is all tall, as T dominates t.

• Law of Segregation: Alleles segregate during gamete formation, each gamete carrying one allele. In the F1 (Tt), self-pollination produces F2 with a 3:1 phenotypic ratio (3 tall: 1 dwarf) and 1:2:1 genotypic ratio (TT:Tt:tt), as T and t segregate independently. The recessive trait (dwarf) reappears in F2 without blending.

Ans: The Law of Independent Assortment states that segregation of one pair of alleles is independent of another pair during gamete formation. In a dihybrid cross between pea plants with round, yellow seeds (RRYY) and wrinkled, green seeds (rryy), the F1 (RrYy) produces four gamete types (RY, Ry, rY, ry). Self-pollination yields F2 with a 9:3:3:1 phenotypic ratio (9 round yellow, 3 wrinkled yellow, 3 round green, 1 wrinkled green), showing independent segregation of seed shape and colour genes.

Ans: The chromosomal theory of inheritance, proposed by Sutton and Boveri, states that genes are located on chromosomes, and their segregation during meiosis explains Mendel’s laws. Chromosomes, like genes, occur in pairs, segregate during gamete formation, and assort independently. Thomas Hunt Morgan verified this using Drosophila, observing sex-linked traits (e.g., white eyes on X-chromosome). His dihybrid crosses showed linked genes on the same chromosome deviate from the 9:3:3:1 ratio, supporting the theory.

Ans:

• Humans (XY Type): Females have XX chromosomes, males have XY, with 22 pairs of autosomes in both. Males produce 50% X-sperm and 50% Y-sperm. Fertilisation of an X-ovum with X-sperm yields XX (female), and with Y-sperm yields XY (male), determined by the sperm’s genetic makeup.

• Birds (ZW Type): Males have ZZ chromosomes, females have ZW, with equal autosomes. Females produce 50% Z-ova and 50% W-ova, determining sex. Z-ovum with Z-sperm yields ZZ (male), and W-ovum with Z-sperm yields ZW (female), showing female heterogamety.

Ans:

• Haemophilia: A sex-linked recessive disorder caused by a mutation in an X-chromosome gene affecting blood clotting proteins. Males (XY) with the mutant allele show non-stop bleeding from minor cuts. Carrier females (XX with one mutant allele) may pass it to sons. Symptoms include excessive bleeding.

• Sickle-cell Anaemia: An autosomal recessive disorder due to a point mutation (GAG to GUG) in the beta-globin gene, substituting glutamic acid with valine. Homozygous (HbSHbS) individuals have sickle-shaped RBCs, causing anaemia and oxygen deficiency. Heterozygotes (HbAHbS) are carriers.
  
  Micrograph of the red blood cells and the amino acid composition of the relevant portion of β-chain of haemoglobin: (a) From a normal individual; (b) From an individual with sickle-cell anaemia