Samacheer Kalvi 10th Science Chapter 17 Long Answer Questions — Heredity with Answers

Answer:

Gregor Johann Mendel (1822–1884), an Austrian monk, conducted breeding experiments on the garden pea (Pisum sativum) from 1856–1863 at a monastery in Brno (now Czech Republic). His work was published in 1866 but was not recognised until 1900. He is called the "Father of Genetics."

Why Mendel Chose Pea Plants:

1. Easy and cheap to grow; can grow several generations in a year 2. Produce both self-pollinating and cross-pollinating flowers 3. Have many clearly contrasting (discontinuous) traits 4. True-breeding varieties were available 5. Short life cycle (3–4 months) 6. Large number of offspring — good for statistical analysis

Seven Pairs of Contrasting Traits Studied by Mendel:

Mendel's Experimental Method:

1. Pure breeding lines: Allowed plants with a particular trait to self-pollinate for several generations → true-breeding plants (homozygous) 2. P generation (Parental): Cross-pollinated two true-breeding plants with contrasting traits (e.g., tall x dwarf) 3. F1 generation (First Filial): All offspring showed only the dominant trait (e.g., all tall) 4. F2 generation (Second Filial): Allowed F1 plants to self-pollinate → dominant and recessive traits appeared in a 3:1 ratio

Mendel's Laws of Inheritance:

Law 1: Law of Dominance When two organisms that are pure-breeding (homozygous) for contrasting traits are crossed, the trait that appears in the F1 hybrids is called dominant and the trait that remains hidden is called recessive. In a heterozygote, the dominant allele masks the expression of the recessive allele.

• Tall (TT) x Dwarf (tt) → F1: All Tall (Tt) — tallness is dominant, dwarfism is recessive

Law 2: Law of Segregation (Law of Purity of Gametes) The two alleles for each trait in an individual are separate (alleles do not blend) and are separated (segregated) during the formation of gametes. Each gamete receives only one allele for each trait. When gametes fuse, the offspring receives one allele from each parent.

• A Tt plant produces two types of gametes: T and t (each gamete has only one allele)
• F1 x F1 (Tt x Tt) → F2: TT, Tt, Tt, tt → 3 Tall : 1 Dwarf

Law 3: Law of Independent Assortment When two pairs of traits (located on different chromosomes) are considered simultaneously, the alleles of each pair assort independently of alleles of the other pair during gamete formation. This gives rise to new combinations of traits.

• This law applies to genes on different chromosomes (non-homologous chromosomes) or on the same chromosome but far apart
• Results in the 9:3:3:1 ratio in dihybrid crosses
• Modern explanation: non-homologous chromosomes align randomly at the metaphase I plate during meiosis I (random orientation)

Answer:

A monohybrid cross is a cross between two individuals that differ in only one pair of contrasting traits.

Example: Tall (TT) x Dwarf (tt) in Pea Plants

Parental Generation (P):

• Tall parent: genotype TT (homozygous dominant)
• Dwarf parent: genotype tt (homozygous recessive)

Gametes from P generation:

• Tall parent produces only T gametes
• Dwarf parent produces only t gametes

F1 Generation:

• All offspring: Tt (heterozygous tall)
• Phenotype: All Tall
• Genotype: All Tt

Self-pollination of F1 (Tt x Tt):

Gametes from F1 plants:

• Each Tt plant produces T gametes and t gametes in equal proportions

Punnett Square for F1 x F1:

F2 Genotypic Ratio:

• TT : Tt : tt = 1 : 2 : 1

F2 Phenotypic Ratio:

• Tall (TT + Tt) : Dwarf (tt) = 3 : 1

Analysis:

• TT — Homozygous Tall (true-breeding tall)
• Tt — Heterozygous Tall (non-true-breeding, will segregate in next generation)
• tt — Homozygous Dwarf (true-breeding dwarf)
• 3 Tall (1 TT + 2 Tt) : 1 Dwarf (tt)

Test Cross:

To determine if a tall plant is TT or Tt, Mendel performed a test cross — crossing the tall plant with a homozygous recessive dwarf (tt).

• If Tall plant is TT (TT x tt): All offspring are Tt — All Tall
• If Tall plant is Tt (Tt x tt): Offspring are Tt and tt — 1 Tall : 1 Dwarf

The test cross reveals the genotype of the unknown dominant individual.

Significance:

• Demonstrated that traits are controlled by discrete units (genes)
• Showed that dominant and recessive traits are heritable
• Proved that traits do not blend — they segregate cleanly in a fixed ratio
• Laid the foundation for all of modern genetics

Answer:

A dihybrid cross is a cross between two individuals that differ in two pairs of contrasting traits.

Example: Seed Shape (Round/Wrinkled) and Seed Colour (Yellow/Green) in Pea Plants

Dominant traits: Round (R) and Yellow (Y) Recessive traits: Wrinkled (r) and Green (y)

Parental Generation (P):

• Round, Yellow (RRYY) x Wrinkled, Green (rryy)
• Gametes: RY (from RRYY) and ry (from rryy)

F1 Generation:

• All offspring: RrYy (heterozygous — Round Yellow)
• Phenotype: All Round Yellow (both dominant traits expressed)

Self-pollination of F1 (RrYy x RrYy):

Each RrYy parent produces 4 types of gametes due to independent assortment:

• RY, Ry, rY, ry (each in equal frequency — 25%)

Punnett Square (4 x 4):

Genotypic Ratio (9 distinct genotypes):

• 1 RRYY : 2 RRYy : 2 RrYY : 4 RrYy : 1 RRyy : 2 Rryy : 1 rrYY : 2 rrYy : 1 rryy

Key Observations from Dihybrid Cross:

1. The 9:3:3:1 ratio is a combination of two independent 3:1 monohybrid ratios: (3:1) x (3:1) = 9:3:3:1 2. New combinations (Round Green, Wrinkled Yellow) appear that were absent in the parents — result of independent assortment 3. The ratio confirms the Law of Independent Assortment — the alleles for seed shape and seed colour sorted independently

Deviations from 9:3:3:1:

• When genes are on the same chromosome (linked genes), they do not assort independently → different ratios
• Linkage (discovered by Morgan): linked genes tend to be inherited together

Answer:

In Mendel's experiments, one allele was completely dominant over the other. However, not all traits follow this pattern. Incomplete dominance and codominance are exceptions to complete dominance.

Incomplete Dominance:

In incomplete dominance, neither allele is completely dominant over the other. The heterozygote has an intermediate phenotype (a "blend") between the two homozygotes. This was first described by Carl Correns in Snapdragon (Antirrhinum majus).

Example: Flower Colour in Snapdragon (Antirrhinum)

• RR = Red flowers
• rr = White flowers
• Rr = Pink flowers (intermediate — neither red nor white)

The R allele codes for red pigment but in a single dose (Rr) it produces only half the normal amount of pigment → pink colour

Cross: Red x White

P: RR (Red) x rr (White) Gametes: R x r F1: All Rr → All Pink

F1 x F1 (Rr x Rr):

F2 Genotypic Ratio: 1 RR : 2 Rr : 1 rr F2 Phenotypic Ratio: 1 Red : 2 Pink : 1 White (1:2:1)

Note: In incomplete dominance, the phenotypic ratio and genotypic ratio are both 1:2:1 (unlike complete dominance where phenotypic ratio is 3:1 but genotypic is 1:2:1).

Key point: The pink colour is NOT a new allele. The original red and white alleles are preserved — demonstrated by the reappearance of red and white in F2. Mendel's Law of Segregation is still valid.

Other examples of incomplete dominance:

• Four o'clock plant (Mirabilis jalapa): red x white → pink in F1
• Human hair texture: straight (HH) x curly (hh) → wavy (Hh)

Codominance:

In codominance, both alleles are simultaneously and fully expressed in the heterozygote — neither allele is dominant or recessive. Both phenotypes appear side by side.

Example 1: ABO Blood Group System (Human)

The I^A and I^B alleles are codominant with each other (though both are dominant over i).

• I^A I^A → Blood Group A (A antigens on RBCs)
• I^B I^B → Blood Group B (B antigens on RBCs)
• I^A I^B → Blood Group AB (BOTH A and B antigens on RBCs)
• ii → Blood Group O (no antigens)

In I^A I^B individuals, both A and B antigens are produced in equal amounts — neither allele suppresses the other. This is pure codominance.

Example 2: Roan Cattle

• R^R R^R = Red coat
• R^W R^W = White coat
• R^R R^W = Roan coat — a mixture of red and white hairs (not pink — individual hairs are either red or white)

Difference between Incomplete Dominance and Codominance:

Answer:

Sex determination is the process by which the biological sex (male or female) of an individual is established. It is controlled by specific chromosomes called sex chromosomes (allosomes) or by environmental factors in some organisms.

Chromosomal Basis of Sex Determination:

Humans have 46 chromosomes (23 pairs):

• 22 pairs of autosomes (non-sex chromosomes) — same in both sexes
• 1 pair of sex chromosomes — different in males and females

Human sex chromosomes:

• Females: XX — two X chromosomes
• Males: XY — one X chromosome and one Y chromosome

The X chromosome is large and carries many genes. The Y chromosome is small and carries few genes, mainly those involved in male sex determination (SRY gene — Sex-determining Region on Y).

Mechanism of Sex Determination in Humans:

In females (XX):

• During meiosis, all eggs (ova) receive one X chromosome
• All eggs are X-bearing (females produce only one type of egg)

In males (XY):

• During meiosis, half the sperm receive the X chromosome (X-bearing sperm) and half receive the Y chromosome (Y-bearing sperm)
• Males produce two types of sperm in equal proportions

At Fertilisation:

• If an X-bearing sperm fertilises the egg: XX → Female
• If a Y-bearing sperm fertilises the egg: XY → Male

Therefore, the father determines the sex of the child — the mother always contributes an X chromosome. Whether the child is male or female depends on which type of sperm (X or Y) fertilises the egg.

Probability: Equal chance (50%) of producing a male or female offspring.

Cross Representation:

Sex Determination in Other Organisms:

Factors That Do NOT Determine Sex in Humans:

• Diet, behaviour, or lifestyle of the mother do NOT determine sex of child
• The sex is determined solely by the sperm that fertilises the egg

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