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Cell and Heredity

Five Subject Roadmaps

Useful for: CTET, OTET, OSSTET, OSSC RHT — Regular High School Teacher, CSIR NET, CTSA / STSS, Junior Teacher, OAVS

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Overview

Cells carry genetic material that passes traits from parents to offspring; Mendel’s experiments explained inheritance patterns.

Key points

Mendel worked with pea plants.
Monohybrid cross F₂ ratios: phenotypic 3:1, genotypic 1:2:1.
Dihybrid cross F₂ phenotypic ratio: 9:3:3:1.
Dominant traits appear in hybrids; recessive traits are hidden.
In humans the father’s sperm (X or Y) decides the child’s sex.
Genes are segments of DNA on chromosomes.

Explanation

Variation arises from sexual reproduction and mutation.
Acquired traits are not inherited.
Homozygous (TT, tt) vs heterozygous (Tt).
Evolution acts on inherited variation.
Chromosome number is constant for a species.

Step by step

Identify the concept or reaction.
Recall the definition, equation or example.
Check balance, units and direction.

Examples

Tall (TT) × short (tt) → F₁ all tall (Tt).
F₁ × F₁ → 3 tall : 1 short.
A child is male if the father contributes a Y chromosome.

Traps and advanced points

Trap: Saying the mother decides sex.
Trap: Wrong Mendelian ratios.
Trap: Treating acquired traits as inherited.

Quick revision

3:1 and 1:2:1.
9:3:3:1.
XY from father.

Practice MCQs

DNA is found mainly in the —
1. cell membrane
2. nucleus
3. cell wall
4. cytoplasm only
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Answer: (b) nucleus. Chromosomes in the nucleus carry DNA.
Mendel performed his experiments on —
1. pea plants
2. mice
3. maize only
4. fruit flies
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Answer: (a) pea plants. Mendel used garden peas.
The F₂ phenotypic ratio in a monohybrid cross is —
1. 3 : 1
2. 9 : 3 : 3 : 1
3. 1 : 2 : 1
4. 1 : 1
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Answer: (a) 3 : 1. 3 dominant : 1 recessive.
The F₂ genotypic ratio in a monohybrid cross is —
1. 9 : 3 : 3 : 1
2. 2 : 1
3. 3 : 1
4. 1 : 2 : 1
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Answer: (d) 1 : 2 : 1. TT : Tt : tt = 1 : 2 : 1.
The F₂ phenotypic ratio in a dihybrid cross is —
1. 9 : 3 : 3 : 1
2. 1 : 1 : 1 : 1
3. 3 : 1
4. 1 : 2 : 1
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Answer: (a) 9 : 3 : 3 : 1. Mendel’s dihybrid ratio.
In humans, the sex of the child is decided by the —
1. environment
2. mother’s egg
3. father’s sperm
4. both equally
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Answer: (c) father’s sperm. The sperm carries X or Y.
Tall (TT) × short (tt) gives F₁ plants that are —
1. all tall (Tt)
2. half tall
3. medium
4. all short
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Answer: (a) all tall (Tt). Tall is dominant.
Genes are segments of —
1. lipids
2. carbohydrates
3. proteins
4. DNA
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Answer: (d) DNA. Genes are DNA segments on chromosomes.
Acquired traits (such as a muscle built by exercise) are —
1. not inherited
2. always dominant
3. recessive
4. inherited
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Answer: (a) not inherited. They do not change genes.
Tt is described as —
1. haploid
2. homozygous dominant
3. heterozygous
4. homozygous recessive
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Answer: (c) heterozygous. Two different alleles.