Heredity – Concept Booster | Class 10 Science CBSE

Q: A human female has XX chromosomes, so when her body makes eggs (via meiosis), every single egg gets exactly one X chromosome. A human male has XY chromosomes. When his body makes sperm, they split: 50% of the sperm carry an X chromosome, and 50% carry a Y chromosome. If an X-sperm wins the race and fertilizes the X-egg, the result is XX (Female). If a Y-sperm wins, the result is XY (Male). Since the sperm population is a perfect 50/50 split of X and Y, the statistical probability of having a boy or a girl is exactly 50%. This also proves that the biological father’s genetic contribution solely determines the sex of the child.

1. Since all progeny bore violet flowers, the violet trait is dominant and the parent must be pure (homozygous) for violet: WW. 2. Since half the progeny were short, the tall parent must have carried the hidden short gene to pass on to the short offspring. Thus, the parent was hybrid (heterozygous) for height: Tt. Combining these, the genotype of the tall, violet parent is TtWW.

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Home Concept Boosters CBSE CBSE Class 10 Science Heredity

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How to Use This Page

Read each concept carefully, then check the common mistake and exam tip before moving to the next. This page completely covers Heredity for CBSE Class 10 Science, unlocking the biological code that makes you, you. Note: The “Evolution” portion has been rationalized from the current syllabus.

Key Concepts

Class 10 · Science · Biology

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Genetics & Heredity

The transmission of traits from one generation to the next

Class 10 · Ch 8

Heredity and Variation Definition

Heredity: The passing of traits (characteristics) from parents to offspring.
Variation: The differences in the characters among the individuals of a species. Asexual reproduction brings minimal variation; sexual reproduction brings maximum variation.

Genes and Alleles Concept

A Gene is a specific segment of DNA on a chromosome that codes for a particular protein/trait. An Allele is an alternative form of a gene (e.g., the gene for height has two alleles: ‘T’ for tall, ‘t’ for dwarf).

Dominant vs. Recessive Traits Rule

Dominant: A trait that expresses itself even if only one copy of its allele is present (e.g., Tt = Tall). Represented by a capital letter.
Recessive: A trait that remains hidden unless two copies of its allele are present (e.g., tt = Dwarf). Represented by a lowercase letter.

Mendel’s Experiment Material Fact

Gregor Mendel, the “Father of Genetics,” chose the Garden Pea plant (Pisum sativum) because it grows quickly, has a short life span, can self-pollinate or cross-pollinate, and has easily observable contrasting traits (e.g., Tall/Short, Yellow/Green seeds).

Monohybrid Cross Ratios

A cross between two parents taking only one contrasting character into account (e.g., Plant Height). In the F2 generation of a heterozygous cross (Tt × Tt):

$$\text{Phenotypic Ratio (Visible)} = 3 : 1 \quad (\text{Tall : Dwarf})$$

$$\text{Genotypic Ratio (Genetic makeup)} = 1 : 2 : 1 \quad (\text{TT : Tt : tt})$$

Dihybrid Cross Ratio

A cross considering two pairs of contrasting characters simultaneously (e.g., Seed Shape and Seed Color). In the F2 generation of a heterozygous cross (RrYy × RrYy):

$$\text{Phenotypic Ratio} = 9 : 3 : 3 : 1$$

(Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green)

Law of Segregation Law

During gamete formation, the two alleles for a trait separate (segregate) so that each gamete receives only one allele. This ensures that gametes are always “pure” for a trait.

Law of Independent Assortment Law

When two pairs of traits are combined in a hybrid, the segregation of one pair of traits is completely independent of the other pair of traits (e.g., Seed color doesn’t affect seed shape).

How do Traits get Expressed? Pathway

Cellular DNA is the information source for making proteins. A gene provides instructions for a specific protein (usually an enzyme). If the enzyme works efficiently, a lot of product (like growth hormone) is made, expressing the dominant trait (Tall). If the gene is mutated, the enzyme is less efficient, resulting in the recessive trait (Dwarf).

Sex Determination in Humans Concept

Humans have 23 pairs of chromosomes. The 23rd pair are the Sex Chromosomes.
Females: Have a perfect pair (XX). All eggs carry an X.
Males: Have a mismatched pair (XY). Half the sperm carry X, half carry Y.
The sex of the baby is entirely determined by which sperm (X or Y) fertilizes the egg.

Concept Deep Dive

The 50/50 Chance of Human Sex Determination

Why is the ratio of boys to girls 1:1?

Core Biological Process

A human female has XX chromosomes, so when her body makes eggs (via meiosis), every single egg gets exactly one X chromosome.

A human male has XY chromosomes. When his body makes sperm, they split: 50% of the sperm carry an X chromosome, and 50% carry a Y chromosome.

If an X-sperm wins the race and fertilizes the X-egg, the result is XX (Female). If a Y-sperm wins, the result is XY (Male). Since the sperm population is a perfect 50/50 split of X and Y, the statistical probability of having a boy or a girl is exactly 50%. This also proves that the biological father’s genetic contribution solely determines the sex of the child.

Homozygous vs. Heterozygous

Purebreds vs. Hybrids

Crucial Understanding

Because you inherit one chromosome from your mother and one from your father, you have two alleles for every gene.

– If the two alleles are identical (e.g., TT or tt), the organism is Homozygous (purebred) for that trait.
– If the two alleles are different (e.g., Tt), the organism is Heterozygous (hybrid).

In a heterozygous individual, a battle takes place. The Dominant allele (‘T’) completely overpowers the Recessive allele (‘t’), which is why ‘Tt’ plants are just as tall as ‘TT’ plants.

Compare & Contrast

✗ Phenotype

The observable, physical appearance of an organism.
What you actually see (e.g., Tall, Dwarf, Yellow seeds).
Can sometimes be altered by environmental factors.
Example: Two plants can have the same phenotype (Tall) but different genotypes.

✓ Genotype

The genetic makeup or specific allele combination of an organism.
The letters we write on paper (e.g., TT, Tt, tt, RrYy).
Strictly inherited and unaffected by the environment.
Example: A plant with genotype ‘Tt’ has a hidden dwarf allele.

Common Mistakes to Avoid

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Mistake 1

Assuming “Dominant” means “Common”: Just because a trait is dominant does not mean it is the most common trait in a population. For example, having 6 fingers (Polydactyly) is a dominant trait, but it is extremely rare in the human population!

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Mistake 2

Mixing up the Ratios:
– 3:1 is the visible (Phenotypic) ratio of a monohybrid cross.
– 1:2:1 is the genetic (Genotypic) ratio of a monohybrid cross.
– 9:3:3:1 is the phenotypic ratio of a dihybrid cross.
Do not swap them around in MCQs!

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Mistake 3

Writing Gametes Incorrectly: When drawing a Punnett square for a Monohybrid cross, the gametes from a Tt parent are T and t. For a Dihybrid cross of RrYy, the gametes are combinations of ONE shape allele and ONE color allele: RY, Ry, rY, ry. Students often incorrectly write RR or yy as a single gamete!

Exam Tips

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Tip 1

Always define F1 and F2: When answering cross questions, clearly label the Parents (P), their Gametes, the F1 Generation (first offspring), and the F2 Generation (offspring of F1 self-pollination). Use a Punnett square to show your working; it prevents silly mistakes and examiners love it.

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Tip 2

The “Test Cross” Concept: How do you know if a tall pea plant is TT or Tt? You cross it with a dwarf plant (tt). If all offspring are tall, the unknown was TT. If 50% are tall and 50% are dwarf, the unknown was Tt. This is a very common analytical reasoning question.

Expected Exam Questions

Board Pattern Questions

Class 10 · Science · CBSE Exam

Class 10 · Biology

A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. What was the genetic makeup (genotype) of the tall parent? [2 marks]

Answer TtWW (Heterozygous Tall, Homozygous Violet) 📝

Explanation

1. Since all progeny bore violet flowers, the violet trait is dominant and the parent must be pure (homozygous) for violet: WW.
2. Since half the progeny were short, the tall parent must have carried the hidden short gene to pass on to the short offspring. Thus, the parent was hybrid (heterozygous) for height: Tt.
Combining these, the genotype of the tall, violet parent is TtWW.

“It is a matter of chance whether a couple will give birth to a male child or a female child.” Justify this statement with the help of a flow chart showing sex determination in human beings. [3 marks]

Answer 50% chance of X sperm and 50% chance of Y sperm fertilizing the X egg. 📝

Explanation

Females produce eggs containing only X chromosomes. Males produce sperm of two types: 50% carry X chromosomes and 50% carry Y chromosomes.
– If an X-bearing sperm fertilizes the egg (X), the zygote is XX (Female).
– If a Y-bearing sperm fertilizes the egg (X), the zygote is XY (Male).
Because the production of X and Y sperm occurs in an exact 1:1 ratio, and millions of both types are released, it is purely a statistical matter of chance which sperm reaches and fertilizes the egg first. (A flow chart showing XX and XY crossing to produce XX, XX, XY, XY should be drawn).

How do genes control the characteristics or traits in an organism? Explain with an example. [3 marks]

Answer Genes code for enzymes (proteins) that synthesize hormones, which cause the trait. 📝

Explanation

A gene is a section of DNA that provides the “recipe” for making a specific protein in the cell. Many of these proteins are enzymes that catalyze biochemical reactions.
Example (Plant Height): Plant height depends on the amount of growth hormone synthesized. The amount of hormone depends on the efficiency of a specific enzyme. If the gene is normal (Dominant ‘T’), the enzyme is highly efficient, a lot of hormone is produced, and the plant grows Tall. If the gene has a mutation (Recessive ‘t’), the enzyme is less efficient or non-functional, less hormone is produced, and the plant remains Dwarf.

Concept Map

Heredity connects to →

Life Continuity

Reproduction (DNA copying & Variation)

Evolution (Variation over millions of years)

Biochemistry (DNA and Protein synthesis)

Heredity – Concept Booster | Class 10 Science CBSE

Key Concepts

Genetics & Heredity

Concept Deep Dive