Heredity Notes

 

MG SCHOOL SCIENCE • CLASS 10

Heredity and Evolution

Structured chapter notes on variation, inheritance, Mendel’s experiments, genes, and sex determination.

Class 10 ScienceChapter NotesResponsive Study Page

📚 Chapter contents

Introduction 1. Variation 2. Heredity Mendel’s experiments Monohybrid cross Dihybrid cross 3. Genes and traits 4. Sex determination Quick revision Practice quiz

When living things reproduce, they produce new individuals that resemble their parents but are not exact copies. Asexual reproduction generally produces fewer differences, while sexual reproduction produces greater variation. This chapter explains how differences arise and how traits pass from one generation to the next.

1. Accumulation of Variation During Reproduction

• Basic body plan is inherited: Offspring inherit a basic design or body plan from their parents, so many core features remain similar across generations.
• Small changes each generation: Every generation may acquire small changes. Over many generations, these changes can accumulate.
• Inheritance across generations: Offspring carry inherited traits into the next generation.
• New variations continually arise: New differences appear through processes such as mutation and recombination.

Example: In asexual reproduction such as bacterial binary fission, daughter cells are nearly identical to the parent. Occasional DNA-copying errors can create small differences.

Remember: Sexual reproduction mixes genetic material from two parents. Independent assortment and recombination typically produce greater diversity.

Survival likelihood of variations

• Different variations have different chances of surviving and reproducing in a given environment.
• Survival and reproductive success depend on traits and environmental conditions.
• Heat-resistant bacteria are more likely to survive during a heat wave than heat-sensitive bacteria.
• Environmental selection of favourable traits is a core mechanism of evolution.

2. What is Heredity?

Heredity is the process by which parents pass characteristics to their offspring through reproduction. It explains the transmission of traits such as skin colour, hair type, eye colour and height from one generation to the next.

Inherited and acquired traits

Inherited traitsFeatures passed from parents to offspring through genes; examples include eye colour and blood group.

Acquired traitsChanges in the body or behaviour that develop during an organism’s life, such as a scar or language skills. They are generally not transmitted genetically to offspring.

• Genetic variation: Offspring inherit genes in different combinations, producing variation in traits.
• Illustration: If parents carry different eye-colour genes, a child may inherit different combinations depending on which genes are passed on.
• Earlobe example: Free and attached earlobes are used as an example of inherited variation.

Mendel’s Contributions – Rules for Inheritance of Traits

Gregor Johann Mendel, known as the Father of Genetics, established fundamental rules of inheritance through experiments with pea plants.

Mendel selected plants with contrasting traits such as round or wrinkled seeds and tall or short stems. He initially crossed plants differing in one trait at a time.

(a) Monohybrid Cross – Single Trait Inheritance

A monohybrid cross follows the inheritance of one pair of contrasting traits. Mendel crossed a homozygous tall plant (TT) with a homozygous short plant (tt).

F1 generationTT × tt produces all Tt offspring. All are tall, showing that tallness (T) is dominant.

F2 generationWhen F1 plants (Tt) self-pollinate, both tall and short plants appear, approximately three tall for every one short.

F2 cross: Tt × Tt

×

T

t

T

TT

Tt

t

Tt

tt

Genotype: 1 : 2 : 1Phenotype: 3 : 1

Genotypic ratio: 1 TT : 2 Tt : 1 tt.

Phenotypic ratio: 3 tall : 1 short, because TT and Tt are tall while tt is short.

Generation	Genotype ratio	Phenotype ratio
F1	100% Tt (heterozygous)	100% tall
F2	1 TT : 2 Tt : 1 tt (1:2:1)	3 tall : 1 short (3:1)

Mendel’s conclusion: Each trait is controlled by two factors (now called alleles), one inherited from each parent. The alleles segregate during formation of reproductive cells, so each gamete carries one allele for each trait.

(b) Dihybrid Cross – Two Trait Inheritance

A dihybrid cross follows two characteristics at the same time: seed shape and seed colour. Round (R) and yellow (Y) are dominant over wrinkled (r) and green (y).

• Parental cross: RRYY (round yellow) × rryy (wrinkled green).
• F1 generation: All offspring are RrYy, with round yellow seeds.
• F2 generation: Self-pollination of RrYy plants produces four phenotype combinations in the ratio 9:3:3:1.

F2 phenotype	Ratio
Round yellow	9
Round green	3
Wrinkled yellow	3
Wrinkled green	1

Principle of independent assortment: Alleles of different genes segregate independently during gamete formation. This is the basis of dihybrid inheritance.

3. How Do Genes Produce Traits?

• DNA and genes: DNA is present in every cell and contains instructions for making proteins. A segment of DNA that contains instructions for a specific protein is called a gene.
• Proteins determine traits: Proteins, including enzymes and structural proteins, carry out functions that lead to observable traits. For example, plant height may depend on a hormone whose production requires an enzyme.
• Two copies of each gene: Most body cells have two copies of each gene, one from each parent. Gametes (sperm and egg) have only one copy.
• Alleles: Different forms of a gene are called alleles.
• Homozygous: Both copies of a gene are identical.
• Heterozygous: The two copies of a gene are different.
• Dominant and recessive: In many cases, a dominant allele masks the effect of a recessive allele in a heterozygous condition.
• Contribution from parents: Each parent contributes one allele for every gene, so both contribute equally to the offspring’s genotype.
• Role of gametes: Gametes carry a single set of alleles. Their fusion restores the pair of alleles.
• Environmental effects: The environment can influence gene expression and phenotype. Nutrition affects height in humans, while temperature can affect fur colour in some animals.

Traits are determined by proteins produced from genes. These inheritance principles apply to most sexually reproducing organisms. In asexual reproduction, variation arises mainly from mutations and copying errors.

4. Sex Determination

Sex-determination mechanisms vary among organisms.

• Temperature-dependent sex determination: In some reptiles, such as many turtles and crocodiles, the temperature at which eggs develop determines the sex of hatchlings.
• Environmental sex change: Some animals, such as certain snails and fish, can change sex in response to environmental or social conditions.

Sex determination in humans

• Humans have 23 pairs of chromosomes in each somatic (body) cell.
• 22 pairs are autosomes, which carry many genes for body traits.
• The 23rd pair are sex chromosomes, which determine genetic sex.
• Female (XX): Females have two X chromosomes. Each egg carries one X chromosome.
• Male (XY): Males have one X and one Y chromosome. Sperm carry either an X or a Y chromosome.
• The mother contributes an X chromosome. The father contributes either X or Y.

Chromosome contribution	Combination	Genetic sex
Mother X + Father X	XX	Female (girl)
Mother X + Father Y	XY	Male (boy)

Key point: The mother always contributes an X chromosome. The father’s sperm contributes either X or Y, determining whether the combination is XX or XY.

Quick Revision – Important Terms

Heredity

Passing characteristics from parents to offspring through reproduction.

Variation

Differences among individuals of the same species; variation can arise through mutation and genetic recombination.

Gene and allele

A gene is a segment of DNA carrying instructions for a specific protein. Different forms of a gene are called alleles.

Dominant and recessive

A dominant allele can mask the effect of a recessive allele in a heterozygous condition.

Homozygous and heterozygous

Homozygous means two identical alleles; heterozygous means two different alleles.

Monohybrid and dihybrid ratios

F2 monohybrid ratios: genotype 1:2:1 and phenotype 3:1. F2 dihybrid phenotype ratio: 9:3:3:1.

Human sex chromosomes

XX is the female chromosome combination and XY is the male chromosome combination.

Practice Quiz

Choose one answer for each question, then select Check answers.

1. Who is known as the Father of Genetics? Charles Darwin Gregor Johann Mendel Louis Pasteur

2. What is the F2 phenotypic ratio in a monohybrid cross? 1:2:1 9:3:3:1 3:1

3. A segment of DNA that contains instructions for a specific protein is a: Gene Tissue Chromosome pair

4. Which chromosome combination is XY? Female Male Both

5. What is the F2 phenotypic ratio in a dihybrid cross? 3:1 1:2:1 9:3:3:1

MG SCHOOL SCIENCE • Class 10 Science • Heredity and Evolution