HEREDITY AND VARIATIONS (NCERT), Study notes of Biology

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Example:- skin color, eye color, nose shape, etc.,. Example:- Piercing of nose or ear, acquiring weight or knowledge, etc.,.

2. IMPORTANT TERMS :-

★ DNA (Deoxyribonucleic acid) : thread like structure carries

genetic information.

★ CHROMATIN : Scattered form of DNA dispersed throughout

the nucleus.

★ GENES : Segments of DNA that code for a particular protein

or trait.

Each gene has two forms called alleles (e.g., T and t for

height).

★ ALLELES : Variant forms of genes that occupy a specific

location on a particular chromosome and control the same

character.

DOMINANT ALLELES RECESSIVE ALLELES

Stronger one from the two alleles. Weaker one from the two alleles. Denoted by capital letters. Denoted by small letters. Express itself in both homozygous or heterozygous condition. Express itself only in homozygous condition. Trait which appears due to expression of dominant alleles, called DOMINANT TRAIT. Trait which appears due to expression of recessive alleles, called RECESSIVE TRAIT.

3. CHROMOSOME :-

★ CHROMOSOME : Highly coiled and condensed form of DNA

and proteins, found in the Nucleus visible during Cell Division.

Humans have 46 chromosomes (23 pairs).

22 pairs are autosomes.

1 pair is sex chromosomes: XX (female), XY (male).

● Diploid (2n): Cells with two sets of chromosomes (e.g.,

body cells).

● Haploid (n): Cells with one set of chromosomes (e.g.,

sperm or egg cells).

★ GENOTYPE : Combination of genes in an organism.

★ PHENOTYPE : Appearance of an organism.

★ NOTE:- NEWLY FORMED PROGENY SHOWS DIPLOID.

4. Genetics & Mendel's Experiments :-

★ GENETICS :- Branch of biology that deals with heredity and

variation.

★ WILLIAM BATSON coined this term.

★ GREGOR MENDEL is known as the "Father of Genetics".

● He experimented on pea plants (Pisum sativum) and proposed 3 laws

of inheritance.

QUES :- Why Pea Plants?

➔ Short life cycle and Annual plant.

➔ Clear contrasting traits (e.g., tall vs short).

➔ Large number of offspring.

➔ Can self-pollinate and cross-pollinate.

Mendel Studied 7 Traits: TRAITS DOMINANT RECESSIVE Seed shape Round Wrinkled Seed colour Yellow Green Flower colour Violet White Pod shape Full Constricted Pod colour Green Yellow Flower position Axial Terminal

6. DIHYBRID CROSS (Two Traits) :-

➢ Cross in which inheritance of two characters

is studied at a time.

➢ He chooses two characters:-

● Example: Seed Color & Shape

➢ Parents: Round Yellow (RRYY) × Wrinkled

Green (rryy)

➢ F1: All Round Yellow (RrYy)

➢ F2: Phenotypic ratio: 9:3:3:

❖ (9 Round Yellow: 3 Round Green: 3

Wrinkled Yellow :1 Wrinkled Green)

➢ Genotypic ratio: 1:2:2:4:1:2:1:2:

7. MENDEL’S LAWS OF INHERITANCE :-

BASED ON MONOHYBRID CROSS BASED ON DIHYBRID CROSS

LAW OF DOMINANCE : One allele is dominant over the other and expresses in the F1 generation.

LAWS OF INDEPENDENT

ASSORTMENT:

Different gene pairs segregate independently during gamete formation. LAW OF SEGREGATION : Alleles separate during gamete formation; each gamete receives one allele.

8. Sex Determination in Humans :-

Evolution is the gradual change in traits in a species over generations. Variation: Differences in traits among individuals of a species. Natural Selection: Process by which traits that help survival become more common. Speciation: Formation of a new species due to genetic differences. Fossils and homologous structures are evidence of evolution.