Heredity and Evolution Class 10 notes | Class 10 Science Chapter 9 Notes

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Accumulation of Variation During Reproduction and Heredity

Genetics: Branch of science that deals with heredity and variation.
Heredity: It means the transmission of features/ characters/ traits from one generation to the next generation.
Variation: The differences among the individuals of a species/ population are called variations

There are two types of variations:

Somatic variationGametic variation
Takes place in the bodyTakes place in the gametes/cells.
(Reproductive cells).
Neither inherited nor transmittedInherited as well as transmitted.
Also known as acquired traits.Also known as inherited traits.
Example: boring of pinna, cutting of tails in dogsExample: human height, skin colour

Accumulation of Variation during Reproduction

Variations appear during reproduction whether organisms multiply asexually or sexually

Asexual ReproductionSexual Reproduction
Variations are fewerVariations are large
Occurs due to small, inaccuracies in DNA copying
Occurs due to crossing over and separation of
chromosomes, (mutation).

Importance of Variation

  • Depending upon the nature of variations different individuals would have different kinds of advantage. For example, bacteria that can withstand heat will survive better in a heat wave.
  • It increases the chances of its survival in a changing environment. Free ear lobes and attached ear lobes are
  • two variants found in human populations.
  • If one bacterium divides and again the resultant two bacteria divide, the four individual bacteria generated
  • will be highly similar, due to asexual reproduction.
  • There will be only minor differences between them, generated due to small inaccuracy in copying of DNA.
  • If sexual reproduction is involved, greater diversity will be generated.
  • All these variations in the species do not have equal chances of survival.
  • Different individuals would have different kind of advantages.
  • Evolution involves selection of variants by environmental factors.


It is a section of DNA that provides information for one protein in a cell

  • If proteins work better and efficiently, the traits get expressed in a better way. If protein helps in the growth of plants, it will be taller. If the protein does not work properly, the plant will be shorter. It means genes will control the trait. It is a unit of inheritance.


Each gene set is present as separate independent pieces called chromosomes. Each cell will have two copies of each chromosome, one from each parent. The total number of chromosomes in the offspring will be 46, 23 from the mother and 23 from the father.

Mendel and His Work on Inheritance

Gregor Johann Mendel (1822-1884): Started his experiments on plant breeding and hybridization

  • Mendel was known as the Father of Genetics

Plant selected by Mendel:

Pisum Sativum (garden pea). Mendel used a number of contrasting characters for garden pea.

Table of Contrasting Characters (Seven Parts)

CharacterDominant Trait [capital letter]Recessive Trait [small letter]
Flower colourViolet VWhite v
Flower positionAxial ATerminal a
Seed colourYellow YGreen y
Seed shapeRound RWrinkled r
Pod shapeInflated IConstricted i
Pod colourGreen GYellow g
Height of plantTall TDwarf t
  • Seven pairs of contrasting characters in garden pea and total 14 varieties of plant were studied by Mendel.

Genetic Terminology

  • Gene: Mendel used the term factor for a gene. A gene is the unit of DNA responsible for the inheritance of character.
  • Allele: A pair of genes that control the two alternatives of the same character e.g., TT/tt.
  • Heterozygous: The organism in which both the genes of a character are unlike e.g., Tt.
  • Homozygous: The organism in which both the genes of a character are similar e.g., TT, tt.
  • Dominant: The gene which expresses itself in F1 generation is known as dominant gene.
  • Recessive: The gene which is unable to express itself in presence of the dominant gene.
  • Genotype: It is the genetic constitution (genetic makeup) of an organism which determines the characters.
  • Phenotype: It is the physical appearance of an individual. (Tall or short)

Mendel’s Experiments:

Mendel conducted a series of experiments in which he crossed the pollinated plants to study one character (at a time)

TT, ttPure or homozygous condition
TtHeterozygous condition or Hybrid

Phenotypic ratio: Ratio among numbers of individuals with different external features, e.g., 3:1
Genotype → Genetic makeup [TT, Tt or tt]
Genotypic ratio: Ratio among numbers of individuals with different genetic make up, e.g., 1:2:1

More Science Notes

Monohybrid Cross:

Cross between two pea plants with one pair (monohybrid cross) contrasting characters

Heredity and Evolution Class 10 notes
  • Phenotypic ratio = 3 : 1
  • Genotypic ratio = 1 : 2 : 1
  • Both dominant gene TT : One dominant, one recessive gene Tt : Both recessive gene tt.


  • All F1 progeny were tall.
  • F2 progeny ¼ were short and, ¾ were tall.
  • Phenotypic ratio: F2 – > 3 : 1
  • Genotypic ratio: F2 – > 1 : 2 : 1


TT and Tt both are tall plants while tt is a short plant.

  • A single copy of T is enough to make the plant tall, while both copies have to be t for the plant to be short.
  • Character/ Trait like ‘T’ is called dominant trait (because it expresses itself) and ‘t’ is recessive trait (because it remains suppressed).

Dihybrid Cross

  • A cross made between two plants having two pairs of contrasting characters is called dihybrid cross
Heredity and Evolution Class 10 notes

Phenotypic Ratio:

  • Round yellow : 9
  • Round green : 3
  • Wrinkled yellow : 3
  • Wrinkled green : 1

Genotypic Ratio:

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

Ratio = 1 : 2 : 2 : 1 : 4 : 2 : 1 : 2 : 1


  • When RRYY was crossed with rryy in F1 generation all were RrYy, round and yellow seeds.
  • Self pollination of F1 plants gave parental phenotype together with two mixtures (recombinants). Round wrinkled and green yellow seeded plants which appeared in the ratio of 9:3:3:1 in F2 generation.

Conclusions : Round and yellow seeds are Dominant characters

  • Occurrence of new phenotypic combinations show that genes for round and yellow seeds are inherited independently of each other.

Mendel’s laws of inheritance

Law of dominance

  • Some alleles are dominant while others are recessive. An organism with at least one dominant allele will display the effect of the dominant allele.

Law of segregation

  • During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.

Law of independent assortment

  • Genes for different traits can segregate independently during the formation of gametes. Mechanism of Heredity: Most of the characteristics or traits of an organism are controlled by the genes. Genes are actually segments of DNA guiding the formation of proteins by the cellular organelles. These proteins may be enzymes, hormones, antibodies, and structural components of different types of tissues. In other words, DNA/ genes are responsible for structure and functions of a living body. The genotype of an individual controls its phenotype.
Heredity and Evolution Class 10 notes
  • Since we inherit half of our chromosomes[genes] from each of our two parents, hence we resemble both of them or their side of the family members. For the same reasons, siblings resemble each other too. But if there are errors during DNA copying, genes may get altered and variation may appear in the form of a defect, disease, or simply changed phenotype. Sexual reproduction is also a source of variation.

Sex Determination

  • Phenomenon of decision or determination of sex of an offspring.

Factors Responsible for Sex Determination

Environmental: In some animals the temperature at which the fertilised eggs are kept decides the gender.
e.g. in snails, some lizards and turtle.
In some animals like humans gender of individual is determined by a pair of chromosomes called sex

Sex Chromosomes: In human beings there are 23 pairs of chromosomes. Out of these 22 pairs of chromosomes are called autosomes and the last pair of chromosomes that help in deciding gender of that individual are called sex chromosomes. XX – female; XY – male.

Sex determination in Human beings

Heredity and Evolution Class 10 notes

Male produces two genetic types of sperms, half with X and other half with Y chromosome. The female
produces only one genetic type of ova, all carrying X chromosome.

This shows that

  • • All children will inherit an X chromosome from their mother regardless whether they are boys or girls.
  • • Thus sex of children will be determined by what they inherit from their father, and not from their mother

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