Class 10 – Heredity and Evolution – Notes

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Heredity and Evolution – Notes

Heredity:- The transmission of traits/characters from one generation to next generation is called heredity.

Genetics:- The study of heredity and variation is called genetics.

Dominant:- The characteristics which is expressed in an organism is called dominant.

Recessive:- The trait which is not expressed in an organism is called recessive.

Homozygous:- Homozygous means that the organism has two copies of the same allele for a gene.
Ex:- TT, tt

Heterozygous:- Heterozygous means that an organism has two different alleles of a gene.
Ex:- Tt

Phenotype:- The trait which is visible in an organism is called its phenotype.

Genotype:- The composition of genes present in an organism is called its gemotype.

Mendel’s experiment:-

Monohybrid cross:-

The cross between two plants with one pair of contrasting characters is called monohybrid cross.
Ex:- Cross between a tall and dwarf (short) pea plant.

Phenotypic Ratio:- 3 : 1
Genotypic Ratio:- 1 : 2 : 1

Seven pairs of contrasting characters are:-

Character (Trait)	Dominant trait	Recessive trait
Flower colour	Violet (V)	White (v)
Height of plant	Tall (T)	Dwarf/Short (t)
Seed colour	Yellow (Y)	Green (y)
Seed shape	Round (R)	Wrinkled (r)
Pod shape	Inflated	Constricted
Pod colour	Green (G)	Yellow (g)
Flower position	Axial	Terminal

Dihybrid cross:-

The cross between two plants with two pairs of contrasting traits (characters) is called a dihybrid cross.

Mendel’s laws:-

• Law of dominance:- If the set of alleles comes from parent (male and female) to offspring then, the dominant allele is expressed and the recessive allele has no noticeable effect.
• Law of segregation:- The two alleles for a character separate during gamete formation.
• Law of independent assortment:- The two pair of alleles are inherited independently of each other.

Gene:- Functional segment of DNA which carries information for 1 – protein is called gene.

Sex determination:-

→ In human body, there are 23 pairs or 46 chromosomes in which one pair is sex chromosomes.

Evolution:- The sequence of gradual changes that take place in an organism over a million of years and leads to the formation of new species is called evolution.

Speciation:- Origin of new species from the existing one is called speciation.

Speciation can take place through:-

• Genetic drift:- Random change in the frequency of allele is known as genetic drift.
• Natural selection:- The process by which nature selects more suitably adopted organism is called natural selection.
• Geographical isolation:- Geographical isolation leads to reproductive isolation due to which there is no flow of genes between separated group of population. It is caused by mountain ranges , rivers etc.

Homologous organ:- The organs having similar structure but different functions are called homologous organ. Ex:- Forelimbs of bird and human.

Analogous organ:- The organs having different structures but similar function are called analogous organs. Ex:- Wings of bird and bat.

Fossils:- The preserved traces of living organism are called fossils.

Methods to estimate the age of fossils are:-

• Relative method:- On digging, the fossils which are closer to the surface are more recent than the fossils found in deeper layers.
• Dating method:- The method of estimating the age of fossil by detecting the ratios of different isotopes of the same element in the fossil is called dating method.