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.

Difference between Acquired and Inherited traits:-

Acquired traitsInherited traits
These traits are acquired by a person during his/her life time.These traits are transferred to the individual from his/her parents.
These traits are not passed to next generation.These are passed to next generation.
They do not direct evolution.They direct evolution.
Ex:- Knowledge, body weight etc.Ex:- Height, eye colour etc.

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 traitRecessive trait
Flower colour Violet (V)White (v)
Height of plantTall (T)Dwarf/Short (t)
Seed colourYellow (Y)Green (y)
Seed shapeRound (R)Wrinkled (r)
Pod shapeInflatedConstricted
Pod colourGreen (G)Yellow (g)
Flower positionAxialTerminal

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.


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