Heredity And Evolution Class 10 Notes | Science | Term 2 |


Heredity And Evolution Class 10 Notes | Science | 


Genetics is a branch of biology that deals with study of heredity and variation. 

Heredity or Inheritance

The passing of characters from parents to offspring or from one generation to next generation is generally defined as heredity or inheritance.

Some Important Terms


  • These are differences in the individuals of same species.
  • Variations which are passed to next generation are called heritable or germinal variations.
  • Variations which are not passed to next generation are called non inheritable or somatic variations. 

Advantages of variations:

  • The chances of survival of species in changing environment could effectively be increased. 


Chromosome is a thread like structure in the nucleus of the cell formed of deoxyribonucleic acid that carries the genes.


Gene is a specific segment of DNA located on chromosome and is the basic unit of inheritance.


Character of an individual whose inheritance is studied is called trait


Allele are different forms of a gene


Genotype can simply be defined as the genetic constitution of an individual.
Example: Tt or TT or tt


Phenotype is the external appearance of an individual. For Example: Tall, Dwarf


An individual in which both the alleles of a gene are identical. Example: TT or tt.


An individual in which both the alleles of a gene are different. Example: Tt, Yy


A gene is said to be dominant when it expresses itself whether in homozygous or heterozygous condition.
Example: Gene T for tallness is dominant over t for shortness.


Hybrid is produced from crossing the two homozygous individuals which are genetically different.

Inherited Characters

The characteristics that are inherited by the offspring from the parents are called inherited characters. For Example : skin colour, height, weight
Free ear lobe is dominant on attached ear lobe.

Acquired Characters

Certain characters that appear in the individual temporary In response to the environment.
Example: suntanned skin, knowledge, skills

Mendel and His Work on Inheritance

Gregor John Mendel- Father of genetics.
He experiments on garden pea- Pisum Sativum
He worked continuously for 8 years

These principles are

  • Law of dominance
  • Law of segregation
  • Law of independent assortment

Mendel crossed homozygous tall plant with homozygous dwarf plants. These plants are called parents generation.
The seeds produced by cross pollinating these preparing generation plants were collected and grown to produce hybrid plants. This belong to first filial generation.
The pants of F1 generation were self pollinated and seeds were collected. The plants raised from the seeds of F1 plant belonged to F2 generation.

Tall- TT
Tall -Tt
Dwarf- tt

Monohybrid Cross

The cross that Mendel made in order to study the inheritance of one pair of contrasting characters is known as monohybrid cross.

Heredity and Evolution Class 10 Notes

Dihybrid Cross

Crosses which were made to study the inheritance of two pairs of contrasting characters.

Heredity and Evolution Class 10 Notes

Law of dominance

When two homozygous individuals with contrasting characters are crossed the F1 hybrids have both the contrasting genes but only one expresses and do not allow other to express.
TT × tt → Tt
T is dominant and expresses itself.

Law of Segregation

In a heterozygote, the two members of the allelic pair remain together without blending. When gametes are formed from this hybrid, the two genes separate out and only one enters each gamete.
Tt- Heterozygote allelic pair no blending or intermixing
T. t. – separates in gametes

Law of Independent Assortment

In two pairs of contrasting characters the distribution of factors of each pair in the gametes is independent of the distribution of other pairs.

Sex Determination in Human Being

  • Human cell has 23 pair of chromosomes.
  • Out of these, 22 pairs are called autosomes.
  • These autosomes are similar in men and women.
  • 23rd chromosome pair is different and is called sex chromosome.
  • In human female the two sex chromosome are similar or homomorphic ,i.e., YY+ XX
  • All boys and girls receive X chromosome from mother.
  • But boys receive Y chromosome and girls receive X chromosome from father.
  • Thus, male or a father is responsible for the sex of a child.

Heredity and Evolution Questions

Question 1: What is a sex chromosome?

Answer: The chromosomes which are associated with the sex determination are called sex chromosomes.

Question 2: What is the basic function of a gene?

Answer: A gene is the career of the genetic information from one generation to the next.

Question 3: Why did Mendel choose only pea plant for his experiments?

Answer: The reason behind using the pea plant for experiments by Mendel is as follows:

  • Easy to grow
  • Short lifespan
  • Larger size of flower
  • Self pollinated
  • Easily distinguishable characters

Question 4: Why does the off springs differ from parents in certain characters?

Answer: It is due to the biparental percentage. The genes on chromosome switch pass over to the next generation is partially derived from both the parents. During fertilization of egg by the sperm new combination of chromosomes enter the zygote, due to which certain variations occur in the offsprings. This is the reason due to which brothers and sisters show variations in their complexion, habits and behaviour.

Question 5: What the contribution of Mendel to genetics?

Answer: According to Mendel’s observation, garden pea shows the occurrence of contrasting characters in various generations. On this basis, he interpreted that these contrasting characters are controlled by factors. He considered each and every character as a unit which is controlled by a factor. Factors are the carriers of hereditary information. Now they are known as genes.

Question 6: How is the chromosomal difference between sperms and eggs of humans beneficial?

Answer: As we know a male has one X chromosome and one Y chromosome. Hence half the sperms will have X chromosomes and other half will have Y chromosomes.
A female has two X chromosomes. Hence, all the female gametes will have only X chromosomes. If sperm carrying X chromosome fertilizes and ovum then the child born will be a girl. If sperm carrying Y chromosome fertilizes and ovum then the child born will be a boy. Thus, the chromosomal difference between sperm and eggs of human helps in determining the sex of the child.

# Heredity and Evolution Class 10 Notes

# Heredity and Evolution Class 10 Notes with Solutions


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