Class 10 science chapter 8 Heredity and Evolution notes

                                                             Class – 10

                                                         Science Chapter – 8

                                                       Heredity and Evolution

Heredity –

                 The transmission of features /characters /and traits from one generation to the next generation.

Variation –

                   The difference among the individuals of a species /population are called variation.

It takes place due to environmental changes, crossing over and recombination of genes and mutation.

Evolution –

                     Evolution is the sequence of gradual changes which takes place in the primitive organisms, over million of years, in which new species are


# Heredity –  

 -Mendal and his work on inheritance

          Sir Gregor Johann Mendal started his experiments on plant breeding and hybridization.

Mendal was known as father of Genetics

-Mendal choose pea plants for hybridization because pea plants has many contrasting characters.

For Example

           Flower colour – violet   white

           Height of plant – Tall    Dwarf / Short

          Seed Shape – Round   Wrinkled]


In the first place, He crossed the pollinated plants to study one character (at a time)

– Cross between two pea plants with one pair of contrasting characters is called a monohybrid.

Phenotype ratio ─> 3:1

Genotype ratio ─> 1:2:1

Here T  –> dominant trait

          t –>    Recessive trait

. Genotype  –>

          It is the combination of alleles that they possess for a specific gene.

. Phenotype –>

          It is the combination of their observable characteristics or traits. Phenotype is influenced by genotype.

Dihybrid cross –

          It is a cross between two plants having two pairs of contrasting characters is called dihybrid cross.

 Two contrasting characters

  • Round and wrinkled, yellow and green

Self pollination

      Phenotype Ratio

          Round, Yellow: 9

          Round, green: 3                        [ 9: 3: 3: 1]

          Wrinkled, yellow: 3

Wrinkled, green; 1

Genotype Ratio:

       RRYY             RRYy           RRyy          RrYY           RrYy          Rryy             rrYY            rrYy             rryy  

          1        :       2           :       1         :       2         :         4          :        2     :        1         :        2         :         1

  • Here Round and yellow seeds are dominant characters.

How do these traits get expressed

In plants – Plants have hormones that can trigger growth. Plant height can thus depend on the amount of a particular plant hormone. 

The amount of hormone made will depend on the efficiency of the process for making it.

If enzyme works efficiently, a lot of hormone will be tall.

If enzyme works less efficient, the amount of hormone will be less, the plants will be short.

          – In Mendelian experiments, both parents contribute equally to the DNA of the progeny during sexual reproduction.

          If both parents can help determine the traits in the progeny, both parents must be contributing a copy of the same gene. This means that each pea plant must have two sets of all genes, one inherited from each parent.


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