Class 12 Biology Inheritance Variation INHERITANCE OF ONE GENE (MONOHYBRID CROSS)


Mendel crossed tall and dwarf pea plant and collected the seeds from them. Seeds were used to generate plants of first generation (F1 or Filial progeny). Mendel observed that all the first generation plants were tall, none of them were dwarf. He made similar observations for the other pairs of traits. He concluded that F1 generation resembled either one of the parents.



He then self-pollinated the tall F1 plants and he observed that some of them were dwarf. Out of all, ¼th were dwarf and 3/4th were tall.


Pic shows Monohybrid cross.

Similar results were obtained for other traits too. In F2 generation, both the traits were expressed in proportion of 3:1. Dominant trait in F2 is about thrice of the recessive from. These contrasting traits did not show any blending at either F1 or F2 stage.

Based on these observations, he concluded that something was being stably passed from one generation to the other. He named it ‘factors’ which are now called as ‘genes’.

Gene is the unit of inheritance. It contains information that is required to express a particular trait in an organism. Genes which code for a pair of contrasting traits are known as ‘alleles’. They are slightly different for a same gene.

For representing traits using alphabetical symbols, capital letter is used for the trait expressed at F1 generation and small letter is used for the other one.

For example: T for tall trait

t for dwarf.

T and t are alleles of each other. Pair of alleles for height in the plants are TT, Tt and tt.

TT and tt are homozygous. TT and tt are called genotype of the plant while the description terms tall and dwarf are phenotype. Tt represents heterozygous.

Test cross is the cross between an individual with dominant trait and a recessive organism. It helps us to understand whether the dominant trait is homozygous or heterozygous.


Pic shows Test Cross.


The production of gametes by the parents, formation of zygotes can be easily understood by Punnett square. It was given by British geneticist RC Punnett. It is a graphical representation used to calculate probability of all possible genotypes of offspring in a genetic cross.


Pic shows Punnett Square.

It is typically used for monohybrid cross conducted by Mendel between true-breeding tall plants and true-breeding dwarf plants.

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