Brief notes on the Principle of segregation or Mendel’s First Law of Heredity


In any diploid organism the two factors of a given character remain together without mixing and keeping their identity distinct. At the time of gamete formation the two factors or alleles segregate and a gamete receives only one of the two factors of any character randomly as per the principle of probability.

As the gamete receives only one allele of a character the gamete is said to be pure for a chracter and the principle is also known as purity of gamete. Dihybrid cross: \

A cross where two pairs of alternative characters are considered for study is known as dihybrid cross. Two traits or characters such as shape of the seed and colur of the seed can be considered together to understand the dihybrid cross.In these two pairs of alternative characters round seed(R) is dominant over wrinkled seed(r) and yellow seed(Y) is dominant over green(y).Like the monohybrid cross Mendel selected pure breeding plants for both the traits for the cross. Thus true breeding plants having round and yellow seed(both dominant) was crossed with plant having yellow and wrinkled seed(both recessive).


In this dihybrid cross the Fi plants were all having round and yellow seeds. Then after selling in F2 generation parental as well as different combinations appeared in the following ratios: Plants with round yellow seeds – g Plants with round green seeds -3 Plants with wrinkled yellow seeds – 3 Plants with wrinkled green seeds – 1 The phenotypic ratio of the dihybrid cross is 9:3:3:1

Explanation: Taking the genes or factors as the bases of characters or traits and assigning each factor a symbol the dihybrid cross can be explained by chequer board or Punnet square as below.

In this dihybrid cross four types of plants appeared in the F2 generation which is two more than the parental types. The genotypic ratio can be grouped into four categories considering the principle of dominance as follows:

R-Y – group – 9 Round Yellow phcnotypcs rr -Y – group – 3 Wrinkled Yellow phcnotypcs R -yy – group – 3 Round Green phcnotypcs rr-yv – group – 1 wrinkled Green |henotypes


Number of plants with seeds having yellow colour = 9+3=12

Number of plants with seeds having green colour = 3+1 =4

Hence the F2 ratio of Yellow: green is 12:4 or 3:1

Similarly, number of plants with round seeds = 12


Number of plants with wrinkled seeds = 4

Hence the F2 ratio of Round: wrinkled seeds = 12:4 or 3:1

Though the factors for both the characters arc present together, their inheritance ratio is the same as the monohybrid cross.This proves that characters are inherited independent of each other.

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