Mendel's Second law - The Law Of Independent Assortment

Law of independent assortment 

Definition - This law states that the alleles of two different characters located in different pairs of homologous chromosomes are independent of one another during gamete formation and coming together into the offspring by fertilization, both processes occurring randomly.

OR

The principle of Independent Assortment may also stated that during the  formation of gametes in each sex, either one of a pair of alleles may enter the same gamete with either one of another pair.

Explanation with examples

i) Cross between Pea Plants

Cross a pure pea plant having round seeds and yellow cotyledons with a pure pea plant having wrinkled seeds and green cotyledons.

Here , the round form of seeds and yellow colour of cotyledons are dominant over the wrinkled seeds and green colour of cotyledons. The parent plants may beCr represented by RRYY and rryy. During the gametes formation , meiosis separate the two alleles of each trait so that gamete get only one allele of each trait.

The F1 generation formed by the fusion of gametes with alleles RY and ry i.e they have RrYy. They all are hybrid with round seeds and yellow cotyledons.

In F2 generation,  

After crossing the F1 generation four types of gametes are formed and produce four types of plants in F2 generation. They are in ratio of 9:3:3:1 where 9 with round seeds and yellow cotyledons,3 with round seeds and green cotyledons, 3 with wrinkled seeds and yellow cotyledons and 1 with wrinkled seeds and yellow cotyledons . 

ii) Cross between Guinea pig

Mate a black, short haired male guinea pig with a pure white female Guinea pig . Here the black colour is dominant over the white colour and short hair are dominant over the long hair. Each character is represented by two alleles such as BBSS and bbss respectively. Here B stands for the allele of dominant black colour,b for white colour,S for the allele of short hair and s for the allele of recessive long hair. During the formation gamete , two character of each allele segregate so a gamete has only one allele of each character. i.e BS and bs. Now fertilization occurs. The zygote comes to possess two unlike allele for the size the hair it has Bb  Ss . The offspring developed from such zygote will be all hybrid with black colour and short hair.

    Now cross the hybrid black, short haired male and female of F1 generation among themselves. The allele for short and long hair again segregate during gamete formation. B can go with S or s i.e BS or Bs. Similarly b can go with S or s i.e bS or bs. Therefore the gamete show four combinations of allele which are 

a) black short BS

b) black long Bs

C) white short bS

d) white long bs

These gametes on fertilization produce four types of guinea pig in F2 generation. They're in the ratio of 9:3:3:1 i.e

9 black short

3 black long

3 white short

1 white long

The black and white guinea pigs are in ratio of 3:1 and pure black, hybrid black and pure white guinea pigs in ratio of 1:2:1 . Similarly short haired and long haired guinea pigs show ratio of 3:1 and pure short hybrid, hybrid short haired and pure long haired a ratio of 1:2:1 .

   The 9:3:3:1 Dihybrid phenotypic ratio is the product of the two separate 3:1  monohybrid ratios, one for each allele pair.ie 

3:1 X 3:1 = 9:3:3:1

Advantage :-

 This cross shows that the two alleles of different characters do not interfere with one another's behaviour, but sorted out independently of one another

Limitations

The law of independent assortment of genes holds good only if the different pairs of alleles lie in the different chromosomes pair and not individual allele which are segregated during reduction division. 

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