Dihybrid cross experiments of Mendel
Mendel had some Questions while doing breeding experiments. They are, do different for different characters also segregate or seperate? Whether the genes are character would alter f the genes of another character during their inheritance together?
To answer the above Questions, Mendel did dihybrid cross. He also deduced the law of independent assortment from dihybrid cross.
DIHYBRID CROSS -DEFINITION
A cross made to study the inheritance of two characters or two pairs of contrasting forms or two pairs of different alleles is known as dihybrid cross.
Crossing of pure yellow, Round seeded plant with green, Wrinkled seeded pisum plants
Character -1 - seed colour Yellow --- green
Character - 2 - seed shape Round -----wrinkled
2. PHENOTYPIC HYPOTHESIS OF DIHYBRID CROSS
A. Crossing of ‘p’ generation:
Mendel crossed pure yellow, round seeded plant with pure green wrinkled
seeded plant. He got all yellow,Round seeded plants in ‘F1’ generation. These are dihybrids.
The, ‘F1’ generation showed that yellow is dominant over green and round is dominant over wrinkled forms. This dominance was allready observed in monohybrid crosses.
B. Crossing of ‘F1’ individuals:
Mendel allowed ‘F1’ di hybrid yellow, round seeded plants for self pollination. He got different types of individuals in ‘F2’.They are,
1. Yellow, Round (Double dominant)
2. Yellow,Wrinkled (Recombinant-I)
3. Green, Round (Recombinant-Il)
4. Green, Wrinkled (Double recessive)
The above ‘4’ types of inidividuals are in the ratio of 9/1 6: 3/16: 3/16: 1/16 respectively.
‘F2’ ‘dihybrid ratio led mendel to propose the law of independent assortment.
3. GENE HYPOTHESIS OF DIHYBRID CROSS
The yellow colour controlling gene is represented as ‘Y’ . The green colour controlling gene is represented as ‘y’. The round shape controlling gene is represented as ‘R’. The wrinkled shape controlling gene is represented as ‘r’.
A. Genotypes of ‘P’generation:
The genotype of pure yellow round seeded plant is ‘YY RR’. The genotype of double recessive green wrinkled seeded plant is ‘yy rr’
B. Crossing of pure ‘P’-.generation:
Pure yellow round seeded plant (YY RR) produced only one type of gametes (YR) type. All gametes carried ‘Y’ and ‘ R’ genes to the F1 generation. Pure green wrinkled seeded plant (yy rr) also produced one type of gametes (yr). When ‘YR’ type of gametes were fertilized by ‘yr’ type of gametes, all F1 inidividuals produced were dihybrid yellow round seeded plants(YyRr). The F1 generation indicates that yellow colour (Y) and round shape (R) controlling genes are dominant over green colour (y) and wrinkled shape (r) controlling genes respectively like in monohybrid crosses.
C. Crossing of’F1’dyhibrid individuals:
Mendel allowed self pollination in dihybrid yellow round seeded plants (YyRr). The ‘F1’ female produced ‘4’ types of ova. They. are YR, Yr, yR and yr. Like wise male also produced ‘4’types of pollen grains. They are YR, Yr, yR, and yr. When the above ‘4” types of ova were fertilized by the above ‘4’ types of pollen grains, Mendel got ‘4’ different phenotypes and ‘9’ different genotypes in F2 generation.
The ‘F2’generation is represented in punnet’s squares or checker board.
4. ‘F2’ – PHENOTYPIC RATIO
In ‘f2’ generation, two parent phenotypes and two recombinats are
produced. They are,
I. Double dominant (VeIIow,Round) -- 9/16.
ii. Recombinant - I (Yellow, Wrinkled ) --- 3/16.
iii. Recombinant -Il (Green, Round) --- 3/16.
iv. Double rcessive (Green, Wrinkled) --- 1/16.
In ‘F2’ generation, if we observe the phenotypic ratio of one character that is either the ratio of yellow and green or Round and Wrinkled, it is 12/16: 4/16 or 3:1 . So, in dihybrid cross, the phenotypic ratio of one character is not effected by phenotypic ratio of another.
5. ‘F2’ - GENOTYPIC RATIO
In F2 generation the ‘1 6’ individuals of 4 different phenotypes contain ‘9’ different genotypes. They are 1 :2:1 :2:4:2:1 :2:1 or (1 : 2 : j)2 . The ‘ F2’ different genotypes are
The ‘9’ double dominant yellow round seeded plants contain ‘4’ different genotypes. They are 1/9 YYRR, 2/9 YyRR, 2/9 YYRr and ‘. 4/9YyRr. The ‘3’ individuals of recombinant -I (yellow wrinkled) contain ‘2’ different genotypes. They are 1/3 YYrr and 2/3 Yyrr.
The ‘3’ individuals of recombinant-IF (green, round) also contain 2 different genotypes. They are 1/3 yyRR and 2/3 yyRr. The double recessive green wrinkled always contains one genotype only. It is yyrr. If we observe the F2 genotypic ratio of dihybrid cross, the genotypic ratio of one character is not affected by the genotypic ratio of another character.
6 LAW OF INDEPENDENDENT ASSORTMENT’
As the recombinants (yellow wrinkled and green round) were produced in ‘F2’ generation of dihybrid cross, Mendel said that every allele behaves independently. Hence all possible combinations were produced in F2 generation. It is called independent assortment. As alleles behaved independently ‘Y’ is not only inherited along with ‘R’ but also with ‘r’ . Like wise ‘y’ is not only inherited along with ‘r’ but also with ‘R’ . This independent behavour of genes lead to the formation of ‘4’ types of gametes from ‘F1’ and two recombinants in.