Question

Explain Dihybrid ratio with examples.

(4 marks)
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Answer 1

Answer :-

ㅤ

Dihybrid cross :- A dihybrid cross is a cross between two sets of plants involving two pairs of contrasting characters.

For example :- A cross between plants having yellow and round seeds ( YYRR ) with plants possessing green and wrinkled seed ( yyrr ) is a dihybrid cross.

P generation -

Yellow and roundㅤ ×ㅤ Green and winkled

ㅤSeeds (YYRR) ㅤㅤㅤㅤㅤSeeds ( yyrr )

ㅤㅤㅤㅤㅤㅤㅤㅤㅤ↧

F₁ generation

ㅤㅤㅤYellow and round seeds ( YyRr )

ㅤㅤㅤㅤㅤㅤㅤㅤㅤ↧ ( selfing )

F₂ generation

ㅤㅤㅤYellow round ( YR ) -ㅤㅤ9

ㅤㅤㅤYellow wrinkled ( Yr ) - ㅤ3

ㅤㅤㅤGreen round ( yR ) -ㅤ ㅤ3

ㅤㅤㅤGreen wrinkled ( yr ) - ㅤ1

ㅤㅤㅤㅤㅤYYRRㅤ × ㅤㅤyyrr

ㅤㅤㅤㅤㅤ⇙ㅤ⇘ㅤㅤㅤ⇙ㅤ⇘

ㅤㅤㅤㅤYRㅤㅤYRㅤㅤyrㅤㅤyr

( punnett square in the attachment 1 )

Now, we obtained the gametes - YyRr , YyRr , YyRr and YyRr

So, all F₁ plants are yellow and round seeded.

Now, crossing between plants of F₁ -

ㅤㅤㅤㅤㅤYyRrㅤ × ㅤYyRr

Gametes of YyRr -

• YR
• Yr
• yR
• yr

Gametes of YyRr -

• YR
• Yr
• yR
• yr

Crossing between the gametes -

( punnett square in attachment 2 )

In F₂ generation :-

Phenotypic ratio - 9 : 3 : 3 : 1

Genotypic ratio - 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1

9 : 3 : 3 : 1 phenotypic ratio in the F₂ generation is obtained in a dihybrid cross, and is called Dihybrid ratio.

Answer 2

Dihybrid cross : When two allelic pairs are used for crossing. Law of independent assortment is based on inheritance of two genes.

When two pairs of contrasting characters are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters. Thus, the genes randomly rearrange in the offsprings producing both parental and new combination of characters. Therefore, the inheritance of one character does not affect the inheritance of another character and both the characters are sought independently. The Punnett square can be used to understand the independent segregation of the two pairs of genes during meiosis. Linkage is an exception to the law of independent assortment.

Phenotypic ratio :-

• 9 : 3 : 3 : 1 - Dihybrid Ratio

• Round yellow : Round green : Wrinkled yellow : Wrinkled green

Genotypic ratio :-

• 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1

• YYRR : YYRr : YyRR : YyRr : YYrr : Yyrr : yyRR : yyRr : yyrr

$\bold{Hope\;it \; helps\;!}$