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  4. What is the phenotypic ratio in a cross between dihybrid and homozygous recessive individual?

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Q. What is the phenotypic ratio in a cross between dihybrid and homozygous recessive individual?

NTA AbhyasNTA Abhyas 2020

Solution:

When the F1 progeny of a dihybrid cross is crossed with the homozygous recessive parent it is called dihybrid test cross. The ratio for this cross is 1:1:1:1.
For example, Parent - YYRR (yellow, round) X yyrr (green, wrinkled) - This character is for seeds.
F1= YyRr (Yellow, round) - In the question it is mentioned as dihybrid.
Test cross: YyRr X yyrr ; (yyrr - homozygous recessive individual/parent).
In this case, the following gametes will be formed -
YyRr - YR, Yr. yR and yr
yyrr - yr, yr, yr and yr
Now, if we put them in Punnett square -
yyrr
From the above Punnett square, we are getting the Phenotypic ratio as follows -
Hybrid Yellow Round (YyRr) : Yellow-wrinkled (Yyrr) : Green-Round (yyRr) : Green-wrinkled (yyrr) = 4:4:4:4 = 1:1:1:1