A two-gene cross: seed shape and colour
"A two-gene cross: seed shape and colour" is a free, interactive Class 12 Biology simulation for NEET, built on the lesson "Inheritance of two genes: independent assortment" in Principles of Inheritance and Variation. Each RrYy parent makes four kinds of gametes (RY, Ry, rY, ry) in equal numbers, so the grid has 16 equally likely boxes. Tap a bar to find its boxes. Independent assortment: the R/r pair separates independently of the Y/y pair, giving 9 : 3 : 3 : 1 in the F₂. It holds for genes on different chromosomes (or far apart on one); tightly linked genes do not follow it.
From the lesson Inheritance of two genes: independent assortment in Principles of Inheritance and Variation. Change the values and watch what happens.
The idea behind it
NCERT § "Inheritance of Two Genes"
- In a dihybrid cross Mendel crossed round-yellow (RRYY) with wrinkled-green (rryy) peas. All F₁ seeds were round and yellow (RrYy).
- Selfing the F₁ produced an F₂ ratio of 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green, from 16 combinations of gametes.
- Looked at one character at a time, round : wrinkled and yellow : green are each 3:1, showing that each gene still segregates on its own.
- Law of independent assortment: if a hybrid carries two pairs of traits together, one pair segregates independently of the other. So an RrYy plant produces four kinds of gametes, RY, Ry, rY and ry, in equal proportion.
- A dihybrid test cross (RrYy × rryy) gives four phenotypes in a 1:1:1:1 ratio.
- Mendel's work, published in 1865, went unrecognised for years, partly because factors were abstract, not physically observed, and his mathematical approach was new to biologists. It was rediscovered independently in 1900 by de Vries, Correns and von Tschermak.
Questions
2 answers
- What does the "A two-gene cross: seed shape and colour" simulation show?
- Each RrYy parent makes four kinds of gametes (RY, Ry, rY, ry) in equal numbers, so the grid has 16 equally likely boxes. Tap a bar to find its boxes.
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Inheritance of two genes: independent assortment" in Principles of Inheritance and Variation (Class 12 Biology).
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