The Punnett square is a representation of the cross, and expresses the probability of getting offspring with different genotypes and phenotypes. In the exposed example, 50% of the offpring is black and 50% is brown.
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PUNNETT SQUARE
The Punnett square is a representation of a cross between two individuals.
By doing the Punnett square, we can get the probabilities of getting offspring with different genotypes and phenotypes.
We can get the genotypic and phenotypic ratio of the offspring.
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The exposed example proposes the cross between two labrador dogs.
EPISTASIS
Labrador dogs exhibit recessive epistasis.
Epistasis means interruption and refers to interactions between genes located in different loci in the same chromosome.
Different genes can interact at different levels. This interaction affects the phenotypic expression, causes a wide genetic variability.
An epistatic gene can alter, influence, or suppress the expression of a hypostatic gene.
When the epistatic gene is recessive, the interaction is known as recessive epistasis.
There are two genes involved in labrador color.
The Diallelic Gene E affects the color expression
- E dominant allele expresses pigment
- e recessive allele does not express pigment
The diallelic gene B codes for color
- B dominant allele codes for black color
- b recessive allele codes for brown
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CROSS:
In this cross, we have
- Sire is a black male ⇒ BbEE
- Dam is a chocolate female ⇒ bbEE
Both dogs express h0m0zyg0us dominant gentoype for E gene. So color can be expressed.
Sire is heter0zyg0us for B gene, expressing black color.
Dam is h0m0zyg0us recessive for b gene, expressing brown color.
Cross: Sire x Dam
Parentals) BbEE x bbEE
Gametes) BE, BE, bE, bE
bE, bE, bE, bE
Punnett square) bE bE bE bE
BE BbEE BbEE BbEE BbEE
BE BbEE BbEE BbEE BbEE
bE bbEE bbEE bbEE bbEE
bE bbEE bbEE bbEE bbEE
F1) 100% of the progeny will express color - 0% will be yellow ⇒ EE
50% of the progeny expresses black color ⇒ BbEE
50% of the progeny expresses brown color ⇒ bbEE
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