AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

PUNNETT SQUARE

• The genotypes and phenotypes resulting from various combinations of gametes can be easily determenined by Punnett squares, devised by Reginald C. Punnett(1875-1967). Here each of the possible gamete is placed in an individual column or row, with vertical column representing the female and horizontal row the male parent.
• The gametes are arranged in all possible combinations and the resulting genotypes are in the boxes along with the phenotypes.
• It is a convenient method if small numbers of unlinked genes are present.
• Example for a dihybrid crosses:
  • In garden pea plants, Round (RR) and Yellow seeds (YY) are dominant over the Wrinkled (rr) and Green seeds (yy) respectively.
  • When plants of garden pea with round yellow seeds (RRYY) were crossed with plants having wrinkled green seeds (rryy), round yellow seed plants (RrYy) were obtained in F₁. Thus round seed shape over wrinkled seed shape and yellow colour of seed exhibited dominance over green independently.
  • In F₁ plants (RrYy), the possible combinations of alleles in the gametes are RY, Ry, rY, and ry.
  • Below is the Punnett-square which will give all the possible combinations in the F₂ after selfing of the F₁.

	Female gametes
Male gametes		RY	Ry	rY	ry
	RY	RRYY	RRYy	RrYY	RrYy
	Ry	RRYy	RRyy	RrYy	Rryy
	rY	RrYY	RrYy	rrYY	rrYy
	ry	RrYy	Rryy	rrYy	rryy