The different ways that these genes can be passed on from parents to offspring can be explored using a punnett square. In Fig. 11 below, a punnett square is used to explore the possibilities for eye colour in the offspring of a man with blue eyes and a woman with brown eyes. The woman has mixed genes but her eyes are brown because the dominant B gene hides the effect of the recessive b gene.
Man’s Genes: bb Man’s Eyes: Blue
Man’s Gametes
All sperm cells will contain only one of the b genes
This is the only type of male gamete
Woman’s Genes: Bb Woman’s Eyes: brown
Woman’s Gametes
50% of eggs will contain the B gene and 50% will contain the b gene
These are the two types of female gamete
50% chance that a child will have brown eyes
50% chance that a child will have blue eyes
Fig. 11 Possible eye colour in the offspring of a blue-eyed man and a brown-eyed woman.
Punnett squares may also be used to predict what the possible offspring may be in other situations:
B B BB
l l l
l
Both parents have brown eyes. Both parents have the genes BB. Both parents produce only one type of gamete, B.
All offspring will have the genes BB, and eyes that are brown.
B
B BB b Bb
l l
l l
132
Both parents have brown eyes. One parent has the genes BB and produces only one type of gamete, B.
The other parent has the genes Bb and produces two types of gamete, B and b.
All offspring will have brown eyes (with a 50% chance that they have the genes Bb, 50% BB).
b b
l l l
l bb
Both parents have blue eyes. Both parents have the genes bb. Both parents produce only one type of gamete, b.
All offspring will have the genes bb and eyes that are blue.
B Bb b
B BB Bb b
bb
l l l
l
Both parents have brown eyes. Both parents have the genes Bb. Both parents can produce two types of gamete, B and b.
This gives a 25% chance that the offspring will have the genes bb and eyes that are blue.