The Basics...

What is a punnett square?

A Punnett square is a diagram that is used to show the potential genetic makeup of the offspring of a particular cross. The square is particularly useful to determine probabilities of phenotype and genotype ratios in the offspring.

For example, if you were to perform the following cross...

Aa x Aa

The punnett square would look like this...

	A	a
A	AA	Aa
a	Aa	aa

Note the ratios of the offspring. The genotype ratio is 1AA : 2Aa : 1aa while the phenotype ratio is 3 dominant : 1 recessive. So if 'A' stood for 'apple cheeks' and 'a' stood for 'rosy cheeks' then the phenotype ratio would be 3 apple cheeks : 1 rosy cheeks.

The above cross is called a monohybrid cross because it only deals with one trait. If we wanted to talk about two traits at the same time we would perform a dihybrid cross. Suppose we wanted to talk about 'apple/rose cheeks' and about height (tall/short). We would then create a larger punnett square.

For example, suppose we wanted to perform the following cross...

TtAa x TtAa

The punnett square would look like this...

	TA	Ta	tA	ta
TA	TTAA	TTAa	TtAA	TtAa
Ta	TTAa	TTaa	TtAa	Ttaa
tA	TtAA	TtAa	ttAA	ttAa
ta	TtAa	Ttaa	ttAa	ttaa

The top row and left most column denote the possible combinations of genes each parent can give to the offspring (the possible alleles in each sperm or egg). Each parent will give one of each of the possible letters. That way, when you combine the letters from the father and mother you have a total set in the children.

Note that there are 16 offspring boxes in this dihybrid cross. Each offspring has 4 letters (2 pair) just like their parents. If you build a punnett square and the offspring don't have the same number of pairs of chromosomes as their parents, then you did something wrong. This is a good self check when you are solving problems.

You should also note that when you make a dihybrid cross where the parents are heterozygous for both traits the phenotype ratio is 9:3:3:1. More specifically, there are 9 dominant/dominant : 3 dominant/recessive : 3 recessive/dominant : 1 recessive/recessive. If you commit this to memory it will make larger crosses easier to solve.

It is important to understand that these are the probabilistic outcome for the offspring, but in the real world parents could have two children and they are both homozygous recessive (ttaa). The odds of this are 1/16 x 1/16 = 1/256, but it is absolutely possible.

Situations where Punnett squares do not apply (from Wikipedia Punnett Square)

The phenotypic ratios of 3:1 and 9:3:3:1 are theoretical predictions based on the assumptions of segregation and independent assortment of alleles. Deviations from expected ratios can occur if any of the following conditions exists:

• the alleles in question are on the same chromosome and linked
• one parent lacks a copy of the gene, e.g. human males have only one X chromosome, from their mother, so only the maternal alleles have an effect on the organism (see sex linkage)
• the survival rate of different genotypes is not the same, e.g. one combination of alleles may be incompatible with life so that the affected offspring expires in utero
• alleles may show incomplete dominance or co-dominance (see dominance relationship)
• there are genetic interactions (epistasis) between alleles of different genes
• the trait is inherited on genetic material from only one parent, e.g. mitochondrial DNA is only inherited from the mother (see maternal effect)
• the alleles are imprinted

Use the external links below to learn more about punnett squares and practice various crosses.

External Links

Punnett Squares 1 - use this interactive to see how to make punnett squares and solve practice problems with chickens and lemmings.
Punnett Squares 2 - use this site to practice monohybrid and dihybrid crosses.
Punnett Squares 3 - this page is long but well worth it. It has a number of interactive problems that starts with a monohybrid cross and then proceeds to a test cross, incomplete dominance, dihybrid, codominance/multiple alleles, trihybrid, polygenic, linked genes, sex-linked, sex influenced, and epistasis. Wheww, that is a lot of stuff, but if you can do everything on this site then you pretty much get the whole genetic cross stuff.
Punnett Square Calculator - this site has a punnett square calculator where you can enter the parent genotypes and see the resultant square. This is especially good for large crosses or to check your work on smaller crosses.