# Review Chez Punnett

**A
Punnett Square shows all the possible combinations all two sets of alleles**,
where the first set of alleles shows all possible zygote types contributed
by the mother, and the second set shows possible types contributed by the
father. We put the mother across the top and the father across the left
hand side, but you could do it the other way around.

The Punnett Square for a mother that was Tt and a father that was Tt would
yield approximately a 3:1 **ratio of dominant to recessive phenotypes**
(and approximately a 1:2:1 ratio of AA, Aa, aa **genotypes**).
Any other combination of parental genotypes can be figured out similarly,
and will yield different ratios of genotypes and phenotypes (so don't assume
that 3:1 is the answer to every problem!!!)

If you are trying to find the results of **a cross involving 2 genes**
(i.e., fur AND teeth), then you need a bigger Punnett Square. With 2 genes,
each parent can contribute 4 different zygote types, and you need to list
each of the 4 on the top (mother) or side (father) of the Punnett Square.
For example, in a double hybrid cross (AaBb x AaBb):

This gives the famous 9:3:3:1 ratios for phenotypes. Again, note that this
ratio ONLY holds if you specifically have a dihybrid cross. If you cross,
say, AABb x Aabb, you will get a completely different ratio (8:8:0:0, in case
you're curious). **If you are asked to find the phenotypic ratios, check
first whether the problem involves a dihybrid cross -- if not, always work
out the Punnett Square. **

**All of these phenotypic ratios are approximate**, because
they're being produced by random processes -- just like you get approximately
50% heads when you flip a coin.

**The probability of a given phenotype can also be found by using the
laws of probability.** First, you need to identify all possible genotypes
that would produce the desired phenotype. Then you use the Law of AND (multiply
probabilities) to determine how likely each genotype is (i.e., multiply the
probability of getting each of the four alleles in the genotype). Finally,
you use the Law of OR (add the probabilities) to determine how likely it is
that one of the possible genotypes was created.

*If you want a printer-friendly version of this module, you can find it here in a Microsoft Word document. This printer-friendly version should be used only to review, as it does not contain any of the interactive material, and only a skeletal version of problems solved in the module.*

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