A Punnett square is a basic tool in genetics being explored in class . It helps determine the probability of a characteristic showing. To fully use a punnett square, though, you must understand a few terms in genetics, such as traits(characteristic, represented by two alleles), alleles(unit that helps determine trait), homozygous(having two of the same alleles for a trait), heterozygous(having two different alleles for a trait), dominant and recessive [alleles], genotype(type of alleles[can be represented by homozygous and heterozygous]), and phenotype(characteristic showing for a trait). Dominant alleles are alleles whose respective characteristic is are always expressed and recessive alleles are alleles whose respective characteristics are hidden when a dominant allele is present.
Punnett squares come in different levels. A mono-hybrid Punnett square is basically a square split into four parts. The mother's two alleles for a trait go on top of the Punnett square, one on top of each column, and the father's two alleles for the same trait go to the left of the Punnett square, one for each row. After that, in each box, copy the mother's allele from the top and the father's allele from the left. You should get a filled Punnett square. You will see that there is an x/4 probability of a certain genotype or phenotype. For example, if mom has a TT for rolling tongue and dad has Tt, they both can roll their tongue. You can see there is a 2/4, or 50% chance of getting homozygous dominant alleles, 50% chance of getting heterozygous alleles (genotype), and 4/4, or 100 chance that the child can roll their tongue (phenotype).
The mono-hybrid Punnett square shows the probability of different distributions of alleles for a single trait, with each box standing for one trait.
Sometimes, you will get two alleles for a trait where one allele isn't dominent over the other or vice versa. That is a situation of incomplete dominance or codominance. In incomplete dominance, there is a blend of both traits, while in codominance, both traits show partially. Incomplete dominance is represented by a capital letter for one allele(F) and the same capital letter with an apostrophe for another allele(F'). Codominance is represented by two different capital letters(B)(G). You set up the punnett squares for these cases the same way and fill it in the same way. If the alleles are homozygous, there is no difference. Only if it is heterozygous does incomplete/codominance come into play.
We have taken the Punnett squares to the next level as di-hybrid and tri-hybrid. Di-hybrid Punnett squares show the probability of each distribution of TWO traits(four alleles) possible with the parents. If the mother had BB for eye color and Ss for skin color, her alleles in a di-hybrid Punnett square would look like TS Ts TS Ts on top of the 4 columns. Those are all of the possibilities when getting sets of two.You do the same for the father and set up your 16 square Punnett square. Each square contains the alleles for two traits. The probability of getting the desired two trais will be x/16.
The tri-hybrid show the probability of the distribution for three traits. If the mother also had Hh for hair color, you can add on to the di-hybrid method. For the mother, you get TSH, TSh, TsH, Tsh, TSH, TSh, TsH, Tsh as the allele combinations for each column. You do the same for the father and set up the 64-square punnett square, with each box representing three traits. There will be an x/64 chance for a desired result.
I found that if you want to go farther, you can multiply probabilities from mono-hybrid punnett squares. If you want the probability of having two parents heterozygous for brown eyes,
black hair, A blood type(over O), and six fingers produce a baby with blue eyes, blonde hair, O blood type, and five fingers, multiply individual probabilities.
1/4 * 1/4 * 1/4* 1/4 = 1/256 = .390625%
That is the probability of getting that kind of baby.
A shortcut I found is that if you have homozygous alleles, only one needs to be represented. If mom had TT and dad had Tt, you only need to set up one column(the second will be the same).
This works for di- and tri- hybrid punnett squares.
All of this I learned or discovered in class. I feel really interested in genetics. I find I work well with punnett squares.
p.s. I couldn't include punnett squares for the example problems because of the limitations of blogger(it kept moving the Punnett squares). For example:
R r
R RR Rr
r Rr rr
should have been a Punnett square.
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