You can see from the Punnett square that three of the four gamete combinations will contain at least one dominant allele (T) and that there is only one chance out of four that the recessive (t) can be expressed. Mendel's experimental results fit the phenotypic probability ratio of 3:1. The genotypic ratio, which Mendel didn't know about, is not 3:1, but 1:2:1. That is, 1 homozygous dominant (TT):2 heterozygous dominants (Tt):1 homozygous recessive (tt). The Punnett square shows only the possible combinations, not the actual. It provides an easy way to visualize the probabilities of a certain combination occurring. In some inherited traits, whether the allele comes from the male or the female parent can make a difference, but in most traits such information does not matter. After making monohybrid crosses for all the traits and finding that the ratios always approximated
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This note was uploaded on 11/14/2011 for the course BIO 1421 taught by Professor Farr during the Fall '08 term at Texas State.