Biology :: Probability - Rule of Multiplication and Addition: Punnett Squares
One of the easiest ways to calculate the mathematical probability of inheriting a specific trait was invented by an early 20th century English geneticist named Reginald Punnett. His technique employs what we now call a Punnett square.
This is a simple graphical way of discovering all of the potential combinations of genotypes that can occur in children, given the genotypes of their parents. It also shows us the odds of each of the offspring genotypes occurring.Punnett Square shows the genotype two individuals can produce when crossed.
To draw a square, write all possible allele* combinations one parent can contribute to its gametes across the top of a box and all possible allele combinations from the other parent down the left side. The allele combinations along the top and sides become labels for rows and columns within the square.
Complete the genotypes in the square by filling it in with the alleles from each parent. Since all allele combinations are equally likely to occur, a Punnett Square predicts the probability of a cross producing each genotype.
Genotype :: the genes of an organism; for one specific trait we use two letters to represent the genotype. A capital letter represents the dominant form of a gene (allele), and a lowercase letter is the abbreviation for the recessive form of the gene (allele).
Phenotype :: the physical appearance of a trait in an organism
Note: Remember, we don't use "R" for red & "W" for white because that would make it two different genes which would code for two different traits, and color is one trait. What the genotype contains are two codes for the same trait, so we use two forms of the same letter (capital & lowercase).
One more note: A very very helpful thing to memeorize is that the ONLY way for a recessive trait to show up in an organism is if that organism's genotype is homozygous recessive (two little letters, like "rr").