Genetics, the study of heredity, began with peas. Gregor Mendel’s studies with pea plants showed that some factor moved characteristics such as color or smoothness from generation to generation in predictable patterns.

Although Mendel presented and published his studies, his work was ignored until a few years after his death. Once Mendel’s work was rediscovered and its value recognized, the study of genetics moved quickly forward.

Genetics studies the patterns of how traits pass from generation to generation. Inherited traits include hair color, eye color, height and blood type. Different versions of the same gene, such as blue eye color and brown eye color, are called alleles. One version or allele of a gene may be dominant over a different recessive allele, or the two alleles may be equal or codominant.

Alleles usually are represented by the same letter, but the dominant allele is capitalized. For example, brown eye alleles, all other factors being equal, are dominant over blue eye alleles. Blood type alleles are an exception to this standard practice.

Blood type A and Blood type B are codominant, so a person inheriting genes for A and for B blood types will have type AB blood. Blood type O is recessive to A and B, so a person inheriting a gene for blood type A and a gene for blood type O will have blood type A. If both alleles for a trait are the same version of the gene, the organism is homozygous for that trait.

If the alleles for a trait are different alleles, the organism is heterozygous for that trait. If the organism is heterozygous for a trait, usually one gene will be dominant over the other gene.

Genotype refers to the genetic combination of an organism. Phenotype refers to the physical expression of the genetic combination.

Punnett squares use a relatively simple grid format similar to a Tic-Tac-Toe board to predict the possible genetic make-up (genotype) and physical make-up (phenotype) of potential offspring. A simple Punnett square shows the cross of the genetic combination for a single trait.

The two genes for a trait from one parent are placed above the two right columns of the Punnett square with one gene above one column and the second gene above the other column. The two genes for the trait from the other parent will be placed on the left side of the Punnett square, one each for the bottom two rows of the Punnett square.

Like a multiplication or mileage chart, the symbol for the gene at the top of the column and the symbol for the gene at the left side of the row are copied into the intersecting square. This is one possible genotype for a potential offspring. In a simple Punnett square with only one trait, there will be four potential genetic combinations (two genes from each parent, so 2x2 or 4 possible outcomes).

For example, consider a Punnett square for the color of Mendel’s peas. A purebred (homozygous) green (y) pea crossed with a purebred yellow (Y) pea yields four possible combinations for color for the next generation of peas. It happens that each genetic outcome contains one gene for green peas and one gene for yellow peas. The genes are not for the same allele (same trait, different physical expression) so the genetic make-up for color in each potential offspring pea is heterozygous (Yy).

Online Punnett square genetic calculators can be used to find the genetic crosses of simple and complex Punnett squares. (See Resources)

Genotypes are the gene combination of potential offspring. In the pea plant example above, the genotype ratio of the cross of homozygous green (y) and homozygous yellow (Y) peas is 100 percent Yy.

All four squares contain the same heterozygous combination of Yy. The offspring will exhibit yellow color because yellow is dominant. But each of the offspring peas will carry genes for both green and yellow peas.

Suppose two heterozygous pea offspring are crossed. Each parent carries a gene for green (y) and a gene for yellow (Y). Place one parent’s genes along the top of the Punnett square and the other parent’s genes along the left side. Copy the genes down the columns and across the rows.

Each of the four squares now shows a possible genotype combination. One square shows a homozygous yellow (YY) combination. Two squares show a heterozygous green-yellow combination (Yy). One square shows a homozygous yellow (YY) combination.

In a simple Punnett square with only one trait, there are four possible gene combinations. In the pea example, the probability of homozygous green peas is 1:4 because only one of the four squares contain the yy genotype. The probability of heterozygous green-yellow genotype is 2:4 because two of the four squares have the Yy genotype.

The probability of yellow peas is 1:4 because only one of the four squares has the YY genotype. The genotype ratio is therefore 1 YY:2Yy:1yy, or 3Y_:1y. The phenotype ratio is three yellow peas:one green pea.

A dihybrid Punnett square shows the possible crosses of two traits at the same time. Each trait still only has two possible genes, so the dihybrid Punnett square will be a grid with four rows and four columns and sixteen possible outcomes. Again, count the number of each gene combination.

Consider a dihybrid cross of two people who are heterozygous brown hair (H) with recessive blond hair (h) with brown eyes (E) with recessive blue eyes (e). Both parent phenotypes would be brown hair and brown eyes. The dihybrid cross, however, shows possible genotypes HHEE, HhEE, hhEE, HHEe, HhEe, HHee, Hhee, hhEE and hhee.

The genotype ratio is 1 HHEE:2 HhEE:1 hhEE:2 HHEe:4 HhEe: 2 Hhee:1 HHee:2 hhEe:1 hhee, which can also be written as 9 H_E_:3 h_E_:3 H_e_:1 h_e_. The phenotype ratio shows that these heterozygous parents have one chance in sixteen of having a blond haired, blue eyed child.