Sheryl A. DeCaprio
The following is a simple example of dominant and recessive traits. Assume that loving math is passed on genetically and is a dominant trait. The letter ‘M’ will indicate loving , a dominant trait, while a ,m, will indicate hating math, a recessive trait. The following pairings are possible to describe the genotype of an individual.
MM . . . . . . .a math loving individual
mm . . . . . . .a math hating individual
Mm . . . . . . .a math loving individual
By constructing a table called a Punnet square, we can calculate the possible combinations of alleles and determine the possible genotype of the offspring. Punnet squares work much the same way as a multiplication chart and should be easy for the students to understand. The symbol represents the father and the represents the mother. Listed vertically on the left is the genotype of the mother and horizontally at the top is the genotype of the father. The inner squares are then filled in by finding the “cross-product of the two alleles. See Figure I.
Figure I
Father- mm . . . . . . . . . .hates math
Mother- MM . . . . . . . . . .loves math
All of the offspring will be math Lovers.
(figure available in print form)
By crossing each vertical entry with each horizontal entry, the above parings occur. As we can see, 100% of the offspring will love math because the dominant allele M appears in each pairing. Figure II describes the possible outcomes if both father and mother are genotypically Mm.
Figure II
Father—Mm . . . . . . .loves math
Mother—Mm . . . . . . loves math
The ratio of math lovers to math haters is 3:1.
(figure available in print form)
This pairing yields the probability of 75% of the offspring being math lovers as three out of four pairings contain the dominant allele. Students can find the ratios and percentages for all the possible combinations of parents and their possible offspring. (See sample lesson)
Individuals whose alleles are identical. MM or mm, are called
homozygous
, and those whose alleles are different, Mm, are called
heterozygous
. Below is an example of the pairing of a homozygous recessive father (mm, math hater) and a heterozygous dominant mother (Mm math lover).
Figure III
There is a 1:1 ratio of math lovers to math haters.
(figure available in print form)
Offspring of these parents will generally separate into two equal groups of math haters and math lovers. Students should review the outcomes of these standard pairings. This information can be used when studying family pedigree charts, charts used to follow the transference of a genetic trait through a family.
Students can be instructed to develop Punnet squares for various combinations of heterozygous and homozygous parents. Examples can be of a scientific nature. Examples of dominant traits are 1) widow’s peak 2) unattached earlobes 3) the ability to curl one’s tongue, while some recessive traits are 1) albinism 2) attached earlobes and 3) sickle cell anemia.
The next section will describe how we can use this knowledge of genetic transference to study families and trace inherited traits through a family tree by means of a pedigree chart.