Heredity is the passing of traits from parent to offspring. Traits are controlled by genes.
The different forms a gene may have for a trait are called alleles. There are two alleles for every
trait. For example, Mendel's pea plants had two alleles for flower color—an allele for purple and an
allele for white. Mendel crossed a purple-flowered pea plant with a white-flowered pea plant. All
the offspring of this cross had purple flowers. Mendel determined that the purple flower allele was
dominant because it covered up the white flower allele, which he called recessive.
English biologist Reginald Punnett developed a simple method for predicting the ways in
which alleles can combine. It is called a Punnett square. In a Punnett square, dominant and
recessive alleles are represented by letters. An uppercase letter represents a dominant allele, and a
lowercase letter represents a recessive allele. Each cell in an organism's body contains two alleles
for every trait. One allele is inherited from the female parent and one allele is inherited from the
male parent.
An organism is homozygous if it has identical alleles for a particular trait. An organism is
heterozygous if it has nonidentical alleles for a particular trait. There are three possible
combinations of alleles of an organism for a particular trait: homozygous dominant (PP),
heterozygous (Pp), and homozygous recessive (pp).
If only one trait is being considered in a genetic cross, the cross is called monohybrid. A
Punnett square for a monohybrid cross is two boxes tall and two boxes wide because each parent
can pass on two kinds of alleles. For example, a heterozygous parent (Pp) can pass on the (P) allele
or the (p) allele. The first box of the Punnett square is labeled (P) and the second box in the Punnett
square is labeled (p).
If two traits are being considered in a genetic cross, the cross is called dihybrid. A Punnett
square for a dihybrid cross is four boxes tall and four boxes wide because each parent can pass on
four possible combinations of alleles. For example, a heterozygous parent (PpTt) can pass on the
following combinations of alleles: (PT), (Pt), (pT), (pt). Each box of the Punnett square is labeled
with one of these possible combinations of alleles.
Alleles represent a genotype, or the genetic makeup of an organism for a trait. A phenotype
is a physical trait that shows as a result of an organism's particular genotype. Organisms may look
alike but have different genotypes. It is impossible to determine homozygous dominant and
heterozygous genotypes by sight.
Common fruit flies, Drosophila melanogaster, are often used to study genetics because of
their short life cycle (about 2 weeks), their small size (several hundred can be housed in a small jar),
and their high rate of reproduction (a single pair may produce more than a hundred offspring).
When using Punnett squares to analyze the results of genetic crosses involving body color in
Drosophila, “G” represents the dominant allele for gray body color and “g” represents the recessive
allele for black body color. When using Punnett squares to analyze the results of genetic crosses
involving wing length in Drosophila, “L” represents long wing length and “l” represents vestigial
wing length. (Vestigial wings are wings with reduced function that may have been useful to an
earlier evolutionary stage of Drosophila.)
Name ________________________________________ Period _________
How can Punnett squares help predict the traits of offspring?
Questions:
1. For one of the monohybrid crosses you performed in this Investigation, describe how to
use the phenotype ratios to determine the percentage of offspring displaying each trait.
2. Can the genotype for a gray-bodied fly be determined? Why or why not? Describe all of
the possible genotypes for a fly with that phenotype.
3. Explain why an organism with a homozygous dominant genotype has the same
phenotype as an organism with a heterozygous genotype.
4. What genetic information can be obtained from a Punnett square? What genetic
information cannot be determined from a Punnett square?
Scenario
Number
1
2
3
4
5
6
7
8
9
10
Genotype of
Parent 1
Genotype of
Parent 2
Ratio of Offspring
Genotype
Ratio of Offspring
Phenotype