Sex-Linked Inheritance Lab
Background
The relationship between genotype and phenotype in sex-linked genes differs from
that in autosomal genes. A female must have two recessive alleles of a sex-linked
gene to express a recessive sex-linked trait. Just one recessive allele is needed for
the same trait to be expressed in a male.
Purpose
To model the inheritance pattern of sex-linked
genes based on probability
Materials
Coins (2) – prelabeled
Genetic cross (given by teacher)
Procedure
1) One coin represents the egg cell and the other coin represents the sperm cell. In
the Data/Results section of your lab, record the genetic cross you were given.
2) Fill in the Punnett square for the given genetic cross.
3) Determine the expected genotypic and phenotypic ratios.
Use the key: A = does not have trait
a = has the trait
4) Flip the two coins and record the genotype of the “offspring” in the count (tally
marks) row of the data table under the appropriate given possible genotype.
5) Repeat step 4 until you have modeled 50 genetic crosses.
6) Calculate the total number of each genotype and the % of each type in your data
table.
Data/Results
Key: A = does not have trait
a = has the trait
Genetic Cross: ____________ X _____________
Expected Genotypic Ratio
Expected Phenotypic Ratio
Possibilities
Count
(Tally Marks)
Total
Percent
Discussion
1) Do all of the females from the genetic cross show the recessive trait? Do all of the
males show the recessive trait? Why or why not?
2) Do the results from your Punnett square agree with those from your experiment?
Why or why not?