SEX-LINKED INHERITANCE AND COLOR VISION DEFICIENCY
Overview:
In humans, gender is determined by the sex chromosomes, X and Y. Men normally have an X
and a Y combination of sex chromosomes, while women have two X's. Traits controlled by
genes located on the sex chromosomes are called sex-linked traits. The presence of X and Y
chromosomes lead us to wander “What happens with genes that are located on the X
chromosome or on the Y chromosome when thinking about inheritance?" Normal inheritance
(such as the presence of dimples) occurs for genes on the other chromosomes (called
autosomes). In this type of inheritance phenotypes are based upon a dominant and recessive
relationship. But what if genes are on the sex chromosomes?
Since only men inherit Y chromosomes, they are the only ones to inherit Y-linked traits. When
talking about these traits, we do not think of them in terms of dominant and recessive since
there is only one Y chromosome and therefore no dominant/recessive interactions can occur.
Basically, whichever form of the gene a man gets on his Y chromosome, is the one he has, good
or bad. For example, there is a gene on the Y chromosome that regulates the "hair-growth
program" specifically in ears. Most alleles of this gene result in little or no ear hair. Some alleles,
however, activate the hair growth program, resulting in hairy ears. Because the Y chromosome
also carries the gene that determines maleness, the hairy-ear phenotype rarely, if ever, occurs
in women.
On the other hand, both men and women can have X linked traits since both men and women
have an X chromosome. For each of the genes that are exclusively on the X chromosomes,
females, who are XX, would obviously have two alleles. Males, who are XY, would have only one
allele. Thus females with one recessive allele and one dominant allele, for a gene that is unique
to the X chromosome, will always display the dominant phenotype. However, like with the Y
chromosome, a male with a recessive allele for a gene unique to the X chromosome will always
exhibit that recessive trait because there is no other corresponding allele on the Y
chromosome. For example, red-green color blindness or color vision deficiency alters our
ability to perceive certain colors and is a recessive sex-linked trait with the gene being located
on the X chromosome. A male needs only one recessive sex-linked gene to express color
blindness. A female must have two recessive genes to show the trait.
There are many different types of colorblindness, ranging from seeing in black and white to not
seeing one particular hue in the spectrum. Some forms of color blindness are more common in
the population than others but overall, it is estimated that 10 – 12 % of the population has
some type of color blindness. What is colorblindness and how does colorblindness affect ones
daily life? In this activity, students will learn the inheritance of sex linked traits, test themselves
for colorblindness and gain a deeper understanding of color blindness.
Objectives: Students will
 predict the offspring of a genetic cross using a Punnett square

recognize sex-linked genes

learn the genetic basis for colorblindness

use Punnett Squares to determine probabilities for a genotypes and phenotypes
Materials:
 Two pennies/group
 Masking tape
 Pen
Part One – Genetics of Color Blindness
1. What is the genetic difference between men and women?"
2. What is women’s genotype for the X chromosome?
3. Sex-linked genes are genes found on either the X chromosome or the Y chromosome,
men have one X and one Y, what do you think this means for men's alleles?
Directions:
1. Prepare the pennies.
Select one of the pennies to represent the male parent and place a piece of masking tape
on both sides of that penny. On one side of this penny write Xb. On the other side write
Y. This is Dan, and he is color-blind.
Place masking tape on each side of the second penny. On one side of the penny write Xb
and on the other side write XB. This is Cindy, and she has normal vision but carries the
gene, “b”, for color blindness.
2. Use the following scenarios to complete the activity. Write your results on notebook paper.
Dan, who knows he is color blind, would like to start a family with Cindy. Cindy is not
colorblind but knows that she is heterozygous for the trait. She knows this because her
father was color blind which means he had an Xb chromosome and he had to have passed
his Xb chromosome to her (it was the only one he had). Because she is not color blind, she
knows she had to have received an XB chromosome from her mother.
Dan and Cindy have their first child. To determine his alleles for the gene that can give rise
to color blindness, toss the two pennies and allow them to land on the table. The
combination of traits shown on the coins represents the traits of their first child. On your
paper, write the genotype represented by the pennies, determine if the first child a boy or
girl and if the child was color blind.
Flip the coins seven more times and record your results.
Coin Toss Coin 1
Coin 2
Genotype
1
2
3
4
5
6
7
8
Phenotype
3. After you have finished finding the genotypes and phenotypes of the eight children, answer
the following questions:
A. How many boys and girls did Cindy and Dan have?
B. How many of the children were color-blind, and how many had normal vision?
C. Do you think color blindness is more common in boys or girls?
4. Complete a Punnett Square based on Cindy and Dan’s genes for color vision. Determine how
many of the girls and how many of the boys would you expect to be color-blind?
5. Using Punnett Squares, answer the following questions.
A. Can two “normal” color seeing parents produce a color-blind son?
B. Can a normal daughter have a color-blind father?
C. Can two color-blind parents produce a normal son?
D. Can a color-blind daughter have a normal father?