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 dominate 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 taints 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

Florida Sunshine State Standards:
SC.7.L.16.1: Understand and explain that every organism requires a set of instructions that specifies its traits, that this hereditary information (DNA) contains genes located in the chromosomes of each cell, and that heredity is the passage of these instructions from one generation to another.
SC.7.L.16.2: Determine the probabilities for genotype and phenotype combinations using
Punnett Squares and pedigrees.
Materials:
 Two pennies/group
 Masking tape
 Pen
Activity Directions:
Part One – Genetics of Color Blindness
1. Begin by introducing sex-linkage with the following questions.
"Can anyone tell me the genetic difference between men and women?"
Men have XY chromosomes and women have two X chromosomes
"What is women’s genotype for the X chromosome?"
Homozygous
"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?"
Men only have one allele for genes found on the X chromosome and the Y chromosome.

2. Inform students that they will be doing an activity to help them understand how a sex-linked trait, color blindness, can be inherited.
3. Before starting the activity, 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 X b
. 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 X b and on the other side write X
B
. This is Cindy, and she has normal vision but carries the gene, “b”, for color blindness.
4. Give students the following scenario and allow them time to complete it. Students should write 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 X b chromosome and he had to have passed his X b chromosome to her (it was the only one he had). Because she is not color blind, she knows she had to have received an X
B 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.
5. After students have finished finding the genotypes and phenotypes of the eight children, ask the following questions.
How many boys and girls did Cindy and Dan have?
Answers will vary. Usually the boy-girl ratio is close, but not always.
How many of the children were color-blind, and how many had normal vision?
Answers will vary. For a girl to be color-blind, she has to have X b
X b
. For a boy to be colorblind, he would have X b
Y.

Do you think color blindness is more common in boys or girls?
It is more common in boys. They have to inherit only one recessive gene, but girls have to inherit two recessive genes.
6. Have students complete a Punnett Square based on Cindy and Dan’s genes for color vision.
Ask them to determine how many of the girls and how many of the boys would you expect to be color-blind?
7. Using Punnett Squares, have students answer the following questions. a. Can two “normal” color seeing parents produce a color-blind son?
Yes, if the mom is a carrier, she has one X with the normal version of the gene and one X with the allele that gives rise to color vision deficiency. There is a 50/50 chance that any male child will get the X allele that gives rise to color deficiency. b. Can a normal daughter have a color-blind father?
Yes, as long as she gets an X with the normal version of the gene, then the allele she gets from her dad, the one that gives rise to color deficiency, will be “masked” by the normal version of the gene. c. Can two color-blind parents produce a normal son?
No, the X chromosome the son has comes from his mom. If both of her X chromosomes contain the allele for color deficiency, then there is a 100% chance that the son will be color blind. d. Can a color-blind daughter have a normal father?
No, if the daughter is color blind then she has 2 X chromosomes that carry the color deficiency allele. One comes from the mom and the other has to come from her father.