Sex-Linked Inheritance 
Because the X and Y chromosomes determine sex, the genes located on them show a pattern of inheritance called sex-linkage. A sex-linked gene is a gene located on a sex chromosome. As you might expect, genes on the Y chromosome are found only in males and are passed directly from father to son. Genes located on the X chromosome are found in both sexes, but the fact that men have just one X chromosome leads to some interesting consequences.
For example, humans have three genes responsible for color vision, all located on the X chromosome. In males, a defective allele for any of these genes results in colorblindness, an inability to distinguish certain colors. The most common form, red-green colorblindness, occurs in about 1 in 12 males. Among females, however, colorblindness affects only about 1 in 200. Why is there such a difference? In order for a recessive allele, like colorblindness, to be expressed in females, it must be present in two copies—one on each of the X chromosomes. This means that the recessive phenotype of a sex-linked genetic disorder tends to be much more common among males than among females.
MYSTERY CLUE
The presence of two sickle cell alleles is needed to produce sickle cell disease. Males and females develop sickle cell disease in equal frequencies. What do these statements suggest about the location of the gene responsible for the disorder?
Quick Lab
GUIDED INQUIRY
How Is Colorblindness Transmitted?
1 Make a data table with the column headings Trial, Colors, Sex of Individual, and Number of X-Linked Alleles. Draw ten rows under the headings and fill in the numbers 1 through 10 in the Trial column. Label one plastic cup Mother and a second plastic cup Father.
2 The white beans represent X chromosomes. Use a black marker to make a dot on 1 white bean to represent the X-linked allele for colorblindness. Place this bean, plus 1 unmarked white bean, into the cup labeled Mother.
3 Mark a black dot on 1 more white bean. Place this bean, plus 1 red bean, into the cup labeled Father. The red bean represents a Y chromosome.
4 Close your eyes and pick one bean from each cup to represent how each parent contributes to a sex chromosome and a fertilized egg.
5 In your data table, record the color of each bean and the sex of an individual who would carry this pair of sex chromosomes. Also record how many X-linked alleles the individual has. Put the beans back in the cups they came from.
6 Determine whether the individual would have colorblindness.
7 Repeat steps 4 to 6 for a total of 10 pairs of beans.
Draw Conclusions How do human sex chromo-somes keep the numbers of males and females roughly equal?
Calculate Calculate the class totals for each data column. How many females were colorblind? How many males? Explain these results.
Use Models Evaluate your model. How accurately does it represent the transmission of colorblindness in a population? Why?
Analyze and Conclude
Table of Contents
- Formulas and Equations
- Applying Formulas and Equations
- Mean, Median, and Mode
- Estimation
- Using Measurements in Calculations
- Effects of Measurement Errors
- Accuracy
- Precision
- Comparing Accuracy and Precision
- Significant Figures
- Calculating With Significant Figures
- Scientific Notation
- Calculating With Scientific Notation
- Dimensional Analysis
- Applying Dimensional Analysis