Sex Linked Inheritance
Background: The relationship between genotype and phenotype in sexlinked 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. In this lab, you will model the inheritance pattern
of sex linked genes.
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Materials:
 2 coins
 Masking tape


Marker
Index card with genetic cross
Procedures:
1. Use the tape and marker to label two coins with the genetic cross shown on your index card.
2. Determine which person will flip for “Mom” (representing the egg cell) and who will flip for
“Dad” (representing the sperm cell).
3. Continue to flip coins and record the genotype of the offspring until you have modeled 50 genetic
crosses. Make a data table to record each genetic cross that you model.
4. Calculate the genotype and phenotype probabilities for both males and females. Calculate the
frequency of male offspring and female offspring.
Sex
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Genotype
Sex
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
Genotype
Analysis:
1. Do all of the females from the genetic crosses show the recessive trait? Do all of the males show
the recessive trait? Why or why not?
2. Make a Punnett square that shows the genetic cross. Do the results from your Punnett square agree
with those from your experiment? Why or why not?
Teacher Notes
1. Each group should model a different monohybrid cross, but be sure all possibilities are covered.
2. Explain the notation used for sex linked genes. A sex chromosome is shown as a capital X or Y.
The allele carried by that sex chromosome is shown as a superscript.
3. Students must record both the sex and the genotype of the offspring
Expected Results: After 50 tosses, calculated frequencies should approach those listed below:
XAXa  XAY:
25% normal females, 25% carrier females, 25% recessive males, 25% normal males
XAXA  XaY:
50% carrier females, 50% normal males
XaXa  XAY:
50% carrier females, 50% recessive males
XAXa  XaY:
25% recessive females, 25% carrier females, 25% recessive males, 25% normal males
XaXa  XaY:
50% recessive females, 50% recessive males
XAXA  XAY:
50% normal females, 50% normal males
Analysis Answers:
1. Answers will vary depending on the cross, but should follow the expected results above. Females
will be recessive only if they inherit two recessive alleles, males will be recessive if they inherit
one recessive allele.
2. The experimental results should be close to those predicted by the Punnett squares because the
same genetic cross was used.