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SEX LINKED INHERITANCE
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v2/04
Prerequisite: Inheritance of Sex, understanding of homologous chromosomes and karyotypes
Reference: CMR p. 178-179
Introduction
Your previous study has taught you that each chromosome in an autosome pair carries alleles for
the same traits. For instance, located on one particular position on a chromosome may be an
allele for right handedness. In the same position on the other (homologous) chromosome of that
pair is an allele for which hand to use. The allele may be for right handedness or left handedness.
loci (locations) for alleles governing which hand you use (on two homologous chromosomes)
However, you already know that sex chromosomes are not always identical. We will concentrate
on sex-linked characteristics in humans and fruit flies, for which the male sex chromosomes are
symbolized XY and the female XX.
Why do we use this notation? Remember that the Y chromosome is considerably smaller than
the X chromosome. In males, the Y chromosome does have genetic effects, but for many genes
carried by the X chromosome, the Y chromosome is considered to be inert. That is, it apparently
does not have these genes, and the X chromosome gene is left to control the expression of the
trait.
Since they physically consist of genes for different types of traits, the X and Y chromosomes
cannot truly be called homologous (they are only partially so). Additionally, the presence of a
gene for a trait only on the X chromosome has implications for it’s pattern of inheritance.
Traits for which the gene is located on a sex chomosome are said to be sex-linked traits. All
the traits we have considered previously were located on autosomes, and thus are considered to
be autosomal traits. Because the pattern of inheritance is different for sex-linked traits, we use a
different notation to represent the alleles. Go on to the next page to see what this looks like…
Note: In this reading, all of the sex-linked traits which we will be considering are on the X
chromsome, and most/all of them are recessive. (Y-linked traits and dominant X-linked traits
exist, but they are not very common or well documented. Why might this be? Consider the role
of evolution in this… Consider what types of traits geneticists/doctors tend to notice and study.)
Genetics
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Sex-Linked Inheritance
Problems in sex-linked inheritance
In humans, red-green color blindness is about 8 to 10 times more common in men than women.
This is due to the fact that the trait is sex-linked; that is, the Y chromosome carries no gene for
red-green vision, only the X chromosome does. On the X chromosome, the recessive allele is for
color blindness, the dominant allele is for normal color vision.
So how do we represent this in genetic notation? We must simultaneously keep track of two
phenotypes: Gender (male or female) and Vision (colorblind or normal). This is done by using
X and Y to track gender and by adding a superscript to the X chromosomes to type them as to
what vision allele they carry, normal or colorblind. For example:
XB = an X chromosome with the allele for normal vision (dominant trait)
Xb = an X chromosome with the allele for colorblindness (recessive trait)
Y
= a Y chromosome (note there is no “B” or “b” here since this chromosome does not
carry a vision gene at all)
Using this notation, match the following genotypes with their corresponding phenotype:
1. XBXB
a. colorblind male
2. XbXb
b. normal vision female who is a carrier of the colorblindness allele
3. XBXb
c. normal vision female (not a carrier)
4. XBY
d. colorblind female
b
5. X Y
e. normal vision male
(check your answers at bottom of page)
Now, is the XBXb woman colorblind herself? Why or why not?
No, because she also has an allele for normal color vision, symbolized here as B. This normal
vision allele is dominant. She is called a carrier of the color-blindness trait.
Note that a woman who was homozygous for colorblindness (XbXb) would not be called a
carrier because she is colorblind herself and is thus an affected individual. Similarly, males
who are XbY are colorblind and thus affected individuals.
Answers: 1. c
Genetics
Page 2 of 4
2. d
3. b
4. e
5. a
Sex-Linked Inheritance
Sample Problems
1. Show the expected children in a marriage of a colorblind man to a woman who is
homozygous for normal color vision.
genotype of man: XbY
gametes of man: Xb and Y (50% probability of each case)
genotype of woman: XBXB
gametes of woman: all XB
Punnett square:
XB
Fill in the bottom row!
Xb
Y
XB Xb
XB Y
XB
a. Will the sons of this couple be color blind? Why or why not?
b. Will the daughters of this couple be color blind? Why or why not?
Answers:
a. No, These sons would have normal vision as they receive a normal allele from their mother.
b. No, these daughters will have normal vision. Their genotype XBXB indicates that they have
one dominant, normal color-vision allele.
2. What are the offspring expected from the marriage of a colorblind man to a woman who is a
carrier for colorblindness but has normal vision? (set this one up on your own)
Key:
b. What percent of the female offspring are carriers?
Genetics
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Sex-Linked Inheritance
Sample Problem #2 Answers:
a. Key: colorblind = Xb
normal vision = XB
colorblind man XBY
Punnett square:
B
X
Xb
F1 Phenotypes:
50% males with normal vision
50% males colorblind
x
XBXb carrier woman
Xb
XB Xb
Xb Xb
Y
XB Y
Xb Y
50% females with normal vision
50% females colorblind
b. 50% of the females are carriers of the gene for colorblindness
(note: 25% of the offspring are carriers – males with the trait would be colorblind.)
Genetics
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Sex-Linked Inheritance