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BIO 2
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Sex- Linked Traits
3323
Sex-linked traits are carried on the sex chromosomes.
Because males have an X and a Y chromosome instead of 2 X chromosomes,
they are more likely to inherit recessive traits.
Sex-Linked Traits
3323
Upon successful completion of this unit, you should be able to do the following:
1. Explain how sex-linked traits are passed from parents to offspring.
2. Explain how the sex chromosomes, XX and XY, are involved with sex-linked
traits.
3. Explain the genotype that creates females, and the genotype that creates
males.
4. List several human maladies that are caused by sex-linked genes.
5. Define each of the following words:
trait
sex-linked
recessive
genotype
chromosome
phenotype
chromosome pair
dominant
gene
allele
sex chromosome
X chromosome
Y chromosome
XX
XY
3.3.2.3 Sex-Linked Traits
Sex-linked traits are easy to remember. Sex-linked traits are the traits
that are carried on the sex chromosomes, either the “X” or the “Y”
chromosome. This affects how they are passed from parent to offspring. Why is
it that recessive genotypes are more often expressed in the male offspring
than in the female? Answer: there are 23 pairs of chromosomes in each human
cell. The 23rd pair is called the sex chromosome. Females have two X
chromosomes. Males have an X chromosome and a Y chromosome.
REMEMBER THIS!!!
The male sex genotype is XY, the female sex genotype is XX.
Question 1. If an organism has a genotype of XX, is it a male or female?
There are many genes located on the 23rd chromosome pair. Traits that
are produced by these genes are called sex-linked traits. Some genes that are on
the X are missing from the Y because the Y chromosome is shorter than the X
chromosome.
Question 2. Why are some traits found on the “X” chromosome and not on the “Y”.
Predict what effect this might have on the offspring.
For example, the gene for normal color vision is on the X chromosome but it
is not on the Y chromosome. Therefore, since males (XY) only have one X
chromosome, they only receive one gene for normal vision or for colorblind vision.
Therefore, only one gene is needed to determine a trait. As you recall, normally it
takes two genes to control a trait – one from the male and one from the female. In
sex-linked traits, sometimes only one gene is present. When that is the case,
that gene determines the trait.
For example, remember colorblindness is a recessive gene located on the “X”
chromosome. Since all females are XX and all males are XY, then the mother
contributes the X chromosome to the male offspring and the father contributes
the Y chromosome to the offspring. Therefore, it only takes one recessive gene
from the mother (the father contributes the “Y” gene) for the trait to be
expressed. Colorblindness is more common in males because males only need to
receive one recessive allele for colorblindness to inherit the trait. Females would
need to receive two, since they have two “X” chromosomes (one from the mother
and one from the father).
REMEMBER THIS!!!
If the mother has the trait, so will all of her sons. That is why it is called
sex-linked.
Question 3. If the mother is colorblind, will all of her daughter also have to be
colorblind?
The genotype for sex-linked traits is written with the symbols for the 23rd
pair or sex chromosomes, either XX or XY. The gene that is sex-lined is written
as a superscript to each chromosome. If the gene for normal color vision is
represent by N, then the gene for colorblind vision is represented by n.
A normal male’s genotype would be XNY
A colorblind male’s genotype would be XnY.
No letter is written by the Y chromosome because no gene is on the Y
chromosome. Remember the Y chromosome is too short to carry all of the genes
that are found on the X chromosome.
Since females have two X chromosomes, they have two genes for normal or
colorblind vision.
A normal female’s genotype would be XNXN or XNXn.
A colorblind female’s genotype would be XnXn.
REMEMBEER THIS !!!
Because males (XY) only have to receive one recessive allele, and females (XX)
have to receive two, colorblindness is more common in males than in females.
Females (XX) have to inherit the recessive gene from both the mother and
father.
Males (XY) only need to inherit the recessive gene from the mother.
Interesting Scientific Fact: Other sex-linked traits include hemophilia and
muscular dystrophy. They are both recessive, sex-linked traits that occur more
often in males than in females.
Question 4. If the father is colorblind, what are the chances that his daughters
will also be colorblind?
Question 5. If the father is colorblind, what are the chances that his sons will
also be colorblind?
TEST YOURSELF
Matching
_____1. sex-linked trait
a. XX
_____2. sex chromosome
b. XY
_____3. XNXN or XNXn
c. the 23rd pair of human
chromosomes
_____4. XnXn
d. genotype of a colorblind male
_____ 5. XNY
e. genotype of a colorblind female
_____ 6. XnY
f. a trait which is produced by a gene
on the X chromosome
_____ 7. the female sex chromosomes
g. genotype of a male with normal
vision
_____ 8. the male sex chromosomes
h. genotype of a female with normal
vision
True or False
_____ 1. The 23rd pair of chromosomes in a human cell are called sex
chromosomes.
_____ 2. Females have two X chromosomes.
_____ 3. Males have two X chromosomes.
_____ 4. Females have two Y chromosomes.
_____ 5. Males have two Y chromosomes.
_____ 6. Females have one X and one Y chromosome.
_____ 7. Males have one X and one Y chromosome.
_____ 8. Sex-linked traits are produced by genes that are on the X chromosome
but not on the Y chromosome.
_____ 9. Sex-linked traits are produced by genes that are on the Y chromosome
but not on the X chromosome.
_____ 10. There are more genes on the X chromosome than on the Y chromosome
because the X chromosome is longer.
_____ 11. The gene for normal vision or colorblind vision is on the X chromosome.
_____ 12. The gene for normal vision or colorblind vision is on the Y chromosome.
_____ 13. A normal male’s genotype would be XNY.
_____ 14. A normal male’s genotype would be XnY.
_____ 15. A colorblind male’s genotype would be XNY.
_____ 16. A colorblind male’s genotype would be XnY.
_____ 17. A normal female’s genotype would be XNXN or XNXn.
_____ 18. A normal female’s genotype would be XnXn.
_____ 19. A colorblind female’s genotype would be XNXN or XNXn.
_____ 20. A colorblind female’s genotype would be XnXn.
Fill in the Blank
sex chromosomes
sex-linked trait
muscular dystrophy
XnXn
females
XX
colorblindness
males
XNY
XY
hemophilia
XNXN or XNXn
XnY
1. Three examples of sex-linked traits are ____________________,
___________________________, and ________________________.
2. A __________________ is a trait that is produced by genes on the X
chromosome.
3. The 23rd pair of chromosomes, called the ________________________ , are
different in males and females.
4. The sex chromosomes in females are ___________.
5. The sex chromosomes in males are ____________.
6. If the gene for normal color vision is N, and the gene for colorblindness is n,
then the genotype for a colorblind male is ________.
7. The genotype for a colorblind female is _________.
8. The genotype for a male with normal color vision is _________.
9. The genotype for a female with normal color vision is ___________ .
10. _____________ are more likely than ______________ to inherit
colorblindness.
Answer the Following:
1.
What sex chromosomes do females have?
2.
What sex chromosomes do males have?
3.
What are sex-linked traits?
4.
What are three examples of sex-linked traits?
5.
How are the genotypes of sex-linked traits written? Write the genotypes
of a colorblind female. _________ a colorblind male. ______ a female with
normal vision _________ a male with normal vision _____.
6.
Are males or females more likely to inherit sex-linked traits? ___________
Why?