Goal 4 - Sex-Linked Traits
In humans, one pair of chromosomes (the 23rd pair) determines the gender of the individual.
The X chromosome is larger and carries more genes than the Y chromosome (See diagram above.).
- These 2 chromosomes are known as the sex chromosomes.
The X chromosome is larger and carries more genes than the Y chromosome (See diagram above.).
- If two X chromosomes (XX) are present in the embryo, it generally develops into a female.
- If one X and Y (XY) are present in the embryo, it generally develops into a male.
Males determine the sex of the offspring as they can create either an X sperm or a Y sperm.
Females can only create X eggs.
- If a Y sperm fertilizes an egg, the embryo becomes a male. If an X sperm fertilizes an egg, the embryo becomes a female.
Females can only create X eggs.
Sex-Linked Traits
Sex-Linked Traits
If a gene is found only on the X chromosome and not the Y chromosome, it is said to be a sex-linked trait. Because the gene controlling the trait is located on the sex chromosome, sex linkage is linked to the gender of the individual.
Usually such genes are found on the X chromosome.
The Y chromosome is thus missing such genes (See Diagram above.).
The result is that females will have two copies of the sex-linked gene while males will only have one copy of this gene.
If the gene is recessive, then males only need one such recessive gene to have a sex-linked trait rather than the customary two recessive genes for traits that are not sex-linked.
This is why males exhibit some traits more frequently than females.
Examples of Sex-linked Traits:
If a gene is found only on the X chromosome and not the Y chromosome, it is said to be a sex-linked trait. Because the gene controlling the trait is located on the sex chromosome, sex linkage is linked to the gender of the individual.
Usually such genes are found on the X chromosome.
The Y chromosome is thus missing such genes (See Diagram above.).
The result is that females will have two copies of the sex-linked gene while males will only have one copy of this gene.
If the gene is recessive, then males only need one such recessive gene to have a sex-linked trait rather than the customary two recessive genes for traits that are not sex-linked.
This is why males exhibit some traits more frequently than females.
Examples of Sex-linked Traits:
- Red-green colorblindness – Inability differentiate between read and green.
- Male Pattern Baldness
- Hemophilia – Causes the blood not to clot. If get a cut it may take a along time to clot or internal bleeding may result from a bruise.
- Duchenne Muscular Dystrophy – Muscular weakness, progressive deterioration of muscle tissue, and loss of coordination.
Red-Green Colorblindness
In humans, red-green colorblindness is a recessive sex-linked trait (c). It is found on the X chromosome, not the Y.
Because, males only have one X chromosome, they have a much greater chance of having red-green colorblindness.
Females would have to be homozygous recessive in order to have red-green colorblindness.
Colorblindness
Normal Mother XCXC
Normal Mother (carrier) XCXc
Colorblind Mother XcXc
Normal Dad XCY
Colorblind Dad XcY
Examples:
C = Normal vision
c = colorblindness
Because, males only have one X chromosome, they have a much greater chance of having red-green colorblindness.
Females would have to be homozygous recessive in order to have red-green colorblindness.
Colorblindness
Normal Mother XCXC
Normal Mother (carrier) XCXc
Colorblind Mother XcXc
Normal Dad XCY
Colorblind Dad XcY
Examples:
C = Normal vision
c = colorblindness
- If mom is a carrier for colorblindness and dad has normal vision can they have a colorblind child? If so how?
- If mom is normal vision female and dad is a normal vision male can they have a colorblind child? If not why?
- What are the genotypes of the parents that have a colorblind daughter?
Hemophilia
Hemophilia
In humans, hemophilia is a recessive sex-linked trait (h). It is found on the X chromosome, not the Y.
Hemophilia
Normal Mother XHXH
Normal Mother (carrier) XHXh
Hemophilia Mother XhXh
Normal Dad XHY
Hemophilia Dad XhY
Examples:
Mom is a normal carrier, Dad has hemophilia. How many girls have hemophilia? How many boys?
Mom = Dad= Offspring =
Mom has hemophilia and dad is a carrier? What is the number and sex of the children who have hemophilia? Are any of the children carriers of the disorder?
Mom = Dad= Offspring =
Mom is normal and homozygous for hemophilia, dad has hemophilia. How many of the children will have the disorder?
Mom = Dad= Offspring =
In humans, hemophilia is a recessive sex-linked trait (h). It is found on the X chromosome, not the Y.
- Because, males only have one X chromosome, they have a much greater chance of having hemophilia.
- Females would have to be homozygous recessive in order to have hemophilia.
Hemophilia
Normal Mother XHXH
Normal Mother (carrier) XHXh
Hemophilia Mother XhXh
Normal Dad XHY
Hemophilia Dad XhY
Examples:
Mom is a normal carrier, Dad has hemophilia. How many girls have hemophilia? How many boys?
Mom = Dad= Offspring =
Mom has hemophilia and dad is a carrier? What is the number and sex of the children who have hemophilia? Are any of the children carriers of the disorder?
Mom = Dad= Offspring =
Mom is normal and homozygous for hemophilia, dad has hemophilia. How many of the children will have the disorder?
Mom = Dad= Offspring =
Male-Pattern Baldness
Male Pattern Baldness
In humans, male pattern baldness is a recessive sex-linked trait (b). It is found on the X chromosome, not the Y.
Male Pattern Baldness
Normal Mother XBXB
Normal Mother (carrier) XBXb
Bald Mother XbXb
Normal Dad XBY
Bald Dad XbY
Mom is heterozygous for baldness, and dad is bald. What is the sex and the number of children who will be bald?
Mom = Dad= Offspring =
Mom is homozygous normal for baldness, dad is bald. What will their children look like?
Mom = Dad= Offspring =
In humans, male pattern baldness is a recessive sex-linked trait (b). It is found on the X chromosome, not the Y.
- Because, males only have one X chromosome, they have a much greater chance of having male pattern baldness.
- Females would have to be homozygous recessive in order to have male pattern baldness.
Male Pattern Baldness
Normal Mother XBXB
Normal Mother (carrier) XBXb
Bald Mother XbXb
Normal Dad XBY
Bald Dad XbY
Mom is heterozygous for baldness, and dad is bald. What is the sex and the number of children who will be bald?
Mom = Dad= Offspring =
Mom is homozygous normal for baldness, dad is bald. What will their children look like?
Mom = Dad= Offspring =