Sex-Linked Traits — Definition & Examples

Sex-Linked Traits

In sexually reproductive organisms, there are two chromosomes called the sex chromosomes.

In humans, these chromosomes are the X and Y chromosomes. When a person has two X chromosomes they are female (XX), and when they have an X and a Y chromosome (XY) they are male. These chromosomes carry genes that produce most of the differences between males and females.

A trait that is only present on the X or Y chromosome is called a sex-linked trait. Most of these traits are on the X chromosome.

Because females have two X chromosomes and most sex-linked traits are on the X chromosome, they will express these traits following the same rules as recessive/dominant alleles. This means that if a female inherits one allele from her mom and another from her dad, she will express the alleles depending on which is dominant.

However, because males only have one X chromosome, they will have to express the genes present on their X chromosome. They do not get the option to express another allele because they only have one copy. This results in a higher chance for males to express negative sex-linked traits.

Almost all negative alleles (those that cause harm to the individual), such as hemophilia, which makes a person's blood very thin, are recessive alleles. They are not expressed if the "healthy" allele is present. However, because males only have a single X chromosome, they are much more likely to express the negative allele.

Above is a Punnett square for sex-linked genes. The mother's alleles are on the top. She is heterozygous for the sex-linked trait (A). The father, listed on the left of the square, has the dominant allele, but the gene is not present on the Y chromosome. The four possibilities for the children are two female and two male. Both of the females would express the dominant allele. The capital A phenotype overrides the lowercase a phenotype. The one son on the bottom left would also express the dominant allele. The son on the bottom right, however, would express the recessive allele.

If a female expresses the recessive allele, she must have gotten it from both her mother and her father. If a son expresses a recessive allele, he always got it from his mother. He always receives a Y chromosome from his father.

Traits Carried on Sex Chromosomes

Sex-linked traits in genetics are traits that have alleles on an organism's sex chromosomes. In humans, these chromosomes are called the X and Y chromosomes. Males will have an X and a Y chromosome, and a female will have two X chromosomes.

Since these alleles are located on sex chromosomes, sex-linked traits are very different in males and females. Most of these traits are on the X chromosome, simply because the X chromosome is bigger and has more genes on it.

When talking about the inheritance of these traits, we still need to consider recessive and dominant alleles. If a sex-linked disease with its allele on the X chromosome is recessive, the following genotypes would actually have the disease:

Female: X$^{a}$X$^{a}$

Male: X$^{a}$

Since the disease is sex-linked recessive, a female with two copies of the allele would have the disease. But, since males only have one copy of the X sex chromosome, only one copy of the allele is necessary. With only one X chromosome, men don't have a dominant allele to overshadow the recessive one.

For the same disease, the following genotypes would not be affected:

Female: X$^{A}$X$^{A}$, X$^{A}$X$^{a}$

Male: X$^{A}$Y

Since there is one (or more) copy of the dominant allele on an X chromosome, the disease is not present.

With sex-linked traits that are coded for on the X chromosome, a disease can only be passed down from mother to son, not from father to son. A father will always give a Y chromosome to his son making it impossible for the disease to be spread from father to son. This also means that if a mother has the disease, her son will automatically have the disease.

What Are Sex-Linked Traits?

Sex-linked traits are exactly what they sound like; they are traits caused by alleles located on a sex chromosome.

Human men have XY chromosomes and women have XX chromosomes, so traits located on these chromosomes can be expressed differently between sexes. Men get their single X chromosome from their mother, and they may inherit recessive genes that will become expressed. So basically, if men have a recessive trait on their X chromosome, they will probably display it.

Women have two X chromosomes, so it's more likely that they have a dominant allele in addition to the recessive allele. If that's the case, they don't show the recessive trait. In order for a woman to show a recessive sex-linked trait, they need to have a recessive allele on both chromosomes in the same place.

Red-green color blindness is an example of a trait that shows a sex linkage. Only about 0.5% of women are color blind. Men, however, are much more likely to be color blind. Male-pattern baldness is also a sex-linked trait.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Allele}}$ : Genes of the same type that contain different kinds of information. Alleles can be dominant or recessive

$\color{#8925AE}{\text{Dominant}}$ : The stronger of two expressed genes

$\color{#8925AE}{\text{Recessive}}$ : The weaker of two expressed genes. Only shown if a gene is homozygous recessive

$\color{#8925AE}{\text{Heterozygous}}$ : Two different kinds of allele, one dominant and one recessive

$\color{#8925AE}{\text{Homozygous}}$ : Two of the same type of allele, both dominant or both recessive

$\color{#8925AE}{\text{Codominance}}$ : Occurs when two heterozygous alleles are fully expressed. Neither allele can dominate so they both show up, but they do not blend

$\color{#8925AE}{\text{Incomplete Dominance}}$ : Occurs when two alleles are not dominant nor recessive. The alleles are both expressed and show up as a mixture of the two alleles

$\color{#8925AE}{\text{Sex-Linked Traits}}$ : Traits caused by alleles located on a sex chromosome

$\color{#8925AE}{\text{Polygenic}}$ : Traits that are controlled by two or more genes

Sex-Linked Inheritance

Sex-linked traits are those which are inherited via sex chromosomes. Specifically for humans, there are two types of sex chromosomes:

• X chromosome

• Y chromosome

In mammals, females are homozygous while males are heterozygous for sex chromosomes. So, human females have the genotype XX and males are XY.

However, these chromosomes vary not only in shape and size, but also in function.

As in the case of a human male, Y chromosomes are not actually homologous (similar in structure) to X chromosomes. The reason is very simple: Y chromosomes are very small compared to X chromosomes. They lack the alleles which are present in X chromosomes. They even have a separate set of genes. This is why human males are sometimes called hemizygous instead of heterozygous. Hemi- means "half", referring to males having only a single copy of a gene (half of the standard two).

X-Linked and Y-Linked Inheritance

Having two varied sets of genes for X and Y chromosomes, we can divide sex-linked inheritance into two broad categories:

1. X-linked inheritance: Genes on the X chromosome

2. Y-linked inheritance : Genes on the Y chromosome

X-linked alleles can be either dominant or recessive. However, the phenotypes are expressed differently in males and females.

We can write the dominant allele as X$^A$ and the recessive allele as X$^a$.

For an X-linked dominant trait to be expressed, a female can have either of the two genotypes below: $$X^A X^A$$ $$X^A X^a$$

For a male, the genotype X$^A$Y indicates the expression of an X-linked dominant trait.

Fragile X syndrome is an X-linked dominant disorder. Girls will be affected by it if their father has the disease, but the boys won’t be. Boys and girls all have a 50% chance of inheriting the disease if their mother is heterozygous (X$^A$ X$^a$).

For any recessive trait to be expressed, women must have the genotype X$^a$X$^a$.

Now what about the genotype X$^a$Y in a male? Remember, a recessive trait is only recessive if there is a dominant allele present. For this genotype, there is certainly no dominant allele present on the Y chromosome. So, the recessive allele determines the expression.

Color Blindness

Color blindness is a widely known X-linked recessive disorder. Boys are more likely to develop this trait than girls because girls need two copies of this allele for the trait to be expressed, while boys only need one. If a female is heterozygous (a carrier) for the allele, then neither she nor any of her daughters will get the trait. But, there is a 50% chance for her sons to be color blind if their father is unaffected.

Y-linked inheritance, also known as holandric inheritance, occurs only in males. Unless mutation occurs, the traits pass from father to all of the sons. A common example is having excessive hair growth in the ear.

(Video) Punnett Squares and Sex-Linked Traits

by Amoeba Sisters

Many sex-linked disorders show recessive inheritance. This video by Amoeba Sisters discusses one such common disorder: hemophilia. They show how it's passed down from parent to offspring by using a Punnett square, which they show you how to draw as well.