Mendel's laws helped us understand genetics, but it took a while for scientists to accept them. There were lots of exceptions to his laws that became the norm. Even Mendel couldn't replicate his laws in a plant called hawkweed. It wasn't until the 1940s and 1950s that Mendel's work, combined with Charles Darwin's theories, was acknowledged. Today, there are still exceptions to Mendel's laws, but they act as the foundation for new exceptions. One of these exceptions are sex-linked genes, like the gene on the X-chromosome that determines pattern bald Sex-linked traits are determined by genes found on the X and Y chromosomes. Females have two X chromosomes, while males have one X and one Y. Sex-linked genes can follow three inheritance patterns: X-linked dominant, X-linked recessive, and Y-linked. In this section, we will look at how males and females inherit each pattern separately.

X-linked Dominant Genes

X-linked dominant genes work like autosomal dominant genes. If a person has one copy of the allele, they will express the trait. Females have two X chromosomes, so having one X-linked dominant allele is enough to express the trait. For example, a female with XAXA or XAXa will express the trait. But, a female with XaXa won't express the dominant trait. Males only have one X chromosome. If a male is XAY, they will express the trait if they are Xa

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X-linked Recessive Genes

In contrast to X-linked dominant genes, X-linked recessive alleles are masked by a dominant allele. Therefore, a dominant allele must be absent for the X-linked recessive trait to be expressed. Females have two X-chromosomes; therefore, both X chromosomes must have the X-linked recessive allele for the trait to be expressed.

Since males only have one X-chromosome, having a single copy of the X-linked recessive allele is sufficient to express the X-linked recessive trait (Table 2).

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Y-linked Genes

In Y-linked genes, the genes are found on the Y chromosome. Since only males have a Y-chromosome, only males will express the trait of interest. Furthermore, it will be passed from father to son only (Table 3).

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Common Sex-Linked Traits

The most common example of a sex-linked trait is eye color in the fruit fly.

Thomas Hunt Morgan was the first to discover sex-linked genes in fruit flies (Fig. 2). He first noticed a recessive mutation in fruit flies that turned their eyes white. Using Mendel's theory of segregation, he expected that crossing a red-eyed female with a white-eyed male would produce progeny all with red eyes. Sure enough, following Mendel's law of segregation, all offspring in the F1 generation had red eyes.

When Morgan crossed the F1 offspring, a red-eyed female with a red-eyed male, he expected to see a 3:1 ratio of red eyes to white eyes because that is what Mendel's law of segregation suggests. While this 3:1 ratio was observed, he noticed that all the female fruit flies had red eyes while half of the male fruit flies had white eyes. Therefore, it was clear that the inheritance of eye color was different for female and male fruit flies.

He proposed that eye color in fruit flies must be on the X chromosome because patterns of eye color differed between males and females. If we revisit Morgan's experiments using Punnett squares, we can see that eye color was X-linked (Fig. 2).

Sex-Linked Traits in Humans

Humans have 46 chromosomes or 23 pairs of chromosomes; 44 of those chromosomes are autosomes, and two chromosomes are sex chromosomes. In humans, the sex chromosome combination determines the biological sex during birth. Biological females have two X chromosomes (XX), while biological males have one X and one Y chromosome (XY). This chromosome combination makes males hemizygous for the X chromosome, which means they only have one copy.

Hemizygous describes an individual where only one copy of the chromosome, or chromosome segment, is present, rather than both pairs.

Just like autosomes, genes can be found on the X and Y chromosomes. In humans, the X and Y chromosomes are differently sized, with the X chromosome being much larger than the Y chromosome. This size difference means there are more genes on the X chromosome; therefore, many traits will be X-linked, rather than Y-linked, in humans.

Males will be more likely to inherit X-linked recessive traits than females since inheritance of a single recessive allele from an affected, or carrier mother will be sufficient to express the trait. In contrast, heterozygous females will be able to mask the recessive allele in the presence of the dominant allele.

Examples of Sex-Linked Traits

Examples of X-linked dominant traits include Fragile X syndrome and Vitamin D resistant rickets. In both of these disorders, having one copy of the dominant allele is sufficient to display symptoms in both males and females (Fig. 3).

Examples of X-linked recessive traits include red-green color blindness and hemophilia. In these cases, females need to have two recessive alleles, but males will express traits with only one copy of the recessive allele (Fig. 4).

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Since there are very few genes on the Y chromosome, examples of Y-linked traits are limited. However, mutations in certain genes, such as the sex-determining region (SRY) gene and the testis-specific protein (TSPY) gene, can be passed from father to son through Y chromosome inheritance (Fig. 5).

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Sex-Linked Traits - Key Takeaways Sex-linked traits are determined by genes found on the X and Y chromosomes. Biological males have one X and one Y chromosome (XY), while biological females have two copies of the X chromosome (XX) Males are hemizygous for the X chromosome, meaning they only have one copy of the X chromosome. There are three inheritance patterns for sex-linked genes: X-linked dominant, X-linked recessive, and Y-linked. X-linked dominant genes are genes found on the X-chromosome, and having a single allele will be enough to express the trait. X-linked recessive genes are genes found on the X-chromosome, and both alleles are needed for the trait to be expressed in a biological female, but only one allele is needed in biological males. Y-linked genes are genes found on the Y-chromosome. Only biological males will express these traits.  Sex-linked genes do not follow Mendel's laws. Common examples of sex-linked genes in humans include red-green color blindness, hemophilia, and fragile X syndrome.

Sex-Linked Traits

What is a sex-linked trait?

Sex-linked traits are traits that are determined by genes found on the X and Y chromosomes 

What is an example of a sex-linked trait? 

Red-green color blindness, hemophilia, and Fragile X syndrome are all examples of sex-linked traits.

How are sex-linked traits inherited?

Sex-linked traits are inherited in three ways: X-linked dominant, X-linked recessive, and Y-linked

Why are sex-linked traits more common in males?

Males are hemizygous for the X chromosome meaning that they only have one copy of the X chromosome. Therefore, regardless of whether a male inherits a dominant or recessive allele, they will express that trait. In contrast, females have two X chromosomes, therefore, a recessive allele can be masked by a dominant alelle.  

Is baldness a sex-linked trait?

Yes, studies have found a gene on the X-chromosome for pattern baldness.

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