While Mendel's laws have been instrumental to understanding genetics, the scientific community did not accept his laws for a long time. Scientists continued to find exceptions to Mendel's laws; the exceptions became the norm. Even Mendel could not replicate his laws in another plant called hawkweed (it turned out hawkweed could also reproduce asexually, following different inheritance principles).
It wasn't until 75 years later, in the 1940s and 1950s, that Mendel's work, in combination with Charles Darwin's theories, was acknowledged by the scientific body. There continue to be new exceptions to Mendel's laws to this day. However, Mendel's laws act as the foundation for these new exceptions. Exceptions that will be explored in this section are sex-linked genes. One example of sex-linked genes is a gene on the X-chromosome that determines pattern baldness (Fig. 1).
Figure 1: Pattern baldness is a sex-linked trait. Towfiqu Barbhuiya
Definition of Sex-Linked Traits
Sex-linked traits are determined by genes found on the X and Y chromosomes. Unlike typical Mendelian genetics, where both sexes have two copies of each chromosome, sex-linked traits are determined by the inheritance of sex chromosomes which differ between the sexes. Females inherit two copies of the X chromosome, one from each parent. In contrast, males inherit one copy of the X chromosome from the mother and one copy of the Y chromosome from the father.
Therefore, females can be either homozygous or heterozygous for X-linked traits based on their two alleles for a given gene, while males will only have one allele for a given gene. In contrast, females do not have a Y chromosome for Y-linked traits, so they cannot express any Y-linked traits.
Sex-Linked Genes
By convention, sex-linked genes are denoted by the chromosome, either X or Y, followed by a superscript to denote the allele of interest. For example, for gene A which is X-linked, a female might be XAXa, where X represents the 'X' chromosome, 'A' represents the dominant allele of the gene, and 'a' represents the recessive allele of the gene. Therefore, in this example, the female will have one copy of the dominant allele and one copy of the recessive allele.
Sex-linked genes determine sex-linked traits. Sex-linked genes can follow three inheritance patterns:
- X-linked dominant
- X-linked recessive
- Y-linked
We will look at both male and female inheritance for each inheritance pattern separately.
X-linked Dominant Genes
Just like dominant traits in autosomal genes, which only need one copy of the allele to express the trait of interest, X-linked dominant genes work similarly. If a single copy of the X-linked dominant allele is present, the individual will express the trait of interest.
X-linked Dominant Genes in Females
Since females have two copies of the X chromosome, a single X-linked dominant allele is sufficient for the female to express the trait. For example, a female who is XAXA or XAXa will express the dominant trait as they have at least one copy of the XA allele. In contrast, a female who is XaXa will not express the dominant trait.
X-linked Dominant Genes in Males
A male has only one X chromosome; therefore, if a male is XAY, they will express the dominant trait. If the male is XaY, they will not express the dominant trait (Table 1).
Table 1: Comparing Comparing genotypes for an X-linked recessive gene for both sexes
| Biological Females | Biological Males |
| Genotypes that express the trait | XAXAXAXa | XAY |
| Genotypes that do not express the trait | XaXa | XaY |
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.
X-linked Recessive Genes in Females
Females have two X-chromosomes; therefore, both X chromosomes must have the X-linked recessive allele for the trait to be expressed.
X-linked Recessive Genes in Males
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).
Table 2: Comparing genotypes for an X-linked recessive gene for both sexes
| Biological Females | Biological Males |
| Genotypes that express the trait | XaXa | XaY |
| Genotypes that do not express the trait | XAXAXAXa | XAY |
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).
Table 3: Comparing genotypes for an X-linked recessive gene for both sexes
| Biological Females | Biological Males |
| Genotypes that express the trait | N/A | All biological males |
| Genotypes that do not express the trait | All biological females | N/A |
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).
X-linked recessive inheritance. Carrier mothers will pass on the mutation to the son or carrier daughters (left) while affected fathers will pass only have carrier daughters (right)
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).
Y-linked inheritance. Affected fathers pass on the mutations to only their sons
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.
Explanations 
Exams 
Magazine 
