Autosomal Trait: How Do Autosomal Traits Differ from Sex-Linked Traits?

What are autosomal traits? And just what are sex-linked traits? In order to understand how the two are different, it is really important to understand what the words autosomal and sex-linked are. Before all that, however, let’s get into a bit of a genetics primer so you have a bit of a background of which terms we use to describe the aforementioned and are fully equipped to take it all in.

What is a genotype and how does it determine everything?

A genotype is the entire collection of the genes a person carries—the entire heritable identity that controls everything, from regulating metabolism processes to protein expression. Essentially, it is the blueprint of all of the observable and/or visible characteristics (appearance, development, behavior) in an individual or their genomic sequence. On the other hand, it can also refer to one or a set of genes or a combination of alleles (or haplotypes, which are groups of genes that are inherited together) carried by an individual. Genotype examples are the different alleles that code for specific features, or phenotypes, of an individual such as hair color (brown vs. blonde vs. ginger), eye color (brown, blue, green, hazel), height and skin color, to name a few. Genotypes are often labeled with letters such as Tt, whereby T stands for one allele and t for another. This is particularly important when calculating genotype ratios. Uppercase letters denote dominant alleles, while lower case letters represent recessive alleles. Genotypes are described as homozygous if they contain two identical alleles and heterozygous in the case of two different alleles. The process of figuring out a genotype is referred to as genotyping. Dominant alleles always mask the traits of the recessive alleles in a heterozygous organism and traits of recessive alleles are only manifested in a homozygous organism.

A genotype ratio refers to the probability of an offspring receiving certain traits or inherited alleles based on the genotypes of their parents. In order to obtain the probability, a Punnett square is drawn whereby the columns represent the alleles carried by one parent and the rows those that are carried by the other parents. The traits of each column are combined with those in each row—genotype ratio is determined by counting occurrences of each of the combinations. A Punnett square is essentially a test cross between two organisms in order to determine their genotype based on their phenotype.

Genotype vs. Phenotype

While ‘genotype’ refers to all the genes carried by an individual, the observable characteristics or the visible expressed traits that the genes code for are termed ‘phenotypes.’ In other words, the genotype definition refers to the inward while the phenotype definition refers to a person’s outward information. However, a person’s genotype is subject to environmental factors and, as such, the genotype is not always perfectly correlated with the phenotype. Also, the phenotype could be the result of several genotypes.

So, now that you have a bit of a primer about some of the generic terms that are relevant, let’s get into autosomal and sex-linked traits and the difference between the two.

Autosomal vs. Sex-Linked Traits

DNA molecules

Let’s start with putting it out there: chromosomes (or tightly wound DNA molecules that carry all or most of the genetic material). The word chromosome has two parts coming from ancient Greek, namely chroma meaning “color” and soma meaning “body.”In other words, chromosome literally means “colored body.” This is because of the intense staining ability by some dyes. The word was coined over a century ago based on the term chromatin that was termed sometime before. But let’s get back to the initial question at hand. Every healthy individual has a set of 23 chromosomes: 1 is a sex chromosome—or allosome—and 22 are non-sex chromosomes. The non-sex chromosomes are also referred to as autosomes. And there you have it. Traits that are inherited on any of the 22 autosomes are called autosomal traits while those that are passed down on sex chromosomes are called sex-linked traits. It’s really that simple.

The autosomes are typically numbered as 1–22 based approximately on their size in base pairs. Allosomes, on the other hand, are labeled using letters. Females have two X-chromosomes (or XX) and males have one X-chromosome and one Y-chromosome (or XY). So, in terms of traits that are passed on, if a trait is on an X-chromosome, females will have two copies of the allele, while males only have one allele. Y-linked traits, on the other hand, are only carried by males (and are very rare in humans).

So, let’s get a bit more genetic before wrapping it all up. It is not a proper genetics lesson unless we talk a bit more about dominant and recessive traits. With autosomal dominant traits, both sexes are equally affected (with sex-linked traits males are more affected as they need just one copy of the X-linked gene to have a trait in question), there is a possibility of father-to-son transmission (as opposed to with sex-linked traits whereby that is not possible). With autosomal dominance, carrying individuals will pass on the trait to about 50% of their offspring (whereby autosomal recessive traits are passed on to about 25% of the offspring (thanks Mendel)).

Some autosomal diseases include Huntington disease, Marfan syndrome, Achondroplasia, Cystic fibrosis, Sickle-cell anemia, Phenylketonuria, and Tay-Sachs.

Some sex-linked conditions are fragile X syndrome, hemophilia, color blindness, baldness, and Duchenne muscular dystrophy. Generally speaking, males are a lot more prone to inheriting any sex-linked disease simply by virtue of having one X-chromosome (as mentioned above).

So. there you have it, an expansive review that delineates the difference between autosomal and sex-linked traits. The takeaway message is that genetics is not as straightforward Mendelian as it seems. And this is primarily due to just one out of the 23 chromosome pairs that humans carry. Isn’t that ironic?

Laura Day