Have you ever noticed that many people with blonde hair also have blue eyes? Or that color blindness seems to affect more men than women? These interesting observations can be explained by something called linked genes. Even though we know a lot about genetics, there are still exceptions to the rules proposed by a famous scientist named Mendel. In this article, we'll take a closer look at linked genes and sex-linked genes that don't follow Mendel's laws. If you need a reminder about Mendel's genetics, check out our articles Mendelian Genetics and Mendel's Laws of Segregation. And if you want to learn even more about exceptions to Mendel's laws, be sure to read our article on Non-Mendelian Genetics.

Linked Genes Defined

Linked genes are a group of genes that are passed down together because they are located near each other on the same chromosome. This means that they are inherited at much higher rates than what Mendel's law of independent assortment would predict. When we talk about independent assortment, we mean that genes are sorted into gametes independently from each other. But when genes are linked, they don't follow this rule. This is because the law of independent assortment assumes that genes are located on different chromosomes or very far apart on the same chromosome. But if they are close together on the same chromosome, they tend to be inherited together.

Linked vs Unlinked Genes

To compare linked and unlinked genes, we need to look at how chromosome segregation works during meiosis. Unlinked located on chromosomes meaning are sorted into gametes independently of each other. During meiosis, two heterozygous parents, AaBb, will produce four types of gametes in equal proportions (AB, Ab, aB, ab, all with a frequency of 25%). Importantly, each chromatid stays intact, and there is no separation during the process of meiosis.

Linked genes, on the other hand, are located close to each other on the same chromatid. This means that they are more likely to be inherited together into a gamete, as the chromatids tend to remain intact throughout meiosis. For example, if pea color and pea shape were linked, we might see green-colored and wrinkly peas frequently inherited together, while yellow-colored and round peas are also frequently seen together.

However, there is an exception to this rule called crossing over or homologous recombination. During recombination, homologous chromosomes are separated, and fragments of their DNA are exchanged with each other, leading to new alleles on the same chromosome. For example, if a recombination event happens between the pea-color gene and the pea-shape genes that were originally linked, we might see green-colored round peas and yellow-colored wrinkly peas.

Following recombination, the new possible gamete genotypes will be AB, Ab, aB, and ab, just like unlinked genes. However, of a recombination event on a chromosome is random, making it less likely that Ab and aB alleles will be inherited together. Parental chromatids are the original allele pairs (AB and ab), while recombinant chromatids are the less likely Ab and aB alleles. The closer the two genes are, the less likely a recombination event will occur between them. In very tight linkage, the parental chromatids will almost always be seen, with few cases of recombinant chromatids.

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Examples of Linked Genes

Linked genes are genes that are passed down together because they are located close to each other on the example of linked genes is hair color and eye color genes. Although hair color and eye color are not determined by just one gene, there is some evidence that two of the genes involved in hair color and eye color are linked. This might explain why individuals with blue eyes might also have blonde hair, or why a person with red hair might also have light skin.

Linkage can also be useful for determining the position of genes on a chromosome. When we follow the law of independent assortment, parental chromatids are found in the offspring 50% of the time. But with linked genes, parental alleles are observed much more commonly than recombinant alleles. This indicates that the two genes are linked and close together on the same chromosome. By repeating this process for multiple different genes, a genetic map or linkage map can be generated.

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Sex-linked Genes

Sex-linked genes are genes found on the X and Y chromosomes and are biased towards one sex over the other. Thomas Hunt Morgan discovered sex-linked genes during his fruit fly genetics research when he observed that eye color did are chromosome inherited from the mother and one Y chromosome inherited from the father. Females, on the other hand, have two X chromosomes inherited from each parent. This difference in inheritance between males and females makes the expression of traits different. Females can be homozygous or heterozygous for a specific X-linked gene, while males will only have one copy of the X-linked gene. Therefore, if a male inherits a recessive allele from their mother, they will express that recessive trait.

Sex-linked genes are denoted by XB for dominant or Xb for recessive, where B is the gene of interest. The X represents that the gene is located on the X chromosome, while the superscript denotes the allele.

Practice Problems for Sex-linked Genes

Punnett Squares are a useful tool for solving practice problems, especially when dealing with genetics. When working with X-linked recessive disorders like red-green color blindness, it is important to keep in mind that females must have two copies of the recessive allele, XbXb, to be colorblind. Conversely, males only need one copy of the recessive allele, Xb, to be colorblind.

If a female carrier and a normal-visioned male have a child, the child has a 25% chance of being colorblind. However, if the child is a biological female, they have a 0% chance of being colorblind as both XBXB and XBXb have normal-colored vision. If the child is a male, they have a 50% chance of being colorblind, as they only have one X chromosome which may carry the recessive allele.

genes refer to a set of genes that to together because they are located close to each other on the same. Recombination can occur when homologous chromosomes split and exchange fragments of their DNA with each other, unlinking the linked genes. Parental chromatids match the parent's alleles, while recombinant chromatids are produced after a recombination event. Sex-linked genes are genes found on the X and Y chromosomes.

Linked Genes

Sex-linked genes are located on which chromosomes?

The X and Y chromosomes.

What are sex linked genes?

Genes that are found on the X and Y chromosome

What is the reason that linked genes are inherited together?

Genes that are found close together on the same chromosome are likely to be inherited together because chromatids are intact during meiosis and recombination between closely positioned genes is less likely to occur. 

Are linked genes on the same chromosome 

Yes, linked genes are found close together on the same chromosome.  

What are x linked genes?

X-linked genes are genes that are found on the X chromosome.

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