Sex-linked genes are those found on only one sex chromosome. This is called linkage. It means that certain traits and conditions are more likely to occur in one sex than the other, based on the chromosome they inherit. If you want to learn more about how genes and chromosomes work, understanding linkage key to

Sex linkage in genetics

, have  pairs. One of these pairs is the sex chromosomes the other 22 pairs are called autosomal chromosomes. Females have two X chromosomes, while males have one X and one Y chromosome.

When our bodies produce sex cells, called gametes, they only have one set of unpaired chromosomes. This means that females will always produce gametes with one X chromosome, while males will produce gametes with either an X or a Y chromosome.

A child inherits one sex chromosome from each parent, which determines their biological sex. If a child inherits two X chromosomes, they will be biologically female (XX), while a child who inherits one X and one Y chromosome will be biologically male (XY).

So, in summary, autosomal chromosomes are any chromosomes that are not sex chromosomes. Females have two X chromosomes, while males have one X and one Y chromosome, which determines their biological sex.

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The X chromosome is much longer than the Y chromosome, and most regions of the X chromosome do not have a homologous pair in the chromosome. This means that a single recessive allele on the non-homologous portion of the X chromosome is enough for the corresponding trait to be expressed. Since there can be no dominant allele on the Y chromosome to mask it, an individual's sex greatly affects their chances of inheriting certain recessive alleles or defective genes that lead to disease.

Homologous pairs of chromosomes consist of two chromosomes with the same sequence of genes, loci, and length. One chromosome comes from the mother, and the other from the father. A homologue refers to the corresponding chromosome in a homologous pair.

For females, there are three possible phenotypes: normal, carrier (has one allele but doesn't suffer from the disease), and has the disease.

For males, there are only two possible phenotypes: normal and has the disease.

Why can men not be carriers of sex-linked genetic diseases?

That's correct. Men cannot be carriers of sex-linked genetic diseases because they have only one X chromosome, and the Y chromosome does not carry the same genes as the X chromosome. In a male, the genetic make-up of the trait is not twofold, as there is no homologous chromosome to pair with the X chromosome. Therefore, if a male inherits a disease-causing allele on his X chromosome, he will have the disease.

Can males be carriers of diseases?

That's correct. A carrier for a disease has the allele for a trait but usually does not show symptoms. They will be heterozygous (e.g. Aa or Hh) and can pass the allele for the disorder to their offspring. If the offspring is homozygous-re (aa or hh), then the offspring will be affected by the disease.

Males only have one X chromosome, and a single recessive gene on that X chromosome will be expressed in the phenotype because there are no corresponding genes on the Y chromosome. Therefore, males cannot be carriers of sex-linked genetic diseases.

In women, a recessive allele on one X chromosome is usually masked in their phenotype by a dominant allele on the other X chromosome. This is why women are more likely to be carriers of sex-linked genetic diseases and not have the gene expressed phenotype.

How can we predict the inheritance of colour blindness?

Some of the genes for red-green colour blindness are sex-linked and found on the X chromosome. Let the allele B represent healthy vision and b represent colour blindness. The capital (B in this case) always denotes the dominant allele and lower case (b) denotes the recessive allele.

Dominant allele: an allele that is always expressed, even if the individual only has one copy of it.

Recessive allele: an allele that is only expressed if the individual has two copies of it and does not have the dominant allele of that gene.

The following diagram shows how two healthy parents (a healthy male and a carrier female) can have offspring with colour blindness. As the Y chromosome does not have either gene it is just represented as Y.

Parent’s phenotypes: normal vision  normal vision

Parent’s genotypes:        Xᴮ Xᵇ                XᴮY

Gametes:                         Xᴮ Xᵇ                XᴮY

Figure 2. Inheritance of colour blindness from a healthy father and a carrier mother

50% of offspring will be male, half of whom will be colour blind. This means 25% of offspring are colour blind males. On the other hand, none of the females is colour blind. A colour blind male can only have inherited the condition from his mother since he would have only gained a Y chromosome from his father.

How can we predict the inheritance of haemophilia?

Haemophilia is a disease that affects the blood’s ability to clot.

Haemophilia can lead to slow and persistent internal bleeding, particularly in joints, and can be lethal if left untreated. Because many individuals with this disease do not survive until reproductive age, it is relatively rare. The overwhelming majority of its sufferers today are male; only a small percentage are female. This is in part due to the fact that many haemophiliac females die from extreme blood loss when menstruation begins.

Haemophilia can be caused by a number of things, but one is a recessive allele on the X chromosome which codes for a faulty protein involved in the clotting process. When a carrier female and a healthy male have offspring, 25% will be haemophilic males, as predicted by the diagram below. While males can inherit haemophilia from their mothers, they cannot inherit it from their fathers. 25% of the offspring will also be carriers XᴴXʰ. The following diagram shows a cross between a non-carrier male and a carrier female.

Parent’s phenotypes: Carrier mother    Non-carrier father

Parent’s genotypes:        XᴴXʰ                     XᴴY

Gametes:                         XᴴXʰ                     XᴴY

Figure 3. Inheritance of haemophilia from non-carrier father and carrier mother

Haemophilia is a recessive trait. Thus, non-carrier genotypes are depicted as Xᴴ, while diseased genotypes are depicted as Xʰ.

Pedigree charts

Pedigree charts are a useful method of tracing the inheritance of sex-linked traits. They have specific characteristics such as:

Males are represented by squares. Females are represented by circles. The shading in each shape represents the absence or presence of a trait in question – usually the phenotype.Shading indicates the presence of a trait.

The following is an example pedigree of a non-diseased male and a carrier female for the phenotype of colour blindness.

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As shown above, neither parent is colour blind. However, one of the offspring is colour blind. This means that one of the parents is a carrier. Recall that males only have one X chromosome, the female parent must be the carrier.

Autosomal linkage

's correct. Autosomal linkage refers to the situation in which two or more genes are located on the same autosome, where they are inherited together and do undergo independent assortment. This is because the genes are physically located close to each other on the chromosome and are therefore more likely to be inherited together.

In the case of linked genes, phenotypes may deviate from the expected 9:3:3:1 ratio seen in a dihybrid cross. Instead, there may be a 3:1 ratio, as seen in a monohybrid cross. This is because, if two genes are linked, they will tend to be inherited together and will not segregate independently.

For example, if the genes T and B control plant height and flower color, respectively, and are linked, then the offspring resulting from a cross between two heterozygous parents will show a 3:1 ratio of phenotypes. This is because there are only two possible combinations of alleles, TB and tb, which will result in tall, red plants; tall, white plants; short, red plants; and short, white plants.

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That's correct. Here are a few key takeaways:

• Sex-linked genes are located on the sex chromosomes (X and Y) and can affect the inheritance of traits in a sex-dependent manner.
• Examples of sex-linked diseases include color blindness and hemophilia, which are more common in males due to their inheritance of only one X chromosome.
• Autosomal linkage occurs when genes are located on the same autosome and can result in a deviation from the expected Mendelian ratios.
• When the results of a test cross do not align with Mendelian ratios, linkage may be a possible explanation.
• Understanding the inheritance patterns of sex-linked and autosomal linked genes can help predict the likelihood of certain traits or diseases in offspring.

Linkage

What is linkage in biology?

In biology, the linkage simply means that genes are connected to something in a way that affects their patterns of inheritance. When genes are linked to sex chromosomes, their inheritance depends on the sex of the individual. When they are linked to other genes, they have a higher probability of being inherited together with those genes.

What is an example of linkage?

Haemophilia is an example of a disease caused by sex-linked genes. A number of things can cause haemophilia, but one is a recessive allele on the X chromosome, which codes for a faulty protein involved in the blood clotting process. When a carrier female and a non-carrier male have offspring, 25% will be haemophilic males.

What are the types of linkages in biology?

There are two types of genetic linkage: autosomal and sex-linkage. In autosomal linkage, genes on the same chromosome will exhibit a tendency to be inherited together. In sex-linkage, genes are only present on one sex chromosome and not the other. 

What is the importance of linkage?

Understanding linkage is vital for understanding how genes are inherited, which can be important for understanding genetic diseases. For instance, linkage can explain unusual or non-Mendelian patterns of inheritance.

How can linkage be detected?

There are many ways for genetic linkage to be detected. For instance, test crosses can show that genes are linked. When the cross results do not align with those predicted by Mendelian ratios, we can assume that there is a biological reason behind it.

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