As you have learned, sex-linked traits are those that are influenced by genes on the 23rd pair of chromosomes, which are the sex chromosomes. In this lesson, you will explore how alterations to genes on these chromosomes can result in some common traits and disorders.
1. X-Linked Traits
An X-linked transmission pattern involves genes located on the X chromosome. Recall that a male human has one X and one Y chromosome. When a male transmits a Y chromosome, the child is male, and when a male parent transmits an X chromosome, the child is female. A female can transmit only an X chromosome, as both her sex chromosomes are X chromosomes.
When an abnormal allele for a gene that occurs on the X chromosome is dominant over the normal allele, the pattern is described as X-linked dominant. This is the case with vitamin D-resistant rickets, which is a weakening of bones in children (and can be caused by vitamin D deficiency in individuals without this mutation): An affected male would pass the disease gene to all of his female offspring, but none of his male offspring, because the male transmits only the Y chromosome to male offspring (see part a of the image below).
If it is the female parent who is affected, all of the offspring—male or female—would have a 50% chance of inheriting the disorder because the female parent can only pass an X chromosome on to children (see part b of the image below). For an affected female, the inheritance pattern would be identical to that of an autosomal dominant inheritance pattern in which one parent is heterozygous and the other is homozygous for the normal gene.
X-Linked Patterns of Inheritance—A chart of X-linked dominant inheritance patterns differs depending on which parent is affected by the disease. (credit: U.S. National Library of Medicine)
Faulty enamel is a trait that is an example of an X-linked dominant disorder. The enamel that protects your teeth doesn't properly develop in people with this disorder. Their teeth will rot easily because they don't have protective enamel on their teeth.
X-linked recessive inheritance is much more common because females can be carriers of the disease yet still have a normal phenotype. Diseases transmitted by X-linked recessive inheritance include color blindness, the blood-clotting disorder hemophilia, and some forms of muscular dystrophy.
key concept
For an example of X-linked recessive inheritance, consider parents in which the female is an unaffected carrier and the male is normal. None of the female offspring would have the disease because they receive a normal gene from their male parent. However, they have a 50% chance of receiving the disease allele from their female parent and becoming a carrier. In contrast, 50% of the male offspring would be affected.
With X-linked recessive diseases, males either have the disease or are genotypically normal—they cannot be carriers. Females, however, can be genotypically normal, a carrier who is phenotypically normal, or affected by the disease. A female can inherit the allele for an X-linked recessive illness when the female parent is a carrier or affected, or the male parent is affected. Female offspring will be affected by the disease only if they inherit an X-linked recessive gene from both parents. As you can imagine, X-linked recessive disorders affect many more males than females.
EXAMPLE
Color blindness affects at least one in 20 males, but only about one in 400 females.
X-Linked Recessive Inheritance—Given two parents in which the male is normal and the female is a carrier of an X-linked recessive disorder, a male offspring would have a 50% probability of being affected with the disorder, whereas female offspring would either be carriers or entirely unaffected. (credit: U.S. National Library of Medicine)
terms to know
X-Linked
The pattern of inheritance in which an allele is carried on the X chromosome of the 23rd pair.
X-Linked Dominant
The pattern of dominant inheritance that corresponds to a gene on the X chromosome of the 23rd pair.
X-Linked Recessive
The pattern of recessive inheritance that corresponds to a gene on the X chromosome of the 23rd pair.
1a. X-Linked Inheritance
Using hemophilia as an example, it is possible to look at how these types of disorders can be passed on. We know that a mother can pass on one of two X chromosomes. Remember that a father can pass on an X and a Y chromosome. If he passes on an X chromosome, the child will be female. If he passes on a Y chromosome, the child will be male. A father with an X-linked disorder like hemophilia will therefore only pass this trait on to a daughter and never to a son. Let's look at more potential combinations.
In the Punnett square below, the four boxes are potential combinations, each with a 25% probability of occurrence. The alleles listed on the top are the sperm a father can supply, and the alleles listed on the left side are the possible eggs a mother could supply. This example shows the mother as a carrier for hemophilia. The X shown in red is the recessive allele that is affected.
Because the mother has a normal X chromosome, any effect the recessive X would have is probably masked. She is only a carrier. She has a 50% chance of passing this chromosome on to any of her children, creating either a daughter who is a carrier like her or a son who has hemophilia. She also has a 50% chance of passing on an X chromosome that is not affected, creating a daughter or a son that is not affected. Since she also has a 50% chance of having either a daughter or a son, the likelihood that she will have a son with hemophilia is therefore 25%.
This illustrates how X-linked traits can be passed on from parents to offspring and why males are generally more susceptible to inheriting these disorders. If they inherit the X chromosome from their mother that is affected, they will automatically get that disease.
Many X-linked disorders follow common patterns of inheritance. There are lots of different patterns of inheritance like this that geneticists can study to understand these diseases and the patterns in which these diseases are passed through generations.
2. Y-Linked Traits
There are upwards of approximately 200 genes on the Y chromosome, and these genes are also subject to the effects of mutation. Y-linked traits and disorders are always passed from father to son because only males have Y chromosomes. Many Y-linked disorders are associated with infertility in males and are therefore not transmitted to subsequent generations. However, there are some that persist in human populations.
IN CONTEXT
One example of a Y-linked trait in humans includes webbed toes, in which the second and third toes are connected by tissue.
Webbed Toes
Another Y-linked disorder is porcupine man syndrome (also known as ichthyosis hystrix gravior), in which the body is covered with spiny scales.
term to know
Y-Linked
The pattern of inheritance in which an allele is carried on the Y chromosome of the 23rd pair.
summary
In this lesson, you learned about some traits and disorders that are caused by mutations on the sex chromosomes. First, you learned about X-linked traits, in which the dominant or recessive trait of interest is only carried on the X chromosome. You learned that an example of a dominant X-linked trait is faulty enamel of the teeth, and some examples of recessive X-linked traits include color blindness, the blood-clotting disorder hemophilia, and some forms of muscular dystrophy, and you explored some patterns of X-linked inheritance. Finally, you learned that Y-linked traits are carried on the Y chromosome and are typically associated with infertility in males, but some, such as hypertrichosis, persist in human populations.
