Sex-Linked Inheritance

Section 3: Sex-Linked Inheritance

The inheritance of human red-green color blindness and vitamin D-resistant rickets is very similar to the inheritance of eye color in fruit flies. The genes that determine these traits are located on the sex chromosomes, and their inheritance is always associated with gender. This phenomenon is called sex-linked inheritance. Why are some of these diseases more common in males, while others are more common in females? What are the other characteristics of sex-linked inheritance? Let's analyze this through examples.

Human Red-Green Color Blindness

In 1792, the British scientist John Dalton (1766-1844) noticed a very peculiar phenomenon while observing a flower: the flower appeared to have two different colors during the day and at night. What was even more surprising was that only he and his brother could observe this phenomenon, while others insisted that the flower was consistently pink. What was going on? Determined to uncover the reason, Dalton conducted a careful analysis and comparison, eventually discovering that both he and his brother had difficulty distinguishing certain colors, which turned out to be a case of color blindness. He published a paper titled "Extraordinary Facts Relating to the Vision of Colours," becoming the first person to describe color blindness. Later, he pursued research in chemistry and proposed the famous "atomic theory" in the history of chemistry. This illustrates how important it is to be observant and persistent in seeking answers in scientific research!

Human sex is determined by sex chromosomes: females have a pair of homologous sex chromosomes denoted as XX, while males have a pair of heterologous sex chromosomes denoted as XY. The human X and Y chromosomes differ in both size and the number and types of genes they carry (Figure 2-12). The X chromosome carries many genes, while the Y chromosome is only about one-fifth the size of the X chromosome and carries relatively few genes. Therefore, many genes located on the X chromosome have no corresponding alleles on the Y chromosome.

Let's first analyze the inheritance of red-green color blindness.

The inheritance of red-green color blindness in humans typically follows these patterns:

• If a female homozygous for normal color vision marries a male with red-green color blindness, all their sons will have normal color vision. Their daughters will appear normal but will carry the gene for red-green color blindness inherited from their father (Figure 2-13).
• If a female carrier of the red-green color blindness gene marries a male with normal color vision, their sons will have a 50% chance of being normal and a 50% chance of being colorblind. Their daughters will not be colorblind, but 50% of them will be carriers of the colorblind gene (Figure 2-14). In this case, the son's colorblind gene is inherited from the mother.
• If a female carrier of the red-green color blindness gene marries a male with red-green color blindness, half of their sons and daughters will be colorblind. If a female with red-green color blindness marries a male with normal color vision, all their sons will be colorblind, and all their daughters will be carriers.

From this analysis of the inheritance of red-green color blindness in humans, we can observe that the inheritance pattern of recessive genes located on the X chromosome is characterized by a much higher incidence of affected males compared to females. Affected males inherit the gene only from their mother and can pass it on only to their daughters.

Vitamin D-Resistant Rickets

Genes located on the X chromosome can be either recessive or dominant. The previously mentioned vitamin D-resistant rickets is an example of an X-linked dominant genetic disorder (Figure 2-15). This disorder is controlled by a dominant gene (D). When a female has the genotype X*XD or X*X, she will be affected by the disease, although the latter will have a milder form of the disease. In male patients, the only possible genotype is XPY, and the severity of the disease is similar to that of females with the XDX genotype. Therefore, the inheritance pattern of dominant genes located on the X chromosome is characterized by a higher incidence in females compared to males, although some female patients may have milder symptoms. In the offspring of a male patient with an unaffected female partner, all daughters will be affected, while all sons will be unaffected.

Practical Applications of Sex-Linked Inheritance Theory

Sex-linked inheritance is common in the biological world. Besides the aforementioned human traits of red-green color blindness, vitamin D-resistant rickets, and the red and white eye color in fruit flies, other examples include hemophilia in humans, the black and white barred feather pattern in certain breeds of chickens, and the inheritance of certain traits in dioecious plants such as poplars and willows.

The theory of sex-linked inheritance has wide applications in medicine and agricultural production. By understanding the rules of sex-linked inheritance, the probability of offspring inheriting certain conditions can be calculated, thereby guiding eugenic practices. For example, when a male patient with vitamin D-resistant rickets marries a healthy female, analyzing the disease in their offspring can lead to medical advice regarding their reproduction. The theory of sex-linked inheritance can also guide breeding practices. For instance, the method of sex determination in chickens is different from that in humans and fruit flies. In chickens, the female has heterologous sex chromosomes (ZW), while the male has homologous sex chromosomes (ZZ). The black and white barred feather pattern in certain chicken breeds (Figure 2-16) is determined by a dominant gene B located on the Z chromosome. When its allele b is homozygous, the chicken displays a non-barred pattern without black and white stripes. If a barred female chicken (ZPZW) is crossed with a non-barred male chicken (ZZ²), all male chickens in the F1 generation will be barred (ZPZ²), while all female chickens will be non-barred (ZW). This allows for early differentiation of chicks by feather pattern, enabling the selective raising of more hens to increase egg production.

If you're interested, you can further explore relevant materials to understand other practical applications of sex-linked inheritance theory in production.