What is Sex-linked Inheritance and What Are Its Impacts?

Sex-linked inheritance occurs when certain genetic traits are associated with gender. This is due to the fact that sex chromosomes, besides controlling the biological sex of an organism, also carry many other genes that simultaneously regulate genetic traits. Diseases caused by pathogenic genes located on sex chromosomes, typically the X chromosome, are inherited in a sex-linked manner. Examples of such diseases include red-green color blindness and hemophilia. What is sex-linked inheritance and what are its impacts? Let's explore further.

Sex-linked inheritance refers to the phenomenon where certain traits in offspring are controlled by genes located on sex chromosomes. The inheritance of these traits, controlled by genes on sex chromosomes, is always associated with gender. This mode of inheritance, which is linked to gender, is referred to as sex-linked inheritance, also known as sex-linkage or sex-chain. Many organisms exhibit sex-linked inheritance, with the most well-understood examples in humans being red-green color blindness and hemophilia. The inheritance patterns of these diseases, along with the X-linked dominant inheritance of white eyes in fruit flies (such as anti-vitamin D rickets and pendular nystagmus) and X-linked recessive inheritance (such as red-green color blindness and hemophilia), as well as Y-chromosome inheritance (such as webbed duck feet and hairy external auditory canal), all fall under the category of sex-linked inheritance.

1. When homozygous dominant genes are transmitted through sex chromosomes of the same gender (e.g., XX for females in mammals and ZZ for males in birds), both female and male offspring in F1 will display the dominant trait. The segregation of traits in F2 will be 3 dominant: 1 recessive, and the segregation of gender will be 1 female: 1 male. The gender of the recessive individuals will be the same as the recessive individuals in the ancestral generation, meaning that half of the grandchildren will share the same phenotypic characteristics as their grandfather.

2. When homozygous recessive genes are transmitted through sex chromosomes of the same gender, F1 will exhibit crossover inheritance, where the mother's traits are passed to the son and the father's traits are passed to the daughter. In F2, the ratio of traits to gender will be 1:1.

3. Traits determined by genes located in the differentiated region of the Y chromosome (specifically in mammals) or traits determined by genes carried by the W chromosome are exclusively passed from the father (or mother bird) to their son (or daughter bird). This manifests as a special form of Y-linked (or W-linked) inheritance.

4. In X-linked dominant inheritance diseases, there are more female patients than male patients; whereas in X-linked recessive inheritance diseases, there are more male patients than female patients.

Humans typically have 23 pairs (46) of chromosomes, with 22 pairs being autosomes and the remaining pair being sex chromosomes. Females are represented as XX, and males as XY. Each pair of chromosomes carries numerous genes, and each gene occupies a specific locus on the chromosome. Genes are composed of deoxyribonucleic acid (DNA), and when the structure of DNA mutates into a pathogenic gene, a genetic disease clinically manifests.