# Sex Linkage Sex linkage refers to the inheritance of traits controlled by genes located on the sex chromosomes (X or Y). The expression of these traits is linked to the sex of the individual. ## Key Concepts - **Sex-Linked Trait:** A trait controlled by an allele on a sex chromosome. - **X-linked Trait:** A trait controlled by an allele located only on the X chromosome. - **Y-linked Trait (Holandric Trait):** A trait controlled by an allele located only on the Y chromosome. - **X-Y Linked Trait (Pseudoautosomal):** A trait controlled by genes with alleles found on both the X and Y chromosomes. These traits follow an inheritance pattern similar to autosomal traits. - **Wild Type:** The phenotype for a character most commonly observed in natural populations (e.g., red eyes in *Drosophila*). - **Mutant:** The phenotype resulting from a mutation in a gene, differing from the wild type (e.g., white eyes in *Drosophila*). ## Sex Linkage in *Drosophila* The concept of sex linkage was first demonstrated by T.H. Morgan through his experiments on the fruit fly, *Drosophila melanogaster*. ### Morgan's Initial Cross 1. **Parental (P):** Morgan crossed a *white-eyed male* ($X^rY$) with a *red-eyed female* ($X^R X^R$). 2. **F₁ Generation:** All 1,237 offspring had red eyes. 3. **F₂ Generation:** When the F₁ generation was interbred, the F₂ progeny consisted of 3,470 red-eyed flies and 782 white-eyed flies. ### Observations and Deviations from Mendelian Ratios 1. The F₂ phenotypic ratio was close to, but slightly deviated from, the standard 3:1 monohybrid ratio. 2. Crucially, **all white-eyed flies in the F₂ generation were male**. ### Confirmatory Crosses 1. **Test Cross:** Morgan crossed an F₂ red-eyed female ($X^R X^r$) with the original white-eyed male ($X^rY$). The offspring included red-eyed females, red-eyed males, white-eyed females, and white-eyed males. This proved that females could carry the recessive white-eye allele without expressing it. 2. **Reciprocal Cross:** Morgan then mated a *white-eyed female* ($X^r X^r$) with a *red-eyed male* ($X^R Y$). The offspring were all red-eyed females and all white-eyed males. ### Morgan's Conclusion The gene for eye color in *Drosophila* is located on the **X chromosome**. The Y chromosome does not carry an allele for this trait. This explains why a recessive allele on the X chromosome is always expressed in males (who are hemizygous for X-linked genes). ## Sex Linkage in Humans In humans, the mode of inheritance for traits is often traced through pedigrees. Human sex-linked traits can be X-linked (recessive or dominant) or Y-linked. ### X-Linked Recessive Inheritance **Pattern:** - Affected individuals are more often males than females. - Females with one recessive allele are *carriers* and typically do not show symptoms. - An affected father cannot pass the trait to his sons. - An affected father passes the recessive allele to all his daughters, who then become carriers. - A carrier female has a 50% chance of passing the allele to her children (50% of sons will be affected, 50% of daughters will be carriers). **Examples:** Haemophilia A and B, Red-Green Colour Blindness, Testicular feminization. ### X-Linked Dominant Inheritance **Pattern:** - Affected fathers pass the trait to **all** of their daughters but to **none** of their sons. - An affected mother has a 50% chance of passing the trait to each child, regardless of sex. - The trait appears in every generation. **Example:** Hypophosphatemic rickets (Vitamin D resistant rickets). ### Y-Linked Inheritance (Holandric Inheritance) **Pattern:** - The trait is passed directly from father to **all** sons. - Females are never affected. - The genes are located on the non-homologous region of the Y chromosome. **Examples:** Hypertrichosis (hair on the rim of the pinna), Testis Determining Factor (TDF). ## Sex-Linked Disorders in Humans ### Haemophilia - An **X-linked recessive** disorder where blood fails to clot properly due to a lack of specific clotting factors. - It exhibits a "zigzag" inheritance, passing from a maternal grandfather through a carrier daughter to a grandson. **Types of Haemophilia:** | S.No | Type | Cause | Percentage | Inheritance | | :--- | :--- | :--- | :--- | :--- | | 1. | Haemophilia A | Deficiency of clotting factor VIII | 80% | Recessive X-linked | | 2. | Haemophilia B | Deficiency of clotting factor IX | 20% | Recessive X-linked | | 3. | Haemophilia C | Deficiency of clotting factor XI | < 1% | Recessive autosomal | ### Colour Blindness - The genes for red and green light-absorbing proteins (opsins) are located on the X chromosome. Mutations in these genes cause red-green color blindness. - It is an **X-linked recessive** trait, making it more common in men than women. **Types of Colour Vision Defects:** - **Dichromacy:** Inability to perceive one of the three primary colors. - *Protanopia:* Red blindness. - *Deuteronopia:* Green blindness. - *Tritanopia:* Blue blindness (caused by a gene on autosome 7). - **Monochromacy:** True color blindness, where only one color can be perceived. Blue cone monochromacy is an X-linked recessive trait where both red and green cone cells are absent. ## Embryonic Screening Techniques These techniques are used to test a fetus for chromosomal abnormalities and genetic diseases. ### Amniocentesis - A sample of amniotic fluid containing fetal cells is extracted from the amniotic sac using a needle, guided by ultrasound. - The cells are cultured, and their chromosomes are examined (karyotyping) to detect abnormalities like Down syndrome (Trisomy 21). ### Chorionic Villus Sampling (CVS) - A small sample of tissue (chorionic villi) is taken from the placenta, either through the cervix or the abdominal wall. - Since the placenta has the same genetic material as the fetus, these cells can be immediately karyotyped to check for genetic problems. ## Possible Questions and Answers **Q:** What is sex linkage? **A:** The inheritance of traits determined by genes located on the sex chromosomes (X or Y). **Q:** Why are X-linked recessive disorders more common in males? **A:** Males have only one X chromosome (hemizygous). Therefore, a single recessive allele on their X chromosome will be expressed. Females have two X chromosomes, so they must inherit two recessive alleles (one on each X) to express the trait. **Q:** In Morgan's experiment, why were all the F₂ white-eyed flies male? **A:** The F₁ females were heterozygous carriers ($X^R X^r$). They produced two types of eggs: $X^R$ and $X^r$. The F₁ males ($X^R Y$) produced two types of sperm: $X^R$ and $Y$. A white-eyed fly ($X^r X^r$ or $X^r Y$) could only be produced if it received an $X^r$ allele. In the F₂ generation, only the male offspring could receive the $Y$ chromosome from the father and the $X^r$ from the mother, resulting in a white-eyed male ($X^r Y$). **Q:** What is a holandric trait? **A:** A trait that is determined by a gene on the Y chromosome. It is passed from father to all his sons.