The inheritance of traits is controlled by genes located on chromosomes. While most genes are found on autosomes, some are located on the sex chromosomes (X and Y). These genes exhibit unique patterns of inheritance because sex chromosomes behave differently from autosomes during gamete formation and fertilization. Traits controlled by such genes can be categorized as sex-linked, sex-limited, and sex-influenced traits.
1. Sex Linkage
Sex linkage refers to the inheritance of traits controlled by genes located on the sex chromosomes (mostly X chromosome, rarely Y chromosome). Since males (XY) and females (XX) have different numbers of X and Y chromosomes, the inheritance patterns differ between the sexes.
Types of Sex Linkage
- X-linked inheritance: Traits controlled by genes on the X chromosome. Males are hemizygous (only one X chromosome), so recessive alleles are expressed. Example: Hemophilia, Color blindness in humans, White eye in Drosophila.
- Y-linked inheritance (holandric traits): Genes present on Y chromosome are inherited only from father to son. Example: Hypertrichosis (hairy pinna of ear in humans).
Example 1: White Eye in Drosophila
Discovered by Thomas Hunt Morgan in 1910. The white eye mutation is a recessive allele (w) located on the X chromosome.
- Parental cross: Red-eyed female (XX, homozygous) × White-eyed male (XÊ·Y).
- F₁ Generation: All offspring red-eyed (because red is dominant).
- F₂ Generation:
- Females: 50% red-eyed, 50% white-eyed
- Males: 50% red-eyed, 50% white-eyed
The inheritance showed that the gene was X-linked, as the trait appeared more frequently in males.
Example 2: Attached X Chromosome in Drosophila
In certain Drosophila females, the two X chromosomes may become physically attached. During meiosis, these attached X chromosomes move together to the same pole.
Consequences:
- Both X chromosomes pass together into the egg.
- If fertilized by a Y-bearing sperm → XXY (phenotypically female).
- If fertilized by an X-bearing sperm → XXX (generally inviable).
This abnormal inheritance provided strong evidence for the chromosomal theory of inheritance.
Example 3: Non-disjunction of X Chromosome
Sometimes, X chromosomes fail to separate (non-disjunction) during meiosis in female Drosophila. As a result, eggs may contain two X chromosomes (XX) or none (O).
Consequences:
- XXY → Female (but abnormal).
- XO → Male (sterile, called "Turner’s type" in humans).
Morgan and Bridges used this phenomenon to prove that genes are located on chromosomes and sex-linked traits follow chromosomal behavior.
2. Sex-Limited Traits
Traits that are expressed only in one sex, though the genes are present in both sexes. Expression is controlled by sex hormones.
- Milk production in cattle (females only).
- Beard development in humans (males only).
- Plumage pattern in domestic fowls.
Key Point: These traits are autosomal in origin but phenotypically limited to one sex.
3. Sex-Influenced Traits
Traits where dominance of an allele differs between sexes due to hormonal differences. The gene is autosomal, but phenotypic expression varies with sex.
- Baldness in humans: Caused by a dominant allele (B).
In males: B is dominant (BB or Bb → bald).
In females: B is recessive (BB → bald, Bb → normal). - Horn development in sheep:
In males: allele for horns is dominant.
In females: allele for horns is recessive.
- Sex linkage: Trait controlled by genes on sex chromosomes (Example: White eye in Drosophila, attached X, non-disjunction).
- Sex-limited traits: Autosomal traits expressed only in one sex (e.g., milk production, cock feathering).
- Sex-influenced traits: Autosomal traits expressed differently in males and females due to hormones (e.g., baldness in humans).
