Knowlet

UNIT 4: Sex Determination and Sex Linkage

Exam Focus: Differentiate between chromosomal and environmental sex determination. Understand the mechanism of X-inactivation/Dosage Compensation (Barr Bodies). Be prepared to solve simple problems involving X-linked recessive inheritance like Colour Blindness or Haemophilia.

Table of Contents

  1. Sex Determination
  2. Sex-linked Inheritance

1. Sex Determination

Sex determination is the biological system that determines the development of sexual characteristics in an organism.

Mechanisms of Sex Determination (Chromosomal)

The sex chromosomes (X and Y in humans) carry the genes that determine the sex.

Type Organism/Description Female Male
XX-XY Type Humans, Mammals, Drosophila (though determined differently) Homogametic (XX) Heterogametic (XY)
ZZ-ZW Type Birds, Reptiles, some Fishes Heterogametic (ZW) Homogametic (ZZ)
XX-X0 Type Grasshoppers, Cockroaches XX X0 (one X chromosome)

Environmental Factors in Sex Determination

In some organisms, sex is not determined by chromosomes but by external environmental conditions.

  • Temperature-Dependent Sex Determination (TSD): Found in many reptiles (e.g., crocodiles, alligators, many turtles). The temperature during a critical period of egg incubation determines the sex of the offspring.
  • Social Sex Determination: In some fish (e.g., the Bluehead Wrasse), the social structure (e.g., the size or presence of a dominant male) can cause a female to change her sex.

Dosage Compensation and Barr Bodies

In mammals, females have two X chromosomes (XX) while males have one (XY). This difference in X-linked gene dosage is balanced through a process called **Dosage Compensation**.

  • Dosage Compensation: The process by which the expression of X-linked genes is equalized between the sexes. In placental mammals, this is achieved by **inactivating one of the two X chromosomes in females**.
  • Barr Bodies (Sex Chromatin): The inactivated X chromosome condenses into a compact, transcriptionally inactive structure called a Barr Body (or sex chromatin), which is visible near the nuclear envelope of female somatic cells.
  • Lyon Hypothesis: Postulates that the inactivation of one X chromosome in female cells is random and occurs early in embryonic development, leading to a mosaic pattern of gene expression (e.g., the coat color pattern in Calico cats).

Genetic Balance Theory

Proposed by C.B. Bridges for sex determination in **Drosophila** (fruit flies).

  • Sex is determined by the **ratio of X chromosomes to the sets of autosomes (A)**, not simply by the presence of the Y chromosome.
  • Sex determination formula: Sex = Ratio of X / A sets.
    Ratio (X/A) Genotype Sex (Drosophila)
    1.0 (XX/AA) XXAA Female
    0.5 (XY/AA) XYAA Male
    1.5 (XXX/AA) XXXAA Metafemale (or Superfemale)
    0.33 (X0/AAA) X0AAA Metamale (or Supermale)
    The Y chromosome is primarily responsible for male fertility in Drosophila.

2. Sex-linked Inheritance

Sex-linked Inheritance

The pattern of inheritance resulting from genes located on the sex chromosomes (most commonly the **X chromosome**).

  • Criss-cross Inheritance: A male (XY) transmits his X-linked trait to his daughter, who then transmits it to her son (i.e., from grandfather to grandson through an unaffected carrier daughter). This is characteristic of X-linked recessive traits.
  • Males are Hemizygous: Since males (XY) only have one X chromosome, they express any trait coded on the X chromosome, even if it is recessive.

Fragile-X-Syndrome

A genetic condition causing a range of developmental problems, including learning disabilities and cognitive impairment.

  • Cause: It is an X-linked dominant disorder (though with incomplete penetrance) caused by a mutation in the **FMR1 gene** on the X chromosome.
  • Features: Characterized by an unusually long, thin, or "fragile" appearance of the X chromosome near the tip of the long arm in laboratory culture. It is the most common inherited cause of intellectual disability.

Sex-linked Inheritance: Haemophilia

An X-linked recessive bleeding disorder caused by a deficiency in one of the blood clotting factors (usually Factor VIII or Factor IX).

  • Inheritance: Recessive allele 'h' is on the X chromosome.
    • XH Y → Normal Male
    • Xh Y → Haemophiliac Male (Affected)
    • XH XH → Normal Female
    • XH Xh → Carrier Female (Unaffected)
    • Xh Xh → Haemophiliac Female (Rare, requires affected father and carrier/affected mother)

Sex-linked Inheritance: Colour Blindness

The inability to distinguish between certain shades of color (most commonly red-green) due to a recessive gene on the X chromosome.

  • Inheritance: Follows the same pattern as Haemophilia. The recessive allele 'c' for color blindness is on the X chromosome. It is much more common in males due to their hemizygous state.
  • Genotype of an affected male: Xc Y.
  • Genotype of a carrier female: XC Xc.
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