lecture 15: Sex Linked

Linkage and Inheritance Patterns

• Linkage refers to multiple genes that are associated with each other, resulting in altered inheritance patterns.

• Today’s focus remains on basic Mendelian principles, with expectations of complexity next week due to linked genes.

Biological Sex Determination

• Definition: Biological sex is determined by specific chromosomes inherited from parents.

• Biological sex is established early in development based on the genetic information contained within gametes (sperm and egg).

• Historical perspectives on sex determination trace back to thinkers like Aristotle.

Aristotle's Perspective on Sex Determination

• Proposed that men possessed "heat" and women had "coldness."

• The concept related to sex determination suggested that male heat could produce male children, while a female's cold led to female offspring.

• Some modern parallels can be drawn from reptile behaviors, such as crocodiles determining sex based on egg temperature in nests.

Secondary Sexual Characteristics

• Secondary characteristics help differentiate biological sexes, such as:

  • Facial hair in males versus absence or minimal presence in females.

  • Disparate coloration in birds: males often display bright colors to attract females.

• Some organisms (e.g., nematodes) may appear identical; sex is determined uniquely by chromosome composition:

  • One X chromosome = male.

  • Two X chromosomes = hermaphrodite (both male and female traits).

Concepts of Chromosomes and Inheritance

• Chromosomes can be categorized as allosomes (sex chromosomes) and autosomes (non-sex chromosomes).

• Two sex chromosomes present in humans:

  • Homogametic (XX): Females can only produce X gametes.

  • Heterogametic (XY): Males can produce X or Y gametes, considered hemizygous due to different chromosome types.

  • The Y chromosome is critical for male biological sex.

Characteristics of Chromosomes

• Presence of the Y chromosome primarily determines male biological sex in mammals.

• Changes in autosomes lead to severe consequences, while sex chromosome variations often have milder effects (e.g., Klinefelter syndrome, XXY individuals possess male characteristics despite an extra X).

• For fruit flies (Drosophila melanogaster), sex is determined by X chromosome quantity:

  • One X = male.

  • Two or more X chromosomes = female.

Birds and Different Sex Chromosome Designation

• In birds, the sex chromosomes are designated as Z and W:

  • ZZ: Males (homogametic).

  • ZW: Females (heterogametic).

SRY Gene and Sex Determination

• The SRY gene on the Y chromosome is imperative for male sex determination.

• It influences the production of testosterone, guiding the development of male sexual characteristics during early embryonic development.

• The absence of the SRY gene leads to female development.

Pseudoautosomal Region (PAR)

• A small region on the tip of both X and Y chromosomes facilitates pairing during meiosis:

  • Ensures correct segregation during gamete formation.

  • Allows for recognition across differing chromosome types in males and females.

Sex-Linked Traits and Their Inheritance

• The Caltech Fly lab, led by Thomas Morgan, studied inheritance as related to sex, particularly using fruit flies as a model organism.

• The eye color gene in fruit flies exemplifies sex linkage:

  • Red eyes (dominant) linked to the X chromosome versus white eyes (recessive).

  • Reciprocal crosses help elucidate sex linkage:

    • If results differ between crosses, the trait is sex-linked; if not, it’s autosomal.

Color Blindness as an Example of Sex-Linked Trait

• Color blindness serves as a quintessential example of an X-linked trait:

  • Typically more common in males (inherited from mothers).

  • Different degrees of color blindness exist.

Further Traits: Sex Limited, Sex Influenced

• Sex-Limited Traits: Expressed only in one sex and linked to autosomes.

  • Example: Lactation involves enzymes that are expressed in females only.

• Sex-Influenced Traits: Traits that are affected by sex but appear on autosomes.

  • Example: Male pattern baldness is dominated by one allele in males, but differs in expression in females.