Genetic Sex Determination and Sex-Linked Traits

Genetic Sex Determination and Sex-Linked Traits

Introduction to Genetic Sex Determination

• Understanding dominant and recessive alleles.
• Key concepts: incomplete dominance, codominance, and multiple alleles.
• Lethal alleles illustrated with blue foxes.

Sex Chromosomes and Genetic Sex

• Chromosomes Overview:
  • Humans have 22 pairs of autosomes and 1 pair of sex chromosomes (XX or XY).
  • Autosomes carry regular traits; sex chromosomes determine genetic sex.
• Genetic females (XX) have eggs with only X chromosomes; genetic males (XY) produce sperm with either X or Y.
• Terminology:
  • Homogametic: two identical sex chromosomes (XX).
  • Heterogametic: two different sex chromosomes (XY).
• Distinction between genetic sex and gender identity: genetic males and females refer to biological chromosome status, not gender.

Genetic Traits on Sex Chromosomes

• Sex-Linked Traits:
  • Defined as traits on the sex chromosomes, particularly X and Y in humans.
  • X-linked traits have more variability since females have two X chromosomes (XX), whereas males have one (XY).
  • Hemizygous: describes genetic males having only one allele for X-linked traits.

Example of Punnett Squares with X-Linked Traits

• Genetic examples show how X-linked traits are inherited, e.g., color blindness.
• Males inherit X-linked traits from their mothers and Y from their fathers (fathers do not pass X-linked traits to sons).

Specific Examples of Sex-Linked Traits

• Color Blindness:
  • X-linked recessive trait; females need two copies of the mutated allele to express the trait.
  • Male offspring of a colorblind mother must inherit the mother's X chromosome with the mutated allele.
  • Punnett Squares illustrate genetic ratios:
  • For color blind mothers crossed with normal fathers:
    • Offspring: All sons colorblind and all daughters normal vision.
• Drosophila Melanogaster Case:
  • Thomas Hunt Morgan's experiments on red and white eye mutations in fruit flies, demonstrating X-linked inheritance.
  • Example crosses show sex-linked ratios for traits based on reciprocal crosses.

Other Forms of Genetic Sex Influence

• Genetic sex can influence phenotype through:
  • Sex Influenced Traits: Different traits expressed based on sex despite same alleles.
  • Example: Male pattern baldness is more commonly expressed in males than in females.
  • Sex Limited Traits: Traits expressed only in one sex, e.g., fancy feathering in chickens.

Mechanisms of Sex Determination

• Human sex determination relies partly on SRY gene, present on the Y chromosome.
• Drosophila sex determination is dependent on the ratio of X chromosomes to autosomes.
• Alternative sex determination systems, e.g., ZW in birds and environmental factors (temperature in reptiles) influence phenotypes.

Summary of Ratios and Predictions

• Genetic predictions for sex-linked traits may yield unexpected results based on the sex of the parent (red vs. white eye mutations in fruit flies).
• Crosses yield distinct proportions:
  • F1 and F2 generation representations reveal hidden genetic relationships.
  • Reciprocal crosses yield different outcomes illustrating clear distinctions between dominant and recessive traits.

Conclusion

• Understanding sex-linked inheritance is crucial for genetic studies, especially for traits uniquely influenced by sex chromosomes. Knowledge of sex determination and allele interactions is fundamental to genetic biology.