Sex Chromosomes 4-2

Overview of Sex Chromosomes and Traits

  • Traits transmitted through sex chromosomes are distinct from those on autosomes.

  • Autosomes (chromosomes 1-22) are homologous, meaning they pair and contain the same genes from both parents.

Human Karyotype

  • A full set of human chromosomes is shown in the human karyotype, which includes both autosomes and sex chromosomes.

  • Chromosomes 1-22 are homologous chromosomes that match in shape and size.

Sex Chromosomes: X and Y Chromosomes

  • Sex chromosomes:

    • In humans, there are two sex chromosomes: the X chromosome and the Y chromosome.

    • The X chromosome is significantly larger than the Y chromosome and contains approximately 1,000 genes.

    • The Y chromosome contains only about 50 genes.

    • Most genes present on the X chromosome do not have counterparts on the Y chromosome, establishing their unmatched status.

    • The only regions of homology between the X and Y chromosomes are the tips, where they share a small sequence of genes.

  • Sex determination:

    • Females (XX) have two X chromosomes.

    • Males (XY) have one X and one Y chromosome.

Sex-Linked Traits

  • We focus on X-linked recessive traits in this discussion.

  • Recessive and Dominant Traits:

    • This course focuses exclusively on recessive traits in relation to sex linkage.

Example: Hemophilia in the Royal Family

  • The pedigree of the royal family illustrates the transmission of an X-linked recessive trait (hemophilia).

  • Pedigree symbols:

    • Solid colored boxes indicate affected individuals (those with hemophilia).

    • Individuals with a dot in their circle are carriers who do not express the disease but carry the allele.

Characteristics of X-Linked Recessive Traits

  • More males than females are typically affected by X-linked recessive traits.

  • Affected sons are usually born to unaffected mothers (carriers).

  • Approximately half of the sons born to carrier mothers will be affected.

  • X-linked recessive traits are never passed from father to son.

  • Affected fathers pass the X-linked allele to all daughters, making them carriers.

Why are Males More Affected than Females?

  • Males express X-linked recessive alleles because they possess only one X chromosome, while females have two.

  • If a male inherits a recessive allele on their single X chromosome, they express the trait.

  • In contrast, females can possess one normal allele on their other X chromosome, potentially masking the recessive allele.

Meiosis and Sex Chromosome Transmission

  • During meiosis in females, a pair of X chromosomes is segregated into eggs.

  • During meiosis in males, X and Y chromosomes are segregated into sperm, resulting in a ratio of 1:1 for X-bearing and Y-bearing sperm.

Fertilization and Genotype Outcomes

  • Possibilities during fertilization:

    • XX (female): from X of both parents.

    • XY (male): from X of mother and Y of father.

  • Thus, a 50% probability exists for progeny being female (XX) and 50% for being male (XY).

Heterozygous Cross Example

  • In a cross between a heterozygous carrier female (X^hX) and a normal male (X^NY):

    • Gametes from female: 50% X^h (mutant allele), 50% X (normal allele).

    • Male contributes: 50% X (normal), 50% Y.

    • Progeny outcomes:

      • Half daughters are carriers (X^hX): not affected by the disease.

      • Half sons (X^hY) carrying the recessive allele: affected males.

      • Other sons will inherit a normal Y and a normal X: unaffected males.

Characteristics in Different Crosses

  • Affected Male Cross:

    • Affected males (X^hY) and normal females (XX):

      • Daughters (X^hX): carriers.

      • Sons (XY): normal.

  • Normal Female with Affected Male:

    • Daughters receive X^h from the father but a normal X from the mother, making them carriers.

    • Sons will inherit the Y chromosome from the father, receiving a normal X from the mother.

Pedigree Analysis of X-Linked Recessive Traits

  • Patterns in pedigrees often show the skipping of generations due to carriers not expressing traits.

  • Affected male descendants often pair with carrier females, perpetuating recessive traits.

Example of Color Blindness as an X-Linked Recessive Trait

  • In the example of X-linked recessive traits like color blindness, affected males are clearly identifiable, while unaffected females can be carriers.

Problem Solving in Pedigree Analysis

  • Analyze pedigrees for X-linked traits:

    • Determine probabilities of inheritance through descendants and carriers.

    • Conduct analyses regarding potential affected progeny based on parental genotypes, focusing on carrier status.

Key Takeaways

  • X-linked recessive traits affect males more commonly due to the singular contribution of their X chromosome without a corresponding allele on the Y chromosome.

  • Crosses between known carriers and affected individuals allow tracking of the probability of the recessive traits through generations.