Chapter 6/Chapter 7

Sex Chromosomes and Meiosis

• Genotypes:

  • Females: $XX$

  • Males: $XY$

• Chromosome Naming: The letters X and Y are names for the chromosomes, not descriptions of their physical shape.

• Meiosis Process:

  • Before Synthesis: Chromosomes exist as single DNA molecules.

  • After Synthesis: Each chromosome has replicated, consisting of two sister chromatids joined at a centromere.

  • Meiosis I: Homologous chromosomes separate.

    • In a female ($XX$): Each homologous pair of X chromosomes separates, resulting in two cells, each with one replicated X chromosome.

    • In a male ($XY$): The X and Y homologous chromosomes separate, resulting in one cell with a replicated X chromosome and another cell with a replicated Y chromosome.

  • Meiosis II: Sister chromatids separate.

    • In a female: The replicated X chromosomes separate into individual X chromatids. This produces four cells (egg cells), each containing a single X chromosome (one DNA molecule).

    • In a male: The replicated X and Y chromosomes separate into individual chromatids. This produces four cells (spermatozoa): two containing a single X chromosome and two containing a single Y chromosome.

X-linked Traits

• Definition: Traits whose genes are located on the X chromosome.

• Copy Number:

  • Males: Possess only one X chromosome and one Y chromosome. Therefore, they have only one copy of any X-linked gene.

  • Females: Possess two X chromosomes, thus having two copies of any X-linked gene.

• Inheritance Patterns from Males:

  • To Sons: A male contributes his Y chromosome to his sons. The Y chromosome does not carry genetic information for X-linked traits. Therefore, males cannot pass X-linked traits directly to their sons.

  • To Daughters: A male contributes his single X chromosome to all his daughters. If a male is affected with an X-linked recessive trait (e.g., genotype $X^aY$), he will pass his $X^a$ allele to all his daughters, making them at least carriers.

• Inheritance Patterns from Females:

  • To Sons: A female passes one of her two X chromosomes to each son. If she is a carrier ($X^AX^a$), she has a $50\%$ chance of passing the recessive allele ($X^a$) to any given son, who would then express the trait since he only has one X chromosome.

  • To Daughters: A female passes one of her two X chromosomes to each daughter.

• Expression in Females: For a female to express an X-linked recessive trait, she must inherit the recessive allele on both her X chromosomes (e.g., genotype $X^aX^a$). If she inherits one dominant and one recessive allele ($X^AX^a$), she is typically a carrier and does not express the trait.

• Prevalence: X-linked recessive traits are observed more frequently in males than in females because males only need one copy of the recessive allele to express the trait ($X^aY$), whereas females need two copies ($X^aX^a$).

Pedigree Analysis: Determining Inheritance Patterns

To analyze a pedigree and determine the mode of inheritance, a systematic approach using a flowchart of questions is highly effective.

• Question 1: Is the trait Y-linked?

  • Rule: If a trait is Y-linked, it will only affect males and will be passed directly from an affected father to all of his sons. If any female in the pedigree is affected or if an affected male has unaffected sons, the trait cannot be Y-linked.

• Question 2: Is the trait Dominant or Recessive?

  • Rule for Dominant: If the trait is dominant, every affected individual must have at least one affected parent. The trait typically appears in every generation and does not skip generations.

  • Rule for Recessive: If the trait is recessive, affected individuals can have unaffected parents (who are typically heterozygous carriers). This means the trait can