Chapter 15: Inheritance of Sex Chromosomes, Linked Genes, and Organelles

Distinctive Sex Chromosomes: Many organisms exhibit pairs of chromosomes that are distinct between sexes and possess unique inheritance patterns.
- Example: Human X and Y chromosomes.
Crisscross Inheritance of X-Linked Genes: X-linked genes exhibit a pattern in which males inherit their X chromosome exclusively from their mothers.
- Implication: Traits on the X chromosome show different patterns of inheritance in males and females.
Close Genes and Linked Inheritance: Genes that are physically close on the same chromosome tend to be inherited together, deviating from independent assortment.
Y-Linked Gene Inheritance: Y-linked genes are typically passed from fathers to sons only.
Organellar Inheritance: Mitochondria and chloroplasts possess separate DNA and follow unique inheritance patterns distinct from nuclear DNA.

Transmission Differences: Genes on sex chromosomes are inherited differently in males and females, violating the simple models proposed by Mendel.
Lack of Independent Assortment: Genes that are closely linked do not assort independently.

Meiosis I and Sex Chromosome Segregation: The homology (similarity) between X and Y chromosomes facilitates their segregation during anaphase of meiosis I.
Result of Random Fertilization: Typically results in a 1:1 sex ratio of XX (female) and XY (male) offspring.

T.H. Morgan's Experiments: Noted a white-eyed male fruit fly among red-eyed variants, leading to the exploration of X-linked inheritance.
- F1 progeny exhibited a typical dominance pattern; however, further crosses revealed a deviation from Mendelian predictions.
X-Linked Recessive Traits:
- Generally expressed in males (due to single X chromosome) and can result in affected daughters who are carriers.
- Common examples: color blindness and hemophilia.

Nondisjunction Phenomena: Occasionally, chromosomes fail to separate properly in meiosis, leading to gametes with abnormal chromosomal constitutions.
- Example from Bridges: Rare occurrences of offspring (XXY or XO) resulted from this event, impacting traits like white eyes in fruit flies.
- Conclusively demonstrated the chromosomal basis for inheritance patterns.

Recombination and Linked Genes: Crossing over during meiosis introduces recombinants and can change allele combinations, but proximity of genes affects recombination rates.
Genetic Maps: Created using recombination frequencies; one percent recombination equates to a genetic map unit.

Unique Inheritance Pattern: Y-linked genes do not crossover with X chromosomes, transmitted strictly from father to son.
Y Chromosome Haplotypes: Utilization of Y chromosome haplotypes aids in tracing ancestral lineage due to its singular inheritance.

Ancient Organelle DNA: These organelles have been retained and transferred in eukaryotic cells since their initial engulfment by ancestral prokaryotic cells.
Patterns of Inheritance:
- Maternal: Mitochondrial DNA primarily inherited from the mother.
- Paternal and Biparental Inheritance: Occurs in some organisms, though less common.
Tracing Ancestry Using Mitochondrial DNA: It is useful for ancestry tracing as it remains unchanged through generations aside from mutation.

Traits and Mitochondrial Mutations: Defects in mitochondrial DNA can lead to observable traits such as muscle fiber mutations, which while present in both genders, are maternally transmitted, thereby affecting all offspring of affected mothers.