Chapter 15: Chromosomal Basis of Inheritance

15.1 Mendel and Chromosomal Inheritance

• Mendel's Discoveries: Mendel did not know about DNA or chromosomes but described inheritance patterns through pea plants.
• Chromosomal Theory of Inheritance:
  • Mendelian genes have specific positions (loci) on chromosomes.
  • Chromosomes undergo segregation and independent assortment during meiosis.
• Meiosis: Relationship to Mendel’s Laws:
  • Alleles separate during gamete formation.
  • Independent assortment allows combinations of mother’s and father’s chromosomes.
  • Explains why offspring have different phenotypes from parents.
• Morgan's Contributions:
  • Showed genes located on chromosomes using fruit flies.
  • Experimented with eye color genes on X chromosome.
  • Only male offspring had mutant (white eyes) showing sex-linkage.

15.2 Sex-Linked Genes

• Humans and Chromosomal Sex Determination:
  • Larger X and smaller Y chromosomes.
  • X chromosomes have genes not only related to sex.
  • Y chromosome has genes like SRY for male characteristics.
• Inheritance Patterns in Sex-Linked Genes:
  • Males are hemizygous for X-linked genes (one X chromosome).
  • Disorders like color blindness, muscular dystrophy often X-linked.
• Example of X-Linked Inheritance:
  • Different parental allele combinations impact offspring phenotype.
  • Females need two recessive alleles, males need one.

X Inactivation

• Mechanism:
  • One X chromosome in females is inactivated, forming a Barr body.
  • Random inactivation results in mosaic expression in heterozygous females.
• Example:
  • Cats with black and orange patches due to random X inactivation.

Linked Genes

• Definition: Genes located close together on the same chromosome.
  • Tendency to be inherited together.
• Experimentation by Morgan:
  • Looked at fruit flies’ body color and wing size.
  • Linked genes do not assort independently, confirmed by non-parental phenotypes.

Recombination and Genetic Mapping

• Genetic Recombination:
  • Crossing over during meiosis leads to new allele combinations.
  • Recombinants differ from parental types.
• Recombination Frequency:
  • Helps determine linkage; frequencies below 50% indicate linkage.
• Genetic Maps:
  • Based on recombination frequencies to show gene loci on chromosomes.

Chromosomal Alterations

• Non-disjunction:
  • Incorrect chromosomal separation during meiosis.
  • Results in aneuploidy (extra or missing chromosome copies).
• Types of Aneuploidy:
  • Monosomy (one copy), trisomy (three copies).
  • Examples: Down syndrome (trisomy 21), Klinefelter syndrome (XXY).
• Chromosomal Structural Alterations:
  • Deletion, duplication, inversion, translocation.
  • Impact gene expression and can lead to disorders.

Exceptions to Mendelian Inheritance

• Genomic Imprinting:
  • Differential expression of alleles based on parent of origin.
  • Involves methylation, often during gamete formation.
• Organelle Genes:
  • Genes in organelles like mitochondria inherited from mother.
  • Impact seen in conditions affecting ATP production, muscular/nervous systems.