simple inheritance

Simple vs. Complex Inheritance

Simple (Mendelian) Inheritance

• Traits controlled by a single gene with two alleles (dominant and recessive).
• Follows Mendel’s Laws:
  • Law of Segregation: Alleles segregate during gamete formation.
  • Law of Independent Assortment: Genes for different traits are inherited independently.
• Predictable inheritance patterns:
  • Examples:
  • $3:1$ ratio for monohybrid crosses.
  • $9:3:3:1$ ratio for dihybrid crosses.
• Examples of Mendelian disorders:
  • Cystic fibrosis (recessive)
  • Huntington’s disease (dominant)
  • Albinism

Complex (Non-Mendelian) Inheritance

• Traits influenced by multiple genes (polygenic) and/or environmental factors.
• Continuous inheritance patterns: Traits do not obey simple dominant/recessive rules.
• No simple predictable ratios; outcomes can vary.
• Examples:
  • Height, skin color, diabetes, heart disease
  • Often exhibit bell curve distribution in populations.

Autosomes vs. Sex Chromosomes

Autosomes

• Types of chromosomes: Non-sex chromosomes (pairs $1–22$ in humans).
• Carry majority of genes; traits inherited are the same in both sexes.
• Can be dominant or recessive.

Sex Chromosomes

• Types of chromosomes: X and Y (pair $23$ in humans).
• Determine biological sex:
  • XX = Female
  • XY = Male
• Inheritance of traits:
  • X-linked traits (e.g., hemophilia, colorblindness) more common in males.
  • Y-linked traits passed from father to son only.

Biological Sex Determination

Chromosomal Sex Determination in Humans

• Determined by the sperm:
  • Egg always contributes X
  • Sperm contributes either X or Y
• Possible Outcomes:
  • XX = Female
  • XY = Male

SRY Gene

• Location: Y chromosome
• Function: Triggers development of male characteristics (e.g., testes).
• Absence of Y: If no Y, embryo develops female characteristics.

Reciprocal Crosses

• Definition: Two crosses performed, reversing the sexes of the parents for a given trait.
• Example:
  • Cross 1: Female with trait × Male without trait
  • Cross 2: Male with trait × Female without trait
• Purpose:
  • Test if a trait is sex-linked or autosomal; in autosomal, results are consistent regardless of parental sex; differing results indicate sex-linked inheritance (usually X-linked).

Patterns Indicating Autosomal Inheritance

Autosomal Dominant Traits

• Key Features:
  • Present in every generation.
  • Affected individuals have at least one affected parent.
  • Equal frequency in males and females.
  • Affected parents can transmit trait to both sons and daughters.

Autosomal Recessive Traits

• Key Features:
  • Can skip generations; carriers may not show trait.
  • Affected offspring often born to unaffected (carrier) parents.
  • Trait appears when both alleles are recessive.
  • Equal frequency in males and females.

Summary: Spotting Autosomal Inheritance

Sex-linked Inheritance in Model Organisms

Morgan’s Discovery and the White Gene

• Pioneers in genetics: Thomas Hunt Morgan studied Drosophila melanogaster (fruit flies).
• In 1910: Discovered a mutant male with white eyes (normal is red).
• Cross results:
  • White-eyed male × red-eyed female → F1 all red-eyed.
  • F2 showed unique pattern: All females red-eyed, half males white-eyed.
• Conclusion: The white-eye gene is on the X chromosome; first evidence linking gene to chromosome.

X-linked Inheritance and Reciprocal Crosses

• X-linked Traits: Genes on the X chromosome; males (XY) express X-linked recessive traits more often due to one X chromosome.
• Females (XX): Can be carriers without exhibiting traits.
• Example with Drosophila (white-eye mutation):
  1. White-eyed male × Red-eyed female → F1: All red-eyed
  2. Red-eyed male × White-eyed female → F1: All females red-eyed, all males white-eyed
• Key Pattern: Traits are more frequent in males; often skip generations through carrier females.

Confirming the Chromosome Theory of Heredity

Chromosome Theory

• Concept: Genes reside on chromosomes, which undergo segregation and independent assortment during meiosis.
• Morgan’s work with X-linked gene: Provided evidence that indicated:
  • Specific genes are located on specific chromosomes.
  • Inheritance patterns align with chromosomal behavior in meiosis.
• Significance: Connected Mendel’s abstract “factors” with visible chromosomes, cemented the concept of genes having physical locations (loci) on chromosomes.

Pedigree Analysis

Pedigrees and Symbols

• Definition: Family trees showing trait inheritance across generations; useful in genetics.
• Common Symbols:
  • ◯ = Unaffected female
  • ⬛ = Affected male
  • ⬜ = Unaffected male
  • ● = Affected female
  • ◐ = Carrier (typically for X-linked)

Analyzing a Pedigree

1. Identify inheritance pattern (autosomal dominant, autosomal recessive, X-linked dominant/recessive).
2. Note affected individuals, their sex, and family position.
3. Look for:
   • Skipped generations → suggests recessiveness
   • Sex bias → suggests X-linkage
   • Every generation affected → likely dominant
   • Only males affected → possibly Y-linked (rare)

Pedigree Features for Inheritance Types

Assigning Genotypes in a Pedigree

1. Autosomal Recessive: Affected individual (aa); unaffected parents likely heterozygous (Aa).
2. Autosomal Dominant: Affected with unaffected child must be heterozygous (Aa); unaffected (aa).
3. X-linked Recessive: Affected male (XᵃY); carrier females (XᴬXᵃ if affected sons).

Tips for Analysis

• Work from knowns to unknowns.
• Use Mendelian logic: only two alleles, one from each parent.
• Cross-check generations for confirmation.