Non-Mendelian Genetics

Complex Patterns of Inheritance

Overview of Complex Patterns

• Inheritance can be more intricate than simple dominance.

• Major complex inheritance types include:

  • Incomplete Dominance

  • Codominance

  • Multiple Alleles

  • Sex-linked Traits

Incomplete Dominance

• Definition: Neither allele is dominant in heterozygous conditions.

• Result: A third phenotype, which is an intermediate expression between the two homozygous parents.

• Example:

  • Snapdragon Flower Color:

    • RR = red

    • WW = white

    • RW = pink

Example Cross: Pink and White Flower

• Cross Setup:

  • Parents: RW (pink) x WW (white)

• Genotypic Ratio: 0 RR : 2 RW : 2 WW

• Phenotypic Ratio: 0 Red : 2 Pink : 2 White

Codominance

• Definition: Both dominant alleles are fully expressed simultaneously in the heterozygous condition.

• Examples:

  • Roan Horses:

    • RR = chestnut

    • WW = white

    • RW = roan

  • Andalusian Chickens:

    • BB = black

    • WW = white

    • BW = black and white

    • (not gray)

Sickle Cell Anemia

• A genetic condition affecting hemoglobin in red blood cells:

  • Normal Hemoglobin (A) vs. Sickle Hemoglobin (S)

  • Genotypes:

    • AA = only normal cells

    • SS = only sickle cells

    • AS = carrier with both cell types (some sickled, some normal)

Example Cross: Normal RBC and Carrier of Sickle Cell Anemia

• Cross Setup:

  • A (normal) x A (normal) or S (sickle) carrier

• Resulting Genotypes:

  • AA: normal RBC;

  • AS: carrier with mixed RBC types.

Multiple Alleles

• Definition: Traits determined by more than two alleles.

• Example: Blood Type

  • Three alleles: A (dominant), B (dominant), and i (recessive).

  • Genotypes vs. Phenotypes:

    • A: AA or Ai

    • B: BB or Bi

    • AB: AB

    • O: ii

Blood Type and Antigens

• Detailed Blood Type Information:

  • Type A: A antigen, Anti-B antibodies, receives A, O; donates A, AB.

  • Type B: B antigen, Anti-A antibodies, receives B, O; donates B, AB.

  • Type AB: A and B antigens, no antibodies, receives A, B, AB, O; donates to A, B, and AB.

  • Type O: No antigens, both Anti-A and Anti-B antibodies, receives O; donates to all.

Example Cross: Type Ai and Type AB Blood

• Parent Genotypes: Ai x AB

• Possible Offspring Genotypes:

  • AA: 2

  • Ai: 1

  • AB: 1

• Phenotypic Ratio: 2 A : 1 B : 1 AB : 0 O

Example Cross: Type AB and Type O Blood

• Parent Genotypes: AB x O

• Possible Offspring Genotypes:

  • Ai: 2

  • Bi: 2

• Phenotypic Ratio: 2 A : 2 B : 0 AB : 0 O

Sex-Linked Traits

• Definition: Traits located on sex chromosomes (X or Y).

• Common Examples:

  • Hemophilia: clotting issue.

  • Muscular Dystrophy: muscle wasting disorder.

Inheritance of Sex-Linked Traits

• Fathers (XY) transmit X-linked disorders only to daughters (XX).

• Mothers (XX) can pass X-linked disorders to both sons (XY) and daughters (XX).

Example: Red-Green Colorblindness

• Involved partners: Heterozygous female (X^C X^c) and normal male (X^C Y).

• Genotypes: Cross results display ratios of normal vision, carriers, and colorblindness:

  • Genotypic Ratio: 1 X^C X^C : 1 X^C X^c : 0 : 1 X^C Y : 1 X^c Y

  • Phenotypic Ratio: Normal (2): Carriers (1): Colorblind (1).