The Bad News on Inheritance Patterns

  • Inheritance patterns are often more complex than predicted by simple Mendelian genetics.
  • The relationship between genotype and phenotype is rarely simple.

The Spectrum of Dominance

Complete Dominance

  • Defined as when the phenotypes of the heterozygote and dominant homozygote are identical.
  • Example: Purple pea flowers.

Multiple Alleles of a Single Gene in a Population

  • Conceptual analogy: One word can have different spellings (e.g., color (US), colour (UK), couleur (France)).
  • Source: Arises from mutation.
  • Examples:
    • Blood groups in humans have 3 alleles: A, B, or O.
    • Hair color in rabbits involves 4 alleles.
  • Confounding Features of Inheritance (CFI): The Spectrum of Dominance.

Confounding Features of Inheritance (CFI)

Co-dominance

  • Definition: Occurs when two dominant alleles affect the phenotype equally and in distinguishable ways.
  • Example: Human blood groups where both alleles are expressed at the phenotypic level leading to six possible genotypes.

ABO Blood Groups

  • Determined by multiple alleles: IA, IB, i.
  • “I” codes for an enzyme that attaches carbohydrates to blood cells.
    • Allele IA → produces A-type carbohydrates.
    • Allele IB → produces B-type carbohydrates.
    • Allele i → recessive to both IA and IB (no carbohydrates).
  • Universal Recipient: Type AB+ blood can receive from A+, A-, O+, O-, B+, B-, AB+.
  • Universal Donor: Type O- blood can donate to A+, A-, O+, O-, B+, B-, AB+.

Rh Factor Genes

  • Inherited in a dominant/recessive fashion:
    • Rh+ (Rh present) = dominant.
    • Rh- (Rh absent) = recessive.
  • Blood Type Compatibility:
    • Table of who can give and receive blood based on different types.

CFI Example: Multiple Alleles in Rabbits

  • Four different alleles exist for the rabbit coat color (C) gene:
    • C (dominant) > cch (dominant over ch) > ch (dominant over c).

CFI: Incomplete Dominance

Definition

  • Incomplete dominance occurs when the phenotype of F1 hybrids is intermediate between the two parental phenotypes.
Example of Incomplete Dominance in Hair Texture
  • Trait types:
    • Curly Hair (HH)
    • Straight Hair (hh)
    • Wavy Hair (Hh)
  • Heterozygotes (Hh) express a phenotype that is intermediate (wavy hair).
Incomplete Dominance Diagram
  • Represents fertilization results of different gametes leading to F1 Generation displaying wavy hair.

CFI: Pleiotropy

  • Definition: An allele has multiple phenotypic effects.
  • Example: Sickle cell disease has the following effects:
    1. Deformed red blood cells.
    2. Physical weakness.
    3. Pain and organ damage.

CFI: Extending Mendelian Genetics for Two or More Genes

  • Some traits may be determined by two or more genes affecting a single phenotypic trait.
Two Scenarios:
  1. Epistasis: One gene at one locus alters or controls the expression of another gene at a different locus.
  2. Polygenic Inheritance: Additive effect of two or more genes on a single phenotypic character.
    • Example of Epistasis:
      • Gene 1 (B/b): Controls coat color (black or brown).
      • Gene 2 (E/e): Controls pigment deposition.
      • Result: Homozygous recessive (ee) will result in a golden color regardless of B/b alleles.
Epistasis Example
  • The gene for total baldness is epistatic to the genes for blond or red hair.

Polygenic Inheritance - Example

  • Traits like skin color and height show variation along a continuum (quantitative characters), indicating polygenic inheritance.
  • Example offspring ratios corresponding to different phenotypic presentations influenced by multiple genes.

Human Quantitative Traits

  • Study referenced from Nature Genetics which indicates genetic components influencing human height.
  • The study included:
    • 253,288 individuals identified 697 variants related to heritability and phenotypic diversity in height.
    • Causal variants and associated pathways were identified including growth mechanisms.

CFI: Nature and Nurture - Environmental Impact on Phenotype

  • Characters influenced by both genetic and environmental factors termed multifactorial characters.
  • Mendel’s fundamental laws of segregation and independent assortment still apply.

The Norm of Reaction

  • Definition: The range of phenotypes for a given genotype influenced by the environment.
  • Example: Effect of soil pH on Hydrangea flower color.

Genotypes vs. Phenotypes

  • Concept that genotypes are crucial but not sufficient for explaining phenotypes.
  • Influence of environmental vs genetic factors in twin studies displayed various disorders.

Mendelian Inheritance and Chromosomal Theory of Inheritance

  • Chromosomal theory states that:
    • Mendelian genes have specific loci on chromosomes.
    • Chromosomes experience segregation and independent assortment during gamete formation.

Thomas Hunt Morgan's Contributions

  • His experiments with fruit flies led to strong evidence that chromosomes are the location of Mendel’s heritable factors.
  • Distinguished between wild type and mutant phenotypes.

Correlating Gene's Alleles with Chromosome Behavior

  • Morgan's findings on X-linked inheritance demonstrated traits like white eyes occurring only among male offspring.

The Chromosomal Basis of Sex

  • Determined largely by sex chromosomes (X and Y).
    • Typical mammalian composition: 44 autosomes + XY/XX.
    • The Y chromosome contains few genes primarily related to sex determination (SRY gene).

Sex-Linked Genes and Traits

  • Genes located on sex chromosomes; predominantly X-linked traits are common.
  • Example: Eye color in Drosophila - red eye dominant to white.

X and Y Chromosome Inheritance

  • Males inherit both X and Y-linked traits, while females receive two X-linked traits and demonstrate typical dominant-recessive expression patterns.
Y Chromosome and Genetic Recombination
  • The Y chromosome rarely undergoes genetic recombination due to its unique inheritance features.

Genghis Khan’s Genetic Legacy

  • Study showing Y chromosomes and their implications on biodiversity traced back to historical figures like Genghis Khan.

Other Sex Determination Systems

  • Different systems found in other organisms like Z-W and haplo-diploid systems.

Genetic Disorders - Dominantly Inherited Traits

  • Dominant alleles lead to phenotypic expressions not necessarily more common than recessive alleles.
    • Example: Polydactyly, a dominant condition.

Achondroplasia

  • A type of dwarfism caused by a rare dominant allele leading to distinct inheritance patterns.

Recessively Inherited Disorders

  • Disorders exhibited only in homozygous recessive individuals; carriers are phenotypically normal.
Example: Sickle-Cell Disease
  • Conditions exemplifying the effects of sickle cell alleles and their impact on health.

X-linked Disorders

  • Female carriers may not express traits linked to X-recessive alleles due to allele masking from a normal X.
  • Example diseases: Color blindness, hemophilia, etc.

Genetic Testing and Pedigree Analysis

  • Use of pedigrees to describe parent-child relationships can predict future offspring conditions and traits.

Genetic Testing for Disorders

  • Information provided through genetic counseling: carriers, disease likelihood, and diagnostic tests available for various genetic disorders.

Social Issues Surrounding DNA Screening

  • The impact of genetic information raises social concerns regarding privacy and discrimination.
    • GINA (Genetic Information Nondiscrimination Act) protects individuals from genetic discrimination in health contexts.