Module 3.2 - Non-Mendelian Genetics

Central Philippine University

College of Medical Laboratory Science

Topic Focus: Non-Mendelian Genetics

Non-Mendelian Genetics

• Definition: Any inheritance pattern that does not strictly follow Mendelian laws.

  • Law of Segregation (First Law): Maternal and paternal alleles separate during gamete formation and recombine during fertilization.

  • Law of Independent Assortment (Second Law): Alleles for different traits segregate independently during gamete formation.

  • Law of Dominance (Third Law): Dominant alleles can mask the expression of recessive alleles.

  • Exceptions: Genetic interactions may not adhere to complete dominance or show simultaneous expression.

Complexity of Genetics

• Observations: Not all predicted traits align with test crosses; Mendel’s laws are foundational but real genetic scenarios are more intricate than Mendel’s pea experiments.

Outline of Topics

I. Alterations to Gene Expression

A. Lethal Alleles

B. Multiple Alleles

C. Incomplete Dominance

D. Codominance

E. Epistasis

F. Penetrance and Expressivity

G. Pleiotropy

H. Genetic Heterogeneity

I. Phenocopy

II. Non-Mendelian Inheritance

A. Extranuclear Inheritance

B. Gene Linkage

C. Sex-linked Traits

I. Alterations to Gene Expression

• Phenotypes are seldom controlled by single genes alone as suggested by Mendel.

  • Genotypic ratios remain, but various factors impact phenotype.

  • Factors affecting single gene phenotypic ratios include:

    • Lethal Alleles

    • Multiple Alleles

    • Incomplete Dominance

    • Codominance

    • Epistasis

    • Penetrance and Expressivity

    • Pleiotropy

    • Genetic Heterogeneity

    • Phenocopies

A. Lethal Alleles

• Definition: Genotypes that cause death before reproduction, affecting expected progeny class.

  • Example:

    • Achondroplastic dwarfism

    • Mexican hairless dogs

• Genotypes for Achondroplasia:

  • AA = lethal, Aa = achondroplasia, aa = normal height

B. Multiple Alleles

• Genes can present multiple alleles, resulting in various phenotypic outcomes.

  • Example: PKU gene has hundreds of alleles, leading to four variations.

  • CF gene has 1500 alleles with varied effects, such as different health issues.

C. Incomplete Dominance

• One allele is not completely dominant; both traits blend.

  • Example Cross: RR (Red) x WW (White) yields RW (Pink).

  • Familial hypercholesterolemia:

    • Heterozygote has fewer receptors for LDL cholesterol than homozygotes.

D. Codominance

• Both alleles are equally expressed in heterozygotes.

  • Example: In certain chicken breeds, black and white feather colors are codominant.

  • In humans, ABO blood type illustrates codominance with IA and IB alleles.

E. Epistasis

• One gene affects the expression of another gene.

  • Example: Hair color in Labrador Retrievers involves two gene pairs that influence pigment production and deposition.

• Human Example: Bombay phenotype, where hh genotype produces O phenotype regardless of ABO alleles.

F. Penetrance and Expressivity

• Penetrance: Percentage of individuals with a genotype expressing the expected phenotype.

  • Example: Polydactyly (80% penetrance).

• Expressivity: Variable severity of phenotype for individuals with the same genotype.

  • Example: Hypercholesterolemia varies from very high to manageable levels.

G. Pleiotropy

• One gene influences multiple traits or has multiple effects.

  • Example: Marfan syndrome exhibits a variety of symptoms due to a single gene defect.

H. Genetic Heterogeneity

• Different genes can result in the same phenotype, such as in conditions like Leber congenital amaurosis.

I. Phenocopy

• An environmentally caused condition mimicking inherited traits.

  • Examples include teratogen exposure causing birth defects.

II. Non-Mendelian Inheritance

A. Extranuclear Inheritance

• Mitochondrial DNA is passed from mother to child, influencing mitochondrial diseases.

  • Example: Ooplasmic transfer technique allows avoiding mitochondrial disorders.

B. Gene Linkage

• Genes located close to each other on chromosomes tend to be inherited together, violating independent assortment.

C. Sex-linked Traits

• Genes located on sex chromosomes, mainly the X chromosome.

  • More common in males; examples include color blindness and Duchenne muscular dystrophy.

Conclusion

• Understanding non-Mendelian genetics expands upon Mendel's foundational work by examining complex inheritance patterns and their implications for phenotypic expression.