5-1

Overview of Non-Mendelian Genetics

  • The lecture focuses on exceptions to Mendel's principles of inheritance, specifically discussing how various factors affect phenotype expression beyond the Mendelian framework.

Mendel's Principles Recap

  • Mendel's principles of segregation and independent assortment help predict outcomes of genetic crosses.

  • Aim is to explore dominance types and how they affect phenotype.

Alleles Definition

  • Allele: A version of a gene at a specific locus.

  • Dominant and recessive traits/alleles:

    • Dominance defined by Mendel; a heterozygote shows the same phenotype as one homozygote, indicating which allele is dominant.

Types of Dominance

  • Complete Dominance: One allele completely masks the presence of another allele.

  • Incomplete Dominance: Heterozygotes display a phenotype that is intermediate between the two homozygotes.

  • Codominance: Both alleles in a heterozygote contribute to the phenotype distinctly, rather than in an intermediate form.

Complete Dominance

  • Example:

    • Genotypes: A1A1 (red flowers, homozygous dominant) vs. A2A2 (white flowers, homozygous recessive).

    • If a heterozygote A1A2 displays the same phenotype as A1A1, then A1 is dominant.

Incomplete Dominance

  • A heterozygote (e.g., A1A2) shows an intermediate phenotype (e.g., pink flowers).

  • Example of incomplete dominance:

    • Genotype of purple fruit: P P and white fruit: p p produces offspring with an intermediate violet color (P p).

  • F2 generation results in a phenotypic ratio of 1:2:1 (1 purple: 2 violet: 1 white) rather than the typical Mendelian 3:1 ratio.

Codominance

  • Example with chickens:

    • Black chicken (BB) x White chicken (WW) results in heterozygotes (BW) exhibiting a checkered pattern, expressing both black and white.

  • Example with cattle:

    • Roan cattle (crossing red RR and white WW) produces heterozygotes with distinct red and white patches (not intermediate).

Penetrance and Expressivity

  • Incomplete Penetrance: Only a percentage of individuals with a specific genotype express the expected phenotype.

    • Complete penetrance is 100%, incomplete is less than 100%.

  • Expressivity: Degree to which a phenotype is expressed. Variability exists even among individuals with the same genotype.

    • Example: Polydactyly in humans exhibits incomplete penetrance as some genotypes do not manifest the trait.

Examples of Penetrance and Expressivity

  • Polydactyly Pedigree: A genetic trait that may not express in all carriers, showing incomplete penetrance.

  • Ichabod mutants in zebrafish: Despite similar genotypes, phenotypic variability (expressivity) is observed in the patterns of coloration.

Multiple Alleles

  • Traits can have more than two alleles within a population, but individuals still only have two alleles for a given gene.

  • Example: Rabbit coat color influenced by multiple alleles (C+, C^H, C^h), leading to various phenotypes.

  • Wild type allele: Most common allele designated with a plus sign (e.g., C+).

  • Polymorphism: Different alleles in a population present at a frequency greater than 1%.

Mutations

  • Different mutations can affect the same gene leading to different phenotypes.

  • Recessive mutations: Often involve a loss of protein function.

    • Null allele: Complete loss of function.

    • Hypomorphic allele: Partial loss of function.

  • Example of mutations expressed in rabbits: color production affected by specific mutations leads to a spectrum of coat colors from fully functional to absent pigmentation.

ABO Blood Group System

  • An example of multiple alleles and codominance.

  • Blood phenotypes: A (IAIA or IAi), B (IBIB or IBi), AB (IAIB), O (ii).

    • A and B are codominant to each other, while O allele is recessive.

  • Blood compatibility: Type O blood can be given to any type (universal donor) as it lacks A and B antigens. Type AB individuals are universal recipients since they have no antibodies against A or B antigens.

Environmental Influence on Traits

  • Some traits are influenced by environmental conditions, leading to variable expressivity.

    • Example: Temperature-sensitive alleles in fruit flies affecting wing size based on environmental temperature (functional only above 25°C).

    • Himalayan rabbit coat color: Temperature at extremities causes differential pigmentation.

Conclusion

  • This lecture elaborates on how various types of dominance, penetrance, expressivity, and multi-allelic traits can contribute to the complexity of inheritance beyond Mendelian genetics. Further exploration of mutations and environmental effects deepens understanding of genetic expression and its variability.