Extensions of Mendelian Genetics: Key Concepts and Applications

Chapter 4: Extensions of Mendelian Genetics

Introduction

This chapter explores how inheritance patterns extend beyond simple Mendelian genetics. It covers the molecular and phenotypic consequences of different types of alleles, the use of genetic symbols, and the complexities introduced by multiple alleles, codominance, incomplete dominance, lethal alleles, and gene interactions. These concepts are fundamental for understanding genetic diversity and inheritance in populations.

4.1 Alleles Alter Phenotypes in Different Ways

Alleles and Phenotypic Variation

• Alleles are alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.

• Mutations can alter the function of the gene product, leading to new phenotypes by:

  • Eliminating enzyme function

  • Changing enzyme efficiency

  • Altering overall enzyme function

• Wild-type allele: The allele that occurs most frequently in a population, typically conferring the standard phenotype.

Types of Mutations

• Loss-of-function mutations: Reduce or eliminate the function of the gene product, often resulting in a recessive phenotype.

• Gain-of-function mutations: Enhance or confer new activity on the gene product, sometimes resulting in a dominant phenotype.

• Neutral mutations: Do not affect phenotype or evolutionary fitness.

4.2 Geneticists Use a Variety of Symbols for Alleles

Allele Notation

• Dominant alleles: Indicated by italic uppercase letters (e.g., D) or letter combinations.

• Recessive alleles: Indicated by italic lowercase letters (e.g., d).

• Mutant alleles: Indicated by italic letters, sometimes with superscripts.

• Wild-type alleles: Indicated by an italic letter plus a superscript + (e.g., w+).

Example: Drosophila melanogaster Body Color

• Wild-type (gray): e+

• Ebony mutant: e

No Dominance

• If no allele is dominant, uppercase letters with superscripts are used to distinguish alleles.

4.3 Neither Allele Is Dominant in Incomplete, or Partial, Dominance

Incomplete Dominance

Incomplete dominance occurs when the phenotype of the heterozygote is intermediate between the phenotypes of the two homozygotes. Neither allele is completely dominant.

• Each genotype produces a distinct phenotype.

• Phenotypic and genotypic ratios in the F2 generation are the same (1:2:1).

Example: Snapdragons

• Red flower (R1R1) × White flower (R2R2) → Pink flower (R1R2)

Human Example: Tay–Sachs Disease

• Homozygous recessive individuals lack Hexosaminidase A, leading to a fatal lipid-storage disorder.

• Heterozygotes have intermediate enzyme activity, but are typically asymptomatic.

Threshold Effect

• Normal phenotype is expressed as long as a certain threshold of gene product is present (often 50% or less).

• In Tay–Sachs, less than 50% enzyme activity leads to disease.

4.4 In Codominance, the Influence of Both Alleles in a Heterozygote Is Clearly Evident

Codominance

Codominance occurs when both alleles in a heterozygote are fully expressed, resulting in a phenotype that simultaneously displays traits from both alleles.

• No dominance or blending occurs.

• Both gene products are detectable in the heterozygote.

Example: MN Blood Group in Humans

• Individuals can express M, N, or both M and N antigens on red blood cells, depending on their genotype.

4.5 Multiple Alleles of a Gene May Exist in a Population

Multiple Alleles

More than two alleles can exist for a single gene within a population, though any individual can only possess two alleles for each gene.

• Multiple alleles result in more complex inheritance patterns.

Example: ABO Blood Groups in Humans

• Three alleles: IA, IB, and i

• IA and IB are codominant; both are dominant over i.

• Phenotypes: Type A, Type B, Type AB, and Type O blood.

Biochemical Basis: H Substance

• A and B antigens are carbohydrate groups attached to lipids on red blood cells.

• O blood type only has the H substance, lacking terminal sugars found in A or B types.

Bombay Phenotype

• Individuals with the Bombay phenotype appear as type O, even if they have A or B alleles, due to a lack of H substance (resulting from a mutation in the FUT1 gene).

• This prevents the formation of A or B antigens.

4.6 Lethal Alleles Represent Essential Genes

Essential, Dominant, and Recessive Lethal Alleles

• Essential genes are required for survival; mutations can be tolerated if heterozygous, but homozygous recessive individuals do not survive.

• Lethal alleles cause death when present in certain genotypes.

• Dominant lethal alleles cause death even when only one copy is present (e.g., Huntington disease).

Example: Agouti Gene in Mice

• The yellow allele is dominant for coat color but recessive lethal; homozygous yellow mice do not survive.

4.7 Combinations of Two Gene Pairs with Two Modes of Inheritance Modify the 9:3:3:1 Ratio

Gene Interactions and Modified Ratios

When two gene pairs with different modes of inheritance are considered simultaneously, the classic 9:3:3:1 Mendelian ratio can be modified, resulting in more complex phenotypic ratios.

• Example: A cross involving albinism (Mendelian inheritance) and ABO blood type (multiple alleles).

Genotype	Phenotype
AA or Aa	Pigmented
aa	Albino
IAIA or IAi	Type A
IBIB or IBi	Type B
IAIB	Type AB
ii	Type O

Summary Table: Key Extensions of Mendelian Genetics

Concept	Definition	Example
Incomplete Dominance	Heterozygote shows intermediate phenotype	Pink snapdragons
Codominance	Both alleles fully expressed in heterozygote	MN blood group
Multiple Alleles	More than two alleles for a gene in a population	ABO blood groups
Lethal Alleles	Alleles that cause death in certain genotypes	Yellow coat in mice
Gene Interaction	Two or more genes affect a single phenotype	Modified 9:3:3:1 ratios

Additional info: This summary integrates textbook content with expanded academic context for clarity and exam preparation.