Genetics- Chapter 5 Lecture 02-14-24

Concepts of Genetics

• Title: Concepts of Genetics

• Author: Robert J. Brooker, Third Edition

• Copyright: ©McGraw-Hill Education

• Intended Use: Authorized for instructor use only, no reproduction or distribution allowed without consent.

Multiple Alleles in Flowers

• Focus: Understanding the genetic variation in flower colors.

Overview of Simple Inheritance Patterns

• Key topics: Mendelian inheritance patterns involving single genes.

• Mechanisms of inheritance: Involves molecular mechanisms that account for different types of inheritance patterns for single genes.

Mendelian Inheritance

Basic Principles

• Simple Mendelian inheritance follows:

  • Law of Segregation: Alleles segregate into gametes.

  • Law of Independent Assortment: Genes for different traits assort independently.

• Note: Many genes exhibit more complex inheritance patterns.

Types of Mendelian Inheritance Patterns (Table 5.1)

Simple Mendelian Inheritance

• Description: Patterns following strict dominant/recessive relationships.

• Key feature: Some genes can have three or more alleles, complicating relationships.

X-linked Inheritance

• Description: Involves genes on the X chromosome; males have one copy, females have two.

• Dominance: Dominant allele produces a functional protein; heterozygous females need 50% of the protein for expression.

Incomplete Penetrance

• Definition: Occurs when individuals with a dominant allele do not show the dominant phenotype.

• Example: Polydactyly allele carrier with normal digits.

Incomplete Dominance

• Definition: Heterozygotes show an intermediate phenotype.

• Example: Red and white flower cross leading to pink flowers.

Overdominance

• Definition: Heterozygote shows a more beneficial trait than either homozygote.

• Mechanisms: Caused by increased resistance to infection, more functional protein dimers, or wider functional ranges.

Codominance

• Definition: Both alleles express fully in the heterozygote.

• Example: AB blood type with distinct A and B antigens.

Sex-influenced Inheritance

• Definition: The effect of sex on allele expression; alleles may be dominant in one sex and recessive in another.

Sex-limited Inheritance

• Definition: Traits appear only in one sex.

• Example: Breast development in mammals.

Lethal Alleles

• Definition: Alleles that may cause an organism's death; typically loss-of-function alleles.

Dominant and Recessive Alleles

Wild-type Alleles

• Definition: Prevalent version of a gene in populations; contributes to reproductive success.

• Genetic Polymorphism: Existence of multiple common alleles in populations.

Recessive Alleles

• Definition: Less common gene variants due to mutations; usually inherited recessively.

• Characteristics: Produces less functional or defective proteins.

Examples of Recessive Human Disease Genes (Table 5.2)

• Phenylketonuria: Defective metabolism of phenylalanine.

• Albinism: Lack of pigmentation due to tyrosinase defect.

• Tay-Sachs Disease: Lipid metabolism defect causing severe symptoms.

• Cystic Fibrosis: Chloride transport defect leading to mucus buildup.

Incomplete Penetrance and Variable Expressivity

Incomplete Penetrance

• Definition: The allele does not always result in the expected phenotype.

Variable Expressivity

• Definition: Range of phenotypic expression for a given genotype.

• Example: Variability in polydactyly expression.

Environmental Effects on Gene Expression

Role of Environment

• Example: Coat color of arctic fox changes with seasons.

• Importance of diet in managing phenylketonuria symptoms.

Norm of Reaction

• Definition: Range of phenotypes from environmental influences on a genotype.

• Usage: Measured through controlled environmental variations.

Incomplete Dominance, Overdominance, and Codominance

Incomplete Dominance

• Characteristics: Heterozygotes exhibit intermediate phenotypes.

Overdominance

• Explanation: Heterozygote has higher reproductive fitness compared to homozygotes.

• Example: Sickle-cell trait and malaria resistance.

Codominance

• Features multiple alleles and unique expression of each phenotype without dominance.

Lethal Alleles and Gene Interaction

Types of Lethal Alleles

• Essential Genes: Required for survival; mutations often cause lethality.

• Conditional Lethal Alleles: Kill only under certain environmental conditions.

Gene Interactions

• Definition: When multiple genes influence a single trait, leading to various outcomes.

• Example: Flower color in sweet peas influenced by epistasis.