Extensions and Modifications of Basic Principles in Genetics

Autosomal Dominant Inheritance

Definition and Key Features

Autosomal dominant inheritance refers to genetic traits or disorders that are expressed when only one copy of a mutant allele is present on an autosome (non-sex chromosome). These traits typically appear in every generation and affect both males and females equally.

• Affected individuals appear in every generation

• Males and females transmit the trait equally to their offspring

• Unaffected individuals do not transmit the trait

• Affected individuals have at least one affected parent

Examples: Huntington’s disease, Polydactyly, Piebald spotting, Pseudoachondroplasia

Gene Interaction and Extensions of Mendelian Principles

Introduction

Classical Mendelian genetics describes inheritance patterns for single-gene traits, but many traits are influenced by multiple genes or modified by environmental factors. Extensions and modifications of basic principles include gene interactions, multiple alleles, and non-Mendelian inheritance patterns.

Sex Determination and Chromosomal Systems

Mechanisms of Sex Determination

Sex determination is the process by which organisms develop into males or females. Several mechanisms exist, including chromosomal, genic, and environmental systems.

• Chromosomal sex determination: Sex is determined by specific combinations of sex chromosomes (e.g., XX/XY in humans, ZZ/ZW in birds).

• Genic sex determination: Sex is determined by genes on undifferentiated chromosomes, not by distinct sex chromosomes.

• Environmental sex determination: Environmental factors such as temperature influence sex determination in some species (e.g., turtles).

Model Organisms in Genetics

Drosophila melanogaster

Drosophila melanogaster (fruit fly) is a widely used model organism in genetics due to its short generation time, ease of culture, and well-understood genome.

Sex-Linked Inheritance

X-Linked Recessive Traits

X-linked recessive traits are determined by genes located on the X chromosome. Males (XY) are more likely to express these traits because they have only one X chromosome, while females (XX) must inherit two copies of the mutant allele to express the trait.

• Examples: Red-green color blindness, Hemophilia

• Females: Can be homozygous normal, heterozygous carriers, or homozygous affected

• Males: Hemizygous for X-linked genes (only one allele present)

Dosage Compensation and X Inactivation

Mechanisms and Examples

Dosage compensation ensures equal expression of X-linked genes in males and females. In mammals, one X chromosome in females is randomly inactivated (Barr body formation), leading to mosaic expression of X-linked traits.

• Barr body: Inactivated X chromosome in female cells

• Random X inactivation: Leads to mosaic phenotypes, such as tortoiseshell coat color in cats

Extensions of Mendelian Inheritance: Dominance Relationships

Complete, Incomplete, and Codominance

Dominance relationships describe how different alleles interact to produce phenotypes in heterozygotes.

• Complete dominance: Heterozygote expresses the same phenotype as one homozygote

• Incomplete dominance: Heterozygote shows an intermediate phenotype

• Codominance: Heterozygote expresses both phenotypes simultaneously

Multiple Alleles and Codominance: ABO Blood Groups

Some loci have more than two alleles in the population. The ABO blood group system is a classic example, with three alleles (IA, IB, i) showing both codominance and dominance relationships.

• IA and IB are codominant

• Both are dominant to i

Expressivity:

Penetrance:

Compound heterozygotę: an individual who carries two different non functional alleles of a gene that results in a recessive phenotype EX: cystic fibrosis