Genetics Essentials: Extensions and Modifications of Basic Principles

Introduction

• Discusses non-Mendelian inherited traits and their exceptions.
• Focus on sex chromosomes and their impact on inheritance.

Sex Chromosomes and Non-Mendelian Inheritance

• X and Y chromosomes: X is longer with more genes, Y is shorter with SRY gene for maleness.
• Sex chromosomes impact inheritance differently in males and females.
• Meiosis involves pairing of non-homologous X and Y chromosomes.
• Male gametes determine sex of offspring (X or Y sperm).

Genetic Variability and Sex Determination Systems

• Meiosis creates genetic variability through crossing over and independent assortment.
• Not all eukaryotes use XY system; some have other chromosomal systems like grasshoppers (XX for females, X for males).
• In birds and other species, males might have identical chromosomes, while females have different ones.
• Environmental factors like temperature can affect sex determination (e.g., turtles and alligators).

Sex Chromosome Anomalies

• Examples include Turner syndrome (X0), Klinefelter syndrome (XXY), and poly-X females (XXX).
• Presence of Y chromosome directs maleness; absence results in femaleness.

X-linked Characteristics

• Discovery by Thomas Hunt Morgan using Drosophila melanogaster.
• X-linked traits differ from Mendelian ratios due to the location on the X chromosome.
• Example: Color blindness and its higher prevalence in males.

X Inactivation and Barr Bodies

• In females, one X chromosome inactivates, forming a Barr body.
• Related to the patchy distribution of color in tortoiseshell cats.

Y-linked Characteristics and Dominance

• Y-linked traits (hollandric traits) are only present in males.
• Dominance types:
  • Complete Dominance: Heterozygote phenotype same as homozygote dominant.
  • Incomplete Dominance: Heterozygote phenotype intermediate between homozygotes.
  • Codominance: Heterozygote expresses phenotypes of both homozygotes.

Penetrance and Expressivity

• Penetrance: Percentage of individuals with a genotype that express expected phenotype.
• Expressivity: Degree to which a trait is expressed.

Lethal Alleles and Multiple Alleles

• Lethal alleles can cause miscarriages and affect observed ratios.
• Multiple alleles within a population may exist for a single gene.

Gene Interaction and Epistasis

• Gene Interaction: Interaction between genes to produce a novel phenotype.
• Epistasis: One gene masks the effect of another gene.

Sex-Influenced and Sex-Limited Characteristics

• Sex-Influenced: Autosomal genes expressed differently in males and females.
• Sex-Limited: Autosomal genes expressed in only one sex.

Cytoplasmic Inheritance and Genomic Imprinting

• Cytoplasmic Inheritance: Traits encoded by genes in cytoplasm, often maternal (e.g., mitochondrial genes).
• Genomic Imprinting: Differential gene expression based on parental origin.
• Epigenetics: Gene expression changes without altering the DNA sequence.

Polygenic Characteristics and Pleiotropy

• Polygenic Characteristics: Traits influenced by multiple genes.
• Pleiotropy: A single gene affects multiple traits.

Conclusion

• This chapter explored the complexities of genetic inheritance beyond Mendel's initial findings.
• Understanding these concepts provides insight into genetic variation and inheritance patterns.