Genetics: Traits, Genes, and Variation

Gene and Genome Fundamentals

• Genomes are the heritable material passed from parents to offspring and define biological identity.
• Composed mainly of nucleic acids, particularly DNA, genomes contain nucleotide sequences that instruct the synthesis of functional molecules like RNAs and proteins.
• A gene is a segment of DNA that encodes a specific functional molecule; it is the molecular unit of heredity.

Genotype and Phenotype

• The genotype represents the complete genetic makeup of an organism and includes all genes.
• Phenotype refers to the observable traits of an organism, influenced by both genotype and environmental factors.
• Traits range from biochemical and physiological characteristics to visible features and behaviors.

Genome Structure and Inheritance

• Eukaryotic genomes are mainly located in the nucleus (nuclear genome) with some within organelles like mitochondria (extranuclear genome).
• Mitochondrial DNA, inherited exclusively from the mother, is circular, while nuclear DNA is linear.
• Diploid organisms, like humans, possess two copies (alleles) of each chromosome, inherited from each parent and comprising 46 chromosomes organized into 23 pairs.
• Homologous chromosomes carry the same genes but are inherited from different parents, allowing for genetic variation.

Chromosomes and Alleles

• Chromosomes have two alleles per gene; these can be identical (homozygous) or different (heterozygous).
• Autosomes are the 22 pairs of chromosomes common to both sexes, while sexual chromosomes differ (XX in females, XY in males).
• Linkage occurs when genes are located along the same chromosome, affecting inheritance patterns.

Types of Traits

• Discrete traits are easily distinguishable and often determined by a single gene (monogenic). They may exhibit dominant/recessive allele dynamics.
• To express a recessive trait, an individual must be homozygous; a dominant trait expresses with one or two copies of the dominant allele.
• Continuous traits show phenotype variations along a gradient, influenced by multiple genes (polygenic) and environmental/developmental factors—examples include height and weight.

Mendelian Inheritance

• Mendelian inheritance involves the segregation of alleles during gamete formation; each gamete carries only one allele per gene.
• During fertilization, the fusion of gametes restores the diploid state with two alleles per gene.

Types of Mendelian Inheritance

• Autosomal recessive inheritance requires both alleles to be recessive for expression; carriers (heterozygotes) do not exhibit the trait.
• Autosomal dominant inheritance necessitates only one copy of the dominant allele for trait manifestation.
• X-linked recessive inheritance involves genes located on the X chromosome, often leading to different expression patterns in males and females.

Explore the fundamental concepts of genetics including traits, genes, and variation. This quiz covers the important distinctions between genome, genotype, and phenotype, and how these elements contribute to the biological identity of living organisms. Test your understanding of the mechanics behind heredity and genetic encoding.