Genetics Study Notes: Patterns of Inheritance and Chromosomal Basis

Continuity and Change in Organisms: Inheritance

Introduction to Inheritance

Inheritance is the process by which genetic information is passed from parents to offspring, ensuring continuity and allowing for change in populations over time. The study of inheritance reveals patterns that explain how traits are transmitted and how genetic diversity arises.

Haploid and Diploid Cells

Definitions and Processes

• Haploid (n) cells: Cells containing one set of chromosomes, typically gametes (sperm and egg).

• Diploid (2n) cells: Cells containing two sets of chromosomes, one from each parent.

• Fertilization: The fusion of two haploid gametes forms a diploid zygote, which develops into a new organism via mitosis.

Sexually reproducing organisms inherit two copies of every chromosome—one maternal and one paternal.

• Meiosis: The process by which diploid cells produce haploid gametes, ensuring genetic diversity.

• Mitosis: The process by which a diploid zygote divides to form all cells of the organism.

Homologous Chromosomes

Structure and Function

• Homologous chromosomes: Chromosome pairs that carry the same sequence of genes but may have different alleles.

• They have the same shape and size and pair up during meiosis.

• Each homologous chromosome is inherited from a different parent.

• Genes located at the same locus on homologous chromosomes may differ in their DNA sequence, resulting in different alleles.

Example: The gene for eye color may have a brown allele on one chromosome and a blue allele on the homologous chromosome.

Genotype and Allelic Combinations

Homozygous vs. Heterozygous

• Genotype: The genetic makeup of an organism, specifically the combination of alleles at a given locus.

• Homozygous: Both alleles at a locus are identical (e.g., BB or bb).

• Heterozygous: The alleles at a locus are different (e.g., Bb).

Example: For a gene controlling flower color, a plant with genotype RR is homozygous, while one with genotype Rr is heterozygous.

Phenotype and Influencing Factors

Genetic and Environmental Contributions

• Phenotype: The set of observable characteristics or traits of an organism.

• Phenotype results from two basic factors:

  • The organism's genetic code (genotype).

  • The influence of environmental factors.

• Some traits are determined solely by genetics (e.g., blood type), while others are influenced only by environment (e.g., tattoos).

• Most traits result from an interaction between genes and environment (e.g., height, skin color).

Example: Identical twins may have different phenotypes if exposed to different environments.

Phenotypic Plasticity

Definition and Importance

• Phenotypic plasticity: The ability of an individual genotype to produce different phenotypes in response to environmental conditions.

• Changes may involve morphology, physiology, or behavior and are not caused by changes in DNA sequence.

• Plasticity increases the chances of survival in changing environments.

Example: Plants may grow taller in low-light conditions to reach sunlight.

*Additional info: Phenotypic plasticity is a key concept in evolutionary biology, as it allows populations to adapt to environmental changes without genetic mutation.*