Biology - Inheritance and DNA

DNA (Deoxyribonucleic Acid) is the molecule that carries genetic information. It has a double-helix structure made of two strands joined by nitrogenous bases: Adenine ($A$), Thymine ($T$), Cytosine ($C$), and Guanine ($G$).

Base pairing rules state that $A$ always pairs with $T$ ($A-T$) and $C$ always pairs with $G$ ($C-G$) via hydrogen bonds.

A gene is a short section of DNA that codes for a specific protein, which determines a particular characteristic.

Chromosomes are long, coiled molecules of DNA found in the nucleus. Humans typically have $23$ pairs of chromosomes ($46$ in total). The $23^{rd}$ pair determines sex: $XX$ for female and $XY$ for male.

Alleles are different versions of the same gene. An individual can be homozygous (having two identical alleles, e.g., $BB$ or $bb$) or heterozygous (having two different alleles, e.g., $Bb$).

The genotype refers to the genetic makeup of an organism (e.g., $Tt$), while the phenotype is the physical characteristic expressed (e.g., Tall).

A dominant allele (e.g., $B$) is always expressed in the phenotype if present. A recessive allele (e.g., $b$) is only expressed if two copies are present ($bb$).