Genetics - Chromosomes and Genes

DNA ($Deoxyribonucleic acid$) is the molecule that carries genetic information. It consists of two strands forming a double helix, with a backbone of phosphate and deoxyribose sugar.

The four nitrogenous bases in $DNA$ are Adenine ($A$), Thymine ($T$), Cytosine ($C$), and Guanine ($G$). They follow complementary base-pairing rules: $A$ pairs with $T$ and $C$ pairs with $G$.

Chromosomes are long, coiled structures of $DNA$ and proteins (histones) found in the nucleus. Humans typically have $23$ pairs of chromosomes ($46$ total).

A gene is a specific segment of $DNA$ that codes for a particular protein, which determines a specific trait in an organism.

Alleles are different versions of the same gene (e.g., the gene for eye color may have a blue allele or a brown allele).

Genotype represents the genetic makeup (e.g., $AA$, $Aa$, or $aa$), while Phenotype refers to the physical expression or observable trait.

Homozygous organisms have two identical alleles for a trait (e.g., $BB$ or $bb$), while Heterozygous organisms have two different alleles (e.g., $Bb$).

Dominant alleles (denoted by capital letters like $R$) mask the expression of recessive alleles (denoted by lowercase letters like $r$) in a heterozygous genotype.