Biology - Inheritance

Inheritance is the transmission of genetic information from generation to generation via chromosomes made of $DNA$.

A gene is a length of $DNA$ that codes for a specific protein. An allele is an alternative form of a gene, often represented by letters such as $A$ or $a$.

Genotype is the genetic makeup of an organism in terms of the alleles present (e.g., $TT$, $Tt$, or $tt$).

Phenotype is the observable features of an organism, determined by its genotype and its environment.

Homozygous means having two identical alleles of a particular gene (e.g., $TT$ or $tt$). Pure-breeding involves homozygous individuals.

Heterozygous means having two different alleles of a particular gene (e.g., $Tt$). Heterozygous individuals do not breed true.

A dominant allele is one that is expressed if it is present ($T$); a recessive allele is only expressed when no dominant allele of the gene is present ($t$).

Codominance occurs when both alleles in a heterozygous organism contribute to the phenotype, such as in blood groups $I^A$ and $I^B$, resulting in blood type $AB$.

Sex is determined by the sex chromosomes: $XX$ for females and $XY$ for males. The $Y$ chromosome is shorter than the $X$ chromosome.

Continuous variation (e.g., height) is influenced by multiple genes and the environment, while discontinuous variation (e.g., blood groups) is caused by a single gene and has no intermediates.