* Genes and Alleles: Organisms inherit traits from their parents through genes. Each gene comes in different versions called alleles. For example, the gene for eye color might have a brown allele (B) and a blue allele (b).
* Dominant and Recessive: Some alleles are dominant, meaning they will express their trait even if only one copy is present. Recessive alleles only express their trait if two copies are present.
* Genotype and Phenotype: An organism's genetic makeup is called its genotype. The observable characteristics of an organism are its phenotype.
* How Traits are Inherited: Each parent contributes one allele for each gene to their offspring. So, an offspring receives two alleles for each trait.
Therefore, an organism typically has two alleles for each trait, one from each parent. These alleles can be the same (homozygous) or different (heterozygous).
Here's how the dominant/recessive concept applies:
* Homozygous Dominant: If an organism has two copies of the dominant allele (BB for brown eyes), it will express the dominant trait (brown eyes).
* Heterozygous: If an organism has one dominant and one recessive allele (Bb for brown eyes), it will still express the dominant trait (brown eyes). This is because the dominant allele "masks" the recessive allele.
* Homozygous Recessive: If an organism has two copies of the recessive allele (bb for blue eyes), it will express the recessive trait (blue eyes) because there is no dominant allele to mask it.
In summary:
* Organisms have two alleles for each trait, not one for dominant and one for recessive.
* Dominant alleles mask recessive alleles when present together.
* An organism only expresses the recessive trait if it inherits two copies of the recessive allele.
