Genetics Fundamentals: Traits, Genes, and Mendel's Laws

Understanding Heredity

Traits

Traits are characteristics present in an organism that can be inherited by its offspring.

Genes and Alleles

A gene is a section of DNA containing information about a specific trait and controlling that trait. Alleles are the different alternatives that a gene controlling a particular trait can have.

Homozygous and Heterozygous

• Homozygous: If the two alleles of a specific gene are identical (e.g., AA or aa), the individual is homozygous or pure for that trait.
• Heterozygous: If the two alleles of a specific gene are different (e.g., Aa), the individual is heterozygous or hybrid for that trait. (A is the dominant allele and a is the recessive allele).

Genotype and Phenotype

• Genotype: An organism's set of genes.
• Phenotype: The expression of a genotype under specific environmental conditions. It is the set of traits an individual has as a result of the interaction of the genotype and its environment.

Mendel's Laws of Inheritance

Mendel's First Law: The Law of Uniformity

If you cross two pure breeds that differ in only one trait, all the hybrids formed in the first generation (F1) are identical to each other and to the dominant parent plant.

Mendel's Second Law: The Law of Segregation

The traits found together in heterozygous individuals separate without mixing when gametes are formed.

Mendel's Third Law: The Law of Independent Assortment

Different traits are transferred independently from one another, meaning all possible combinations can appear in the offspring.

Exceptions to Mendel's Laws

Non-Dominant/Recessive Relationships

Some genes are not ruled by the dominant/recessive relationship between alleles as described by Mendel (full dominance). Other relationships exist, such as intermediate inheritance and co-dominance.

Intermediate Inheritance

In intermediate inheritance, the F1 heterozygous phenotype exhibits a trait that is an intermediate between the two homozygous parents, as neither allele is dominant over the other.

Co-dominance

Co-dominance occurs when both alleles manifest themselves simultaneously. The F1 filial generation is heterozygous and exhibits traits from both parents.