1 Foundations of Genetics and Mendelian Inheritance

Genes, Alleles, and Traits

Genetics examines inheritance and how genetic variation interacts with the environment. A gene is a unit of heredity: a DNA sequence that contributes to a biological function or trait. Different versions of a gene are called alleles. In many sexually reproducing organisms, an individual carries two alleles at a gene location, one inherited from each parent.

An organism’s genotype is its genetic makeup for a gene or set of genes. Its phenotype is its observable characteristics. Phenotype can be influenced by both genotype and environment, so an inherited allele combination does not always tell the whole story of an observable trait.

For a simple Mendelian example, an individual is homozygous when its two alleles match and heterozygous when they differ. In complete dominance, the dominant allele determines the phenotype of a heterozygote, while the recessive phenotype appears only when both alleles are recessive. Dominance describes how alleles affect a particular phenotype; it does not mean an allele is more common or better. Many real traits do not follow this simple pattern.

Takeaway: Genes have different alleles; an organism’s genotype contributes to its phenotype, alongside environmental influences.

Mendel’s Crosses and Segregation

Gregor Mendel investigated inheritance by breeding garden pea plants. He chose characteristics with clear contrasting forms, such as yellow and green seeds, and used plants that bred true for a characteristic. By controlling which plants crossed and comparing offspring across generations, he could identify patterns of inheritance.

When true-breeding plants with contrasting forms of a trait were crossed, the first-generation offspring, called $F_1$, showed one form. When the $F_1$ plants reproduced, the alternate form reappeared in the second generation, called $F_2$. The return of the hidden form showed that inherited information does not simply blend away: an allele not visible in an $F_1$ plant can still be passed to its offspring.

Mendel’s law of segregation explains this pattern. The two alleles an individual carries for a gene separate during gamete formation, so each gamete receives one. At fertilization, alleles from two gametes come together in the offspring.

Takeaway: The reappearance of a trait in a later generation is consistent with inherited alleles remaining distinct and separating into gametes.

Predicting Pea Seed Color

Let $Y$ represent the dominant allele for yellow seeds and $y$ the recessive allele for green seeds. A true-breeding yellow plant with genotype $YY$ crossed with a true-breeding green plant with genotype $yy$ produces $F_1$ offspring with genotype $Yy$. Under complete dominance, these offspring have yellow seeds.

When two $F_1$ plants are crossed, $Yy \times Yy$, each can pass on either $Y$ or $y$. The expected $F_2$ genotype ratio is:

Because $YY$ and $Yy$ plants have yellow seeds while $yy$ plants have green seeds, the expected phenotype ratio is:

These ratios describe expected outcomes across many offspring. They are probabilities, not a guarantee that every small group of offspring will match the ratios exactly.

Takeaway: Use the allele combinations to predict genotypes first, then apply the dominance pattern to predict phenotypes.