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In the world of genetics, alleles are different forms of a particular gene present on a chromosome. Each individual inherits two alleles for each gene, one from each parent. The concepts of dominant and recessive alleles are fundamental to understanding how traits are passed down through generations.
Dominant alleles are those that express their trait even when only one copy is present. This means that if an organism has at least one dominant allele (\(T\)) for a trait, the dominant trait will be visible. In our pea plant example, the allele for tallness (\(T\)) is dominant over the allele for dwarfness (\(t\)).
Recessive alleles require two copies to express their trait. If an organism has two recessive alleles (\(tt\)), the recessive trait will be expressed.
In simple terms, when you have a dominant allele in the mix, it masks the effect of a recessive allele when they occur together. That's why in our \(\mathrm{F}_{1}\) generation, all plants appear tall because they have at least one \(T\) allele.

Genotypic ratio is a crucial concept that provides insight into the genetic makeup of offspring. It refers to the frequency of the different genotypes that appear in the offspring after a genetic cross.
Let's refer to the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) generations of pea plants as an example.

• In the \(\mathrm{F}_{1}\) generation, the cross is between a homozygous tall plant (\(TT\)) and a homozygous dwarf plant (\(tt\)). This results in offspring that all have the same genotype: \(Tt\). Therefore, the genotypic ratio is all \(Tt\).
• In the \(\mathrm{F}_{2}\) generation, when two \(Tt\) plants are crossed, the genotypic ratio becomes more varied. The outcomes are:
  • 1 \(TT\) (homozygous dominant)
  • 2 \(Tt\) (heterozygous)
  • 1 \(tt\) (homozygous recessive)
  This makes the genotypic ratio 1:2:1.

Genotypic ratios are significant because they reveal not just what we see, but the possibilities of what could manifest depending on allele combinations.

Phenotypic ratio describes the observable traits or characteristics—known as phenotypes—that result from various genotypes in offspring. Identifying the phenotypic ratio involves analyzing what traits are expressed visually.
For our pea plants, the key trait is height: tall versus dwarf.

• In the \(\mathrm{F}_{1}\) generation, since all offspring are \(Tt\), which includes a dominant allele for tallness, the phenotypic outcome is entirely tall. Thus, the phenotypic ratio is 100% tall.
• In the \(\mathrm{F}_{2}\) generation, the scenario changes. There is a mixture of genotypes: 1 \(TT\), 2 \(Tt\), and 1 \(tt\).
  • Plants with \(TT\) and \(Tt\) genotypes appear tall.
  • Only \(tt\) manifests as dwarf.
  This results in a phenotypic ratio of 3 tall to 1 dwarf.

Understanding phenotypic ratios helps to predict how visible traits can vary depending on underlying genetic makeup.