Predicting Trait Probability

In genetics, we use a Punnett Square to predict the possible outcomes of a cross between two parents. To do this, we look at the Genotype, which is the actual genetic code (the letters like $BB$ or $Bb$). Because each parent gives one allele to their offspring, a square with four boxes represents a $100\%$ total probability. Each individual box represents a $25\%$ chance. When we talk about the Genotype Ratio, we are counting how many of each letter combination appears. For example, if we have one $BB$, two $Bb$, and one $bb$, our ratio is written as $1:2:1$.

While the genotype is the code, the Phenotype is the physical trait that shows up (like being tall or having blue eyes). Because of Dominance, different genotypes can result in the same phenotype. For example, both $BB$ (homozygous dominant) and $Bb$ (heterozygous) will show the dominant trait. Only $bb$ (homozygous recessive) will show the recessive trait. When calculating the Phenotype Ratio, we group the dominant-looking boxes together and compare them to the recessive-looking boxes.

A common mistake is thinking that if a couple has four children, one must have the recessive trait if the probability is $25\%$. However, genetics works like a coin flip. Just because you flipped 'heads' once doesn't mean the next flip has to be 'tails.' Each child is an independent event. The Punnett square tells us the probability for each individual birth, not the guaranteed order of a whole family. To find the probability of two independent events happening together, we multiply their individual probabilities: $P(A) \times P(B)$.