Dihybrid Cross Introduction

When studying genetics, dihybrid crosses track two traits at once, like fur color and texture in guinea pigs. Black fur (B) is dominant over white fur $b$, and rough fur (R) is dominant over smooth fur $r$. When both parents are heterozygous for both traits (BbRr), we need to find all possible combinations.

The FOIL method helps us determine all possible gametes each parent can produce. For a parent with BbRr genotype, the possible gametes are BR, Br, bR, and br. Since both parents have identical genotypes, we use these same combinations on both sides of our Punnett square.

After completing the 16-box Punnett square, we can calculate the probability of different phenotypes. Guinea pigs with black rough fur appear in 9/16 of outcomes, black smooth fur in 3/16, white rough fur in 3/16, and white smooth fur in 1/16. This creates the classic 9:3:3:1 ratio typical of dihybrid crosses.

💡 Remember: In dihybrid crosses, if both genes show complete dominance, the phenotypic ratio will always be 9:3:3:1 when crossing two heterozygous parents!