Q: What is the difference between genotype and phenotype?

The genotype is the biological genetic makeup of an organism, represented by allele letters (e.g., BbTt). The phenotype is the physical trait resulting from that genotype (e.g., Purple Flowers and Tall Stems).

Q: How do you count genotypes in a dihybrid cross?

We place maternal and paternal gametes into a 4x4 grid (16 cells total). We then count unique allele combinations. Crossing double heterozygotes produces 9 unique genotypes grouped into 4 distinct phenotypes.

Q: What is a genetic testcross?

A testcross crosses a dominant-expressing individual with a homozygous recessive individual (e.g., bbtt) to determine if the dominant parent is homozygous or heterozygous. If any recessive offspring appear, the dominant parent must be heterozygous.

Parental Genotypes

Parent 1 Genotype

Parent 2 Genotype

Use letters B (dominant, e.g. Purple Flower) and b (recessive, e.g. White Flower). Example: BB, Bb, bb.

Introduction to Mendelian Genetics

Mendelian genetics is the system of laws governing the inheritance of biological traits, discovered by Gregor Mendel in 1865. Using pea plant hybridization, Mendel proved that traits are inherited as discrete "factors" (which we now call genes) consisting of alternative forms called alleles.

Fundamental Laws of Inheritance

Law of Segregation: During gamete formation, the two alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.
Law of Independent Assortment: Genes for different traits assort independently of one another during the formation of gametes (applicable to dihybrid crosses, assuming the genes are on separate chromosomes).
Law of Dominance: In a heterozygote, one allele masks the phenotypic expression of another allele for the same gene. The expressing allele is dominant (capitalized), and the masked allele is recessive (lowercase).

Monohybrid vs. Dihybrid Crosses

A monohybrid cross examines the inheritance of a single trait. Crossing two heterozygotes (e.g. Bb × Bb) yields a genotypic ratio of 1:2:1 and a phenotypic ratio of 3:1.

A dihybrid cross tracks two traits simultaneously. Crossing double heterozygotes (e.g. BbTt × BbTt) yields a classic Mendelian phenotypic ratio of 9:3:3:1.

Frequently Asked Questions

What is the difference between genotype and phenotype? +

The genotype is the exact genetic makeup of an organism, represented by allele symbols (e.g., BbTt). The phenotype is the physically observable trait resulting from that genotype combined with environmental factors (e.g., Purple Flowers and Tall stems).

How do you count genotypes in a dihybrid cross? +

We combine maternal and paternal gametes into a 4×4 grid (16 offspring cells total). We then count unique allele combinations. Standard homozygous double-heterozygous dihybrid crosses produce 9 unique genotypes grouped into 4 distinct phenotypes.

What is a genetic testcross? +

A testcross is performed to determine the unknown genotype of a dominant-expressing individual. The individual is crossed with a homozygous recessive partner (e.g., bbtt). If any recessive offspring appear, the dominant parent must be heterozygous.

Punnett Square

Parental Genotypes

Interactive Punnett Square

Standard Practice Crosses

Genotypic Ratios

Phenotypic Ratios

Introduction to Mendelian Genetics

Fundamental Laws of Inheritance

Monohybrid vs. Dihybrid Crosses

Frequently Asked Questions