Monohybrid Crosses

A monohybrid cross is a breeding experiment involving two organisms that differ in a single trait controlled by one gene. Each parent carries two alleles for that trait. The cross tracks how alleles segregate and recombine in offspring, revealing inheritance patterns and whether traits are dominant or recessive.

A Punnett square is a grid showing all possible allele combinations from two parents. List each parent's alleles on the grid axes, then fill cells with offspring genotypes. This visual tool predicts the probability of each genotype and phenotype in the next generation, typically showing a 3:1 ratio for dominant to recessive traits.

Genotype is the genetic makeup—the specific alleles an organism inherits, such as homozygous dominant or heterozygous. Phenotype is the observable characteristic resulting from the genotype, like flower color or seed shape. A monohybrid cross reveals how genotypes produce different phenotypes across generations.

In a cross between two heterozygous parents, three-quarters of offspring inherit at least one dominant allele, expressing the dominant phenotype. One-quarter inherits two recessive alleles, expressing the recessive phenotype. This 3:1 ratio emerges because dominant alleles mask recessive ones in heterozygous individuals.

While a monohybrid cross predicts genotypic ratios, the interaction between genotype and environmental factors can modify how phenotypes appear. Environmental conditions like temperature, light, or nutrition may influence trait expression, causing observed phenotypes to deviate from expected Mendelian ratios in real populations.

Homozygous describes an organism carrying two identical alleles for a trait—either two dominant or two recessive alleles. Homozygous dominant individuals express the dominant phenotype, while homozygous recessive individuals express the recessive phenotype. Heterozygous organisms, by contrast, carry two different alleles.

Monohybrid crosses show that allele pairs separate during reproduction, with each gamete receiving only one allele. When two heterozygous parents mate, their alleles recombine randomly in offspring, producing the predictable 3:1 phenotypic ratio. This segregation pattern confirms that inheritance follows predictable genetic rules.