1.2
Monohybrid Crosses
In the 1850s and 1860s, Gregor Mendel investigated inheritance by performing monohybrid crosses in pea plants. He crossed two plants that were true-br…
Using the pea plant, Gregor Mendel originally performed monohybrid crosses, breeding experiments that involve a single trait, such as pod color.
For example, in the original parental generation, or P zero, one parent has only green pods.
Its genotype is homozygous for the green pod color allele, denoted by an uppercase G.
The other parent has only yellow pods and is homozygous for the yellow pod allele, written as lowercase g.
After crossing, the first-generation offspring, known as the filial or F1 generation, appear identical and show the same green pod phenotype.
This result shows the Principle of Uniformity, where all F1 offspring show the same phenotype.
It also indicates that green is the dominant trait over yellow. These F1 plants are heterozygous Gg.
When these F1 plants are allowed to self-fertilize, the yellow trait re-emerges in the F2 generation. The F2 offspring inherit homozygous dominant, heterozygous, and homozygous recessive genotypes in a 1:2:1 ratio, resulting in a 3:1 phenotypic ratio of green to yellow pods.
This result shows that yellow is the recessive trait, which comes from the original parent with yellow pods.
It stays hidden in the F1 generation and appears in the F2 generation only when both alleles are recessive.
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Q1: What is a monohybrid cross in genetics?
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.
Q2: How do you use a Punnett square to predict monohybrid cross outcomes?
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.
Q3: What is the difference between genotype and phenotype in a monohybrid cross?
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.
Q4: Why do monohybrid crosses typically show a 3:1 phenotypic ratio?
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.
Q5: How does the interaction between genotype and environmental factors affect monohybrid cross results?
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.
Q6: What does homozygous mean in the context of monohybrid crosses?
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.
Q7: How do monohybrid crosses demonstrate Mendel's law of segregation?
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.