Principles of Inheritance and Variation - Inheritance of One Gene (Monohybrid Cross)
Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
A Monohybrid Cross is a genetic cross between two individuals involving a single pair of contrasting characters (e.g., stem height: Tall vs Dwarf).
Mendel's Law of Dominance states that in a heterozygote (), one allele (dominant) masks the expression of the other allele (recessive). Thus, the generation always expresses the dominant trait.
The Law of Segregation (Purity of Gametes) states that the two alleles for a trait separate during gamete formation, such that each gamete receives only one allele. This is a universal law with no exceptions in diploid organisms.
The Punnett Square, developed by Reginald C. Punnett, is a graphical representation used to calculate the probability of all possible genotypes of offspring in a genetic cross.
A Test Cross is performed to identify the genotype of an organism showing a dominant phenotype. The organism is crossed with a homozygous recessive parent (). If the offspring are all tall, the parent was ; if the ratio is , the parent was .
In the generation of a monohybrid cross, the phenotypic expression follows a ratio, while the underlying genetic makeup follows a ratio.
📐Formulae
💡Examples
Problem 1:
In a monohybrid cross between a pure tall pea plant () and a pure dwarf pea plant (), calculate the number of dwarf plants expected in the generation if the total number of offspring produced is .
Solution:
The phenotypic ratio in the generation is (Tall:Dwarf). The fraction of dwarf plants is . Total plants = . Number of dwarf plants = .
Explanation:
According to the Law of Segregation, the generation () produces two types of gametes, and , in equal proportion. Selfing results in homozygous tall (), heterozygous tall (), and homozygous dwarf ().
Problem 2:
A pea plant with violet flowers (dominant) is crossed with a plant with white flowers (recessive). If the resulting progeny shows violet and white flowers, determine the genotype of the parent violet plant.
Solution:
The ratio indicates a Test Cross. Let be the allele for violet and be the allele for white. Since white flowers are , and some offspring are white (), the violet parent must be heterozygous ().
Explanation:
If the parent were homozygous dominant (), all offspring would be (violet). The appearance of the recessive trait in of the offspring proves the parent is a heterozygote: .