Monohybrid crosses Knowledge-Based Study

Study notes

1. A monohybrid cross is a genetic cross between two individuals that tracks the inheritance of a single trait governed by one genetic locus. Gregor Mendel established the foundational principles of inheritance by performing monohybrid crosses in pea plants (Pisum sativum).

   Key Terminology

   • Gene: A unit of hereditary information located at a specific position on a chromosome that determines a particular characteristic.
   • Allele: Alternative forms of a gene that account for variations in inherited traits.
   • Dominant Allele: An allele that expresses its phenotypic effect even in the presence of a different allele (conventionally represented by an uppercase letter, such as A).
   • Recessive Allele: An allele whose phenotypic effect is masked in the presence of a dominant allele and is only expressed when two copies are present (conventionally represented by a lowercase letter, such as a).
   • Genotype: The specific genetic makeup or combination of alleles of an organism (e.g., AA, Aa, or aa).
   • Phenotype: The observable physical or physiological traits of an organism resulting from its genotype and environmental influences.
   • Homozygous: Having two identical alleles for a given gene (homozygous dominant: AA; homozygous recessive: aa).
   • Heterozygous: Having two different alleles for a given gene (Aa).

   Mendel's Law of Segregation

   Mendel's First Law, the Law of Segregation, states that diploid organisms possess pairs of alleles for any given trait. During the formation of gametes (meiosis), these two alleles segregate (separate) from each other so that each gamete carries only one allele for each gene. Fertilization then restores the paired condition in the offspring.

   The Parental (P) Generation

   In a standard monohybrid experiment, Mendel began with true-breeding (pure-breeding) parental lines. True-breeding individuals are homozygous for the trait of interest:

   • Parent 1 (Homozygous dominant): Genotype AA (produces only A gametes)
   • Parent 2 (Homozygous recessive): Genotype aa (produces only a gametes)

   The First Filial (F1) Generation

   When these parental lines cross (AA × aa), every offspring in the first filial generation receives one dominant allele (A) and one recessive allele (a):

   • F1 Genotypic Ratio: 100% heterozygous (Aa)
   • F1 Phenotypic Ratio: 100% dominant phenotype

   Although the recessive trait completely disappears in the observable phenotype of the F1 generation, the underlying recessive allele remains intact and unaltered.

   When two heterozygous individuals from the F1 generation are crossed (Aa × Aa), the recessive phenotype reappears in the second filial (F2) generation. A Punnett square is a visual tool used to predict the genotypic and phenotypic ratios resulting from such a genetic cross.

   Constructing the Punnett Square

   Each heterozygous parent produces two types of gametes in equal proportions: 50% carry the A allele and 50% carry the a allele.

   • Top axis (Male gametes): A, a
   • Left axis (Female gametes): A, a

   Combining these gametes yields four equally probable combinations:

   • A combined with A produces AA (Homozygous dominant)
   • A combined with a produces Aa (Heterozygous)
   • a combined with A produces Aa (Heterozygous)
   • a combined with a produces aa (Homozygous recessive)

   Ratios in the F2 Generation

   • Genotypic Ratio: 1 AA : 2 Aa : 1 aa (or 25% homozygous dominant, 50% heterozygous, 25% homozygous recessive)
   • Phenotypic Ratio: 3 dominant : 1 recessive (or 75% dominant, 25% recessive)

   An individual displaying a dominant phenotype may have either a homozygous dominant genotype (AA) or a heterozygous genotype (Aa). Because these two genotypes produce identical outward appearances under complete dominance, simple visual inspection cannot distinguish between them.

   Purpose and Method of a Testcross

   A testcross is an experimental cross designed to determine the unknown genotype of an organism displaying a dominant phenotype. The individual of unknown genotype (A_) is crossed with a known homozygous recessive individual (aa).

   Analyzing Testcross Outcomes

   • Outcome 1: All offspring display the dominant phenotype.
     If the unknown parent is homozygous dominant (AA), every gamete it provides carries allele A. When combined with the recessive parent's a gametes, 100% of the progeny are Aa and express the dominant trait.
   • Outcome 2: A 1:1 ratio of dominant to recessive offspring.
     If the unknown parent is heterozygous (Aa), it produces A and a gametes in equal proportions. The resulting progeny are 50% heterozygous (Aa, dominant phenotype) and 50% homozygous recessive (aa, recessive phenotype). The appearance of even a single recessive offspring confirms that the unknown parent carries a recessive allele.