Monohybrid cross is a term for a pairing in which two members of a parent generation share a genetic trait that is affected by two alleles, or DNA sequences. These parents are typically both heterozygous and the way in which the trait will be expressed in members of an offspring generation can be expressed through a simple analysis in how the alleles can pair off. The two alleles will usually indicate dominant and recessive properties of this single trait. A monohybrid cross compares only a single trait and the alleles involved, and can typically begin with a homozygous pairing, that gives way to heterozygous offspring.
The simplest way to explain a monohybrid cross scenario is with a fairly basic example. Begin with two pea pods, one green and the other blue, determined by a chromosome representing the coloration trait. The green coloration allele is dominant and expressed as “G,” while the blue allele is recessive and expressed as “g." These two parent plants are homozygous diploid organisms, which means they each have two alleles on the chromosome that indicates coloration. Homozygous means that both of these alleles are the same, and diploid indicates that two alleles exist to establish this trait.
One is purely green dominant, expressed as “GG,” each “G” representing an allele; the other is completely blue recessive, “gg,” which means this parent is actually blue in color. Since any offspring they have gets one allele from each parent, all of their offspring would have coloration chromosomes made up of “Gg.” In this pairing, all of the offspring are green, since the dominant allele is present, though they still contain the recessive possibility for blue coloration.
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These resulting generations are called heterozygous since their alleles are not the same, unlike the parent generation. If two heterozygous offspring are then bred together, the resulting possibilities make up a monohybrid cross. Since the two parents in a second breeding would both be “Gg,” the possibilities for the coloration trait in offspring is easy to anticipate. There are four possible outcomes: "GG," "Gg," "Gg," and "gg."
By utilizing this sort of monohybrid cross breeding experiment, it becomes apparent how a recessive trait can continue to exist even when not visible. While the odds are certainly against it, there is a 25% chance from this type of pairing that offspring would be blue in color and become homozygous with regard to this trait. The offspring from this future parent would have an increased chance of being blue when paired with a heterozygous mate, though with a homozygous dominant parent, once again “GG,” the offspring would again be heterozygous. A monohybrid cross is often the easiest way to begin learning basic genetics, since it only compares a single trait, unlike a dihybrid cross that considers two traits.