Monohybrid Cross: Videos & Practice Problems

A monohybrid cross is a genetic cross between two organisms that are heterozygous for a single trait. It is used to study the inheritance of one specific gene. By using a Punnett square, we can predict the possible genotypes and phenotypes of the offspring. For example, crossing two heterozygous purple-flowered plants (Aa) can result in three genotypes: AA, Aa, and aa, and two phenotypes: purple and white. This helps in understanding how traits are passed from parents to offspring and the concept of dominance and recessiveness in alleles.

To set up a Punnett square for a monohybrid cross, draw a grid with four boxes. Place one parent's alleles on the top and the other parent's alleles on the side. For example, if both parents are heterozygous (Aa), place 'A' and 'a' on the top and side. Fill in the boxes by combining the alleles from the top and side. The resulting genotypes will be AA, Aa, Aa, and aa. To interpret, count the frequency of each genotype and phenotype. In this case, the genotypes are 1 AA, 2 Aa, and 1 aa, and the phenotypes are 3 purple (dominant) and 1 white (recessive).

In a monohybrid cross between two heterozygous organisms (Aa x Aa), the possible genotypes of the offspring are AA, Aa, and aa. Using a Punnett square, we find that the genotypic ratio is 1:2:1 (1 AA, 2 Aa, 1 aa). The phenotypes depend on the dominance of the alleles. If 'A' is dominant and 'a' is recessive, the phenotypic ratio will be 3:1, with three showing the dominant trait and one showing the recessive trait. For example, if 'A' represents purple flowers and 'a' represents white flowers, the phenotypes will be 3 purple and 1 white.

Dominance affects the phenotypes in a monohybrid cross by determining which trait is expressed in the offspring. In a dominant-recessive relationship, the dominant allele (represented by a capital letter, e.g., 'A') masks the expression of the recessive allele (represented by a lowercase letter, e.g., 'a'). For example, in a cross between two heterozygous organisms (Aa x Aa), the phenotypes will be determined by the presence of the dominant allele. If 'A' is dominant for purple flowers and 'a' is recessive for white flowers, the phenotypic ratio will be 3 purple (AA and Aa) to 1 white (aa).

In a monohybrid cross, the genotype refers to the genetic makeup of an organism, specifically the combination of alleles it possesses (e.g., AA, Aa, or aa). The phenotype, on the other hand, refers to the observable traits or characteristics of the organism that result from the interaction of its genotype with the environment. For example, in a cross between two heterozygous purple-flowered plants (Aa), the genotypes of the offspring could be AA, Aa, or aa. The phenotypes would be the physical appearance of the flowers, which could be purple (dominant) or white (recessive).