Plant Purity: Mendelian Genetics Quiz Mastery

In a monohybrid cross between two heterozygous parents, the F2 generation follows a 3:1 phenotypic ratio, where three offspring show the dominant trait and one shows the recessive trait. This pattern is a foundational principle of Mendelian genetics and is observed consistently in traits governed by a single gene with complete dominance.

Independent assortment occurs when two genes are located on different chromosomes or are far apart on the same chromosome. Genes on the same chromosome tend to be linked and do not assort independently. Mendels Law of Independent Assortment specifically applies to genes on separate chromosomes, producing the classic 9:3:3:1 ratio in F2 offspring.

The F2 generation of a monohybrid cross between two heterozygous parents produces a genotypic ratio of 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive. This ratio differs from the 3:1 phenotypic ratio because the two heterozygous individuals appear identical to the homozygous dominant in their outward traits.

The Law of Segregation states that each organism carries two alleles per trait, these alleles separate during the formation of gametes, and each gamete carries only one allele. Offspring then inherit one allele from each parent. Dominant alleles do not eliminate recessive alleles but simply mask their expression in heterozygous individuals.

When a homozygous dominant plant is crossed with a homozygous recessive plant, all F1 offspring are heterozygous and express the dominant phenotype. This is because the dominant allele masks the expression of the recessive allele. This cross is commonly used to demonstrate the concept of dominance in classical genetics.

A test cross is performed by mating an organism displaying the dominant phenotype but with an unknown genotype with a homozygous recessive organism. If any offspring show the recessive phenotype, the unknown parent must be heterozygous. This method is a standard approach in genetics to identify whether a dominant-phenotype plant is purebred or a carrier of the recessive allele.

The F2 generation of a dihybrid cross produces offspring in a 9:3:3:1 phenotypic ratio. This means 9 offspring show both dominant traits, 3 show one dominant and one recessive, 3 show the opposite combination, and 1 shows both recessive traits. This ratio is a direct result of independent assortment and is a cornerstone of Mendelian inheritance in plants.

Recessive phenotypes can appear when both parents are homozygous recessive, when two heterozygous parents are crossed producing a 3:1 ratio, or when a heterozygous parent is crossed with a homozygous recessive parent producing a 1:1 ratio. A cross between homozygous dominant and homozygous recessive parents produces all heterozygous dominant offspring with no recessive phenotype visible.

Incomplete dominance occurs when neither allele is fully dominant, resulting in a blended phenotype in heterozygous offspring. A cross between red and white-flowered plants produces pink flowers in F1, as neither allele completely masks the other. This differs from complete dominance and shows that the expression of traits can vary depending on the relationship between alleles.

In codominance, both alleles are fully expressed simultaneously in the heterozygous individual, rather than blending into an intermediate phenotype. A classic example is AB blood type in humans, where both A and B antigens are expressed. In plants, codominance can result in offspring showing patches or sectors of both parental phenotypes rather than a single blended trait.

A dihybrid cross involves parents that differ in two distinct traits, each controlled by a separate gene. Mendel performed dihybrid crosses with pea plants to study the simultaneous inheritance of two traits, leading to his Law of Independent Assortment. The resulting F2 offspring display four distinct phenotypic classes in a 9:3:3:1 ratio under conditions of complete dominance.

True-breeding plants are homozygous for the trait under study, meaning both alleles are identical. When self-fertilized, they consistently produce offspring with the same phenotype across generations. True-breeding lines do not carry hidden recessive alleles since they are homozygous, making them essential as parental lines in controlled hybridization experiments to observe how traits are passed on.

A 1:1 phenotypic ratio in offspring results from a cross between a heterozygous individual and a homozygous recessive individual. Half the offspring inherit the dominant allele and show the dominant phenotype, while the other half inherit the recessive allele and show the recessive phenotype. This ratio is a key indicator used to confirm that a parent with the dominant phenotype is heterozygous.

Mendel selected pea plants for his hybridization studies because they possess several clearly distinguishable traits such as seed color, seed shape, and plant height. Pea plants also reproduce quickly and can be self-fertilized or cross-fertilized with ease. These characteristics made them ideal for controlled breeding experiments that allowed Mendel to track the inheritance of specific traits across multiple generations.

A cross between two heterozygous parents for a single gene produces offspring in a 1:2:1 genotypic ratio, consisting of one homozygous dominant, two heterozygous, and one homozygous recessive individual. While the phenotypic ratio appears as 3:1 under complete dominance, the genotypic ratio reveals the true underlying genetic composition of the F2 generation offspring.