Gregor Mendel studied genetics through the crossbreeding of pea plants. Through his studies, he proposed laws of heredity (the law of segregation, the law of independent assortment, and the law of dominance), that are now called the laws of Mendelian inheritance. Darwin famously studied finches on the Galapagos Islands.

Heterozygous organisms carry one dominant allele and one recessive allele. The dominant allele is expressed over the recessive allele, giving the organism the dominant phenotype. If the heterozygous plants in the question are yellow, then we can conclude that yellow is dominant to some other phenotype (not given).

The cross for these two plants would be:

Parents: Yy (yellow) x Yy (yellow)

Offspring: YY (yellow), Yy (yellow), Yy (yellow), yy (other/unknown)

Three of the four possible offspring will show the dominant yellow phenotype, leading to the answer: 75%.

The overall idea that Mendel was studying was that organisms have two alleles per trait, and that each parent passes down one allele. The other answers refer to Mendel’s laws: the Law of Segregation, the Law of Independent Assortment, and the Law of Dominance. Mendel was unaware of genetic linkage, which is an exception to the Law of Independent Assortment. We know this to be true because chromosomes and DNA had not yet been discovered in his time.

Gregor Mendel's famous work on pea plants built our first understandings of inheritance. He identified that "discrete particles", which we now call genes and alleles, are passed to offspring in numerous of combinations. These different combinations create variation in a population.

A population is composed of numerous individuals, each carrying a common set of genes with a unique combination of genetic alleles. The gene pool is the sum of all of these alleles.

The allele frequency for any given gene is the relative proportion of each allele of that gene in a population. This value can be found by dividing the number of a specific allele by the total number of alleles in a population.

Evolution is any change in the proportions of different genotypes in a population from one generation to the next. Mutation, geneflow, nonrandom mating, and natural selection all contribute toward favoring certain alleles over others within a population. This leads to changes in allele frequency, and subsequent evolution.

The Hardy-Weinberg principle is a mathematical model that states that, under certain conditions, the allele frequencies and genotype frequencies in a sexually reproducing population will remain constant over generations. This consistency means that evolution is not occurring, as evolution (by definition) requires a change in allele frequency.

Requirements for Hardy-Weinberg equilibrium include: large population size, no mutation, no migration, random mating, and no natural selection.

Genetic drift is a change in the allele frequencies of a small population purely by chance.

The bottleneck effect occurs when allele frequencies are affected by a cataclysmic event. Evolution refers to a change in allele frequency, but is not limited to small populations or random chance. Natural selection refers to changes in allele frequency due to specific conditions, as opposed to random chance.

An adaptation is a characteristic of an organism that helps it survive and reproduce in a particular environment. Adaptations are the result of random mutations that have favorable outcomes. The favorability of these traits enables offspring that inherit them to thrive, thus increasing their prevalence in the population.