A test-cross is used to determine the genotypes of unknown organisms, but can also be valuable to distinguish dominant and recessive traits. A test-cross is when an unknown organism is crossed with a homozygous recessive organism. If the unknown organism is homozygous dominant, then all offspring will show the dominant phenotype.

Crossing an homozygous tall plant with a homozygous short plant, and observing all tall offspring, would prove that the tall trait is dominant.

One answer option suggests crossing a homozygous tall plant with a heterozygous short plant; this is impossible because all heterozygotes will show the tall phenotype. The remaining two answers will result in some combination of tall and short offspring. Though the ratios of the offspring phenotypes could be helpful in confirming that the tall trait is dominant, a test-cross is more useful and more definite.

If both parents are homozygous and the F1 generation only resembles one of the parents, then that parent must have been homozygous for the dominant gene. This means that the yellow plant has genes that are dominant over the green plant, making all offspring yellow.

Parents: AA (yellow) x aa (green)

Offspring: all Aa (yellow)

All of the offspring will be heterozygous and show the dominant phenotype (yellow).

A test-cross would cross the F1 offspring with a homozygous recessive individual. The result would be half Aa (yellow) and half aa (green) offspring.

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.