Incomplete dominance results in phenotypic blending in heterozygous organisms. In this example, homozygous flowers will be either white or red, and heterozygous flowers will be pink.

We can determine the outcome of this cross by looking at the parental genotypes:

Parents: Pp (pink) x PP (red)

Offspring: half Pp (pink) and half PP (red)

Half of the offspring will be heterozygous, showing the pink phenotype, and half will be homozygous for the red allele. None of the offspring will be homozygous for the white allele.

We are told that the two parents are heterozygous. This allows us to set up the given cross fairly easily.

Parents: Aa x Aa

Offspring: AA, Aa, Aa, aa

Now, however, we need to determine the phenotypes of these offspring. Since the alleles exhibit incomplete dominance it is irrelevant which allele is represented by A and which is represented by a. AA will show one extreme phenotype, aa will show the other extreme phenotype, and Aa will show an intermediate blended phenotype. In this case, the phenotypic ratio will be 1 black (AA) : 2 gray (Aa) : 1 white (aa).

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.