Genes are determined by sequences of DNA that code for certain proteins. Sometimes, mutations to the gene can result in a modified protein that maintains the same or similar functions as the original. When this modified gene is passed down, it is known as an allele. Most accurately, an allele is a variation of a given gene.

Most alleles can be considered dominant or recessive, with respect to one another; however, instances of codominance and incomplete dominance mean that there is a spectrum of dominance. Defining all alleles by these parameters is not very accurate. Some alleles code for wild genotypes, while others code for mutated genotypes. Recombination is the transfer of genetic material between homologous chromosomes, and does not result in new alleles. New alleles require a mutation event in order to increase genetic diversity.

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 green.

The cross for these two plants would be:

Parents: Yy x Yy

Offspring: YY (yellow), Yy (yellow), Yy (yellow), yy (green)

Of four possible offspring, one will be green, leading to the answer: 25%.

For this question we are not given any information about dominant and recessive phenotypes. We know only that we are working with two heterozygous plants. Using only this information, we can determine the genotypes of the offspring.

Parents: Hh (dominant) x Hh (dominant)

Offspring: HH (dominant), Hh (dominant), Hh (dominant), hh (recessive)

We do not know if the plants will be tall or short, but we know that three of the four offspring will show the dominant phenotype. This leads to the answer of 75%.

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 plant in the question has green peas, then we can conclude that green peas are dominant to yellow peas. The yellow pea plant must be homozygous recessive.

The cross for these two plants would be:

Parents: Pp (green) x pp (yellow)

Offspring: half Pp (green) and half pp (yellow)

Half of the offspring will be heterozygous, displaying the dominant green phenotype, and half will be homozygous recessive, displaying the recessive yellow phenotype.

The red leaf phenotype represents a new allele in the population. None of the other plants have this trait and there are no other known red-leaf plants in the region. Most likely, the new phenotype is the result of a mutation. All alleles start as mutations and spread as the mutation is inherited by more individuals in the population.

Recombination (crossing over) can result in new combinations of existing alleles, but cannot create new traits. Incomplete dominance can result in an unpredicted phenotype, but will be present in all organisms with a heterozygous phenotype. It would be highly unlikely that all other plants in the population were homozygotes. Though there is an extraordinarily small chance that the red leaves result from a recessive allele, this is not likely the case considering the sample size.

Mendel's work focused heavily on identifying mechanisms and patterns in genetic inheritance. He is credited with three essential laws: the law of segregation, the law of independent assortment, and the law of dominance.

The law of segregation states that a parent organism will pass only one copy of each allele to its offspring. No parent will pass two copies of the same gene. The law of independent assortment states that the alleles to be passed down are not linked by past modes of inheritance, and will separate independent of one another. The law of dominance simply states that dominant traits will mask recessive traits.

The law of natural selection is credited to Charles Darwin for is work in evolution. The law of genetic variability and the law of diploidy are not recognized scientific principles.

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.