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 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 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%.

Mendel's "factors" are today's genes. From his limited knowledge of cell biology, Mendel was able to observe the effects of genes by observing phenotype. Genes come in multiple forms, known today as alleles, which control dominant and recessive inheritance.

Chromatin is the term used to describe DNA packaged by proteins. Centromeres are the area of chromosomes where sister chromatids are attached. Chromosomes would not be an acceptable description of a "factor" because it is the specific gene that controls the phenotype, not the entire chromosome.

Cystic fibrosis occurs in individuals who are homozygous recessive for a single gene, following Mendelian inheritance patterns.

Down Syndrome is caused by a trisomy and is not conferred via a specific allele. The disorder is the result of a nondisjunction event during meiosis. Fragile X Syndrome occurs when a portion of the X-chromosome in men is extended due to dozens or hundreds of repeats. The number of repeats changes between generations, making this non-Mendelian. Type I diabetes is most often caused by a poorly understood autoimmune condition, wherein the immune system attacks the cells in the pancreas responsible for insulin production. The underlying autoimmune response is thought to be partially genetic and partially environmental.

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.

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.

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 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%.