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Mendel's experiments with pea plants revealed that there are two sets of instructions for each characteristic, known as inherited trait genes. Each parent gives one set of genes to the offspring, resulting in two versions of the same gene for every characteristic. These versions are known as alleles, with dominant alleles causing the dominant trait and recessive alleles causing the recessive trait Genes affect an organism's appearance, known as its phenotype. In pea plants, the dominant allele for purple flowers (P) is required, and a plant with two dominant or two recessive alleles is homozygous. A plant with the genotype Pp is heterozygous. Punnett squares are used to predict the possible genotypes of offspring in a specific cross. In his second experiment, Mendel allowed first-generation plants to self-pollinate, revealing that the possible genotypes are Pp, PP, and pp. The ratio of dominant alleles to recessive alleles is 3:1, as calculated from Mendel's data. Probability is the mathematical chance of something happening, often expressed as a fraction or percentage. It is often used to calculate probabilities, such as the probability of tossing two heads in a row or the probability of inheriting two p alleles from a pea plant. Traits are often more complex than initially thought, with exceptions to Mendel's principles. One gene can determine one trait, but sometimes it influences more than one trait. For example, the white tiger's fur color is caused by a single gene that influences eye color. Genetic disorders can result from an error in one allele of one gene, affecting many traits. Some traits, such as skin color, hair, and eyes, are the result of several genes acting together. Multiple genes also affect many internal traits, such as how people break down food for energy. The environment plays a significant role in determining traits in organisms, both internal and external. Nutrition influences height, while exposure to the sun affects skin color. Genetic variation, which is the difference in alleles between individuals in a population, is another factor influencing traits. Humans have approximately 30,000 genes, with each individual having two alleles. This variation is found in many populations, such as the corn snake population, where skin color is determined by alleles. Genes also affect internal traits, such as enzymes, hormones, and chemicals. Even when two people look similar, their alleles for each gene may be very different.