ABO blood types exist as three alleles, which are represented as (IA, IB, IO).įurther, some traits are polygenic, meaning that they are controlled by more than one gene. A common example of this in humans is ABO blood type. While most genes contain two allele forms, some have multiple alleles for a trait. For example, when red tulips are crossed with white tulips, the resulting offspring can have flowers that are both red and white. This results in a third phenotype that displays distinct characteristics of both alleles. In co-dominance, both alleles are fully expressed. For example, a red snapdragon plant that is cross-pollinated with a white snapdragon plant produces pink snapdragon offspring. This results in a third phenotype that is a mixture of the phenotypes observed in the parent alleles. In incomplete dominance, one allele does not completely dominate the other. Some patterns of inheritance do not exhibit regular Mendelian segregation patterns. For example, plants with the genotype of (rryy) expressed the phenotype of wrinkled, green seeds. The specific combination of alleles that comprise the genotype determines which phenotype is observed. There were nine different genotypes in the F2 plants resulting from the dihybrid cross. The phenotypic ratio in the F2 plants was 9:3:3:1. The F2 generation pea plants expressed either round or wrinkled seed shape with either yellow or green seed color. Self-pollination in the F1 plants resulted in a different phenotypic ratio in the F2 plants. The phenotypes (observable physical traits) in the F1 plants were the dominant traits of round seed shape and yellow seed color. Genotype determines which traits are expressed in the phenotype. In Mendel's experiment with seed shape and color, the genotype of the F1 plants was RrYy. Inherited allele combinations determine an organisms genotype (gene composition) and phenotype (expressed traits). They are separated during meiosis (process for the production of sex cells) and united at random during fertilization.ĭiploid organisms inherit two alleles per trait, one from each parent. These different forms are called alleles, which are positioned at specific locations on specific chromosomes.Īlleles are transmitted from parents to offspring by sexual reproduction. Each gene is located on a chromosome and can exist in more than one form. Genes are segments of DNA that determine distinct traits. Wikimedia Commons/CC BY-SA 3.0 How Genes and Alleles Determine Traits This means that the dominant traits of round seed shape and yellow color completely masked the recessive traits in the F1 generation. The resulting offspring (or F1 generation) were all heterozygous for round seed shape and yellow seeds (RrYy). Wrinkled seed shape (rr) and green seed color (yy) are recessive.

In this cross, the traits for round seed shape (RR) and yellow seed color (YY) are dominant. For example, a plant that had round seeds and yellow seed color was cross-pollinated with a plant that had wrinkled seeds and green seed color. Mendel performed dihybrid crosses in plants that were true-breeding for two traits. Mendel's Independent Assortment Experiment Heterozygous alleles exhibit complete dominance as one allele is dominant and the other recessive.These alleles separate during meiosis, leaving each gamete with one allele for a single trait.Organisms inherit two alleles (one from each parent) during sexual reproduction.Genes exist in more than one form or allele.The law of segregation is based on four main concepts: It was during earlier experiments that Mendel formulated this genetics principle. Would both traits be transmitted to the offspring together or would one trait be transmitted independently of the other? It is from these questions and Mendel's experiments that he developed the law of independent assortment.įoundational to the law of independent assortment is the law of segregation. Mendel began to wonder what would happen if he studied plants that were different with respect to two traits. These cross-pollination experiments were performed with pea plants that differed in one trait, such as the color of the pod. Mendel arrived at this conclusion by performing monohybrid crosses. These allele pairs are then randomly united at fertilization. The law of independent assortment states that the alleles for a trait separate when gametes are formed.

Mendel formulated this principle after discovering another principle known as Mendel's law of segregation, both of which govern heredity. Independent assortment is a basic principle of genetics developed by a monk named Gregor Mendel in the 1860s.

Mendel principle of dominance quizlet