HER - Mendelian Inheritance [LESSON]

Mendelian Inheritance

Gregor Mendel: Gregor Johann Mendel was a German-speaking Moravian scientist and Augustinian friar Genetics is the study of heredity or the passing of traits from parents to offspring. Gregor Johann Mendel set the framework for genetics long before chromosomes or genes had been identified, at a time when meiosis was not well understood. For his work, Mendel is often referred to as the "father of modern genetics." Mendel selected a simple biological system, garden peas, and conducted methodical, quantitative analyses using large sample sizes. 

Mendel grew and studied around 29,000 garden pea plants in a monastery's garden, where he analyzed seven characteristics of the garden pea plants: flower color (purple or white), seed texture (wrinkled or round), seed color (yellow or green), stem length (long or short), pod color (yellow or green), pod texture (inflated or constricted), and flower position (axial or terminal). Based on the appearance, or phenotypes, of the seven traits, Mendel developed genotypes for those traits.

Based on Mendel's experiments, the genotype of the pea flowers could be determined from the phenotypes of the flowers.

Mendel performed crosses, which involved mating two true-breeding individuals that have different traits. In the pea, which is a naturally self-pollinating plant, this is done by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety. In plants, pollen carries the male gametes (sperm) to the stigma, a sticky organ that traps pollen and allows the sperm to move down the pistil to the female gametes (ova) below. To prevent the pea plant that was receiving pollen from self-fertilizing and confounding his results, Mendel painstakingly removed all of the anthers from the plant's flowers before they had a chance to mature.

Mendel crossed plants that were true-breeding for violet flower color with plants true-breeding for white flower color

Plants used in first-generation crosses were called P0, or parental generation one, plants. Mendel collected the seeds belonging to the P0 plants that resulted from each cross and grew them the following season. These offspring were called the F1, or the first filial (filial = offspring, daughter or son), generation. Once Mendel examined the characteristics in the F1 generation of plants, he allowed them to self-fertilize naturally. He then collected and grew the seeds from the F1 plants to produce the F2, or second filial, generation. Mendel's experiments extended beyond the F2 generation to the F3 and F4 generations, and so on, but it was the ratio of characteristics in the P0−F1−F2 generations that were the most intriguing and became the basis for Mendel's postulates.

Probabilities are mathematical measures of likelihood. The empirical probability of an event is calculated by dividing the number of times the event occurs by the total number of opportunities for the event to occur. Empirical probabilities come from observations such as those of Mendel. An example of a genetic event is a round seed produced by a pea plant. Mendel demonstrated that the probability of the event "round seed" was guaranteed to occur in the F1 offspring of true-breeding parents, one of which has round seeds and one of which has wrinkled seeds. When the F1 plants were subsequently self-crossed, the probability of any given F2 offspring having round seeds was now three out of four. In other words, in a large population of F2 offspring chosen at random, 75 percent were expected to have round seeds, whereas 25 percent were expected to have wrinkled seeds. Using large numbers of crosses, Mendel was able to calculate probabilities and use these to predict the outcomes of other crosses.

What is the chance that all three children of a couple will be male?

1/2*1/2*1/2 = 1/8 because each event is independent of the others. This means that the probabilities are multiplied.

One-Trait Punnett Squares

Watch the One-Trait Punnett Squares video below to learn how to complete a one-trait Punnett square.

Click through the slide to learn how to complete a One-trait Punnett Square. 

Two-trait Punnett Squares

According to the law of segregation, only one of the two gene copies present in an organism is distributed to each gamete (egg or sperm cell) that it makes, and the allocation of the gene copies is random. When an egg and a sperm join in fertilization, they form a new organism, whose genotype consists of the alleles contained in the gametes.

Mendel's law of independent assortment states that the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene. If there are two heterozygous organisms, AaBb, the chance of A traveling with B into the same gamete during meiosis is the same as the chance of A traveling with b into a gamete.  All four gametes (AB, Ab, aB, and ab) have an equal chance of forming. 

If the genes assort independently, we can complete a two-trait Punnett square to predict phenotype ratios in the offspring. 

Watch the Two-trait Punnett Squares video below and take notes:

 Give the Heredity Practice learning object a try below to check your knowledge.

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