Introduction to Genetics Chapter 11

Mendelian Genetics Gregor Mendel was an Austrian monk was born in 1822. After becoming a monk, he spent many years studying science and mathematics He was then a high school teacher and he was in charge of the monastery gardens. His work changed our understanding of Biology forever!

Gregor Mendel He carried out his work with ordinary garden peas What did he know? Part of each flower produces pollen, which contains the plant’s male reproductive cells-or sperm The female portion of the flower produces egg cells During sexual reproduction, male and female reproductive cells join-a process called fertilization. Fertilization produces a new cell, which develops into a tiny embryo encased within a seed. Pea flowers are normally self-pollinating—the sperm cells in pollen fertilize the egg cells in the same flower. These seeds inherit all of their characteristics from the single plant that bore them.

Mendelian Genetics In other words— A tall pea plant would reproduce into another tall pea plant A short, bushy pea plant would reproduce another short, bushy pea plant A plant with purple flowers would produce another plant with purple flowers A plant with white flowers would produce another plant with white flowers. So what would happen if a tall plant pollinated/fertilized a short, bushy plant?

Mendelian Genetics A trait is a specific characteristic, such as seed color or plant height, that varies from one individual to another. So if Mendel crossed plants with contrasting characteristics for the same trait, the resulting offspring only had 1 of the characteristics. For example, crossing a pea plant with purple flowers with a pea plant with white flowers didn’t produce a lighter purple flower—it produced 3 purple flower plants and 1 white flowered plant. When you cross (fertilize) plants, the original pair of plants is called the P (parental) generation. Their offspring are the F1 (first filial) generations. Filial stand for “son or daughter” in Latin. The offspring of crosses between parents with different traits are called hybrids.

Mendelian Genetics So Mendel determined that each character/trait was a different allele of the same gene. (purple flower vs. white flower) From these experiments, Mendel concluded that some alleles are dominant and others are recessive or the Principle of Dominance The trait that showed was considered the dominant allele of a gene, the one that did not show was considered the recessive allele of a gene. With only 1 dominant allele, that trait will be seen. You have 2 have 2 recessive alleles in order for that trait to be seen.

Mendelian Genetics Mendel’s next question was , had the recessive alleles disappeared, or were they still present in the F1 generation? To answer this questions, he allowed his hybrid plants in the F1 generation to produce and F2 generation by self-pollination. The results were remarkable! In the F2 generation, the recessive traits had reappeared. Roughly ¼ of the F2 generations showed the trait controlled by the recessive allele. So his question was—Why did the recessive allele seem to disappear in the F1 generation and reappear in the F2 generation?

Mendelian Genetics Mendel concluded: When each F1 plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene. Therefore, each F1 plant produces two types of gametes, those with the allele for tallness and those with the allele for shortness. (Or purple flowers and white flowers)

Probability and Punnett Squares The likelihood that a particular event will occur is called probability. As an example of probability, consider a simple event like a coin toss. There are two possible outcomes—heads or tails. So the probability of either outcome is equal-50/50. If you flip a coin 3 times: ½ X ½ X ½= 1/8, so you have a 1/8 chance that you will flip heads 3 times. The principles of probability can be used to predict the outcomes of genetic crosses.

Probability and Punnett Squares The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square Punnett squares can be used to predict and compare the genetic variations that will result from a cross.

Alleles of Genes Organisms that have 2 identical alleles for a particular trait—SS or ss in our example below—are said to be homozygous. Organisms that have 2 different alleles for the same trait-Ss-are heterozygous. All of the peas shown on our Punnett square have phenotype, or physical characteristics but the do not have the same genotype, or genetic make-up. The genotype of the green peas is either SS or Ss. The genotype of the 4th pea, the wrinkled one has the ss genotype. Three of the plants have the same phenotype but different genotypes. Watch Bozeman Biology Mendelian Genetics.

Independent assortment Mendel was using more characteristics of pea plants in his experiments-green or yellow seed color; tall or short plant; round seeds or wrinkled; purple or white flowers. Each trait has a specific allele. On the Punnett square we use capital letters to indicate a dominant gene and lower case letters to indicate a recessive gene. Punnett squares can get quite complicated:

Independent assortment Mendel wondered if all of the alleles were connected on the same genes or if they sorted independently. After further experimenting, he determined that each of the traits that he was testing were their own alleles and none were related. So he concluded: the principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms.

Lastly… Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes. Summary of Mendel’s principles: The inheritance of biological characteristics is determined by individual units knows as genes. Genes are passed from parents to their offspring In cases in which 2 or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and other may be recessive. In most sexually reproducing organisms, each adult has 2 copies of each gene- one from each parent. These genes segregated from each other when gametes were formed. The alleles for different genes usually segregate independently of one another.