Lesson Script NOTE: Uploaded are files with Mendelian Genetics Vocabulary Word Flashcards that students can write the definitions or examples, in their own words, as the lesson is conducted. This is a great accommodation for those students that require it. If you lost a dog and wanted to put up posters around town but couldn’t put a picture on the poster, what would you write so people would know it was your dog if they see one? Think about a particular breed of dog. What types of features indicate a particular breed? Can you tell individuals apart within a breed? These features you described are called traits. When traits are passed from generation to generation it is called inheritance, or heredity. The scientific study of inheritance is called genetics. The study of genetics began with a man named Gregor Mendel. Because of this, he is called the father of genetics. Mendel was an Austrian monk. His monastery sent him to the University of Vienna to study botany and mathematics. Mendel spent a great deal of time studying garden pea plants, which are true-breeding plants. This means that they consistently produce offspring with only one form of a trait. Pea plants normally reproduce by self-fertilization, which is when the male gamete (sperm or pollen) of a flower combines with the female gamete (egg) of the same flower. A gamete is a sex-cell that has half the number of chromosomes (the structures that contain the genetic material that is passed from generation to generation) of an organism. So, this means that a gamete has half the information that will determine the traits of the offspring. In plants that self-fertilize, both the male and female gametes are in the same flower or plant. When they reproduce in this way, the offspring are nearly identical to the parent plant. Pea plants can also be cross-pollinated, which is when the male gamete of one flower is combined with the female organs of a different flower of a different plant. In this case, half of the chromosomes, or genetic material, comes from one flower of one plant, and the other half comes from a different plant, so the offspring will have some variance from the parents. Mendel noticed that the pea plants had specific traits generation after generation. One such trait was green seeds versus yellow seeds. He wanted to understand how this happened, so he cross-pollinated the male gamete of a true-breeding green seed plant with the female organs of a true-breeding yellow seed plant. To be sure the plants didn’t self-pollinate, he removed the male organs from the yellow seed plants. He called these two plants the parent or P generation. Mendel found that all of the offspring of these two plants had yellow seeds. He called these offspring the first filial, or F1 generation. The word filial comes from the Latin words for “son” (filius) and “daughter” (filia). It seemed like the green-seed trait had disappeared. Mendel decided to cross these to see if the green-seed trait really had disappeared. He planted the seeds from the F1 generation and then let them grow and self-fertilize. Then he examined the seeds that were produced. He called these offspring the second filial, or F2 generation. Of the seeds he collected, 6022 were yellow and 2001 were green. This is an almost perfect 3:1 ratio of yellow to green seeds. Mendel concluded that there were two forms of the seed color trait (yellow and green) and each are controlled by a factor we now know as alleles. An allele is an alternative form of a gene passed from generation to generation. Alleles come from the genes in the chromosomes of the parents. Mendel called the form of the trait seen in the F1 generation dominant, and the form of the trait that was masked in the F1 generation recessive. The dominant trait has the ability to mask the recessive trait. When modeling inheritance, the dominant allele is represented by a capital letter, and the recessive allele is represented by the same letter in lower case. Each parent has two alleles for a trait. One allele is inherited from each parent. When an organism has two of the same alleles, they are considered homozygous. When they have two different alleles, they are called heterozygous. If at least one dominant allele is present, the organism will exhibit that dominant trait. In the case of the seed color for the pea plant, we’ll use the letter Y because yellow was the dominant color. The homozygous alleles will be YY and yy. The heterozygous allele will be Yy. When writing both alleles of an organism, this is called the genotype. We can look at the genotypes to determine which trait will be visible. The visible trait is called the phenotype. Let’s look back at the genotypes of the seed color for the pea plant. We already determined that there are three possible combinations of alleles, two for homozygous (dominant and recessive) and one for heterozygous. Therefore, there are 3 possible genotypes for this cross. Now let’s determine the phenotypes. Remember, if at least one dominant allele is present in the genotype, the dominant trait will be displayed. YY – yellow; yy – green; Yy – yellow If you remember, in meiosis the chromosome number is divided in half, so gametes only have one of the pair of alleles. During fertilization, two alleles for that trait unite. This is called the law of segregation. In simpler terms, if we take a pea plant with the genotype YY, in meiosis these alleles split into two Ys. During fertilization, one of these Ys would unite with one allele (half a genotype) from the other parent. Now we can use this knowledge to predict the possible traits in the offspring if we know the genotypes of the parents. Let’s look back at Mendel’s pea plants again. We can take two genotypes and predict what the offspring will look like (phenotype) using a simple table known as a Punnett Square. We’ll take at the homozygous dominant plant and cross it with the homozygous recessive, as Mendel did in his experiments. Split the genotype of the male into two alleles and write them across the top. If you remember, the male pea plant produced green seeds, so it is the homozygous recessive genotype (yy). Now separate the genotype of the female, the homozygous dominant (YY), into alleles and write them vertically along the left side. Now we cross the gametes by matching the parent alleles that are above and to the left of each square to determine the possible genotypes for the offspring. If you have only one dominant allele, it must be written first in the genotype. Look in each square and list all the different genotypes present. Notice in this cross that they are all Yy, or heterozygous. What is the phenotype for this? So, all of the offspring have yellow seeds. If you remember, this is the same as what Mendel found in his experiment. Now let’s try a different cross. This time, we will cross a heterozygous male with a homozygous dominant female. We’ll set it up together, then I want you to practice the cross on your own white board (or paper). What genotypes did you get? (YY, Yy) What phenotypes? (all yellow) Does anyone notice anything significant about both of these crosses? (They all have the yellow phenotype) What was the same in both sets of parents? (Homozygous dominant female) Since the offspring get half of its genotype from each parent, when at least one parent is homozygous dominant, the offspring will always get at least one dominant allele. Since the dominant trait masks the recessive one, these offspring of a homozygous dominant parent will always show the dominant trait. Now I want you to practice a few more on your white board. YY x YY yy x yy yy x Yy Yy x Yy Mendel also studied 6 other traits in his pea plants: Flower color (purple dominant over white); Seed pod color (green dominant over yellow); Seed shape/texture (round dominant over wrinkled); Seed pod shape (round dominant over constricted); Stem length (tall dominant over dwarf); Flower position (axial – along stem – dominant over terminal – at top of stem). Let’s practice predicting offspring using a different trait, seed shape. Remember, round is dominant over wrinkled. Cross a wrinkled seed plant with a purebred (another name for homozygous) round (round dominant over wrinkled) Cross a hybrid (another name for heterozygous) with a purebred wrinkled. Now cross 2 hybrids. Now that you have a better understanding of this, you are going to practice on the computer with some different scenarios. NOTE: Teachers, the Mendelian Genetics Website uploaded files will contain the locations for the computer practice.