Chapter 6 Notes Sections 6.4, 6.5 & 6.6 Mendel’s “discrete units” are now called genes. Genes are pieces of DNA that provides a set of instructions to a cell to make a certain protein. Most genes exist in many forms When looking at Mendel’s experiments this is evident: Yellow or green seed color Tall of short plant height Any alternative form of a gene is an allele and we represent these alleles with letters. T = tall t= short Every trait has 2 genes (alleles), one from mom and one from dad. When both of the alleles are the same (TT) or (tt), we call this a homozygous condition. When the alleles are different (Tt), we call this a heterozygous condition. When we talk about an organism’s gene make-up, we are talking about its genotype. While both (TT) or (Tt) plants would be tall, they have a different genotype. This refers to the actual alleles that make-up a trait. When we talk about an organisms physical appearance, we are talking about its’ phenotype. In the previous example, both TT and Tt would be tall, so we don’t care that the alleles are different We only care about the physical appearance when talking about phenotype. In Mendel’s studies, he found that when a trait was in the heterozygous condition, the phenotype of the plant showed only one of the two traits ◦ NOT BOTH AND NOT A MIXTURE OF BOTH! Mendel quickly learned from his crosses that certain genes masked other genes. He called genes that masked other genes dominant genes. The genes that are being masked are the recessive genes. Trait Dominant Recessive Seed Shape Round Wrinkled Seed Color Yellow Green Pod Shape Smooth Constricted Pod Color Green Yellow Flower Position Axial Terminal Flower Color Purple White Plant Height Tall Short •Remember- dominate alleles are not necessarily better or occur more. It simply means when 2 different alleles are together, one masks the other. We use an uppercase letter for dominant traits and a lowercase letter for recessive traits. Because some alleles are dominant over others, 2 different genotypes can produce the same phenotype! ◦ TT= tall and Tt =tall As you know not all plants are tall or short or just have purple or white flowers. Environmental factors like water and sun affect plants as well. Mendel soon concluded after some of his initial findings that through probability laws he could determine possible outcomes. A scientist by the name of R.C. Punnett took some of Mendel’s work and developed a grid system to figure possible outcomes. The punnett square is a grid system used to predict all possible genotypes and phenotypes resulting from a cross. By using this grid system, possible outcomes of certain crosses can be predicted. Remember the punnett square is a prediction of possible outcomes, it is not what will actually happen. The first type of cross is called a monohybrid cross or one-factor cross. ◦ This deals with only one trait!! The traits can be either homozygous or heterozygous. Remember, every trait ALWAYS has 2 genes, you get one gene from mom and one gene from dad. When doing actual crosses, it is important to know what type of organism you are starting with. If you don’t know the genotype of an organism, you can do a test cross ◦ A test cross involves crossing an organism with an unknown genotype with a recessive organism. If you cross an unknown plant with a recessive plant and get offspring that are recessive, than you know your unknown is hybrid! Dihybrid crosses involve 2 traits! Up until now we have dealt with crosses involving one trait. You can, however, cross two traits at a time- this is called a dihybrid cross. Mendel did dihybrid crosses and wondered: ◦ “would traits always stick together or if they would express themselves independently?” In other words- Mendel wanted to know if you took a plant that was both tall and axial and crossed it with a short, terminal plant would you only get offspring that look like the parents, or would you get recombination (tall & terminal) and (short & axial). When Mendel did his first dihybrid cross, his results were similar to the results he got in his first monohybrid crosses ◦ Crossing a purbred (Tall, Axial) x (Short, Terminal) = ALL tall, axial He then let the F1 fertilize themselves (TtAa x TtAa) ◦ His results showed that all possible combinations are possible Tall, axial Short, axial Tall, terminal Short, terminal This led him to his 2nd law which is the Law of Independent assortment. This law states that given 2 traits each of these traits separate independently so all combinations are possible. ◦ Traits don’t stick together! After years of study, Mendel started to figure out that genetics has a probability component to it. Probability is the likelihood that a particular event will occur ◦ Probability= # of desirable outcomes/total # of all possible outcomes This type of probability can be applied to gamete formation. Suppose you have a plant hybrid for height (Tt). The likelihood of a (T) is 1 over the total possible outcomes (T) & (t)is two ◦ So probability = ½ Genetic variation is the essence of all life. Sexual reproduction allows for different genetic variations daily. When chromosomes line up in the middle during metaphase I of meiosis, there is about 8 million different combinations that can be formed. Since an egg & sperm combine, the possible different combinations increase to about 70 trillion In other words: ◦ A human couple can produce a child with one of about 70,000,000,000 different combinations of genes! Genetic diversity is affected by other things as well. A lot of variation comes from a process called crossing-over This is when chromosomes exchange parts during metaphase I of meiosis. As you can see, genes have exchanged places on like chromosomes. What this does is increase the number of different combinations possible now that genes have crossed-over. What Mendel didn’t know was that some genes are on the same chromosomes! ◦ How will that affect crosses??? FIND OUT NEXT CHAPTER!!!!