Patterns of Inheritance Part 1 CHAPTER 9

Why Study Genetics? Cystic fibrosis is the most common fatal genetic disorder in the United States. Cystic fibrosis is occurs due to a mutation in the CFTR gene. This gene codes for a protein that moves chloride ions out of epithelial cells. When chloride ions move out of epithelial cells, water follows by osmosis. This causes a thin film of water to always remain on the surface of epithelial tissues. This film of water allows mucus on epithelial tissues to easily glide across the tissues and be expelled from the body.

Why Study Genetics? When the CFTR gene is mutated however, the protein that allows chloride ions to be transported across the membrane is not able to reach the cell surface. The result is that water does not leave the cells as normal, causing epithelial to not be as wet as they should be, resulting in sticky globs of mucus building up and clogging passageways of the lungs and the intestines. Since the CFTR protein also serves as a receptor to alert the immune system when bacteria are present, when it is nonfunctional as it is in cystic fibrosis, this results in bacteria not being disposed of by the immune system and bacterial infections also become a special concern for cystic fibrosis patients.

Why Study Genetics? More than 10 million people carry the CFTR mutation that causes cystic fibrosis in one of their two copies of this gene, but most of them never realize it. Cystic fibrosis occurs in about 1 out of every 3,300 births. While this is a prevalent disease, why does it not affect every person who carries at least one copy of the mutated gene that causes it? Why is it that people with only one copy of the mutated gene are healthy while those with two copies become ill with cystic fibrosis? Why is the mutation that causes CF so common if its effects are so deadly? Studying genetics helps us to answer questions like these.

Tracking Traits An Austrian monk named Gregor Mendel was the first person who ever applied math to the study of inheritance of traits. Up until Mendel, some people believed that fluids from both parents “blended” during fertilization to produce ‘blended” traits in the child. However, this was obviously not the case in many instances. (Ex, eye color) Mendel carefully documented the result he obtained from studying how inheritance works in pea plants. What characteristics made pea plants an ideal organism for Mendel’s study of inheritance?

Tracking Traits Mendel found that some pea plants breed true, meaning that the offspring have the same form of the trait as the parents, generation after generation. Mendel bred these true breeding pea plants with each other and found that the hybrids that resulted occurred in predictable patterns. The patterns in phenotypes (observable traits) that he witnessed in the hybrids were clues as to the genotypes (actual alleles carried by an organism) that the hybrids possessed. Mendel referred to the particular form of a trait that showed up in the first generation (F 1 ) produced from the true breeding peas as the dominant form (or allele) for the trait, while the form of the trait that was masked (or hidden) in the first generation but reappeared in the second generation (F 2 ), he referred to as the the recessive form (or allele) for the trait. We usually indicate a dominant allele on our paper with an uppercase letter (A) and a recessive allele with a lowercase letter (a).

Tracking Traits: Mendel’s Experiments True-breeding Parents Dominant = purple Recessive = white First Generation Hybrids Second Generation Hybrids Homozygous Dominant (AA) Homozygous Recessive (aa) Heterozygous (Aa) Homozygous recessive (aa) AAAa

Mendelian Inheritance Patterns Mendel performed many monohybrid crosses. In a monohybrid cross, crosses are performed with hybrid individuals (Aa x Aa) to check for the dominance relationship between the alleles for ONE gene. In the hundreds of monohybrid crosses that Mendel performed, he always discovered that about three out of every four offspring produced would have the dominant form of the trait, while about one out of every four had the recessive form of the trait. This phenotypic ratio of 3:1 indicated that the traits that Mendel studied were specified by alleles with a strict dominant-recessive relationship. (We will see later that this is not always the case!!)

Mendelian Inheritance Patterns Mendel always got approximately a 3:1 dominant:recessive ratio, no matter what pea plant trait he studied. (The blue numbers indicate the number of pea plants Mendel counted with that particular form of the trait.)

Mendelian Inheritance Patterns We can understand this 3:1 ratio by looking at the possible number of outcomes of a particular cross. To do this we can draw a Punnett square to allow us to determine the probability of the genetic outcomes of a cross. For Example, When using hybrid (Pp) individuals to do a cross, there are only two possible types of gametes, P and p. These two types of gametes can meet up in only four possible ways at fertilization: (Also see Punnett Square on next slide) Possible EventOutcome Sperm P meets Egg POffspring is PP Sperm P meets Egg pOffspring is Pp Sperm p meets Egg POffspring is Pp Sperm p meets Egg pOffspring is pp In this example, three out of four possible outcomes include a genotype with an “P” allele, making the phenotype of those individuals the dominant phenotype, while one out of four has only recessive “p” alleles, making the phenotype of that individual the recessive phenotype. We say that this monohybrid cross has a phenotypic ratio of 3:1.

Mendelian Inheritance Patterns: Monohybrid Cross Male  Gametes Female Gametes  ¾ Purple ¼ White

Mendelian Inheritance Patterns If smooth pods are dominant (B) to wrinkled pods (b), illustrate with a Punnett square the results of crossing two heterozygous plants with smooth pods. If 1000 offspring plants were produced, how many would be expected to be smooth? How many would be expected to be rough?

Mendelian Inheritance Patterns A dihybrid cross is a test for dominance relationships between alleles of TWO genes. In this kind of cross, two hybrids are crossed (AaBb x AaBb) and the traits that show up among the offspring are observed. With these two parents, four combinations are possible in the offspring: AB, Ab, aB, and ab. This means that there are 16 possible combinations of gametes at fertilization and four different phenotypes possible. (Also see Punnett Square on next slide.) 9 offspring would be dominant for both traits tested. 3 offspring would be dominant for trait 1 and recessive for trait 2 3 offspring would be recessive for trait 1 and dominant for trait 2 1 offspring would be recessive for both traits tested. We say that this dihybrid cross has a phenotypic ratio of 9:3:3:1

Mendelian Inheritance Patterns: Dihybrid Cross Female Gametes  Male  Gametes 9/16 Purple and Tall 3/16 White and Tall 3/16 Purple and Short 1/16 White and Short Both parents: Heterozygous Purple and Tall

Mendelian Inheritance Patterns If smooth pods are dominant to wrinkled pods and yellow peas are dominant to green peas, illustrate with a Punnett Square the results of a cross between two pea plants that are both heterozygous for both smooth pods and yellow peas (RrTt x RrTt). If 1000 offspring plants were produced, how many would be expected to be smooth with yellow peas? How many would be expected to be smooth with green peas? How many would be expected to be wrinkled with yellow peas? How many would be expected to be wrinkled with green peas?

Mendelian Inheritance Patterns Mendel never really knew what his results meant. Few read his work and no one understood it. He died in 1884, never knowing that his experiments were the beginning of modern genetics. Johann Gregor Mendel