AP Biology Chapter 14 Notes Mendel and the Gene Idea I. Chapter 14.1: Mendel used the scientific approach to identify two laws of inheritance. II. Chapter 14.2: The Laws of Probability Govern Mendelian Inheritance. a. Overview i. The probabilities of all possible outcomes for an event must add up to 1. 1. Probability for flipping a coin : a. Head = ½ b. Tails = ½ 2. Probability for drawing an ace of spaces out of a deck of cards: a. Ace of spades = 1/52 b. The Multiplication and Addition Rules Applied to Monohybrid Crosses. i. Multiplication rule: states that to determine this probability, we multiply the probability of one event by the probability of the other event. 1. Probability that tossing two coins and both landing on heads: a. ½ x ½ = ¼ ii. Addition rule: the probability that any one of two or more mutually exclusive events will occur is calculated by adding together their individual probabilities. 1. The probability for one possible way of obtaining an F2 heterozygote- the dominant allele from the egg and the recessive allele from the sperm is ¼. 2. The probability for the other possible way the recessive allele from the egg and the dominant allele from the sperm is ¼. 3. The probability of an F2 heterozygote is: a. ¼ + ¼ = ½ c. Solving Complex Genetics Problems with the Rules of Probability. i. Dihybrids are the equivalent of two monohybrids occurring at once. ii. Trihybrids are the equivalent of three monohybrids occurring at one iii. Using probability can make calculating dihybrids and trihybrids a lot easier. III. Chapter 14.3: Inheritance Patterns are Often More Complex than Predicted by Simple Mendelian Genetics. a. All the traits Mendel tested were determined by one gene, with the exception of flower position which is controlled by two genes.
i. This relation between genotype and phenotype is not always so simple. b. Extending Mendelian Genetics for a Single Gene. i. The Spectrum of Dominance 1. Alleles can show different degrees of dominance and recessiveness in relation to each other. a. Complete dominance- some alleles are completely dominant over the other allele i. Mendels F1 generation for flower color were all purple ii. Cannot phenotypically distinguish between heterozygous dominant and homozygous dominant b. Codominance: the phenotype is affected by both alleles i. Human blood c. Incomplete dominance: the phenotypic expression falls between the two alleles i. Flower color in snapdragons 1. Red flower crossed with white flower produces a pink flower. ii. The Relation Between Dominance and Phenotype: 1. Dominant alleles do not subdue recessive alleles a. There is no interaction between dominant and recessive alleles in a heterozygote b. Dominance and recessiveness come into play in the pathway to expression or the phenotype. i. Example: round vs. wrinkled seeds in peas. Dominance codes for an enzyme that helps convert sugars to starch where the recessive version of this enzyme is defective causing sugar accumulation in the seed coat. 1. Excess sugar causes high osmotic uptake of water causing the seed to first swell then wrinkle as it dries ii. For any character, the observed dominance/recessive relationship of alleles depends on the level at which we examine the phenotype. 1. Tay sachs: on chromosome 15 there is a gene that controls the production of an enzyme called hexosaminidase which breaks
down gangliosides, particularly the GM2 ganglioside. 2. If this gene is defective there is no enzyme to break down the GM2 ganglioside which builds up in the brain- gangliosides are glycolipid transport proteins that play a role in the immune system 3. Lipid accumulation results causing seisures, blindness and degeneration of motor skills 4. Symptoms start in the womb 5. Death usually occurs at a very young age 6. On the level of the organism, Tay Sachs is recessive a. The heterozygote at the biochemical level is incomplete dominance- which does not lead to symptoms because half the normal activity is enough to prevent lipid accumulation 7. Also, on the molecular level, a heterozygote pro duces equal amounts of the normal and defective hexosaminidase making the Tay Sachs allele codominant. 2. Whether alleles appear to be completely dominant, incompletely dominant, or codominant relative to each other depends on which phenotypic trait is considered. iii. Frequency of Dominant Alleles. 1. Dominant alleles are not necessarily more frequent in a population a. Poldactyly- dominant trait that affects only one our of 400 born in US. 2. Multiple Alleles: 3. Pleiotropy: 4. Epistasis; a. One the biochemical level one gene can inhibit the transcription of other genes b. One the phenotypic level, one gene expression can mask another, example; albinism(white hair) masks the pigment of normal hair color 5. Polygenic Inheritance:
IV. 6. Nature and Nurture: the Environmental Impact on Phenotype. a. Norm of reaction: genotype is not associated with a rigidly defined phenotype. i. Rather a range of possible phenotypes due to environmental influences b. Multifactional: many factors both genetic and environmental influence phenotype. c. Environmental factors influence many traits both directly and indirectly i. Hydrangeas- flower color is influenced by soil ph ii. Artic fox, tamarin, baby seals- fur color is influenced by season iii. Height and weight in humans iv. Sex determination in reptiles d. An organism s adaptation to the local environment reflects a flexible response of its genome i. Darker fur color in cooler regions of the body of certain animals ii. Alterations in timing of flowering due to climate change Chapter 14.4: Many human traits follow Mendelian patterns of inheritance: a. Pedigree Analysis: genetic analysis of the results of matings that have already occurred in a family s history. i. Pedigree- information about a family s history for a particular trait and assembling this information into a family tree describing the interrelationships of parents and children across the generations b. Recessively inherited disorders: i. Examples: 1. Albinism- lack of pigment- relatively mild 2. Cystic fibrosis- life threatening ii. Genetic disorders are caused by alleles that code for a malfunctional protein or for no protein at all. iii. Heterozygotes for genetic disorders are normal in phenotype because there is usually a sufficient amount of the specific protein. 1. Heterozygous are considered carriers iv. The recessive form is always homozygous, one recessive allele inherited from each parent. 1. Homozygous recessive individuals have parents that are usually carriers v. Cystic fibrosis:
1. Affects one out of very 2500 of European descent a. Membrane protein that transfers the chloride ion between certain cell (lysosomes) and the extracellular fluid b. Protein channels are either absent or defective c. Causes build up of extracellular chloride which results in the accumulation of mucus d. Mucus coats cells of the lungs, pancreas, digestive tract and other organs e. Results= poor absorption of nutrients, bronchitis, bacterial infections i. Suppresses immune system f. Life can be prolonged: i. Pounding on the chest usually can release mucus from lungs ii. Daily doses of antibiotics iii. Individuals can live to the 20 s or 30 s vi. Sickle cell disease: 1. Recessive genetic disease 2. Single nucleotide substitution in the gene for the β- globulin subunit of hemoglobin. a. Alters a single amino acid in the polypeptide, changing a glutamic acid to valine i. Forms hemoglobin S which crystalizes when deoxygenated ii. Leads to reduced number of erythrocytes iii. Leads to anemic state- lack of oxygen supply to tissue due to lack of oxygen carrying hemoglobin iv. Fragments can cause blockages b. Alters characteristics of hemoglobin c. Same molecular weight(or very similar) difference in charge causes a shift in the isoelectric point and consequently electrophoretic motility i. Isoelectric point is the difference in the number of acid amino acids and basic amino acids ii. Neutral amino acids have isoelectric points around ph of 6.0 iii. Acidic amino acids have an isoelectric point around ph 3.0 iv. Basic amino acids have isoelectric points around ph 10 v. Isoelectric point of normal hemoglobin is 6.2.
vi. Isoelectric points of hemoglobin variants is different depending on the type d. makes it detectable in gel electrophoresis 3. normal hemoglobin has two α- polypeptide chains (141aa) and two β- polypeptide chains (146 aa) vii. Mating of close relatives c. Dominantly Inherited Disorders: i. Achondroplasia- dwarfism ii. Huntington s disease 1. 1 out of 10,000 2. nerve degenerative disease 3. no obvious phenotype until its too late 4. scientist have found its locus near the tip of chromosome 4- which can now be tested and detected d. Genetic Testing and Counseling: i. Amniocentesis: 1. Needle is inserted into uterus and amniotic fluid is extracted (10ml) a. Chemical test will help detect certain disorders such as Tay Sachs b. Karyotyping can detect trisomy 21 ii. CVS- chorionic villus sampling: 1. Narrow tube is inserted through the cervix into the uterus 2. Small sample of placenta is taken- cells of the chorionic villi a. Karyotyping is carried out 3. Advantage- cells proliferate rapidly and karyotyping can occur quickly, more so than with amniocentesis 4. CVS can be performed as early as eight to ten weeks iii. Imaging techniques: 1. Ultrasounds 2. fetoscopy