BIO 101 1st EditionLecture 18Outline of Last LectureI. Reproductiona. Asexualb. Sexuali. Meiosis1. Meiosis I2. Meiosis IIii. FertilizationII. Sources of Genetic Variationa. Independent Assortment of Chromosomesb. Crossing OverOutline of Current LectureI. 17th Century geneticsII. Mendel’s Experimentsa. Law of Segregationb. Law of Independent AssortmentIII. Human Geneticsa. Recessive Disordersb. Dominant DisordersIV. Complications of Mendelian GeneticsChapter 14: Mendelian GeneticsInheritance- how traits get from one generation to the nextPangenesis- (Aristotle)- pangenes come from all parts of the body to make egg and sperm17th Century: spermists- little person in the sperm, female is only an incubatorOvists- little person in the egg, sperm only stimulates growthModern genetics began in 1860 with the work of an Augustian monk named Gregor Mendel- did experiments and used the results to figure out the basis of genetics- much of that still stands todayMendel’s ExperimentsMethods: garden peas as experimental organism.Advantages: easy to grow, strict control over mating, easily distinguished varietiesTerms: character- heritable feature (today we call characters, GENES)Trait- each character occurs in 2 variant forms, which Mendel called traits (today we call traits, ALLELES)Mendel chose 7 different characters to study, each of these characters had 2 variant forms (traits).BIO 101 1st EditionExample: For each character, Mendel obtained “true breeding” plants this means, that if the plant self fertilized, the offspring ALWAYS had the parental traitExperimentsMendel’s experiments began with crosses between parent plants that only differed from one another in ONE TRAIT= MONOHYBRID CROSSESExample: - These same kind of results were obtained with all 7 characters- The F1 generation always had ONLY offspring with ONE of the 2 traits- But when self-fertilized, the other trait would reappear in a portion of the offspring- The trait was not LOST- it was only masked in the F1 offspringMendel’s hypothesis to explain his data= LAW OF SEGREGATION1. Inheritable features (characters/genes) exist in two forms (traits/alleles) ex: pod color (gene)- green or yellow (alleles)2. Organism has 2 copies of each character, one inherited from each parent3. The two copies SEGREGATE (separated) in the gametes (each gamete gets only one or the other) and RECOMBINE in the zygote (first cell of new offspring) 4. One trait (allele) is dominant over the other- The dominant allele is always seen (even if only one copy is present)- The other allele is called the recessive allele- that trait is only seen if both copies are the recessive copyGenotype- what genes (and alleles of those genes) an organism has= genetic makeup of individualBIO 101 1st EditionPhenotype- what an individual looks like (what traits are expressed)Terminology for genetics problemsDominant allele- is given a name indicated as a CAPITOL LETTER ex: green pod allele= dominant allele= GRecessive allele- is named the same, only the letter is LOWER CASE ex: yellow pod= recessive alleles= gAll individuals have 2 copies of each gene because they are DIPLOID=2N, there are 2 possibilitiesHomozygous dominant= both copies are dominant alleleHomozygous recessive= both copies are recessive alleleHeterozygous= one copy is dominant allele, one copy is recessive alleleEx: Pod ColorGenotype Phenotype GametesGG Green All have G allelegg Yellow All have g allelesGg Green ½ have G allele, ½ have g alleleBIO 101 1st EditionMendel’s Law of Independent Assortment- Mendel figured out the rules for his Law of Segregation using monohybrid crosses (parents differed only in 1 trait)- If you look at more than 1 trait, how do they sort out with regard to one another?- To examine this, Mendel did dihybrid crosses- parents differed with regards to 2 different traits- He found that the 2 traits segregated independently of each other- Today we know that independent assortment is due to independent assortment of homologous chromosome pairs during meiosis- So Mendel’s Law of Independent assortment is only true when the traits being examined are on different chromosomesHuman GeneticsReasons why humans are not a good experimental organism for genetics:- Generation time is long (~20 years)- Few offspring- Social constraints (can’t do breeding experiments)To study human genetics- use family pedigrees (results of matings that have already occurred)TWO MAJOR KINDS OF GENETIC DISORDERS recessive alleles and dominant allelesRecessive Disorders- Much more common than dominant disorders- Because the detrimental allele can be carried by an individual without being expressed (a recessive allele must be homozygous for phenotype to show)- In a cross between 2 heterozygous individuals:~ ¼ of offspring will be homozygous normal~ ½ of offspring will be heterozygous CARRIERSBIO 101 1st Edition~ ¼ of offspring will be homozygous recessive  show the disorder- If a recessive disorder is lethal before reproductive age, it tends to be rare (person dies before passing on gene)Examples1. Cystic fibrosis- excess mucus in lungs, lethal before 5 years old if untreated1/2500 Caucasians 2. Tay-Sachs disease- brain unable to metabolize lipids properly, lethal before 6 years old1/3600 Central European Jews3. Sickle cell anemia- altered hemoglobin (protein that carries O2 in blood)- Red blood cells take on a shape like a sickle (get caught in capillaries), don’t carry O2 as well- Despite this detrimental phenotype in homozygous individuals, the allele is maintained at high level in African-American population (~1/500 have the disorder, 1/10 carry allele)What explains the high level of this allele?- Pleiotrophy- sickle cell allele affects more than one trait- It confers sickle cell disorder when homozygous but also confers RESISTANCE to malaria- Resistance to malaria occurs even when the allele is heterozygous- Malaria is a serious disease that is endemic to tropical AfricaDominant Disorders- Much less common than recessive disorders- always expressedExamples1. Dwarfism (Acondondroplasia)- heterozygous individuals are dwarfs (~1 in 10,000 people), homozygous individuals die before birth2. Huntington’s disease- late acting disorder, symptoms begin at 35-45 years, affects nervous system, degenerative, lethal, often these people have already reproduced before they know, 50% chance of transmitting to children*Now there are