BIO 101 1st Edition Lecture 18 Outline of Last Lecture I Reproduction a Asexual b Sexual i Meiosis 1 Meiosis I 2 Meiosis II ii Fertilization II Sources of Genetic Variation a Independent Assortment of Chromosomes b Crossing Over Outline of Current Lecture I 17th Century genetics II Mendel s Experiments a Law of Segregation b Law of Independent Assortment III Human Genetics a Recessive Disorders b Dominant Disorders IV Complications of Mendelian Genetics Chapter 14 Mendelian Genetics Inheritance how traits get from one generation to the next Pangenesis Aristotle pangenes come from all parts of the body to make egg and sperm 17th Century spermists little person in the sperm female is only an incubator Ovists little person in the egg sperm only stimulates growth Modern 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 geneticsmuch of that still stands today Mendel s Experiments Methods garden peas as experimental organism Advantages easy to grow strict control over mating easily distinguished varieties Terms 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 Edition Example For each character Mendel obtained true breeding plants this means that if the plant self fertilized the offspring ALWAYS had the parental trait Experiments Mendel s experiments began with crosses between parent plants that only differed from one another in ONE TRAIT MONOHYBRID CROSSES Example 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 offspring Mendel s hypothesis to explain his data LAW OF SEGREGATION 1 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 parent 3 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 copy Genotype what genes and alleles of those genes an organism has genetic makeup of individual BIO 101 1st Edition Phenotype what an individual looks like what traits are expressed Terminology for genetics problems Dominant allele is given a name indicated as a CAPITOL LETTER ex green pod allele dominant allele G Recessive allele is named the same only the letter is LOWER CASE ex yellow pod recessive alleles g All individuals have 2 copies of each gene because they are DIPLOID 2N there are 2 possibilities Homozygous dominant both copies are dominant allele Homozygous recessive both copies are recessive allele Heterozygous one copy is dominant allele one copy is recessive allele Ex Pod Color Genotype GG gg Gg Phenotype Green Yellow Green Gametes All have G allele All have g alleles have G allele have g allele BIO 101 1st Edition Mendel 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 chromosomes Human Genetics Reasons 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 alleles Recessive 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 of offspring will be homozygous normal of offspring will be heterozygous CARRIERS BIO 101 1st Edition 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 Examples 1 Cystic fibrosis excess mucus in lungs lethal before 5 years old if untreated 1 2500 Caucasians 2 Tay Sachs disease brain unable to metabolize lipids properly lethal before 6 years old 1 3600 Central European Jews 3 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 Africa Dominant Disorders Much less common than recessive disorders always expressed Examples 1 Dwarfism Acondondroplasia heterozygous individuals are dwarfs 1 in 10 000 people homozygous individuals die before birth 2 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 specific tests for many genetic disorders can detect carriers as well advised for anyone with family history BIO 101 1st Edition Complications of Mendelian Genetics 1 Intermediate inheritance incomplete dominance ex Cross red flower