MCB 181 1st Edition Lecture 23Outline of Previous Lecture I. Meiosis - The Race to Create LifeII. Process of MeiosisIII. Clicker QuestionsOutline of Current Lecture I. Mendel and the Gene IdeaII. Mendel’s ConceptsIII. Clicker QuestionsCurrent LectureI. Mendel and the Gene Ideaa. Mendel chose the pea plant for genetic studies because the mating can be controlled, pea plants have sperm producing organs (stamens) and egg producing organs (carpels), and cross-pollination is possibleb. Mendel could prevent self-fertilization by removing the male reproductive organs from the flower to allow cross-fertilization suing pollen from a different plantII. Mendel’s Conceptsa. The Firsti. Alternative versions of genes account for variations in inherited characteristics1. For example, the gene for flower color in pea plants exists in two versions: purple and white2. These alternative versions of a gene are now called alleles3. Each gene resides at a specific locus on a specific chromosomeb. The Secondi. The second concept is that for each characteristic, an organism inherits two alleles, one from each parent1. Mendel made this deduction without knowing about the role of chromosomesii. The two alleles at a locus on a chromosome may be identical like the true-breeding plants of Mendel’s P generationiii. Alternatively, the two alleles at a locus may differ such as with the F1 hybridsc. The Thirdi. The third concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearanced. The Fourthi. The fourth concept, now known as the law of segregation, states that the two alleles for a heritable characteristic separate (segregate) during gamete formation and end up in different gametesii. Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organismiii. This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosise. Homozygous – an organism with two identical alleles for a characteristicf. Heterozygous – an organism that has two different alleles for a genei. Unlike homozygotes, heterozygotes are not true-breedingg. Phenotype – the physical appearance of an organismh. Genotype – the genetic composition of an organismi. Testcross – breeding a mystery individual with a homozygous recessive individuali. If any offspring display the recessive phenotype, the mystery parent must be heterozygousj. Monohybrids – individuals that are heterozygous for one characterk. Dihybrids – cross between parents that are both heterozygous for two traitsIII. Clicker QuestionsTay-Sachs disease causes nerve cells to malfunction and results in death by age 4. Two healthy parents know from blood tests that each parent carries a recessive allele responsible for Tay-Sachs. If their first three children have the disease, what is the probability that their fourth child will not? 75%Assuming they have not yet had a child, what is the probability that, if they have four children, all four will have the disease?