Mendelian Genetics The Laws of Inheritance Morpho plelides Mark T. Hark, MD BioEdWaugaman& Online Lathrop Important Terms Heredity: is the passing on of characteristics from parents to offspring. These characteristics are called traits Mendel was the first person to predict which traits would be passed from parents to offspring. The study of heredity is called GENETICS. Waugaman - Bierfeldt www.BioEdOnline.org 1 Review Parts and Pieces: Waugaman - Bierfeldt www.BioEdOnline.org Need to know… Heredity is the passing of genes from parents to offspring. Genes contain the recipe (Code) for a trait. Genes are portions of the DNA code and are on Chromosomes. Chromosomes are complete DNA strands with many genes each. Chromosomes are found in the Nucleus. Waugaman - Bierfeldt www.BioEdOnline.org 2 Genes and Alleles Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting varieties. These gene variations produced different expressions, or forms, of each trait. The different forms of a gene are called alleles. These two genes Waugaman - Bierfeldt are alleles!!!!!!!!! www.BioEdOnline.org Analogy Place these terms where they belong: Bases ATGC, Chromosome, Nucleus, Gene Waugaman - Bierfeldt www.BioEdOnline.org 3 Manipulating Flowers Waugaman - Bierfeldt www.BioEdOnline.org Gregor Mendel “Father of Genetics” Parents were farmers…lived in Austria Studied science and mathematics at the University of Vienna He became a monk He began to study heredity in the 1850’s He choose pea plants as his experimental subjects, mainly because they were easy to cross and showed a variety of contrasting traits Purple flowers vs. white ones Tall plants vs. short ones Round seeds vs. wrinkled seeds Waugaman - Bierfeldt www.BioEdOnline.org 4 Blending Model of Inheritance vs. Particulate Model of Inheritance Blending Model: Offspring are a blend of the parents. The blend of characteristics is passed on to the next generation. Variation is washed out over time. Particulate Model: Offspring are a combination of the parents. Parental contributions are passed to the next generation as separate entities. Variation is maintained over time. Waugaman - Bierfeldt www.BioEdOnline.org Mendel’s Evidence for Particulate Theory of Inheritance A plant with purple flowers is crossed with another plant that has purple flowers. Some of the offspring have white flowers (HOW!). Mendel set out to discover how this could happen. Made pure breeding (parental = P) plants for different phenotypes of pea plants: Pisum sativum (pea plant) Waugaman - Bierfeldt www.BioEdOnline.org 5 The Experiments of Gregor Mendel Mendel carried out his work with ordinary garden peas, partly because peas are small and easy to grow. A single pea plant can produce hundreds of offspring. Today we call peas a “model system.” By using peas, Mendel was able to carry out, in just one or two growing seasons, experiments that would have been impossible to do with humans and that would have taken decades—if not centuries—to do with other large animals. “Click the Pea People to watch” Waugaman - Bierfeldt www.BioEdOnline.org Pea Plant Traits Mendel Observed Seed Seed Flower Flower Pod Height Color Shape Color Position Pod Shape Color Tall Yellow Round Purple Axial Inflated Green (Side) Short Green Wrinkled White Terminal Constricted Yellow (End) Waugaman - Bierfeldt www.BioEdOnline.org 6 Mendel’s Experiments Mendel choose true-breeding lines of each plant/trait he studied. True Breeding individuals always produced offspring of the same type. Crossed the pure plants of the opposite trait (purple vs. white). He called this the Parental (P) Generation. He recorded data on the offspring of this cross to get F1 generation (First Filial) All of the F1 individuals looked the same He self pollinated F1 offspring to get F2 generation ¾ of the F2 individuals looked like one of the parental (P) plants. ¼ of the F2 individuals looked like the other parental (P) plants. Waugaman - Bierfeldt www.BioEdOnline.org Mendel’s Experiments Waugaman - Bierfeldt www.BioEdOnline.org 7 Mendel’s Results Waugaman - Bierfeldt www.BioEdOnline.org More Important Terms Allele: Form of a Gene that codes for a trait. Most genes have 2 or more alleles. Example: Gene for Eye color has 2 Alleles – Brown and Blue Phenotype: The way an organism looks and behaves. The phenotype for a tall plant is tall…the plant could have an allele combination of TT or Tt. Genotype: The allele (or gene) combination of an organism. (The actual DNA code) The genotype cannot always be determined even if you know the phenotype. Waugaman - Bierfeldt www.BioEdOnline.org 8 Phenotype vs genotype Waugaman - Bierfeldt www.BioEdOnline.org Mendel’s Analysis The F1 generation always displayed one trait (he later called this the dominant trait). The F1 generation must have within it the trait from the original parents – the white trait (even though they only exhibit the purple trait). The F2 generation displayed the hidden trait, ¼ of the F2 generation had it (he later called this hidden trait the recessive trait). Each individual has 2 “factors” that determine what external appearance the offspring will have (we now call these factors genes or alleles) Mendel’s Principle of Dominance and Recessiveness One trait is masked or covered up by another trait Waugaman - Bierfeldt www.BioEdOnline.org 9 What is Dominance? Mendel called the observed trait dominant and the trait that “disappeared” recessive. A dominant allele will mask, or cover up, a recessive allele. The plants that had a purple and white allele were all purple because purple is dominant over white. These individuals are called heterozygous because they have two different alleles. The plants with two alleles for purple were all purple. These individuals are called homozygous dominant because they have two of the same dominant alleles. The plants with two alleles for white were all white. These individuals are called homozygous recessive because they have two of the same recessive alleles. Waugaman - Bierfeldt www.BioEdOnline.org Working Out Mendelian Genetics A Punnet square is bookkeeping tool. It is used to determine the probability of having a certain type of offspring given the alleles of the parent. Allele capital letter T confers tallness. In this case, the tall parental line has all T’s. This is the dominant allele. Allele lower case letter t confers shortness. The short parental line has all t’s. This is the recessive allele. Cross a TALL male with a short female. Each of these individuals is considered a purebred because they have the same type of allele (both dominant alleles or both recessive alleles) Waugaman - Bierfeldt www.BioEdOnline.org 10 Crossing the Parental Lines: Male Contribution First, the male passes on his alleles, one per gamete (pollen cell). Waugaman - Bierfeldt www.BioEdOnline.org Crossing the Parental Lines: Female Contribution Then, the female passes on her alleles, one per egg. Crossing these two parents yields all Tt offspring. Since T is dominant to t, all of the offspring are tall pea plants. These offspring are considered HYBRIDS because they have two different forms of alleles. Waugaman - Bierfeldt www.BioEdOnline.org 11 Crossing the F1 Generation Crossing two homozygous individuals (purebreds), TT and tt, yields all heterozygous offspring (hybrids), Tt. This generation is referred to as F1. Now, cross two individuals from the F1 generation (Tt x Tt). First, the male pea plant passes on its alleles, one per gamete. Waugaman - Bierfeldt www.BioEdOnline.org Crossing the F1 Generation Now, the female passes on her alleles, one per egg. Mendel found that: 3 out of 4 (3/4 or 75%) of the offspring were tall 25% of them will be homozygous tall (both dominant alleles). 50% of them will be heterozygous tall (one allele for tall and one allele for short) 1 out of 4 (1/4 or 25%) of the offspring were short. These are homozygous short (both of their alleles are recessive) Waugaman - Bierfeldt www.BioEdOnline.org 12 Back to Mendel’s Experiment & Punnet Squares Genotypes of the F2 ¼ of the offspring are TT. ½ of the offspring are heterozygotes (one T and one t). ¼ of the offspring are tt. Phenotypes of the F2 ¾ are tall (at least one T). ¼ are short (homozygous for t). Waugaman - Bierfeldt www.BioEdOnline.org Different Gene – Seed Color Parental Cross: F1 – Offspring Genotypes Pure Green with a Pure Yellow Genotypes: _ _ x _ _ F1 – Offspring Phenotypes Waugaman - Bierfeldt www.BioEdOnline.org 13 Different Gene – Seed Color F1 Cross: F2 – Offspring Genotypes Heterozygous Green with a Heterozygous Green Genotypes: _ _ x _ _ F2 – Offspring Phenotypes Waugaman - Bierfeldt www.BioEdOnline.org Another Example – Guinea Pig Color Parental Cross: F1 – Offspring Genotypes Pure Black with a Pure White Genotypes: _ _ x _ _ F1 – Offspring Phenotypes Waugaman - Bierfeldt www.BioEdOnline.org 14 Another Example – Guinea Pig Color F1 Cross: F2 – Offspring Genotypes Heterozygous Black with a Heterozygous Black Genotypes: _ _ x _ _ F2 – Offspring Phenotypes Waugaman - Bierfeldt www.BioEdOnline.org Mendel’s First Law of Heredity Mendel took the facts that he learned from his experiments and created rules or laws to explain heredity. The first of his laws is: Law of Segregation – every organism has two alleles of each gene and when gametes are produced the alleles separate. Each gamete receives one of these alleles. During fertilization, these gametes randomly pair to produce four possible combinations of alleles. Waugaman - Bierfeldt www.BioEdOnline.org 15 Waugaman - Bierfeldt www.BioEdOnline.org Punnett Square Practice A homozygous tall plant is A heterozygous tall plant is crossed with a homozygous crossed with a heterozygous short plant. tall plant. P - Genotypes: _ _ x _ _ P - Genotypes: _ _ x _ _ Offspring Genotype? ______ Offspring Genotype? ______ Offspring Phenotype?_______ Offspring Phenotype?_______ Waugaman - Bierfeldt www.BioEdOnline.org 16 Punnett Square Practice A homozygous purple flower A heterozygous yellow seed plant is crossed with a plant is crossed with a heterozygous purple flower homozygous green seed plant.