Lecture 8: Mendelian Genetics PDF

Lecture 8: Mendelian Genetics BIO1101 Biology Department College of Science IMSIU Overview: Drawing from the Deck of Genes • What genetic principles account for the passing of traits from parents to offspring? • The “blending” hypothesis is the idea that genetic material from the two parents blen...

Lecture 8: Mendelian Genetics BIO1101 Biology Department College of Science IMSIU Overview: Drawing from the Deck of Genes • What genetic principles account for the passing of traits from parents to offspring? • The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green) Copyright © Pearson Education Ltd. • The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes) • Mendel documented a particulate mechanism through his experiments with garden peas Copyright © Pearson Education Ltd. Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance • Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments Copyright © Pearson Education Ltd. Fig. 14-2 TECHNIQUE 1 2 Parental generation (P) Stamens Carpel 3 4 RESULTS First filial generation offspring (F1) 5 Fig. 14-2a TECHNIQUE 1 2 Parental generation (P) Stamens Carpel 3 4 Fig. 14-2b RESULTS First filial generation offspring (F1) 5 • Mendel chose to track only those characters that varied in an either-or manner • He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate) Copyright © Pearson Education Ltd. • In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization • The true-breeding parents are the P generation • The hybrid offspring of the P generation are called the F1 generation • When F1 individuals self-pollinate, the F2 generation is produced Copyright © Pearson Education Ltd. Fig. 14-3-1 EXPERIMENT P Generation (true-breeding parents)  Purple flowers White flowers Fig. 14-3-2 EXPERIMENT P Generation (true-breeding parents) F1 Generation (hybrids)  Purple flowers White flowers All plants had purple flowers Fig. 14-3-3 EXPERIMENT P Generation (true-breeding parents) F1 Generation (hybrids)  Purple flowers White flowers All plants had purple flowers F2 Generation 705 purple-flowered 224 white-flowered plants plants • Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids • Mendel called the purple flower color a dominant trait and the white flower color a recessive trait • Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits • What Mendel called a “heritable factor” is what we now call a gene Copyright © Pearson Education Ltd. Table 14-1 Mendel’s Model • Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring • Four related concepts make up this model • These concepts can be related to what we now know about genes and chromosomes Copyright © Pearson Education Ltd. • The first concept is that alternative versions of genes account for variations in inherited characters • For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers • These alternative versions of a gene are now called alleles • Each gene resides at a specific locus on a specific chromosome Copyright © Pearson Education Ltd. Fig. 14-4 Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers • The second concept is that for each character an organism inherits two alleles, one from each parent • Mendel made this deduction without knowing about the role of chromosomes • The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation • Alternatively, the two alleles at a locus may differ, as in the F1 hybrids Copyright © Pearson Education Ltd. • 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 appearance • In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant Copyright © Pearson Education Ltd. • The fourth concept, now known as the law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes • Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism • This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis Copyright © Pearson Education Ltd. • The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup • A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele Fig. 14-5-1 P Generation Purple flowers White flowers Appearance: Genetic makeup: PP pp Gametes: P p Fig. 14-5-2 P Generation Purple flowers White flowers Appearance: Genetic makeup: PP pp Gametes: p P F1 Generation Appearance: Genetic makeup: Gametes: Purple flowers Pp 1/ 2 P 1/ 2 p Fig. 14-5-3 P Generation Purple flowers White flowers Appearance: Genetic makeup: PP pp Gametes: p P F1 Generation Appearance: Genetic makeup: Gametes: Purple flowers Pp 1/ 2 1/ 2 P Sperm F2 Generation P p PP Pp Pp pp P Eggs p 3 1 p Useful Genetic Vocabulary • An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character • An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character Copyright © Pearson Education Ltd. • Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition • Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup • In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes Copyright © Pearson Education Ltd. Fig. 14-6 3 Phenotype Genotype Purple PP Purple (homozygous) 1 Pp (heterozygous) 2 Purple 1 White Ratio 3:1 Pp (heterozygous) pp (homozygous) Ratio 1:2:1 1 The Behavior of Recessive Alleles • Recessively inherited disorders show up only in individuals homozygous for the allele • Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented) • Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair Copyright © Pearson Education Ltd. Fig. 14-16 Parents Normal Aa  Normal Aa Sperm A Eggs A AA Normal Aa a Normal (carrier) a Aa Normal (carrier) aa Albino