Lecture 4 On Mendelian Inheritance PDF

1 Lecture 4 2 3 Course contents II. Classical Genetics At the end of this part, you should be able to: Mendelian inheritance Mendel's experiments Types of dominance 4 Lecture Objectives At the end of this Lecture, you shoul...

1 Lecture 4 2 3 Course contents II. Classical Genetics At the end of this part, you should be able to: Mendelian inheritance Mendel's experiments Types of dominance 4 Lecture Objectives At the end of this Lecture, you should be able to: A. Mendelian inheritance. 5 Key concept Mendel’s research showed that traits are inherited as discrete units. 6 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) The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes) This hypothesis can explain the reappearance of traits after several generations  Gregor Johann Mendel (1822-1884) 7 “Father of Genetics” Mendel was an Austrian monk and scientist. Mendel's work was not recognized until the turn of the 20th century 8 Gregor Johann Mendel Began the field of genetics in the 1860s, Deduced the principles of genetics by breeding garden peas, and Relied upon a background of mathematics, physics, and chemistry. Mendel died in 1884. Sixteen years later, in 1900, his work was rediscovered by Hugo de Vries and others looking for clues into the puzzle of heredity.  9 Mendel laid the groundwork for genetics Traits are distinguishing characteristics that are inherited. Genetics is the study of biological inheritance patterns and variation. Mendel showed that traits are inherited as discrete units. 10 Mendel’s data revealed patterns of inheritance Mendel made three key decisions in his experiments: – use of purebred plants – control over breeding – observation of seven “either-or” traits 11 Pea plants happened to be a good choice to study because: They are self-pollinating. He had different pea plants that were True-breeding. True-breeding - means that varieties result when self-fertilization produces offspring all identical to the parent. – EX. if the plants self-pollinate they produce offspring identical to each other and the parents.  Produce True-breeding plants: 12 Mendel produced True-breeding by allowing the plants to self-pollinate for several generations 13 Pea plants happened to be a good choice to study because: Can be grown in a small area. Produce lots of offspring. Several generations. Can be artificially cross-pollinated Many traits known. Above all, easy to grow. 14 When discussing generations’ traits, we label them as following: The True-breeding parental generation is called the “P generation”. The offspring of the two parental plants is called the “F1 generation”. A cross between F1 generation would be called “F2 generation.” 15 Mendel’s Pea Plant Experiments  Mendel used pollen to fertilize 16 selected pea plants When Mendel wanted to cross-fertilize plants, he ✓ prevented self-fertilization by cutting off the immature stamens of a plant before they produced pollen and ✓ dusted its carpel with pollen from another plant to cross-fertilize the stamenless flower. After pollination, the carpel developed into a pod, containing seeds (peas) that he later planted.  1 Stamen removal 2 Pollen transfer 17 Mendel used pollen to fertilize Parents Stamens selected pea (P) Carpel plants 3 Carpel matures into pod 4 Seeds from pod planted Offspring (F1)  Mendel used pollen to fertilize 18 selected pea plants 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 or cross- pollinate with other F1 hybrids, the F2 generation is produced.  Mendel used pollen to fertilize 19 selected pea plants P generation crossed to produce F1 generation Interrupted the self-pollination process by removing male flower parts Mendel controlled the He then fertilized the female fertilization of his pea plants part, or pistil, with pollen from by removing the male parts, a different pea plant. or stamens.  Mendel allowed the resulting 20 plants to self-pollinate Among the F1 generation, all plants had purple flowers F1 plants are all heterozygous Among the F2 generation, some plants had purple flowers, and some had white flowers.  Character Traits Dominant Recessive The seven pea 21 Flower color characters studied Purple White by Mendel Flower position Axial Terminal Seed color Yellow Green Seed shape Round Wrinkled Pod shape Inflated Constricted Pod color Green Yellow Stem length Tall Dwarf 22 Key concept The inheritance of traits follows the rules of probability.  Punnett squares illustrate 23 genetic crosses The Punnett square is a grid system for predicting all possible genotypes resulting from a cross. – The axes represent the possible gametes of each parent. A a – The boxes show the A possible genotypes a of the offspring. The Punnett square yields the ratio of possible genotypes and phenotypes. 24 How to Use a Punnett Square…cot.  Inside each square, take the letter from the top and combine it with the letter from the left. A a a A a aa a Aa aa 25 How to Use a Punnett Square…cot.  Heterozygous Red flower crosses with a Homozygous White flower (Rr x rr)  Write one parent on the top and the other on the left side of the square. R r r R r rr r Rr rr 26 How to Use a Punnett Square…cot.  To find the genotype probability, write the genotypes of the offspring; then calculate the probability as a fraction or percent (each square represents 25%).  Rr 2/4 or 50% R r  rr 2/4 or 50% r R r rr r Rr rr 27 How to Use a Punnett Square…cot.  To find the phenotype probability, write the phenotypes of the offspring; then calculate the probability. R r  Red 2/4 or 50% r R r rr  White 2/4 or 50% r Rr rr  It’s Your Turn! 28  Short hair (S) is dominant to long hair (s) in mice.  What is the genotype and phenotype ratio of a cross between two heterozygous short-haired mice? Genotype: SS: ¼ or 25% S s Ss: 2/4 or 50% ss: ¼ or 25% S SS Ss Phenotype: Short: 3/4 or 75% s Ss ss Long: ¼ or 25%