Heredity The Experiments of Gregor Mendel Genetics is

Heredity The Experiments of Gregor Mendel

• Genetics is the scientific study of heredity • Study of patterns of inheritance & variations in organisms

History of Genetics • Gregor Mendel • Austrian monk • Notorious for his work with pea plants • • He was a priest He studied science & math at University of Vienna • He spent 14 years working in the monastery & teaching high school • He was in charge of the monastery garden

Gregor Mendel’s Experiment • Fertilization produces a new cell • Develops into tiny embryo encased in seed • Pea plants are self-pollinating • Pollen contains the male gametes and will fertilize the eggs of the same plant • Seeds produced by self-pollination inherit all characteristics from the single plant • Only have 1 parent

Gregor Mendel’s Experiment • Mendel’s garden had many pea plants: • Peas were true-breeding: • If allowed to self-pollinate, would produce offspring identical to themselves • Mendel wanted to produce seeds with 2 “parents” • By joining male & female reproductive cells from 2 different plants • Needed to prevent self-pollination

Gregor Mendel’s Experiment • To prevent plants from self-pollinating: • He cut off pollen-bearing male parts • Then dusted pollen from another plant onto the flower • This process is known as cross-pollination • It produces seeds that have 2 different “parents”

Mendel Cross Pollinated

Gregor Mendel’s Experiments • Mendel studied 7 different plant traits • Trait: specific characteristic that varies from 1 individual to another • Each trait had 2 contrasting characters (p. 264 in text) • Mendel crossed plants with each of the 7 contrasting characters • studied their offspring

Gregor Mendel’s Experiments • P (parental) generation • original pair of plants • F 1 (first filial) generation • offspring • FYI: Filius & filia: Latin words for “son” & “daughter” • Hybrids • offspring of crosses between parents with different traits

Gregor Mendel’s Experiments • After crossing: • all offspring had character of only 1 parent • Character from other parent “disappeared” • Mendel came up with 2 conclusions

Mendel’s Conclusions 1. Biological inheritance determined by factors passed from 1 generation to next • Chemical factors that determine traits =genes • Each trait is controlled by 1 gene • Each gene occurred in 2 contrasting forms • The different forms of a gene are alleles

Mendel’s Conclusions 2. Law (Principle) of dominance: • Some alleles are dominant, others are recessive • Dominant: the primary or strong allele • Recessive: the secondary or weak allele • In Mendel’s experiments: • • Plant height: tall=Dominant, short=Recessive Seed color: yellow-Dominant, green-Recessive

Mendel’s 2 nd experiment • Mendel’s question: • “Did the recessive alleles disappear or were they still present in F 1 plants? ” • Mendel’s experimental design: • allow the F 1 plants to produce an F 2 generation by self-pollination

Mendel’s nd 2 experiment • Mendel’s results: • Traits controlled by the recessive alleles appeared! • Approx. ¼ of F 2 plants showed traits controlled by recessive alleles

Mendel’s Conclusion • Alleles for tallness & shortness segregated from each other during formation of gametes • When each F 1 plant produces gametes: • • • 2 alleles separate Each gamete carries single copy of each gene Each F 1 plant produces 2 types of gametes • 1 with allele for tallness • 1 with allele for shortness Law (principle) of Segregation

Homozygous vs. Heterozygous • Homozygous • Organism with 2 identical alleles for trait • Both chromosomes have same form of gene • Either both dominant or both recessive • Ex. MM or mm • Heterozygous • • • Organism with 2 different alleles for trait Each chromosome has a different form of gene 1 dominant & 1 recessive • Ex. Mm

An example:

Genotype vs. Phenotype • Genotype: • Genetic makeup • Reveals type of alleles organism has inherited • Represented by a letter: • Capital-dominant • Lowercase-recessive • Examples: • TT = homozygous dominant genotype • tt = homozygous recessive genotype • Tt = heterozygous genotype

Genotype vs. Phenotype • Phenotype: • Physical characteristics • Description of way trait is expressed in organism • The way organism LOOKS • If genotype is: • TT or Tt phenotype = tall • tt phenotype = short

Phenotype vs. Genotype

Mendel’s 3 rd experiment • Mendel’s question: • “Does the segregation of one pair of alleles affect the segregation of another pair of alleles? ” • “Does the gene for seed shape have anything to do with the seed for seed color? ” • Mendel’s experimental design: • He performed a two-factor (dihybrid) cross

Mendel’s 3 rd experiment • Mendel’s results: • F 1: RRYY x rryy (all offspring were round and yellow) • This wasn’t a surprise to Mendel • The real question was: in the gametes of these offspring would the dominant alleles stay together or would the segregate independently? • F 2: Rr. Yy x Rr. Yy • 315/556 were round & yellow • 32/556 were wrinkled & green • 209/556 were other combinations

Mendel’s Conclusion • Segregation of alleles of 1 trait does not affect segregation of alleles of another trait • Genes on separate chromosomes separate independently during gamete formation (meiosis) Law (principle) of Independent Assortment

Summary of Mendel’s Principles • Inheritance of biological characteristics is determined by individual units called genes • Genes are passed from parents to offspring

Summary of Mendel’s Principles • When 2 or more forms for a single trait exist: (alleles) of the gene • Some forms of gene may be dominant • Others may be recessive Law (Principle) of Dominance

Summary of Mendel’s Principles • In most sexually producing organisms: • Each adult has 2 copies of each gene • 1 from each parent • These genes are separated from each other when gametes are formed Law (Principle) of Segregation

Summary of Mendel’s Principles • Alleles for different genes usually segregate independently of one another Law (Principle) of Independent Assortment