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 F1 (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 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 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 2nd experiment Mendel’s question: “Did the recessive alleles disappear or were they still present in F1 plants?” Mendel’s experimental design: allow the F1 plants to produce an F2 generation by self-pollination

Mendel’s 2nd experiment Mendel’s results: Traits controlled by the recessive alleles appeared! Approx. ¼ of F2 plants showed traits controlled by recessive alleles

Mendel’s Conclusion Law (principle) of Segregation Alleles for tallness & shortness segregated from each other during formation of gametes When each F1 plant produces gametes: 2 alleles separate Each gamete carries single copy of each gene Each F1 plant produces 2 types of gametes 1 with allele for tallness 1 with allele for shortness Law (principle) of Segregation

Homozygous vs. Heterozygous 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 3rd 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 3rd experiment Mendel’s results: F1: 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? F2: RrYy x RrYy 315/556 were round & yellow 32/556 were wrinkled & green 209/556 were other combinations

Mendel’s Conclusion Law (principle) of Independent Assortment 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 (alleles) of the gene for a single trait exist: 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