The Origins of Genetics Genetics – the study of biological inheritance patterns and variation in organisms Traits – distinguishing characteristics that are inherited Heredity – the passing of characters (traits) from parents to offspring
Mendel’s Breeding Experiment Gregor Johann Mendel Austrian monk Experiments with garden pea plants led to our modern understanding of heredity
Why are Pea Plants Good Subjects? Several characters (traits) of the plant exist in two clearly different forms Flower color Seed color Seed shape Pod color Pod shape Flower position Plant height Easy to grow Short life cycle
Mendel’sExperiment Monohybrid cross – involves one pair of contrasting traits Step 1 – allowed each variety of garden pea to self-pollinate for several generations to ensure they are genetically uniform, or purebred.
Mendel’s Experiment Step 2 – cross-pollinated first two parent plants (P generation) that had contrasting traits The offspring of these plants were called the first filial generation (F1 generation) He examined each F1 plant and recorded the number of plants expressing each trait
Mendel’s Experiment Step 3 – allowed the F1 generation to self- pollinate The offspring are called the F2 generation Each F2 plant was examined and the traits recorded
Mendel’s Results In F1 generation, all plants were purple In F2 generation, the ratio of purple to white plants was 3 to 1 Ratio: 3:1 or 3 to 1 or 3/1
Mendel’s Hypotheses1. There are alternative versions of genes Example: the gene for flower color in peas can exist in a “purple” version or a “white” versionToday the different versions of a gene are called its alleles.
Mendel’s Hypotheses2. When two different alleles occur together, one of them may be completely expressed, while the other may have no observable effect on the organism’s appearance.Expressed form of the character – dominantTrait that is not expressed - recessive
Mendel’s First Law orLaw of Segregation Organisms carry two copies of each gene, one from each parent When gametes are formed, the alleles for each gene in an individual separate. Thus, gametes carry only one allele for each inherited character.
The same gene can have manyversions Gene – a piece of DNA that holds the instructions to make a certain protein Allele – any of the alternative forms of a gene that are at a certain locus on a chromosome
Homozygous vs. Heterozygous If two alleles of a particular gene present in an individual are the same, the individual is said to be homozygous for that trait. If the alleles of a particular gene present in an individual are different, the individual is said to be heterozygous for that trait.
Genotype vs. Phenotype Genome – all of an organism’s genetic material Genotype – the set of alleles that an individual has Phenotype – the physical appearance of a character Genotype Phenotype PP Purple flower Pp Purple flower pp White flower
Punnett Squares A diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in the cross.
Outcomes of Crosses Probability – the likelihood that a specific event will occurProbability = # of 1 kind of outcome total # possible outcomes