Continuing with Mendel

A recap from last class: Mendel found that we get one factor from each parent The F1 generation of a cross between a true-breeding purple- flower plant and a true-breeding white-flower plant yielded a generation of purple plants The F2 generation showed a 3:1 distribution of purple to white flowered plants The plants must have ‘white’ factors and ‘purple’ factors X F1 F2

Factor = Allele What Mendel termed a ‘factor’ is what we now call an allele We each get one ALLELE from each parent – one copy of a gene from each parent These two unique copies combine at fertilization to give us a pair of genes

Dominant and Recessive The purple factor was shown to be DOMINANT Dominant allele: the allele that, if present, is always expressed The white factor was shown to be RECESSIVE Recessive Allele: The allele that is expressed only if the dominant allele is not present

Let’s take a look…

Phenotype: An individual’s outward appearance with respect to a specific trait The characteristic being ‘expressed’ Genotype: The genetic makeup of an individual The set of alleles carried by an individual

Homozygous: An individual carries two of the SAME allele for a given characteristic E.g. PP, pp, aa, DD, X A X A etc…. Can be homozygous dominant (BB), homozygous recessive (bb) etc. Heterozygous: An individual carries two DIFFERENT alleles for a given characteristic E.g. Pp, Aa. Dd, X A X B etc…

Punnett Square A diagram that summarizes every possible combination of each allele from each parent; a tool for determining the probability of a single offspring having a particular genotype

How can we use it? Mr. Sheps has two true breeding pea plants, one with round seeds which is a dominant trait, and one with wrinkled seeds. How many plants will have wrinkled seeds in the F 1 generation? Approximately how many will have wrinkled seeds in the F 2 generation if he has 500 seedlings?

How to do a Punnett Square? 1. Assign the trait a letter, and write LET statements 2. Write out the genotypes and phenotypes (if available) of the individuals that are being crossed 3. Complete a Punnett Square 4. Analyze the Punnett Square to answer your question and determine genotypic and phenotypic frequencies and ratios (as necessary). 5. Answer the question in a complete sentence.