Continuing with Mendel

A recap from last class:Mendel found that we get one factor from each parentThe 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 = AlleleWhat 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 parentThese two unique copies combine at fertilization

to give us a pair of genes

Dominant and RecessiveThe purple factor was shown to be DOMINANTDominant allele: the allele that, if present, is

always expressedThe 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 traitThe characteristic being ‘expressed’

Genotype: The genetic makeup of an individualThe set of alleles carried by an individual

Homozygous: An individual carries two of the SAME allele for a given characteristicE.g. PP, pp, aa, DD, XAXA etc….Can be homozygous dominant (BB),

homozygous recessive (bb) etc.

Heterozygous: An individual carries two DIFFERENT alleles for a given characteristicE.g. Pp, Aa. Dd, XAXB etc…

Punnett SquareA 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 F1 generation?

Approximately how many will have wrinkled seeds in the F2 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 Square4. 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.