 Used pea plants to study the way characteristics are passed from one generation to the next

 grow and reproduce quickly  lots of traits that could be studied  distinct characteristics  easy to cross-pollinate

 Mendel’s experiments  short x short = short (no surprise)  tall x tall = mostly tall, but some short (SURPRISE)  If the offspring are identical to both parents, the parents are called TRUE BREEDERS  What if he crossed true breeding tall plants with true breeding short plants?  ALL PLANTS WERE TALL – shortness disappeared

 What if he crossed those offspring?  Shortness reappeared!

 Gene – unit of heredity  Allele – two alleles make up one gene (one allele from mom, one from dad)-different forms of a gene  TT – true breeding tall plant-homozygous dominant  tt – true breeding short plant-homozygous recessive  Tt – hybrid plant (grows tall, short allele is hidden)-heterozygous

 A DOMINANT allele always hides a recessive allele  A recessive allele is always hidden by a DOMINANT allele  We use letters to indicate dominant and recessive alleles  capital letters = dominant  lowercase letters = recessive

 Mendel came up with four laws:  Law of Inheritance: factors are passed from parents to offspring  Law of Dominance: alleles are either dominant or recessive  Law of Segregation – one allele from each from each pair is passed to the sex cells (egg or sperm)  Law of Independent Assortment – each allele is passed independently of the other alleles (example – a tall plant won’t always have green peas, they could be yellow)

IN SUMMARY  Traits are passed from one generation to the next  Traits are controlled by genes  Organisms inherit genes in pairs (each part is an allele)  Some genes are DOMINANT, others are recessive  DOMINANT genes hide recessive gene when both are present

 Phenotype: physical characteristics of an organism  Genotype: genetic makeup of an organism

 A tool to predict the probability of certain traits in offspring that shows the different ways alleles can combine  A way to show phenotype & genotype  A chart that shows all the possible combinations of alleles that can result when genes are crossed

 To set up a Punnett square, draw a large square, and then divide it into 4 equal sections (also squares). It should look something like this:

Now you need two parents to mate, ones with a known genotype For example, a red flower (genotype Rr) and a white flower (genotype rr). Rr x rr Place one of the parents on top, and one on the left. You should get a something similar to this:

 Finally, take each letter in each column and combine it with each letter from each row in the corresponding square. You should now have a picture close to this:

 The two-letter combinations are the possible genotypes of offspring  They are: › Rr, Rr, rr, and rr  From this it is possible to determine the probability (chance) that a flower will have a red phenotype (2/4 or 50%) or a white phenotype (2/4 or 50%)