Packet 6

Mendelian Genetics

A) GREGOR MENDEL (1860s)

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

Why Pea Plants?

   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!

B) Dominant and Recessive Genes 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

C) Inheriting Traits

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

D) What is a punnett sqaure?

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%)