Mendel’s Theory Notes

Who is Mendel? Mendel: an Austrian monk who completed a series of genetics experiments on pea plants

Mendel’s 4 Hypotheses 1.For an inherited trait, an individual has 2 copies (one from each parent)

Mendel’s 4 Hypotheses 2. There are alternative forms of genes called alleles Example: brown hair vs. blonde hair

Mendel’s 4 Hypotheses 3. When 2 alleles occur together, one is completely expressed, one is NOT expressed -dominant: expressed allele (Ex: D) -recessive: allele NOT expressed (ex: d)

Mendel’s 4 Hypotheses 4. Gametes only carry 1 allele for each trait because of meiosis

Homozygous vs. Heterozygous *Individuals can be homozygous or heterozygous for each trait. -homozygous…Example: DD or dd -heterozygous….Example: Dd

Practice The allele for hair color is expressed by the letter D. D= brown hair d= blonde hair

Practice What will be expressed? Homozygous or Heterozygous? DD Dd dd

Practice The allele for freckles is expressed by the letter F. F= has freckles f= does not have freckles

Practice What will be expressed? Homozygous or Heterozygous? FF Ff ff

The Laws of Heredity 1.Law of Segregation: 2 alleles for the same trait separate during meiosis. Ex: Ff Goes in 1 gamete Goes in another gamete

The Laws of Heredity 2.Law of Independent Assortment: the inheritance of 1 trait does not affect the inheritance of another! Ex: traits for eye color and hair color may be on the same chromosome, but each trait is passed on on its OWN This is why not all blond people have blue eyes.

Genotype vs. Phenotype For the following examples, B is brown hair, b is blond hair Genotype: Alleles a person HAS Ex: Bb or bb or BB Phenotype: What trait we see on that person Ex: Bb (brown hair) vs. bb (blonde hair)

What is a PUNNETT SQUARE? 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

Using a PUNNETT SQUARE It should look like this:

Using a PUNNETT SQUARE 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

Using a PUNNETT SQUARE Place one of the parents on top, and one on the left. You should get a something similar to this:

Using a PUNNETT SQUARE Drag and drop! Possible Genotypes: ____________, ______% Phenotype: ______ ____________, ______% Phenotype: ______

Punnett Square Practice In cows, black (B) is a dominant color to white (b) which is recessive. If two heterozygous black cows mate, what are the possible genotypes and phenotypes of their offspring? Genotypes of parents: _____ & _____

Punnett Square Practice Possible Genotypes: ____________, ______% Phenotype: ______ ____________, ______% Phenotype: ______

Dihybrid Crosses Dihybrid crosses are those where we consider the inheritance of two characteristics at the same time.

Dihybrid Crosses Big feet (F) are dominant to small feet (f) Hairy big toe (H) is dominant to smooth big toe (h). Complete a dihybrid cross for Mary & Joe. Step 1: Figure out the parents’ genotypes.

Dihybrid Crosses Mary is heterozygous for big feet, and also for hairy toes. Her genotype is ______ Joe is heterozygous for big feet and for hairy big toes. His genotype is _______

Dihybrid Crosses Step 2: Figure out which combinations of alleles can be present in each parents’ gametes. Mary _______Joe _______

Dihybrid Crosses Step 3: Set up a Punnett Square

Results ____ ::: Big Feet Hairy Big Toe Big Feet Hairless Big Toe Small Feet Hairy Big Toe This is the typical ratio expected in a dihybrid cross. Small Feet Hairless Big Toe

Incomplete Dominance We have only discussed cases of complete dominance where one trait always shows over another.

Incomplete Dominance Incomplete dominance occurs when two or more alleles influence an organism’s appearance (its phenotype). The resulting phenotype is something between (a blend) the two alleles.

Incomplete Dominance Ex: Consider the four o’clock (a flower) -Normally they are red or white If the flower receives an allele for red (R) and an allele for white (r) then the resulting flower is pink!

Incomplete Dominance Practice: The Common Hardbarger Blossom is a flower normally occurring in blue or yellow. Give the homozygous genotypes of the normal colored flowers. _________ X ________ (blue) (yellow)

Incomplete Dominance

The blossom shows incomplete dominance, so what color is the special Hardbarger Blossom? What is its genotype?

Codominance Some traits do not exhibit true dominance. This is codominance (dominance is shared) In this case there is NO blending of traits and both alleles are expressed in the organism’s phenotype.

Codominance Ex: How now roan cow? You’ve seen red cows, you’ve seen white cows. Roan is a combo of red and white. The expression of its alleles is different… *R = Red *W = White The genotype for roan is RW