Mendelian Genetics
Genetics = the study of heredity
Heredity = the passing of traits from one
generation to the next
Genetics
• Genetics is the study of heredity.
(how things are passed down
from one generation to the next.)
• The founder of genetics is
Gregor Mendel
Gregor Mendel
• Austrian Monk born in 1822
• Spent 7 years experimenting with pea plants
• From these experiments he came up with the basic laws of
heredity
Why peas?
• Mendel experimented with peas for two reasons
– easily identifiable traits.
– The can self- pollinate.
The experiment
• used true-breeding plants (plants that offspring are always
identical to the parent)
• crossed purple flower with a white flower (P generation)
• Got all purple flowers!
The experiment
The experiment
• he crossed two of the purple offspring (F1 generation)
• He got a 3 to 1 ratio of purple to white.
The experiment
• 1,000 crosses were done and it
was the same result every time.
75% of the offspring had purple
flowers and 25% had white
flowers
Mendel's Conclusions
• Genes control traits.
• There are two alternate forms for every gene,
these are called alleles. You inherit one from
your mom and one from your dad.
• There are dominant alleles (capital letter)
• There are recessive alleles (lowercase letter)
Mendel's Conclusions
• Law of Segregation – Alleles
for a trait separate during
fertilization
Mendels conclusions
• Law of Independent Assortment – Genes will separate
during fertilization; Traits are inherited independently of
each other, not in pairs
• (Just because you have your dad’s hair color doesn’t
mean you will have his eye color)
Vocab you need to know
• Homozygous- Same allele
– Homozygous dominant (two capital letters) HH
– Homozygous recessive (two lowercase letters) hh
• Heterozygous-Different alleles (one capital, one lower case) Hh
Vocab you need to know
• Genotype – genetic makeup
of an organism (alleles)
• Phenotype- physical
appearance (the way an
organism looks)
• 1. For each genotype, indicate whether it is
heterozygous (HE) or homozygous (HO)
AA ____Bb ____Cc ____Dd ____ Ee ____
ff ____GG ____ HH ____ Ii ____Jj ____
• 2. For each of the following genotypes below,
determine the phenotype.
Purple flowers are dominant to white flowers
PP ________ Pp________ pp _________
Brown eyes are dominant to blue eyes
BB _________ Bb_______ bb__________
Punnett Squares
• A monohybrid cross is used when your predicting ONE
trait – also called a punnett square
How to do a Punnett Square
Step 1. Read the problem and figure out what the genotype of each parent is. Only 3 possibilities
(homozygous dominant, heterozygous, or homozygous recessive) Use the same letter.
Example Problem: A homozygous purple flower is crossed with a white flower. What percentage of the
offspring will be purple?
Parent 1
Step 2. Draw Punnett square. Place the genotype of the first parent
On the top and the genotype of the second parent on the side.
Parent 2
Parent 1 (Mom egg cell)=_______________________
Parent 2(Dad sperm cell)=_______________________
Punnett squares for Monohybrid crosses
• gametes from one
parent are listed on top
and bottom of the
square
How to Draw a Punnett Square
Step 4: Determine the phenotype of the offspring
Based on the different genotypes inside the squares.
Answer: All of the offspring are__________ (genotype)
So the phenotype is ______________
Parent 2
Step 3: Write the genotype of the different ways the gametes can
be combined inside the square. (This shows you the possible
genotypes of the offspring)
Parent 1
P
P
p
Pp Pp
p
Pp
Pp
Sample Problem #1
Problem: In rabbits Brown fur is dominate and white hair is recessive. A heterozygous brown rabbit is crossed with
A white rabbit. What percent of the offspring will be white? What percent of the offspring will be brown?
Parent 1____________
Parent 2____________
Parent 2
______________ will be white
______________ will be brown
Parent 1
Sample Problem #2
Problem: In humans Brown eyes is dominate to Blue eyes. Two parents that are heterozygous for brown
eyes have kids. What is the chance their children will have brown eyes? Blue eyes?
Parent 1_____________
Parent 2_____________
_______ will be brown
_______ will be blue