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Classical Genetics
The Legacy of Gregor Mendel
Or
The Monk with the Missing “Peas”
The Big Question
 Why
do children
look like their
parents?
That’s GENETICS!
 The
scientific study
of heredity
 Heredity:
the
passing down of
traits from parents
to offspring via
genes and
chromosomes
 Gregor
Who started genetics?
Gregor Mendel of course!!!
Mendel
 Austrian monk in
1860’s
 Studied
different
traits of the
garden pea
 Discovered
the
basic laws of
genetics
 “Father of
Genetics”
More Mendel!!!
 1st
person to
succeed in
predicting how
traits are passed
from one
generation to
next
Why is Mendel SO important?
 Studied
one trait
at a time
 Analyzed his data
mathematically
 Looked at
multiple traits
 Used multiple
trials
Why peas?
 Quick
growing
 Lots of different traits
So what did Mendel find out?
 Genes
and Alleles
 GENE:
section of DNA
that determines a trait
(Mendel called them
factors)
 ALLELE:
particular form of
a trait (represented by
letters)
 EX: plant size
 “T”
is the tall allele
 “t”
is the short allele
Combinations of Alleles
We
get one allele from each
parent
Three
TT:
possible combinations
homozygous dominant
Tt: heterozygous
tt: homozygous recessive
Dominant/Recessive
 One
allele is dominant over the other
(capable of masking the recessive allele)
PP = purple
Pp = purple
pp = white
New Vocabulary
 Genotype
- combination of alleles that an
individual has. (TT, tt, Tt)
 Phenotype
- physical appearance of a
trait (Tall, Tall, Short)
So how do we write this?
 Remember
T = Tall and t = short
 So if a plant is..
Genotype
 Homozygous Dominant:
TT
 Heterozygous:
Tt
tt
 Homozygous Recessive:
Phenotype
Tall
Tall
Short
Punnett Square
Father’s Genes
way to show
which genes can
combine when an
egg and sperm join
 Letters are used in
place of genes
 Large letter =
dominant gene (T)
 Small letter =
recessive gene (t)
Mother’s Genes
A
Punnett Square
 Large
letter goes
first in heterozygous
organisms (Ff)
 Let’s do a cross
between a pure
dominant male
(sperm) for free
earlobes (FF) and a
female (egg) who is
heterozygous for
free earlobes (Ff)
F
f
F
FF
Ff
F
FF
Ff
F is dominant over f
What are the expected results?
F
F
F
f
FF
Free
Earlobes
Ff
Free
Earlobes
FF
Free
Earlobes
Ff
Free
Earlobes