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GENETICS
&
THE WORK OF MENDEL
Gregor Mendel

Modern genetics began in the mid1800s in an abbey garden, where a
monk named Gregor Mendel
documented inheritance in peas
used experimental method
 used quantitative analysis
 collected data & counted them
 excellent example of scientific method

Mendel’s work


Mendel bred pea plants to
study genetics
Generations
 Parent
P
generation = P
 Offspring
of P generation =
F1 generation (F = filial)
of F1 generation
= F2 generation
F1
 Offspring
F2
Looking closer at Mendel’s work
P
F1
true-breeding
X
purple-flower peas
100%
purple-flower peas
generation
(hybrids)
F2
generation
true-breeding
white-flower peas
Where did
the white
flowers go?
White
flowers came
back!
75%
purple-flower peas
25%
white-flower peas
What did Mendel’s findings mean?

Allele – alternate form of a gene
 Example:
purple vs. white flower color
purple-flower allele &
white-flower allele are two DNA
variations at flower-color locus
different versions of gene at
same location on homologous
chromosomes
Traits are inherited as units

For each trait expressed, an organism inherits 2
alleles, 1 from each parent
 diploid
organism
 inherits
2 sets of chromosomes,
1 from each parent
 homologous chromosomes
 like having 2 different types of social media
 Instagram
 Snapchat
What are the
advantages of
being diploid?
Dominant vs. Recessive
 Dominant
alleles
 will
ALWAYS cover up recessive allelles
 Represented by a CAPITAL letter
 Dominant allele is always written First (Pp)
 Recessive
homgous
chromoloosomes
allele
 will
only show up when 2 are present (pp)
 Represented by a lower case letter
 Scientist
use the same letter for the different alleles
of the same trait
 Flower


color of pea plants
Purple = P (dominant)
White = p (recessive)
Genotype vs. Phenotype
 Genotype
 Refers
to the “gene” allele combination that is producing
the phenotype
 Pea flower color (PP, Pp, pp)
 Phenotype
 Refers
to what you “PHysically” see
 The expression of the genotype
 You can NOT always determine genotype
 Pea flower color (purple or white)
X
P
purple
Explain Mendel’s results using
…dominant & recessive
…phenotype & genotype
white
F1
all purple
Genotypes
•Homozygous = same alleles (PP, pp)
•True breed
•Pure breed
homozygous
dominant
heterozygous
homozygous
recessive
•Heterozygous = different alleles (Pp)
•Hybrid
Phenotype

2 organisms can have the same phenotype but
have different genotypes
purple
PP
purple
Pp heterozygous
homozygous dominant
How do you determine the
genotype of an individual with
with a dominant phenotype?
Can’t tell
by lookin’
at ya!
st
Mendel’s 1 law
 Law of segregation
 all
of heredity
organisms have a pair of alleles for each trait
 each parent passes on 1 randomly selected allele
for trait to the offspring
 The offspring in turn has its own pair of alleles for
the trait
Mendel’s

nd
2
law of heredity
Law of independent assortment

Separate genes for separate traits are passed down
independently of one another from parents to offspring

For example, just because you have brown hair does not mean
you will have blue eyes
Review: Mendel’s laws of heredity

Law of segregation

monohybrid cross



single trait
each allele segregates into separate gametes
Law of independent assortment

dihybrid (or more) cross


2 or more traits
genes on separate chromosomes
assort into gametes independently
Punnett squares: (used to make predictions)
Probability
%
genotype
Pp x Pp
P
p
PP
25%
75%
Pp
P
PP
%
phenotype
50%
Pp
Pp
p
Pp
pp
pp
ratio
25% 25%
1:2:1
3:1