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Mendelian
Genetics
1
Particulate Inheritance
•Mendel stated that physical
traits are inherited as
“particles”
•Mendel did not know that
the “particles” were actually
genes located on
chromosomes
2
Inheritance
 We inherit genes from
our parents
 Different versions of a
gene are called alleles
 How can genes differ,
and how do alleles
affect an organism?
3
Alleles in an Organism
 How many alleles does an
organism have for a single trait?
 Genes come in pairs because
chromosomes come in pairs
(homologues)
4
5
Where does each allele come from?
• Half from mother
• Half from father
• What process divides
the genome in half?
• How does the cell
ensure each sex cell
receives one allele for
each trait?
6
Anaphase I
Anaphase II
Homologous
Pairs Separate
Replicated
Chromosomes
Separate
7
Genes Come in Pair
•An individual
can only have
two alleles of a
given gene
•A population
can have more
than two alleles
of a given gene
8
Two Types of Alleles
 Dominant - stronger of two alleles
that masks the trait of other allele;
represented by a capital letter (R)
 Recessive – allele that’s trait is
masked by other alleles;
represented by a lowercase letter
(r)
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10
Genetic Make-Up
 Genotype - allele combination for a trait
(e.g. RR, Rr, rr)
 Phenotype - the physical feature
resulting from a genotype ( seven traits
of pea plants)
 Genotype dictates phenotype!
11
Two Types of Genotypes
•Heterozygous – genotype of two
different alleles (Rr)
•Homozygous – genotype of the same
two alleles (rr or RR)
Which allele’s trait will we see in a
heterozygote?
12
Classifying Homozygotes
RR
rr
Homozygous
Dominant
Homozygous
Recessive
13
14
Crosses (Breeding)
15
Odds of Inheritance
•What are the odds of inheriting each
allele from a parent? Why?
50%
16
Practice Cross
• Trait: Seed Shape
• Alleles: R – Round
r – Wrinkled
• Cross: Round seeds x Wrinkled seeds
•
RR
x
rr
r
r
R
Rr
Rr
R
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Ratio: All alike
Phenotypic
Ratio: All alike
17
Practice Cross
• Trait: Seed Shape
• Alleles: R – Round
r – Wrinkled
• Cross: Round seeds x Round seeds
•
Rr
x
Rr
R
R
r
RR
Rr
r
Rr
rr
Genotype: RR, Rr, rr
Phenotype: Round &
wrinkled
Genotypic Ratio: 1:2:1
Phenotypic Ratio: 3:1
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Practice Cross
• Trait: Seed Shape
• Alleles: R – Round
r – Wrinkled
• Cross: Round seeds x Round seeds
•
RR
x
Rr
R
r
R
RR
Rr
R
RR
Rr
Genotype: RR, Rr
Phenotype: Round
Genotypic
Ratio: 1:1
Phenotypic
Ratio: All alike
19
Practice Cross
• Trait: Seed Shape
• Alleles: R – Round
r – Wrinkled
• Cross: Wrinkled seeds x Round seeds
•
rr
x
Rr
R
r
r
Rr
rr
r
Rr
rr
Genotype: Rr, rr
Phenotype: Round &
Wrinkled
Genottypic Ratio: 1:1
Phenotypic Ratio: 1:1
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Dihybrid Cross
RY
RY RRYY
Ry RRYy
rY RrYY
ry
RrYy
Ry
rY
ry
RRYy
RrYY
RrYy
RRyy
RrYy
Rryy
RrYy
rrYY
rrYy
Rryy
rrYy
rryy
Round/Yellow:
Round/green:
9
3
wrinkled/Yellow: 3
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
21