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Biology 12
Unit 3: Heredity
Mendelian Genetics
Genes and Heredity
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•
•
•
Mendel’s Experiments
Alleles
Punnett Square
Monohybrid & Dihybrid Crosses
Mendelian Genetics
• Traits are controlled by genes
• Law of Dominance: dominant and recessive
• Law of Segregation: genes separate during
meiosis
• Law of Independent Assortment: genes for
one trait will separate independently of how
other traits separate
More Mendelian
• Mendel worked with pea plants
• True breeding tall plants are homozygous
for the tall (T) allele
• True breeding dwarf plants are homozygous
for the dwarf (t) allele
• When the parental generation (P1) was
made up of true breeding tall and true
breeding dwarf plants all offspring were tall
P1
F1
F2
phenotype:
genotype:
gametes:
phenotype:
genotype:
gametes:
phenotype:
genotype:
Tall
X Dwarf
TT
X tt
T
t
Tall
X Tall
Tt
X Tt
T t
T t
3 Tall : 1 dwarf
1 TT : 2 Tt : 1 tt
Punnett Square
• This F1 generation of tall plants were all
heterozygous: one tall allele and one dwarf
allele
• When F1 individuals were crossed Mendel
found a 3:1 ratio of tall to short plants
T
T
t
Tt
Tt
t
Tt
Tt
T
t
T
TT
Tt
t
Tt
tt
Possible Crosses
A
A
A
AA
AA
A
AA
AA
D
d
D
DD
Dd
d
Dd
dd
B
B
B
BB
BB
b
Bb
Bb
E
e
e
Ee
ee
e
Ee
ee
C
C
c
Cc
Cc
c
Cc
Cc
f
f
f
ff
ff
f
ff
ff
Monohybrid Cross
Test Cross
• When an individual showing the dominant
phenotype is crossed with a homozygous
recessive individual to determine the
genotype
t
t
T
Tt
Tt
?
?t
?t
If dwarf individuals are
observed in the
offspring, then the
unknown genotype is
heterozygous
Dihybrid Crosses
• Mendel looked at two traits at a time to
determine the Law of Independent
assortment
• True breeding Yellow Round Seed cross
with true breeding green wrinkled seed
Let Y be yellow, y is green
Let R be round, r is wrinkled
P1: genotype
gametes
F1: genotype
gametes
YYRR
X
yyrr
YR
yr
YyRr
X
YyRr
YR Yr yR yr
YR
F2:
YR
Yr
yR
yr
Yr
yR
yr
Dihybrid Crosses
YR
Yr
yR
yr
YR
YYRR
YYRr
YyRR
YyRr
Yr
YYRr
YYrr
YyRr
Yyrr
yR
YyRR
YyRr
yyRR
yyRr
yr
YyRr
Yyrr
yyRr
yyrr
Mendel’s First & Second Law
The Law of Segregation &
The Law of Independent Assortment
Non-Mendelian Inheritance
• Multiple Alleles
– Example: Eye colour in Drosophelia
– Wild Type > Apricot > Honey > White
• Incomplete Dominance
– Example: Snap dragons, red X white => pink
• Co-Dominance
– Example: Shorthorn cattle, red X white => roan
E2
E3
CR
CW
E1
E1E2
E1E3
CR
E4
E2E4
E3E4
CW CRCW CWCW
R W
CRCR C C
Hr
Hw
Hr
r w
HrHr H H
Hw
HrHw HwHw
• Polygenic Inheritance
– Gene interaction: Example Chicken Comb
Rose (R-pp)
Walnut (R-P-)
Pea (rrP-)
Single (rrpp)
• Pleiotropic genes
– One gene with multiple effects
– Example: Sickle-cell anemia
• Lethal Genes
– When a specific genotype is not viable
– eg. if AA is lethal, a heterozygous cross would
give a 2:1 ratio (2 Aa : 1 aa – the AA is dead)
Non-Mendelian Genetics
Clip 2
Clip 1
Linkage
• Mendel demonstrated his Law of
Independent Assortment using simple traits.
• Some traits do not follow this Law.
• When genes are on the same chromosome
they do not separate independently