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AHL Genetics
I. Dihybrid Crosses
•  Monohybrid Aa genotype – one gene / trait
•  Dihybrid AaBb genotype – two genes / traits
•  Recombinant – the reassortment of genes or traits into
combinations different from those of the parents
•  Dihybrid cross parents that are both heterozygous.
–  Example: Mendel’s peas
•  Yellow vs. green
&
Tall vs. short
–  So we are crossing two plants heterozygous for both traits,
Yellow and Tall:
YyTt X YyTt
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I. Dihybrid Crosses
YT
Yt
yT
yt
YT
YYTT YYTt
YyTT YyTt
Yt
YYTt
YYtt
YyTt
Yytt
yT
YyTT YyTt
yyTT
yyTt
yt
YyTt
yyTt
yytt
Yytt
Dihybrid Cross – Genotypes?
YT
Yt
yT
yt
YT
YYTT
YYTt
YyTT
YyTt
Yt
YYTt
YYtt
YyTt
Yytt
yT
YyTT
YyTt
yyTT
yyTt
yt
YyTt
Yytt
yyTt
yytt
Recombinants?
Genotypes:
YYTT- 1
YYTt - 2
YYtt - 1
YyTT- 2
YyTt - 4
Yytt - 2
yyTT - 1
yyTt - 2
yytt - 1
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9:3:3:1 Phenotypes
YT
Yt
yT
yt
YT
YYTT YYTt
YyTT YyTt
Yt
YYTt
YYtt
YyTt
Yytt
yT
YyTT YyTt
yyTT
yyTt
yt
YyTt
yyTt
yytt
Yytt
Dihybrid cross
•  Know this ratio! 9:3:3:1
– 
– 
– 
– 
Yellow Tall
Yellow short
green Tall
green short
•  Can give other ratios IF…
– 
– 
– 
– 
Either parent is homozygous for one or both traits
Either gene has codominant alleles
Either gene is sex-linked
There is an interaction between the two genes
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Dihybrid cross
Try this!
In rabbits, black is due to a
dominant gene (B) and brown
is due to a recessive allele (b);
short hair is a dominant gene
(S) while long hair is recessive
(s).
A black, long-haired rabbit
(whose mother was brown),
was crossed with a brown
rabbit with short hair (whose
father had long hair).
A black, long-haired rabbit (whose mother was brown), was crossed
with a brown rabbit with short hair (whose father had long hair).
1.  Key:
B – black, b – brown
S – short, s – long
2.  Parents Phenotypes:
Black long, brown Short
3.  Parents Genotypes:
Bbss, bbSs
4.  Gametes:
  Bs, bs
  bS, bs
5.  Punnett Square:
Bs
bs
bS
BbSs bbSs
bs
Bbss bbss
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Key:
B – black, b – brown
S – short, s – long
6.  Offspring genotypes:
25% each: BbSs, bbSs, Bbss,
bbss
7.  Offspring Phenotypes:
25% each: Black Short, brown
Short, Black long, brown
long
8.  Any Recombinants?
Yes: BbSs and bbss
Bbss X bbSs
Bs
bs
bS
BbSs bbSs
bs
Bbss bbss
II. Autosomes & Sex
Chromosomes
•  Autosomes:
chromosomes 1-22.
•  Sex chromosomes:
X and Y
–  they determine the sex
of the individual
–  XX female & XY male
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III. Linkage Group
Definition
–  Linkage group – all the genes on a chromosome.
–  Crossing over between non-sister chromatids of a
homologous pair in prophase 1 can result in random
orientation in metaphase 1
B. Linked Genes
•  Let’s compare 2
examples:
•  UNLINKED:
 If A and B are
linked, we write,
AB
ab
–  AaBb X aabb
ab
AB
Ab
aB
ab
AaBb
Aabb
aaBb
aabb
•  Recombinants:
–  Aabb & aaBb
ab
ab
X
ab
AB
AB
ab
ab
ab
ab
Recombinants: No!
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Linked Genes
•  If A & B linked
AB
ab
ab
ab
X
ab
ab
AB
AB
ab
ab
ab
ab
•  What if you breed them and
actually get the following?
•  37%
AB
•  37%
ab
ab
•  13%
Ab
ab
•  13%
aB
ab
•  So, 50% and 50%
AB
ab
ab
ab
Linked Genes
•  There must have been crossing over!
•  The probability of crossing over increases,
the further apart the genes are on the
chromosome.
•  This used to be used to map chromosomes
(the old way).
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Linked genes
•  Try these:
1. 
2. 
Ab
aB
Ab
aB
(What are the recombinants?)
X
X
ab
ab
Ab
aB
All recombinants
50% recombinants
IV. Polygenic Inheritance
•  More than one gene
codes for a trait
•  Example – height
•  Continuous variation
•  The environment also
is part of variation:
good nutrition or poor
nutrition.
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Polygenic Inheritance
•  Example: skin color
•  Up to 7 genes (each
“dark” codominant
allele contributes a
certain amount of
melanin)
•  Continuous variation
•  The environment also
contributes melanin
(sun causes skin to
make extra melanin)
Polygenic Traits: skin
•  UV light
–  Causes skin to make
Vitamin D  strong bones
–  May cause sunburn  skin
cancer
–  Destroys folic acid 
neural tube defects which
can kill fetuses
•  Melanin blocks UV light
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Polygenic Inheritance
•  Example: skin color
•  Natural Selection:
–  Dark skin has an advantage in direct
sun (equator)
–  Light skin has an advantage further
from the equator
–  Original dark-skinned people
travelled north where light skin was
selected for
–  In southern India, people have dark
skin, but came from lighter skinned
people
–  Color change in as little as 2500 yrs.
Skin color mirrors UV radiation levels
That’s it!
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