Back to Basics
What are
their
phenotypes?
How would
you describe
their
genotypes?
A is a dominant characteristic –
orange feathers.
a is a recessive characteristic – blue
feathers.
This bird has two genes for orange
feathers.
Its genotype is AA.
This bird has two genes for blue
feathers.
Its genotype is aa.
Punnet Squares
Which one is the
female?
How do you know?
What if we wanted to look at two
features at once?
Feather color -- ORANGE or blue
Crest? -- CREST or no crest
A – Orange
What are the possible
genotypes of these two
birds?
a - blue
C – Crest
c – no crest
Which
alleles are
dominant?
Dihybrid Cross
Feather color -- ORANGE or blue
Crest? -- CREST or no crest
Genotype: Aacc
Possible gametes: Ac; Ac; ac; ac
Genotype:
aaCc
Possible
gametes:
aC; ac; aC;
ac
Dihybrid Cross
Feather color -- ORANGE or blue
Crest? -- CREST or no crest
Genotype: Aacc
Possible gametes: Ac; Ac; ac; ac
Genotype:
aaCc
Possible
gametes:
aC; ac; aC;
ac
Dihybrid Cross
What are the
possible
genotypes of the
offspring?
What are the
phenotypic
ratios?
What are the
possible
phenotypes of the
offspring?
Another example
MA = lots of
melanin
mA = small amount
of melanin
MB = lots of
melanin
mB = small amount
of melanin
mA mB
MAmB
mAMB
MAMB
Linked Genes

These are:
• Genes that have loci that are
physically close to each other on
the same chromosome
• less likely to be independently
assorted (separated from each
other) during crossing over in
meiosis
Linked Genes

These are:
• Genes that have loci that are
physically close to each other on
the same chromosome
• less likely to be independently
assorted (separated from each
other) during crossing over in
meiosis
Because…

A crossing over
point is more
likely to occur on
a chromosome
between two
genes that are
widely separated
compared to
genes that are
closer together
For example:
So what??

This means that:
• Linked genes are almost always
inherited together as a single unit
from one of the two parents
• A dihybrid cross cannot be used to
estimate the probability of
inheriting different combinations of
traits
So how can we tell if genes are
linked?

By looking at the results of a test
cross:
• ABab x abab = ABab; Abab; aBab; abab
• If all offspring occur in equal amounts,
genes are not linked
• If linked, the vast majority of the
offspring will be:


ABab or abab
Because AB or ab will be inherited as single
units from one parent whilst only an ab
combination an be inherited from the other.
Test
crosses
always
include
one homozygous
recessive
parent
and one
heterozygous
parent
So how can we predict outcomes
of linked gene crosses?



This depends on the distance
between the linked genes on
the chromosome
This is measured by map
units
The number of map units
correlates to the percentage
of offspring that have
recombined traits
For example


In the cross
ABab x abab
If A and B are 6
map units apart
the recombined
alleles Ab and
aB will only
have a
probability of
making up 6%
of the total
offspring (3%
each)
Parent Gametes
AB
ab
Ab
aB
ab
AaBb
aabb
Aabb
aaBb
ab
AaBb
aabb
Aabb
aaBb
ab
AaBb
aabb
Aabb
aabb
ab
AaBb
aabb
Aabb
aabb
AaBb
47%
aabb
47%
Aabb
3%
aaBb
3%
Offspring
Additional examples






In the cross CDcd x cdcd
8 map units apart
recombined alleles Cd
and cD will only have a
probability of making up
8% of the total offspring
(4% each)
In the cross EFef x efef
12 map units apart
recombined alleles Ef
and eF will only have a
probability of making up
12% of the total
offspring (6% each)
Parent Gametes
cd
Offspring
CD
cd
Cd
cD
CdDd
ccdd
Ccdd
ccDd
46%
46%
4%
4%
Parent Gametes
ef
Offspring
EF
ef
Ef
eF
EeFf
eeff
Eeff
eeFf
44%
44%
6%
6%
Pedigree Charts


Pedigrees are diagrams which
demonstrate the inheritance of a
particular trait within a family
Pedigrees can indicate the
inheritance modes of particular
traits
Pedigree Symbols
I, II, III, IV – 1st, 2nd,
3rd, 4th generations
respectively
Are males and
females equally
affected?

Pedigrees of
x-linked
recessive
traits:
• Males affected
more
commonly
• All sons of
affected
female will
also be
affected
• All children of
2 affected
individuals
will also have
the trait
Are males and
females equally
affected?
Do all affected
individuals have
at least one
affected parent?
Autosomal Dominant

Pedigrees of autosomal dominant
traits:
• Males and females affected equally
(autosomal)
• Affected individuals must have at
least one affected parent

Traits cannot ‘skip’ generations
• No carriers
Are males and
females equally
affected?
Do all affected
individuals have
at least one
affected parent?
Autosomal Recessive

Pedigrees of autosomal recessive
traits:
• All children of 2 affected parents
will also have the trait
• Some affected individuals might
have 2 unaffected parents if both
are carriers:

Can ‘skip’ generations
X-linked Dominant

Pedigrees of x-linked dominant
traits:
• All daughters of affected males will
also have the trait
• Female with the trait can pass it on
to both daughters and sons
• No carriers