Chapter 15 (Part 6)
The Chromosomal Basis of
Inheritance
GENE MAPPING
AP Biology/ Ms. Gaynor
Linked Genes
• Remember…
–Linked genes tend to be inherited
together because they are located
near each other on the same
chromosome
–CLOSER genes are linked LESS
crossing over  tend to “travel”
or segregate together into gamete
How Linkage Affects
Inheritance
• Morgan did other
experiments with fruit flies to
see how linkage affects the
inheritance of 2 different
characteristics (genes)
–Body color and wing size
Morgan’s Dihybrid Cross
• In flies,
– gray body (b+) = dominant
– black body (b) = recessive
– long (normal) wings (vg+) = dominant
– vestigial wings (vg) = recessive
• What is expected phenotype ratio from a
cross between a heterozygous gray, normal
winged fly with a black, vestigial fly?
– b+b vg+vg x bb vgvg
(BbNn x
bbnn)
• ¼ : ¼ : ¼ : ¼ ratio or a 1:1:1:1 ratio
But what he got was…..
Recall…What is Mendel’s Law of
Independent Assortment?
• states that allele pairs separate
independently during meiosis
• Therefore, traits are transmitted to
offspring independently of one another
What explains the nonMendelian results?
• There was NO independent assortment!
– Traits are linked on the same chromosome
• If traits are linked, then all of offspring
should resemble the parental types, black
vestigial or gray normal.
• How did we get the recombined traits of
black, normal and gray vestigial?
Looks like
parents
Crossing Over
• Linked genes are not inherited together
every time.
• Chromosomes exchange homologous
genes during meiosis
• Produces offspring with
combinations of traits different from
those found in EITHER parent
How does genetic recombination
occur???
...CROSSING OVER!!!
Linked genes can cross over and travel
TOGETHER!!!
Morgan determined…
–Genes close together on SAME
chromosome are linked and do
not assort independently
–Unlinked genes are either on
separate chromosomes or FAR
apart on the same chromosome
and assort independently (if
crossing over separates the
genes)
• Recombinant offspring
– offspring that show NEW
combinations of traits
• When 50% of all offspring are
recombinants
–Geneticists say that…
• there is a 50% frequency of
recombination
• This is related to the distance
between linked genes
Linked vs. Not
Linked???
• Linked genes
–Produce recombination
frequencies less than
50%
• Frequency = # of recombinants x 100
total # of offspring
Linkage Mapping
• A genetic map
–Is an ordered list of the
genetic loci (positions)
along a particular
chromosome
–Can be developed using
recombination frequencies
Using Recombination
Frequency to Map Traits
• The farther apart genes are on a
chromosome
–The more likely they are to be
separated during crossing over
• Therefore…
–Recombination frequency is
greater between genes further
apart on a chromosome
PRACTICE…
Using Recombination
Frequency to Map Traits
• The following 3 traits are linked to
chromosome #1
– Recombination frequency between black (b) and
vestigial wing (vg) traits is 17%
– Recombination frequency between cn (cinnabar
eyes) and black is 9%
– Recombination between cn and vg is 9.5%
• Map these traits on the chromosome.
Chromosome #1
Distance between genes are called
MAP UNITS
Let’s Practice Again…
• Determine the order of genes along a
chromosome based on the following
recombination frequencies:
• A - B 8%
A - C 28%
A - D 25%
B - C 20% B - D 33%
__D___ __25mu___A_8 mu_B _ _20 mu__ C
Eukaryotic
ChromosomesHow Do You Solve
Gene Mapping
Problems?
How do you tell if genes
are linked or unlinked?
• Linked = no independent assortment
• Genes are on the SAME chromosome
• Unlinked = independent assortment
occurs
• Genes are on SEPARATE chromosomes or VERY far
apart on the same chromosome
–Use “expected” phenotype and
genotype ratios (null hypothesis)
• If you do NOT get expected Punnett Square
ratios, the genes are probably LINKED!
If genes are linked, how do you tell
which are the parental linkages?
– Parental genotypes should be found MORE
frequently (more often) in the offspring than the
recombinant linkages
– 2 ways alleles can be linked in a heterozygous
parent (--- = chromosome)
-----A------B--------a------b----
• Cis
 AaBb
• 2 dominant alleles on 1st chromosome homologue 2
recessive alleles on 2nd chromosome homologue
OR…
• Trans
-----A------b--------a------B----
 AaBb
• 1 dominant, 1 recessive allele on 1st chromosome
homologue (ex: A, b) 1 dominant, 1 recessive allele
on 2nd chromosome homologue (ex: a, B)
Example #1…
• Mating : AaBb
x
A= Long antennae
a = Short antennae
aabb
(this is a testcross  )
B = Green eyebrows
b = Blue eyebrows
• If genes are UNLINKED, what do you expect to see in the
offspring (progeny)?
• 25% Long antennae, Green eyebrows
• 25% Long antennae, Blue eyebrows
• 25% Short antennae, Green eyebrow
• 25% Short antennae, Blue eyebrow
• Assume your offspring look like:
Long, Green
850
Are the genes linked? _________
Long, Blue
150
Parental genotypes are
Short, Green
150
_________ x _________
Short, Blue
850
Example #2…
A= feathers, a= no feathers
B= horns, b= no horns
D= whiskers, d= no whiskers
Starting with pure breeding lines, Cross
AA BB DD x aa bb dd (P)
mom
dad
AaBbDd (F1)
The parental chromosomes in the F1
have to be ABD and abc
From mom
-----A------B------D---------a------b------d-----
From dad
F1 Genotype = AaBbDd
Cross (ABD abd) F1 progeny with (abd abd)
testcross
AaBbDd x aabbdd
You expect
1/8 (12.5 %) for each phenotype (½ x ½ x ½)
BUT….You get the F2 offspring below…
Feathers, horns, whiskers
580
Feathers, horns, no whiskers
3
No feathers, no horns, whiskers
5
No feathers, no horns, no whiskers 592
Feathers, no horns, whiskers
45
Feathers, no horns, no whiskers
89
No feathers, horns, whiskers
94
No feathers, horns, no whiskers
40
So…genes are probably linked!
How do you tell SINGLE
from DOUBLE
recombinants?
• Remember…recombinants do NOT look
like parents
– Should be found MORE frequently in the
offspring than the double recombinant
linkages but LESS frequently in the
parental linkages
Feathers, horns, whiskers
Feathers, horns, no whiskers
No feathers, no horns, whiskers
No feathers, no horns, no whiskers
Feathers, no horns, whiskers
Feathers, no horns, no whiskers
No feathers, horns, whiskers
No feathers, horns, no whiskers
580
3
5
592
45
89
94
40
*ABD
ABd
abD
abd
AbD
Abd
aBD
aBd
A= feathers, a= no feathers
B= horns, b= no horns
D= whiskers, d= no whiskers
**NOTE: the 2nd allele has to be recessive for each gene because one
parent is a testcross
So…ABD really means AaBbDd; Abd means Aabbdd
Which is which genotype?
Parental genotypes
Single recombinants (1 crossing over event)
Double recombinants (2 crossing over events)
A---B
A---D
B---D
-----A------B----
-----A------D----
-----B------D----
-----a------b----
-----a------d----
-----b------d----
•Ab
•aB
•Ad
•aD
•Bd
•bD
Feathers, horns, whiskers
Feathers, horns, no whiskers
No feathers, no horns, whiskers
No feathers, no horns, no whiskers
Feathers, no horns, whiskers
Feathers, no horns, no whiskers
No feathers, horns, whiskers
No feathers, horns, no whiskers
580
3
5
592
45
89
94
40
ABD
ABd
abD
abd
AbD
Abd
aBD
aBd
A---B
A---D
B---D
-----A------B----
-----A------D----
-----B------D----
-----a------b----
-----a------d----
-----b------d----
•Ab
45 + 89
•aB
94 + 40
•Ad
3 + 89
•aD
5 + 94
•Bd
3 + 40
•bD
5 + 45
Feathers, horns, whiskers
Feathers, horns, no whiskers
No feathers, no horns, whiskers
No feathers, no horns, no whiskers
Feathers, no horns, whiskers
Feathers, no horns, no whiskers
No feathers, horns, whiskers
No feathers, horns, no whiskers
580
3
5
592
45
89
94
40
ABD
ABd
abD
abd
AbD
Abd
aBD
aBd
A---B
A---D
B---D
-----A------B----
-----A------D----
-----B------D----
-----a------b----
-----a------d----
-----b------d----
•Ab
45 + 89
•aB
94 + 40
•Ad
3 + 89
•aD
5 + 94
•Bd
3 + 40
•bD
5 + 45
= 268
= 191
= 103
Linked genes
Produce recombination frequencies less than 50%
Frequency = # of recombinants x 100
total # of offspring
A---B
= 268 x100
1448
= 18.5 %
= 18.5 mu
B---D
A---D
= 191 x 100
1448
= 6.4 %
= 6.4 mu
= 103 x 100
1448
= 13.2 %
= 13.2 mu
Frequency = # of recombinants x 100
total # of offspring
TOTAL # of offspring = 1448
Feathers, horns, whiskers
Feathers, horns, no whiskers
No feathers, no horns, whiskers
No feathers, no horns, no whiskers
Feathers, no horns, whiskers
Feathers, no horns, no whiskers
No feathers, horns, whiskers
No feathers, horns, no whiskers
580
ABD
3
5
592
45
89
94
40
ABd
abD
abd
AbD
Abd
aBD
aBd
Now…map the genes
on a chromosome!
A---B = 18.5 mu
Chromosome
A----D = 6.4 mu
B----D = 13.2
B-----------------D------A
13.2 mu
6.4 mu
18.5 mu