1. The Chromosomal Basis of Inheritance GENE MAPPING AP Biology/ Ms. Day
Chapter 15 (Part 6)
The Chromosomal Basis of Inheritance
GENE MAPPING
AP Biology/ Ms. Day

2. Linked Genes Remember…
tend to be inherited together located near each other on same chromosome
The CLOSER genes are linked LESS crossing over  tend to “travel” or segregate together into gamete

3. 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

4. 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 fly (for both trait with a recessive fly for both traits?
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…..

6. 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

7. What explains the non-Mendelian 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

8. 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

10. 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)

11. Produce recombination frequencies less than 50%
Linked vs. Not Linked???
Linked genes
Produce recombination frequencies less than 50%
Frequency = # of recombinants x 100
total # of offspring

12. Linkage Mapping A genetic map
an ordered list of the genetic loci (positions) of genes along a particular chromosome
Can be developed using recombination frequencies

13. 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

14. Practice Problem #1
Suppose you cross a fly that is completely heterozygous with a completely recessive fly.
AaBb (Wildtype) x aabb (mutant)
These are your results.
Expected Observed
Wildtype
Mutant
Short wings, Wild
Wild, Black Body
What is the recombination frequency btw these genes? Are they linked?
How far apart are these 2 alleles if linked?

15. Practice Problem #2
In pea plants, flower color and seed color are on the same chromosome. A plant w/ purple flowers and green seeds (AaBb) is crossed with one recessive for both traits (aabb) AaBb x aabb
These are your results.
Expected Observed
Purple, green
White, green
purple, yellow
white, yellow
What is the recombination frequency btw these genes? Are they linked?
How far apart are these 2 alleles if linked?

16. Practice Problem #3… 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

17. Distance between genes are called MAP UNITS = recombination frequency

18. 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

19. Eukaryotic Chromosomes- How Do You Solve Gene Mapping Problems?

20. 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!

21. If genes are linked, how do you tell which are the parental offsr?
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)
Cis
2 dominant alleles on 1st chromosome homologue 2 recessive alleles on 2nd chromosome homologue
OR…
Trans
1 dominant, 1 recessive allele on 1st chromosome homologue (ex: A, b) 1 dominant, 1 recessive allele on 2nd chromosome homologue (ex: a, B)
-----A------B----
-----a------b----
 AaBb
-----A------b----
-----a------B----
 AaBb

22. Example #1… P Mating : AaBb x aabb (this is a testcross  )
A= Long antennae B = Green eyebrows
a = Short antennae 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 F1 offspring look like:
Long, Green 850
Long, Blue 150
Short, Green 150
Short, Blue 850
Do these genes appear linked? ____
Parental genotypes are
_________ x _________

23. 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

24. Example #2… P Mating : AaBbCc x aabbcc (this is a testcross  )
A= Long antenna B = Green eyebrows C= Red eyes
a = Short antennae b = Blue eyebrows c = White eyes
If genes are UNLINKED, what do you expect to see in the offspring (progeny)?
EQUAL # OF ALL OFFSPRING PHENOTYPE POSSIBILITIES
 Assume your F1 offspring look like:
Long, green, red 850
Long, green, white 175
Long, blue, red
Long, blue, white 150
Short, blue, white 850
Short, blue, red 50
Short, green, white 175
Short, green, red 150
Who are the single recombinants?
Who are the double recombinants?

25. Example #3… Starting with pure breeding lines, Cross
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

26. 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
Feathers, horns, no whiskers
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!

27. Which is which genotype?
Feathers, horns, whiskers *ABD
Feathers, horns, no whiskers 3 ABd
No feathers, no horns, whiskers 5 abD
No feathers, no horns, no whiskers abd
Feathers, no horns, whiskers AbD
Feathers, no horns, no whiskers 89 Abd
No feathers, horns, whiskers aBD
No feathers, horns, no whiskers 40 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)

28. A---B A---D B---D -----A------B---- -----a------b---- Ab aB
Feathers, horns, whiskers ABD
Feathers, horns, no whiskers 3 ABd
No feathers, no horns, whiskers 5 abD
No feathers, no horns, no whiskers abd
Feathers, no horns, whiskers AbD
Feathers, no horns, no whiskers 89 Abd
No feathers, horns, whiskers aBD
No feathers, horns, no whiskers 40 aBd

29. A---B A---D B---D -----A------B---- -----a------b---- Ab 45 + 89 aB
A---D
-----A------D----
-----a------d---- Ad
3 + 89 aD
5 + 94
B---D
-----B------D----
-----b------d---- Bd
3 + 40 bD
5 + 45
Feathers, horns, whiskers ABD
Feathers, horns, no whiskers 3 ABd
No feathers, no horns, whiskers 5 abD
No feathers, no horns, no whiskers abd
Feathers, no horns, whiskers AbD
Feathers, no horns, no whiskers 89 Abd
No feathers, horns, whiskers aBD
No feathers, horns, no whiskers 40 aBd

30. A---B A---D B---D Linked genes Frequency = # of recombinants x 100 aB
= 268
A---D
-----A------D----
-----a------d---- Ad
3 + 89 aD
5 + 94
= 191
B---D
-----B------D----
-----b------d---- Bd
3 + 40 bD
5 + 45
= 103
Linked genes
Produce recombination frequencies less than 50%
Frequency = # of recombinants x 100
total # of offspring

31. A---B A---D B---D Frequency = # of recombinants x 100
1448
= 18.5 %
= 18.5 mu
A---D
= x 100
1448
= 6.4 %
= 6.4 mu
B---D
= x 100
1448
= 13.2 %
= 13.2 mu
Frequency = # of recombinants x 100
total # of offspring
TOTAL # of offspring = 1448
Feathers, horns, whiskers ABD
Feathers, horns, no whiskers 3 ABd
No feathers, no horns, whiskers 5 abD
No feathers, no horns, no whiskers abd
Feathers, no horns, whiskers AbD
Feathers, no horns, no whiskers Abd
No feathers, horns, whiskers aBD
No feathers, horns, no whiskers aBd

32. Now…map the genes on a chromosome!
A---B = 18.5 mu A----D = 6.4 mu B----D = 13.2
B D------A
13.2 mu mu
18.5 mu
Chromosome