1. Genes that do not obey Mendel’s Second Law
AUTOSOMAL LINKAGE
Genes that do not obey Mendel’s Second Law
© 2016 Paul Billiet ODWS

2. Reminder
In monohybrid inheritance the F2 generation gives a 3:1 ratio because of Mendel 1
A test cross with a heterozygote (Aa) gives a 1:1 ratio
In dihybrid inheritance the F2 generation gives a 9:3:3:1 ratio because of Mendel 2
A test cross with a heterozygote for both pairs of alleles (AaBb) gives a ratio of 1:1:1:1
© 2016 Paul Billiet ODWS

3. Genes which do not obey Mendel's Second Law
Sweet Peas
Characters
Traits
Alleles
Flower colour
Purple F
Red f
Pollen grain shape
Long L
Round l
© 2016 Paul Billiet ODWS

4. Looked at from a simple dihybrid cross perspective
Phenotypes
Purple Long X
Red Round
(Pure Bred)
Genotypes
FFLL
ffll F1
All purple long
FfLl
(Selfed)
© 2016 Paul Billiet ODWS

5. Sweet peas, the second generation
Phenotypes
Purple Long X
Red Round
(Pure Bred)
Genotypes
FFLL
ffll F1
All purple long
FfLl
(Selfed) F2
Purple Round
Red Long
Expected Ratios
9/16
56,25%
3/16
18,75%
1/16
6,25%
© 2016 Paul Billiet ODWS

6. What was expected? F2 Phenotypes Purple Long Purple Round Red Long
Red Round
Expected Ratios
9/16
56,25%
3/16
18,75%
1/16
6,25%
Parental combination
© 2016 Paul Billiet ODWS

7. Not quite what was expected!F2
Phenotypes
Purple Long
Purple Round
Red Long
Red Round
Expected Ratios
9/16
56,25%
3/16
18,75%
1/16
6,25%
Observed numbers
296 19 27 85
Total = 427
Observed Ratios
69,32%
4,45%
6,32%
19,91%
Parental combination
© 2016 Paul Billiet ODWS

8. Linked genes
The parental combinations of alleles (purple long and red round) inherited as almost a 3:1 ratio
As though they were behaving as a single character
These genes are called LINKED GENES
That is the LOCI for these genes are linked on the same chromosome.
© 2016 Paul Billiet ODWS

9. Crossing over and recombinants
BUT if these genes were perfectly linked together they would stay in their parental combinations (purple & long or red & round)
There would be no Purple Round or Red Long
These combinations have come about because of CROSSING OVER between the linked alleles on their chromosomes during Meiosis 1
These are called RECOMBINANTS.
© 2016 Paul Billiet ODWS

10. Meiosis & Crossing over
Early prophase 1
Anaphase 1
Telophase 2
© 2016 Paul Billiet ODWS

11. Are there any other recombinants?
Yes, hidden amongst the Purple Long plants
The genotype FfLl does not tell us enough about linked genes.
Genotypes for linked genes can be shown as:
This is an example of a parental combination
This genotype would give the same phenotype as:
This is an example of a recombinant.
© 2016 Paul Billiet ODWS

12. Recombinants
Note: recombinants are any combination of alleles that are not the same as the parental combinations
This is not exclusive to the crossing over of linked genes.
© 2016 Paul Billiet ODWS

13. The Genetic Diagram for Linked GenesP
Phenotypes
Purple Long X
Red Round
(Pure Bred)
Genotypes
Gametes
F L
f l F1
All Purple Long
© 2016 Paul Billiet ODWS

14. The Genetic Diagram for Linked Genes
Phenotypes
All Purple Long
(Selfed)
Genotypes
crossing over in meiosis I
Gametes
F L
f l
F l
f L
Parental Combinations
Recombinants
© 2016 Paul Billiet ODWS

15. The Genetic Diagram for Linked Genes
Genotypes
F L
f l
F l
f L
© 2016 Paul Billiet ODWS

16. The ratios Phenotypes Purple Long Red Round Purple Round Red Long
Expected Ratio if the genes were only linked
3/4
75%
1/4
25%
© 2016 Paul Billiet ODWS

17. The ratios Phenotypes Purple Long Red Round Purple Round Red Long
Expected Ratio if the genes were only linked
3/4
75%
1/4
25%
Observed Ratio
69,32%
19,91%
4,45%
6,32%
© 2016 Paul Billiet ODWS

18. COMBINATIONS OR RECOMBINANTS?
How do we know which of the Purple Long plants are Parental Combinations and which are Recombinants?
How often does crossing over occur?
Answer: Test Cross the F1 with a double recessive for both characters.
© 2016 Paul Billiet ODWS

19. CALCULATING THE CROSS OVER VALUE
Drosophila
Characters
Traits
Alleles
Wing shape
Normal B
Bent b
Body colour E
Ebony e
© 2016 Paul Billiet ODWS

20. CALCULATING THE CROSS OVER VALUE
Phenotypes
Heterozygous wild type X
Ebony Bent
Genotypes
BbEe
bbee
Gametes
BE, Be, bE, be be
If these genes are linked Be and bE could only be produced by crossing over.
© 2016 Paul Billiet ODWS

21. CALCULATING THE CROSS OVER VALUE
Phenotypes
Wild type
Normal Ebony
Bent Normal
Bent Ebony
Genotypes
BbEe
Bbee
bbEe
bbee
Numbers 83 82 76 71
Approx. Ratio
25%
Parental combination
Recombinants = 50% of the offspring
© 2016 Paul Billiet ODWS

22. CALCULATING THE CROSS OVER VALUE
These results are typical of non-linked genes
The recombinants are in the same frequency as the parental combinations
Note: Bent wing flies are a bit crippled so their offspring are not so viable. This accounts for their low numbers.

23. CALCULATING THE CROSS OVER VALUE
Drosophila
Characters
Traits
Alleles
Eye colour
Red P
Pink p
Body colour
Normal E
Ebony e
© 2016 Paul Billiet ODWS

24. Heterozygous wild type
CALCULATING THE CROSS OVER VALUE
Phenotypes
Heterozygous wild type X
Pink Ebony
Genotypes
PpEe
ppee
Gametes
PE, Pe, pE, pe pe
Wild type
Red Ebony
Pink Normal
Ppee
ppEe
© 2016 Paul Billiet ODWS

25. Parental combinations
CALCULATING THE CROSS OVER VALUE
Phenotypes
Wild type
Red Ebony
Pink Normal
Pink Ebony
Genotypes
PpEe
Ppee
ppEe
ppee
Numbers
601 3 4
584
Parental combinations
Recombinants < 50%
The frequency of the recombinants is less than 50%
This is an example of linkage.
© 2016 Paul Billiet ODWS

26. THE CROSS OVER VALUE
The % recombination in a test cross is called the CROSS OVER VALUE (cov)
The cross over value between ebony and pink =
This value tells us how far apart the loci of the genes are
Cross over values from several pairs of genes permit a geneticist to plot a gene map of the chromosome.
© 2016 Paul Billiet ODWS