1. Chapter 7 – Linkage, Recombination, and Eukaryotic Gene Mapping

2. Genetic Principles Principle of Segregation
Diploid organisms have 2 alleles for each gene
Separate during meiosis – only one gamete enters each gamete
Principle of Independent Assortment
2 alleles of a gene separate independently from alleles at other loci/other genes

3. Chromosomes Chromosomes follow independent assortment IF:
Genes are located of different chromosomes BUT:
If genes are on the same chromosome, they tend to travel together
Linked genes – close together on the same chromosome

4. Sweet peas – dihybrid cross
P generation purple, long x red, round
F1 generation – all purple,long
Prediction for F2 generation – ratio of 9:3:3:1

5. Sweet pea – dihybrid cross cont
Expected F2 phenotype ratios is not observed
Conclusion – genes for flower color and pollen shape must be located close together on the same chromosome
Why are any recombinant progeny seen?

6. Crossing over
If 2 genes are on the same chromosome, but far apart, crossing over can allow for recombination of gametes
Genes very far apart on the same chromosome will always be separated by crossing over, and are not considered to be linked

7. Notation for linked genes
Horizontal lines indicate actual chromosome
A_________B
a b
*individual heterozygous for 2 different genes where both dominant alleles are on one chromosome, and both recessive alleles are on its homologous chromosome
Can be abbreviated by AB/ab

8. Testcross for linkage
For determination if two genes are linked (close together on the same chromosome) or not
Set-up:
One individual heterozygous for both traits x individual homozygous recessive for both traits

9. Testcross for linkage cont
MmDd x mmdd
If not closely linked, alleles will assort independently
MmDd individual can form 4 different types of gametes
50% recombinant offspring/50% non-recombinant offspring

10. Testcross for linkage cont
MD/md x md/md
If closely linked, 2 alleles will always travel together
all offspring are non-recombinant

11. Testcross for linkage cont
Can be separated by crossing over
Small number of recombinant progeny/chromosomes is seen

12. Crossing over
Single cross over produces 50% nonrecombinant chromosomes (same configuration as parental chromosome) and 50% recombinant chromosomes (new allelic combination)

13. Recombination frequency
= number of recombinant progeny x 100
total number of progeny
Values from slide #11
= 123 = 12.2% or .122
Smaller the recombination frequency = more closely linked

14. Coupling and Repulsion
For heterozygous individuals
Cis configuration/coupling
Both wildtype alleles are on one chromosome; both mutant alleles are on the homologous chromosome
Trans configuration/repulsion
Each chromosome has one wildtype allele and one mutant allele

16. Recombination Interchromosomal Intrachromosomal
Between genes on different chromosomes
Independent assortment/random segregation during Metaphase/Anaphase I
Produces 50% recombinant/50% non-recombinant gametes
Intrachromosomal
Between genes on same chromosome
Crossing over during Prophase I
Usually produces recombinant gametes less than 50%
Unless very far apart on the same chromosome

17. Genetic mapping
Relative position of different genes based on recombination rates
Does NOT state actual chromosome, or position (locus)
Distance measured in map units or centimorgans (cM)
1 m.u. (or cM) = 1% recombination

18. Genetic mapping example
A and B = 5 m.u.
A and C = 15 m.u.
B and C = 10 m.u.
A and D = 8 m.u.
B and D = 13 m.u.
C and D = 23 m.u.
Any genes with 50% recombination are either on different chromosomes, or very far apart on the same chromosome (crossing over always separates them)

19. Physical mapping
Locates gene to a specific chromosome/region of chromosome
Deletion mapping
Chromosome deletion studies – how phenotype is affected/what genes may be missing
Duchenne m.s.
X linked disease – but where on X?
Some affected males have small deletions – common deleted area must be where gene is located

20. Somatic cell hybridization
Fusion of 2 cell types (altered by viruses or tumor cells to allow cell lines – uninhibited growth)
Somatic cells
Heterokaryon – 2 distinct nuclei
Eventually fuse
Most chromosomes are lost (differentially from one type)
Human chromosomes usually lost, only a few remain
Human genes expressed in hybrid cell lines must be located on retained chromosomes
deletion studies can give more specific location on chromosome

22. Molecular Analysis Fluorescence In Situ Hybridization (FISH)
Probe complementary to gene sequence will bind to DNA
Gene sequence/partial sequence must be known
DNA sequencing
Yields base pair distance between two genes