1. Linkage Mapping
AP BIO

3. # of genes in a cell >> # of chromosomes
Recall:
# of genes in a cell >> # of chromosomes
Each chromosomes has 100’s of genes (except Y, ~78)
Genes on the same chromosome are inherited together
We call these linked genes
Sex linked vs. linked gene
Sex linked is single gene on sex chromosome
Linked gene is 2+ genes on same chromosome
Linked genes produce results in genetic crosses that vary from the expected Mendelian ratio’s and laws

4. Genetic Map: an ordered list of the genetic loci along a particular chromosome Loci: the positions of a genes or DNA sequences on a chromosome

5. Can you explain this theory?
Alfred Sturtevant:
Constructed the 1st genetic map
Predicted that:
“the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency”
Can you explain this theory?
The greater the distance b/t two genes, the more points there are b/t them where crossing over can occur.

6. Recombination Frequency (RF): % of recombinant offspring 𝑹𝑭= # 𝒐𝒇 𝒓𝒆𝒄𝒐𝒎𝒃𝒊𝒏𝒂𝒏𝒕𝒔 𝒕𝒐𝒕𝒂𝒍 # 𝒐𝒇 𝒐𝒇𝒇𝒔𝒑𝒓𝒊𝒏𝒈

7. If RF ≥ 50% genes are far apart or on different chromosome (usually unlinked/assort independently) If RF < 50% genes are linked

8. Example 1: 2 plants have 100 offspring, 5 are red and short, 10 are white and tall, 40 are red and tall, and 45 are white and short. The red short and white tall plants exist because the first parent's chromosomes underwent recombination. Calculate the RF and determine if the genes are linked.

9. Linkage Map: genetic map based on recombination frequencies
RF determines the map distance b/t the genes on a chromosome
𝑴𝒂𝒑 𝑫𝒊𝒔𝒕𝒂𝒏𝒄𝒆= # 𝒐𝒇 𝒓𝒆𝒄𝒐𝒎𝒃𝒊𝒏𝒂𝒏𝒕 𝒑𝒓𝒐𝒅𝒖𝒄𝒕𝒔 𝒕𝒐𝒕𝒂𝒍 # 𝒐𝒇 𝒑𝒓𝒐𝒅𝒖𝒄𝒕𝒔
Measured in map units:
1 map unit = 1 centiMorgan (cM) (named after Thomas Hunt Morgan)
1 map unit is equivalent to a 1% RF
RF in adjacent intervals (for multiple genes) is additive

10. Genetic maps are measured in cM
Physical maps are measure in bp (base pairs)

11. Genetic and physical map of the X chromosome in humans
A reference marker is a segment of DNA for which the location is known
Used to track inheritance

12. Example 2: The RF for 3 Drosphila genes pairs are b-cn 9%, cn-vg 9
Example 2: The RF for 3 Drosphila genes pairs are b-cn 9%, cn-vg 9.5 %, and b-vg 17%. Draw a linkage map for these genes. (b-black body, cn- cinnabar eyes, and vg-vestigial wings)

13. These are a few of the genes mapped on chromosome 2 for Drosphila
# at each locus is the # of map units b/t that locus and the locus for aristae length
Notice that eye colour is about halfway b/t body colour and wing type

14. Linkage Mapping Practice….