1. Crossing Over

2. Meiosis: Law of segregation

3. Law of Independent AssortmentB A a A b B b a B a b

4. Genes located on same chromosome…
If A and B are “linked” to the same chromosome, they should stay together… A b A a b B a B

5. This limits your choices on Punnett Squares…Ab aB
Instead of… AB aB Ab ab

6. Crossing Over…
Genes are exchanged between homologous chromosomes during Prophase I of Meiosis I (remember?)
Forms recombinants… with new combinations of alleles

7. How do we know if linkage occurs?
Get an unusual distribution of phenotypes that wouldn’t follow the “normal” Mendelian ratios…
Drosophila melanogaster (fruit flies)
P = normal eye; p = purple eye
B = normal body; b = black body PB Pb pB pb
PpBb
Ppbb
ppBb
ppbb

8. Predicted… Out of 300 flies, you’d expect: 75 normal eye, normal body
75 normal eye, black body
75 purple eye, normal body
75 purple eye, black body

9. What we actually got… 151 normal eye, normal body
10 normal eye, black body
8 purple eye, normal body
131 purple eye, black body
Linked genes? PB, pb
But, if linked, expect 150 normal eye, normal body and 150 purple eye, black body…
What happened?
Recombinants? pB, Pb

10. Percent crossing over?
18/300 = 6%

11. Chromosome mapping?
The relative distance between genes on a chromosome can be measured using crossover frequencies…
Map units = relative distance between genes
If crossover > 50%, likely not linked on same
chromosome
The closer 2 genes are, less likely they are to cross over

12. Sample problem…
Four genes, J, K, L, and M are on the same chromosome. Given that the crossover frequency between K and J is 3, between K and L is 8, between J and M is 12, and between L and M is 7, what is the order of the genes on the chromosome? 3 5 7 K J L M

13. Sex-linked inheritance
Alleles carried on sex chromosomes
Gender influences phenotype

14. Symbols of Sex-linked inheritance
XH = dominant allele
Xh = recessive allele
Y = no allele for this trait XH Xh Y

15. Examples:
Hemophilia
Red-green color-blindedness
Male pattern baldness

16. X-inactivation…
If a male is XY, female is XX, how can females get “double” the amount of “X” chromosome DNA?
The answer? The second “X” is turned off in females
= “Dosage compensation” or “X inactivation”