1. The problems: Crossing over
Meiosis
The problems:
Crossing over

2. Where does crossing over occur?
Happens in Prophase 1
Every somatic cell has pairs of each chromosomes (homologous pairs)
Line up in prophase to form bivalents
Bivalent formation called synapsis
Pairing allows chiasma to form
Chromosomes exchange segments
New combinations of genes formed recombination

3. Linkage Refers to genes on same chromosome
Means two genes cannot be seperated
Results in skewed inheritance
More of one phenotype seen than expected as chromosomes don’t mix through crossing over

4. Recombination
Referring of exchange of alleles between homologous chromosomes
Offspring from crossed over chromosomes called recombinants
Recombinants are different from both parents
Use proportion of recombinants to calculate amount of crossing over
Then used to produce gene maps
Independent assortment of chromosomes give greater degree of variation

5. Chromosome mapping
Amount of crossing over of linked genes measures distance between the genes
Distant genes cross more frequently because there are more crossover points between them
Results collected by test crosses

6. To create a map you must know
the order of the genes
Relative distances between them
You should do the exercise on pg 97. it’s the kind of thing they like to put in the exam
Do pg 99 as well so we are all operating with the same information, it should be revision

7. Genetic Crossing
Tracking the alleles

8. Dominance of alleles
What you knew before is just going to get more complicated!
You had dominant and recessive, now we add incomplete dominance and codominance

9. Incomplete dominance Neither allele has complete control over a trait
Heterozygous offspring is intermediate
Phenotype and genotype ratios identical
Example red flower x white flower = pink flower

10. Codominance Both allele are indendepently and completely expressed
Example red cow x white cow = roan cow
Roan has red hair and white hair mixed not pink!
Blood types

11. Russ has B antigen
Toni has A antigen
Connor is AB
Connor has A type blood molecules and B type blood molecules mixed together. No intermediates!, just A’s and B’s
Don’t worry about rhesus factors now

12. Your Homework Revision
Do the work on pgs 100 – 103
Read up the Bayley book on this section, she’s really good at explaining this stuff

13. Lethal Alleles These kill OBVIOUSLY!
Always kill in homozygote for lethal gene
May kill in heterozygote
May confer no effect in heterozygote
May have special phenotype in heterozygote

14. Sex Determination
Pg 105
Note we are not ‘normal’. Our xy system is just one way of making sure we have similar numbers of boys and girls
Should be revision, let me know if problems

15. Sex Linkage Linkage of alleles to sex chromosome
Expression in heterogametic sex
Homogametic sex are carriers
Human males are heterogametic (xy)
Example is tortoishell cats which are nearly always female

16. Inheritance patterns
Pg 108, try this at home

17. Pedigree Analysis Very popular basis for exam questions!
We’ll spend a lesson on this
You do the cat analysis at home

18. Gene-Environment Interaction
Genes respond to their environment
Factors effecting genes include
Nutrition
Temperature
Altitude/latitude
Other organisms

19. Temperature
Males or females develop in eggs depending on the temperature eggs are incubated at
Tuatara
Turtles
alligators

20. Other organisms
Presence of others of same species determine what sex an individual will be
Wrasse fish
Presence of predators may change phenotype
Insects Daphnia

21. Altitude
Can determine tree shape

22. Gene Interactions Pleiotropy Polygeny Epistasis
One gene controls/effects several phenotypic traits
Polygeny
One phenotypic trait controlled by several genes
Epistasis
Two genes at different loci where action of one gene masks another gene

23. Collaboration Genes work together to produce phenotype
Four phenotypes possible

24. Polygenes A phenotype that is determined by a number of genes
Recognised by phenotype have a wide and continuous range
Skin colour in humans
Not just black white and middle, but a continuous range of colour from black to white

25. Complementary genes