1. Crossing over Linkages Sex linkages
How to shuffle the genes

2. Mendel’s 2 laws (what we know that he didn’t know)
The law of segregation.
When crossing one trait (monohybrid cross)
Each trait is made up of two factors
These two trait must get separated so that
One can come from the mother and one from the father.
Proof: 3:1 phenotypic ration in F2 generation, Mechanics (Meiosis)
The Law of Independent Assortment
When crossing two traits at the same time (Dihybrid cross)
The two traits assort themselves randomly.
Proof : 9:3:3:1 phenotypic ratio in the F2 generation
Assumption (the traits are on different chromosomes)

3. A quick reminder Genes are segments of DNA
They are a series of base pairs
1 gene codes for 1 protein
We need many proteins we use them for enzymes, hormones , structural body parts, pigments, lots of stuff.
Each chromosome has hundreds of genes
Each gene has a special place like beads on a string.

4. The linkage Each chromosme has hundreds of genes on it.
Each gene is found in a certain place or locus
If the two genes we are looking at are on the same chromosome then they are linked.

5. Linked genes and the Law of Independent Assortment
If two genes are on different chromosomes they sort independently and inherited combinations show up in a Phenotypic ratio of 9: 3: 3: 1
If two genes are one the same chromosome they are linked and give a phenotypic ratio of 3:1 just like a monohybrid cross.

6. Let A= antibody, a=No antibody B= Big flower, b= little flower
Let A and B be alleles on the same chromosome.
To show that they are linked we put a bar across the top and treat them like 1.
Homozygous pure parents for both traits genotype would be
__ __ r __ __
AB AB ab ab

9. The tetrad
In Metaphase 1 the two sets of homologous chromosomes line up at the equator of the cell. This is called a tetrad because there are 4 side by side.

10. The chiasma
Sometimes the chromosomes are sticky and the homologous pairs in the middle cross over each other
This is called
A chiasma
The DNA can
Switch chromosomes
And shuffle the genes.

11. Crossing over or Recombination
When there is an equal trade between non- sister chromotids at the chiasma you can get new combinations of genes

12. How we can map chromosomes
The farther apart genes are on a chromosome the more likely they are to cross over.
We count the frequency of crossovers between all the genes on a chromosome and find out their relative positions.

13. Sex chromosomes and Autosomes
You have 23 pairs of chromosomes, one from your mom and one from your Dad. 22 pairs are homologous. These are your autosomes.
The 23rd pair is not homologous. They determine your sex. If you have XX chromosomes you are a girl. XY you are a boy. The Y is very small and doesn’t carry any genes other than sex determination. These are your sex chromosomes.

14. Sex linkages
The last pair are your sex chromosomes. They aren’t the same size X is bigger and carries more genes. The y is small and doesn’t have many genes. In humans the Y determines the sex. If the gene in question is on the X chromosome we call that SEX-Linked.
It poses and interesting problem. The male has only 1 X. He either gets the dominant or the recessive. He only gets one chance. This is called hemizygous.

15. To do Punnet squares traits are written as superscripts on the X chromosome.
Male is always hemizygous XY
He is either dominant XH Y
Or recessive Xh Y
The female is XX
She can be homozygous dominant XH XH
Homozygous recessive Xh Xh
Or Heterozygous XH Xh

16. Sex linked traits are Hemophilia, colour blindness, white eyes in fruit flies
Let XH = normal blood and
Xh = hemophilia
If Josh has a normal blood type and Mariam is a carrier for hemophilia,
what is the phenotypic ratio of the F1
What are the chances their boys will have hemophilia?
How many of their daughters will be carriers of the disease?

17. example Punnet: Answer: F1 have 3:1 Ratio normal : hemophilia
Boys have 50% chance
of hemophilia
all girls normal
50% girls are carriers XH Y XH
XHXH
XHY Xh
XHXh
XhY

18. Polygenic Inheritance
So far we’ve only looked at traits that are caused by 1 gene.
Some traits are caused by 2 or more genes.
This is called polygenic inheritance.
Eye colour ,skin colour, and hair colour are all polygenic.
This can get really complicated.