2. The Chromosome Theory of Inheritance
“Traits determined by genes are inherited through the movement of chromosomes during meiosis”
During gamete formation (sperm or eggs) alleles separate as homologous pairs of chromosomes are separated.
So we know that genes are carried on chromosomes
When chromosomes are separated to make haploid gametes, that means that they get one or the other allele for a trait (parent carries two)

3. Review – 9:3:3:1 for Dihybrid crosses
That is when F1 generation heterozygous for traits
Crossed together in F2 generation to make offspring that occur in this ratio

4. This 9:3:3:1 is a phenotypic ratio
1 offspring displays recessive form of both traits (green and wrinkled)
3 display recessive for one and dominant for other (green and round)
3 display dominant for one and recessive for other
(yellow and wrinkled)
9 display dominant forms of both traits

5. What about the genotypic ratio?

6. What about other crosses
Different combinations
Wolf problem on p. 215
Allele for brown eyes is dominant to blue eyes
B= brown, b= blue
Grey coat is dominant to black coat
G= grey, g= black
Blue eyed grey wolf (had a black coated mother)– what does this tell us?

7. Mates with one that is heterozygous for both traits
Blue eyed grey wolf
Genotype bb Gg
Mates with one that is heterozygous for both traits
BbGg (phenotype= brown eyed grey wolf)

8. BbGg * bbGg
--notice that second allele set only has two possibilities for passing on alleles (bG & bg)
alleles BG Bg bG bg

9. Questions from homework?

10. Autosomal Inheritance
P. 220 in text
Remember, autosomes are chromosome pairs 1-22 in humans chromosome pair 23 are the sex chromosomes
Most of the traits we talk about are traits coded for on the autosomes (ex: curly hair, freckled skin, etc)
For each of these, one allele is dominant over the other
Many disorders are linked to autosomal genes
They can be dominant or recessive

11. Autosomal dominant
A disease causing allele is dominant, so that if a person has one copy of the gene, they have the disease (and if they have two copies, they also have the disease)
Ex: “H” –Huntington’s disease allele
Hh or HH --- both have the disease

12. Autosomal recessive
The disease allele is recessive, so a person only has the disorder when they have both copies of the allele
Ex: cystic fibrosis
C– normal gene, c= disease gene
Cc– does not have disease (is a carrier)
CC- does not have disease
cc- has disease

13. Sex linked traits/ disorders
alleles carrying traits on the x or y chromosome mean that these traits are “sex linked” (occur on the x or y chromosome)
Usually traits are x- linked as y codes for very little
This means that some conditions are rarely expressed in females, as females have two X chromosomes
But these conditions expressed in males whether or not the trait is dominant, as males only have 1 X and 1 Y
For example, colour blindness
It is recessive
Therefore if a female is X© and X – disease will not be expressed (one normal X)
But if a male is X © Y , he is in trouble, because this recessive trait will be expressed (he only has the recessive allele
Read p for more info

14. disorder inheritance and genetic testing
Read pages to understand autosomal inheritance and for an introduction to pedigree charts
Read page 224 – 226 for basic information on genetic testing, and genetic counselling

15. Gene therapy
“a technique aimed at treating genetic disorders by introducing the correct form of a defective gene into the patients genome”
Read p. 226 for a basic introduction to gene therapy. Then reflect:
How do you get the right gene into the cells of a patient?
Once you get the correct gene into some cells, how does it spread to other cells?
Problems with uptake of new gene– sometimes the chromosomes do not accept/ incorporate this new gene into the DNA, so it is not copied into new cells (treatment is then ineffective)