1. Monday 31st October COPY LO: To describe and explain sex linkage
Starter
Explain how the sex of a human is determined

2. BATS Describe how sex is determined genetically
Explain what is meant by sex linkage
Describe how sex linked diseases such a haemophilia are inherited

3. How many chromosomes do humans have?
23 pairs - 22 of these have partners that are identical in appearance, whether in male or female
The remaining pair are the sex chromosomes

4. In females the 2 sex chromosomes appear the same and are called the X chromosomes
In the human male - there is a single X chromosome like that in the female
but the second one is smaller in size and shaped differently
This is the Y-chromosome
Hand out sheet ‘Sex Linkage’ - go through first paragraph - draw in sex chromosomes.

5. Explain Karyotype preparation and amniocentesis - use sheets + sex linkage sheet

6. female Gametes - egg cells all contain X chromosomes
Unlike other features in an organism , sex is determined by chromosomes rather than genes
female Gametes - egg cells all contain X chromosomes
male Gametes - sperm - 1/2 have an X chromosome and 1/2 have a Y chromosome

7. Draw a genetic diagram and punnet square to show the inheritance of sex (Fig 1 Page 118)

8. A gene is describes as being sex-linked if it is found on one of the sex chromosomes.
There are many genes controlling features on these chromosomes. The most common are red/green colour blindness, haemophilia and Duchennes muscular dystrophy.

9. Explain why characteristics that are controlled by recessive alleles will appear more frequently in males
Because there is no homologous portion on the Y-chromosome that might have a dominant allele

10. The X chromosome carries many genes - e.g. haemophilia? - what is this?
Blood clots slowly
May be slow and persistent internal bleeding
Lethal if not treated
Why are diseases like this selected against?

11. About 1 in 20 000 people in Europe have the gene
Although there are a few haemophilic females nearly all are men

12. One cause of haemophilia - a recessive allele that has altered DNA nucleotides - mutation - so the required protein is not coded for.
This results in the person not being able to produce one of the blood clotting proteins.
How are haemophiliacs treated?

13. How do we draw sex linked genetic diagrams?
We use capital letters and lower case letters to represent dominant and recessive as before but we attach these to capital X
e.g XH and Xh
Why do we not attach them to the capital Y in the same way?
Human sex linkage : Haemophilia sheet

14. Copy fig 3 - top of page 119
Remember the boys X chromosome ALWAYS comes from their mother as the Y can only come from a man
Read Hint and Examiners tip on Page 119

15. So - the defective allele must always come from the mother - She will either have the disease or be a ‘carrier’
Explain why haemophiliac males cannot pass on the haemophilia allele to their sons but they can to their daughters

16. Go through rest of sheet and do colour blind example.

17. Pedigree charts - rules
Male = square
Female = circle
shading = presence of a character in the phenotype
dot in a circle = woman has a normal phenotype but carries the defective allele

18. Q 1-5 Bottom page 119 Application Page 120
Royal Family haemophilia sheet

19. Glenn Toole, Susan Toole (2008). A2 Biology. Cheltenham: Nelson
Other useful Sources - summary of Mendelian inheritance The best A Level Biology Website
References
Glenn Toole, Susan Toole (2008). A2 Biology. Cheltenham: Nelson