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

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

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

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.

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

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

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

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.

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

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?

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

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?

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

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

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

Go through rest of sheet and do colour blind example.

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

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

Glenn Toole, Susan Toole (2008). A2 Biology. Cheltenham: Nelson Other useful Sources http://www.youtube.com/watch?v=NgXOLU3uiHg - summary of Mendelian inheritance http://www.biologymad.com/master.html?http://www.biologymad.com/A2Biology.htm The best A Level Biology Website References Glenn Toole, Susan Toole (2008). A2 Biology. Cheltenham: Nelson