Name the disorder for each description, and say whether it is autosomal or X-linked…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Name the disorder for each description, and say whether it is autosomal or X-linked.
- The red cells have too little of the beta chain of haemoglobin, although the chain that is made is normal.
- A boy bleeds for a long time from a small cut, and his maternal uncle has the same problem.
- One amino acid in the beta chain is wrong. Under low oxygen the red cells become sickle shaped.
- An enzyme that turns phenylalanine into tyrosine is missing. The urine contains phenylpyruvic acid.
Need a reminder? The key ideaShow
In an autosomal recessive disorder, two carriers (Aa × Aa) have children in the ratio 1 AA : 2 Aa : 1 aa. The chance of an affected child is , of a carrier , and each child is an independent event. Thalassaemia is a problem of the amount of globin, and sickle-cell anaemia is a problem of one amino acid of the globin.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) Beta thalassaemia. The amount of globin is too small. It is autosomal recessive.
Step 2. (b) Haemophilia. The boy and his maternal uncle are males related through the mother, and this fits an X-linked recessive allele.
Step 3. (c) Sickle-cell anaemia. The wrong amino acid is valine in place of glutamic acid. It is autosomal recessive.
Step 4. (d) Phenylketonuria. It is autosomal recessive.
Answer: (a) thalassaemia, autosomal; (b) haemophilia, X-linked; (c) sickle-cell anaemia, autosomal; (d) phenylketonuria, autosomal.
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