Polydactyly and cystic fibrosis, how carriers pass conditions on, and the arguments for and against embryo screening.
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1.What is an inherited disorder?
A disorder caused by the inheritance of certain alleles rather than by a pathogen or lifestyle.
2.What is polydactyly?
An inherited disorder in which a person has extra fingers or toes.
3.Is polydactyly caused by a dominant or recessive allele?
A dominant allele.
4.What does it mean that polydactyly is dominant?
Only one parent needs to carry the allele for a child to inherit the condition.
5.What is cystic fibrosis?
An inherited disorder of the cell membranes that causes thick sticky mucus in the lungs and digestive system.
6.Is cystic fibrosis caused by a dominant or recessive allele?
A recessive allele.
7.What must be true of both parents for a child to inherit cystic fibrosis?
Both must carry the recessive allele, either as carriers or as sufferers.
8.Two carriers of cystic fibrosis have a child. What is the probability it has the disorder?
One in four, which is 25 per cent.
9.Two carriers of cystic fibrosis have a child. What is the probability it is a carrier?
Two in four, which is 50 per cent.
10.Why can carriers be unaware they carry the allele?
They are heterozygous, so the dominant healthy allele is expressed and they show no symptoms.
11.What is embryo screening?
Testing the cells of an embryo, often during IVF, to see whether it carries alleles for a genetic disorder.
12.Give an argument in favour of embryo screening.
It stops children being born with serious disorders, reduces suffering, and reduces the long term cost of treatment.
13.Give an argument against embryo screening.
Embryos with the alleles are usually destroyed, which some people object to, and it implies that people with the disorder are less valuable.
14.What worry do people raise about where screening could lead?
That it could be used to select for desirable characteristics rather than to avoid disease, producing designer babies.
15.Why is embryo screening expensive?
It requires IVF and specialist laboratory testing of each embryo.
16.Why might a couple choose genetic counselling?
To understand the risk of passing a disorder to their children before deciding whether to have them.
17.How can a family tree help predict risk?
It shows who in the family has had the disorder, which allows the pattern of inheritance and the likely genotypes to be worked out.
18.Why does a dominant disorder appear in every generation of a family tree?
Only one copy of the allele is needed, so anyone with it shows the condition and can pass it directly to a child.
19.Why can a recessive disorder skip generations?
It is carried silently by heterozygous individuals and only appears when two carriers have a child together.
20.What is meant by a genetic test being a probability, not a certainty?
It shows the chance of inheriting the alleles. Whether a particular child does so depends on which gametes happen to fuse.
21.Give a social reason some people oppose embryo screening.
They feel it discriminates against people living with disabilities by treating their existence as something to prevent.
22.Give an economic argument used in favour of screening.
Treating a lifelong inherited disorder is very expensive, so preventing it saves money that can be spent elsewhere.
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