Revise
Inherited Disorders
Inherited Disorders Revision
Inherited Disorders
Some disorders are inherited from parents. Embryos can be screened to find out if they contain alleles that will cause these diseases but many people are opposed to this process. Family trees can be used to determine the probability that new offspring will have specific traits or diseases.
Examples of Inherited Disorders
Cystic fibrosis is a disorder that is inherited from parents. It affects cell membranes and causes the body to produce a thick mucus in the air passages and pancreas. It is caused by a recessive allele which means individuals can only get the disorder if the parents are carriers (heterozygous) or have the disorder (homozygous recessive).
Polydactyly is a disorder that causes people to have extra fingers and toes and is caused by a dominant allele. As it is dominant, it is expressed even if they only have one dominant allele.
Embryo Screening
Cells can be taken from embryos created by IVF and screened for genetic disorders. If the screening shows that the cell contains alleles that will cause a disorder, another healthier embryo may be chosen instead and the original embryo may be destroyed.
Cells taken from embryos in the womb can also be screened. People may wish to terminate the pregnancy if alleles are found that will cause a serious disorder.
Embryo screening is very controversial because of these decisions that are made in response. Some people believe all embryos are potential new lives and it is wrong to destroy them or terminate pregnancies.
| Advantages of embryo screening | Disadvantages of embryo screening |
| Avoids suffering as children that may suffer with the disorders will not be born. | Implies that people with disorders are undesirable which is incorrect and unfair. |
| Saves money in the long term as disease treatments can be expensive. | Screening is very expensive. |
| Several laws prevent it being abused by parents that may want their offspring to have specific desired traits. | People could take advantage of the screening process and choose to use embryos that will produce desired characteristics. |
Gene therapy is where faulty genes are replaced with normal genes. It is also opposed by some people that believe it is unnatural.
Family Trees
Family trees can help trace inherited traits and diseases through generations and predict the likelihood of different family members inheriting certain alleles. Gender and phenotypes of individuals are represented by different colours and shapes. Horizontal lines show that the two individuals have produced offspring together.
Example:
In this family, Polly has polydactyly and is homozygous dominant. Using the family tree, calculate the probability that Stuart and Liza’s child will have polydactyly.
We know that Polly is homozygous dominant so her genotype will be AA. Craig must be homozygous recessive (aa) as he is unaffected.
We can then work out, using a punnet square, that all of their offspring (Mitchel and Stuart) are heterozygous for polydactyly (Aa).
Liza must also be homozygous recessive (aa) because she is unaffected.
So when Liza and Stuart reproduce, there will be a 50\% chance that the offspring will have polydactyly and a 50\% chance they will be unaffected.
Inherited Disorders Example Questions
Question 1: Explain how it is possible for an offspring to have cystic fibrosis when neither of their parents have the disease?
[3 marks]
Cystic fibrosis is caused by a recessive allele.
Both of the parents must be heterozygous for the allele / carriers of the disease.
So there would be a 25\% chance of the offspring being homozygous recessive / having cystic fibrosis.
Question 2: Give an advantage and disadvantage to screening IVF embryos for genetic diseases.
[2 marks]
Advantage (any one from):
- Prevents suffering.
- Saves money in long term.
- protected by laws to stop process being abused.
Disadvantage(any one from):
- Implies that people with disorders are undesirable which is unfair/incorrect.
- Screening can be expensive.
- People may take advantage of screening and pick embryos based of other desired characteristics.
- Opposed by people who believe embryos are potential new lives and don’t agree with defective embryos being destroyed.
Question 3: A genetic disorder is controlled by a dominant allele (G).
Using the family tree below, determine the genotype of person 1 and explain how you know this.
[2 marks]
Genotype of person 1: Gg
Person 4 does not have the disorder so person 1 must be heterozygous.
Inherited Disorders Worksheet and Example Questions
Inherited Disorders Questions
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