GCSE Biology revision

Inherited disorders

Reproduction

AQA 4.1.1.1

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AQA student objectives

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ALL 🎯

Identify and describe What Are Inherited Disorders?, Polydactyly and Cystic Fibrosis in the context of inherited disorders.

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Explain the relationships between What Are Inherited Disorders?, Polydactyly and Cystic Fibrosis and how they contribute to inherited disorders.

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Apply and analyse knowledge of What Are Inherited Disorders?, Polydactyly and Cystic Fibrosis to interpret unfamiliar information about inherited disorders.

Revision summary

Key knowledge

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What Are Inherited Disorders?

Inherited disorders are conditions encoded within a person's genome and passed down from parents to offspring through the inheritance of different alleles.
Unlike disorders acquired during a person's lifetime (e.g. through injury), genetic disorders are present from birth due to mutated genes.
Alleles are different versions of the same gene, and it is specific mutated alleles that cause inherited genetic disorders.

1. What Are Inherited Disorders?AQA

Notes

Inherited disorders are conditions encoded within a person's genome and passed down from parents to offspring through the inheritance of different alleles.
Unlike disorders acquired during a person's lifetime (e.g. through injury), genetic disorders are present from birth due to mutated genes.
Alleles are different versions of the same gene, and it is specific mutated alleles that cause inherited genetic disorders.

Polydactyly

Polydactyly is a genetic disorder characterised by the presence of extra fingers or toes.
It is caused by a dominant allele, meaning that inheriting just one copy of the allele is enough for the disorder to present in the phenotype.
Because the allele is dominant, any offspring who inherits it will develop polydactyly (barring further mutation).
Polydactyly is not life-threatening and can often be corrected surgically after birth.

2. PolydactylyAQA

Notes

Polydactyly is a genetic disorder characterised by the presence of extra fingers or toes.
It is caused by a dominant allele, meaning that inheriting just one copy of the allele is enough for the disorder to present in the phenotype.
Because the allele is dominant, any offspring who inherits it will develop polydactyly (barring further mutation).
Polydactyly is not life-threatening and can often be corrected surgically after birth.

Cystic Fibrosis

Cystic fibrosis is a genetic disorder caused by a recessive allele that leads to a mutation affecting chloride channels in the body.
This mutation results in the production of abnormally thick mucus, particularly in the lungs, airways, and pancreas, causing severe breathing difficulties.
Because it is recessive, a person must inherit two copies of the faulty allele (one from each parent) to develop cystic fibrosis.
Unaffected heterozygous parents (carriers) each carry one dominant and one recessive allele and can pass the recessive allele on to their children without having the disorder themselves.
In a genetic cross between two heterozygous carriers, there is a 25% chance that an offspring will inherit both recessive alleles and develop cystic fibrosis.

3. Cystic FibrosisAQA

Notes

Cystic fibrosis is a genetic disorder caused by a recessive allele that leads to a mutation affecting chloride channels in the body.
This mutation results in the production of abnormally thick mucus, particularly in the lungs, airways, and pancreas, causing severe breathing difficulties.
Because it is recessive, a person must inherit two copies of the faulty allele (one from each parent) to develop cystic fibrosis.
Unaffected heterozygous parents (carriers) each carry one dominant and one recessive allele and can pass the recessive allele on to their children without having the disorder themselves.
In a genetic cross between two heterozygous carriers, there is a 25% chance that an offspring will inherit both recessive alleles and develop cystic fibrosis.

Dominant vs Recessive Alleles in Genetic Disorders

A dominant allele always expresses itself in the phenotype, even if only one copy is inherited (heterozygous).
A recessive allele only expresses itself in the phenotype when no dominant allele is present, i.e. the individual is homozygous recessive.
Carriers of a recessive disorder are heterozygous individuals who do not show symptoms but can pass the faulty allele to their offspring.

4. Dominant vs Recessive Alleles in Genetic DisordersAQA

Notes

A dominant allele always expresses itself in the phenotype, even if only one copy is inherited (heterozygous).
A recessive allele only expresses itself in the phenotype when no dominant allele is present, i.e. the individual is homozygous recessive.
Carriers of a recessive disorder are heterozygous individuals who do not show symptoms but can pass the faulty allele to their offspring.

Embryonic Screening

Embryonic screening is the process of testing a developing fetus for the presence of risk alleles associated with genetic disorders.
Amniocentesis involves inserting a needle to collect a sample of amniotic fluid surrounding the fetus, which contains fetal cells that can be genetically analysed.
Chorionic villus sampling (CVS) involves taking a tissue sample directly from the placenta, which contains chorionic villi, for genetic testing.
The DNA extracted from either sample is then analysed to determine whether the embryo carries alleles for inherited disorders.
It is important to know which sample is taken from where, as exam questions may ask you to identify the screening method based on the source of the sample.

5. Embryonic ScreeningAQA

Notes

Embryonic screening is the process of testing a developing fetus for the presence of risk alleles associated with genetic disorders.
Amniocentesis involves inserting a needle to collect a sample of amniotic fluid surrounding the fetus, which contains fetal cells that can be genetically analysed.
Chorionic villus sampling (CVS) involves taking a tissue sample directly from the placenta, which contains chorionic villi, for genetic testing.
The DNA extracted from either sample is then analysed to determine whether the embryo carries alleles for inherited disorders.
It is important to know which sample is taken from where, as exam questions may ask you to identify the screening method based on the source of the sample.

Screening in IVF

In IVF (in vitro fertilisation), embryos can be screened for genetic disorders before being implanted into the uterus.
If an embryo is found to carry a serious genetic disorder during IVF screening, it is not implanted, which means it will be destroyed — a highly controversial outcome.
Pre-implantation genetic diagnosis in IVF allows parents to select embryos free from inherited disorders before pregnancy begins.

6. Screening in IVFAQA

Notes

In IVF (in vitro fertilisation), embryos can be screened for genetic disorders before being implanted into the uterus.
If an embryo is found to carry a serious genetic disorder during IVF screening, it is not implanted, which means it will be destroyed — a highly controversial outcome.
Pre-implantation genetic diagnosis in IVF allows parents to select embryos free from inherited disorders before pregnancy begins.

Ethical Issues Surrounding Screening

Some people believe embryonic screening is unethical because it may lead to the termination of a pregnancy or destruction of an embryo, denying it the chance of life.
Others argue that screening allows parents to make informed decisions and be better prepared to support a child born with a genetic disorder.
The option to terminate a pregnancy is generally only considered in cases where the disorder is severe or life-threatening, not for conditions like polydactyly that can be surgically treated.
False positive or false negative screening results can cause significant emotional trauma to families, raising concerns about the reliability of testing.

7. Ethical Issues Surrounding ScreeningAQA

Notes

Some people believe embryonic screening is unethical because it may lead to the termination of a pregnancy or destruction of an embryo, denying it the chance of life.
Others argue that screening allows parents to make informed decisions and be better prepared to support a child born with a genetic disorder.
The option to terminate a pregnancy is generally only considered in cases where the disorder is severe or life-threatening, not for conditions like polydactyly that can be surgically treated.
False positive or false negative screening results can cause significant emotional trauma to families, raising concerns about the reliability of testing.

Economic and Social Considerations of Screening

Genetic screening is expensive, which raises questions about the cost-effectiveness and accessibility of these procedures.
The cost of screening must be weighed against the long-term healthcare costs of supporting a child born with a serious genetic disorder.
Social impacts include the psychological burden on families who receive positive results and must make difficult decisions about the pregnancy.

8. Economic and Social Considerations of ScreeningAQA

Notes

Genetic screening is expensive, which raises questions about the cost-effectiveness and accessibility of these procedures.
The cost of screening must be weighed against the long-term healthcare costs of supporting a child born with a serious genetic disorder.
Social impacts include the psychological burden on families who receive positive results and must make difficult decisions about the pregnancy.

Key Terminology Recap

Genotype refers to the genetic make-up of an individual, whilst phenotype refers to the physical expression of that genotype.
A mutation is a change in the sequence of DNA bases, which can result in a faulty allele being produced and potentially causing a genetic disorder.
Homozygous means having two identical alleles for a gene, whilst heterozygous means having two different alleles for the same gene.

9. Key Terminology RecapAQA

Notes

Genotype refers to the genetic make-up of an individual, whilst phenotype refers to the physical expression of that genotype.
A mutation is a change in the sequence of DNA bases, which can result in a faulty allele being produced and potentially causing a genetic disorder.
Homozygous means having two identical alleles for a gene, whilst heterozygous means having two different alleles for the same gene.