Inheritance and genetic screening (2.13, 2.15, 2.16)
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A gene is a sequence of DNA that codes for a polypeptide or functional RNA molecule. The specific position of a gene on a chromosome is called its locus.
A gene can exist in two or more forms at the same locus, on homologous chromosomes. These forms are known as alleles.
A diploid individual can only carry two alleles (one on each homologous chromosome).
Multiple alleles increase genetic variation and can lead to different phenotypes. Sexual reproduction further increases genetic variation by producing new combinations of alleles through meiosis and fertilisation.
An organism’s genotype is its genetic makeup, expressed as the set of alleles it possesses. It determines the potential for particular traits or characteristics.
A phenotype is the observable expression of the genotype. It can be influenced by environmental factors. Individuals with the same phenotype may have different genotypes.
A dominant allele is expressed in the phenotype even if only one copy is present (e.g., A in Aa).
A recessive allele is expressed only when two copies are present (e.g., aa).
Incomplete dominance occurs when neither allele is fully dominant, resulting in an intermediate phenotype in the heterozygote (e.g., a blue-flowered plant crossed with a white-flowered plant producing offspring with light blue flowers).
The interaction between alleles contributes to an organism’s phenotype.
Diploid organisms have two copies of each gene – one on each homologous chromosome. The combination of alleles at a locus determines how a trait is expressed in the phenotype.
- Homozygous: Both alleles at a locus are the same (e.g., AA or aa).
- Heterozygous: The two alleles at a locus are different (e.g., Aa).
Monogenic inheritance refers to the inheritance of a characteristic controlled by a single gene.
Monohybrid inheritance describes the inheritance pattern of a single gene in a genetic cross. It is often represented using Punnett squares or genetic diagrams to predict offspring genotypes and phenotypes.
Pedigree diagrams illustrate inheritance within a family across generations. They can be used to determine whether a trait is dominant or recessive and to identify individuals who are carriers of a recessive allele.
Genetic screening involves testing for genes that may cause disease before any symptoms appear. It uses DNA analysis to identify mutations or chromosomal abnormalities.
- Carrier testing identifies individuals carrying one copy of a faulty gene (e.g., CF carriers) to inform reproductive decisions.
- Pre-implantation genetic diagnosis (PGD) is used during in-vitro fertilisation (IVF) to test embryos for genetic disorders before implantation.
- Prenatal testing tests an embryo or foetus for genetic conditions during pregnancy.
Prenatal genetic testing includes:
- Amniocentesis involves taking a sample of amniotic fluid (usually at 15–20 weeks of pregnancy), which contains foetal cells for testing.
- Chorionic villus sampling (CVS) involves taking a sample of placental cells (at 10–14 weeks); it allows earlier diagnosis but has a slightly higher miscarriage risk.
There are varied implications associated with undertaking prenatal genetic testing:
- Medical: Informs parents about potential genetic conditions, enabling informed decisions about treatment or pregnancy termination.
- Ethical: Raises concerns about embryo selection and potential discrimination.
- Emotional: Can cause stress, anxiety, or difficult decisions for parents.
- Social: May affect perceptions of disability and create pressure to screen.