Welcome to Inheritance, Mutation, and Genetic Screening!

Ever wondered why some traits seem to skip a generation, or why certain health conditions run in families? This chapter is all about the "instruction manual" of life—our DNA—and what happens when those instructions are changed or passed down. We will explore how genes determine who we are, how mutations can lead to conditions like Cystic Fibrosis, and the big ethical questions surrounding genetic screening.

Note: This topic builds on what you learned in the "DNA, RNA and Protein Synthesis" chapter. If you remember that DNA codes for proteins, you are already halfway there!

1. The Language of Genetics

Before we dive into the math of inheritance, we need to speak the language. Genetics has a lot of "key terms" that sound similar, but they mean very different things. Don't worry if they feel confusing at first—most students take a little time to get these straight!

  • Gene: A sequence of bases on a DNA molecule that codes for a specific polypeptide (protein). Think of it as a single recipe in a cookbook.
  • Allele: A different version of a gene. For example, the gene for eye color has alleles for blue, brown, or green. We usually represent them with letters like \(B\) or \(b\).
  • Genotype: The specific alleles an organism has (e.g., \(Bb\)).
  • Phenotype: The physical characteristic that shows up because of the genotype (e.g., Brown eyes).
  • Homozygote: An individual where both alleles for a gene are the same (e.g., \(BB\) or \(bb\)).
  • Heterozygote: An individual where the two alleles for a gene are different (e.g., \(Bb\)).
  • Dominant: An allele that is always expressed in the phenotype, even if only one copy is present (represented by a capital letter, \(B\)).
  • Recessive: An allele that is only expressed if there are two copies present (represented by a lowercase letter, \(b\)).
  • Codominance: A situation where both alleles in a heterozygote contribute to the phenotype. Neither one "hides" the other; they both show up.

Quick Tip: Always use the same letter for the same gene (e.g., \(H\) and \(h\)), and make sure your capital and lowercase letters look very different so you don't confuse yourself during an exam!

2. Mutations: When the Code Changes

A mutation is a change in the base sequence of DNA. Since DNA provides the template for mRNA, which then tells the ribosome which amino acids to link together, a change in DNA can change the final protein.

Types of Mutations

There are three main types you need to know for your Unit 1 exam:

  1. Substitution: One base is swapped for another. Example: \(ATG\) becomes \(ACG\). This might change one amino acid, or it might have no effect at all (because the genetic code is degenerate—meaning multiple codons can code for the same amino acid).
  2. Insertion: An extra base is added into the sequence.
  3. Deletion: A base is removed from the sequence.

Why are Insertions and Deletions so dangerous?
Because the genetic code is read in triplets (groups of three), adding or losing a single base shifts the entire reading frame. This is called a frameshift. Every single amino acid after the mutation point will likely be wrong, resulting in a protein that cannot function.

Key Takeaway: A mutation in a gene changes the primary structure of a protein, which changes how it folds (secondary/tertiary structure), which ultimately stops it from doing its job.

3. Monohybrid Inheritance and Pedigree Diagrams

Monohybrid inheritance looks at how a single gene is passed from parents to offspring. We use a Punnett Square to predict the probability of certain traits.

The Genetic Cross Step-by-Step:

If two carriers of a recessive disease (Genotype: \(Nn\)) have a child:

  1. Identify the gametes: Each parent can pass on either \(N\) or \(n\).
  2. Draw the square:
    \( \begin{array}{c|c|c} & N & n \\ \hline N & NN & Nn \\ \hline n & Nn & nn \end{array} \).
  3. Analyze the results: There is a \(25\%\) chance of being unaffected (\(NN\)), a \(50\%\) chance of being a carrier (\(Nn\)), and a \(25\%\) chance of having the disease (\(nn\)).

Pedigree Diagrams

These are family trees used to track a trait through generations.
- Squares = Males; Circles = Females.
- Shaded = Has the trait; Unshaded = Does not have the trait.
- If two unaffected parents have an affected child, the trait must be recessive (the parents were both carriers).

4. Sex Linkage (X-Linkage)

Some genes are located on the sex chromosomes (\(X\) and \(Y\)). Humans have two sex chromosomes: females are \(XX\) and males are \(XY\).

Red-green colour blindness is a famous example of an X-linked recessive disorder. The gene is carried on the \(X\) chromosome, but the \(Y\) chromosome is much smaller and does not carry a corresponding allele.

  • Females (\(XX\)) need two copies of the recessive allele to be colour blind. If they have one, they are just carriers.
  • Males (\(XY\)) only have one \(X\) chromosome. If they inherit the recessive allele, they will be colour blind. There is no second \(X\) to "mask" the faulty gene.

Did you know? This is why red-green colour blindness is much more common in men than in women!

5. Cystic Fibrosis (CF): A Case Study

Cystic Fibrosis is caused by a mutation in the gene that codes for the CFTR protein (Cystic Fibrosis Transmembrane Conductance Regulator). This is a recessive condition.

  • Normal CFTR: Acts as a channel protein, pumping chloride ions out of cells. Water follows by osmosis, keeping mucus thin and slippery.
  • Mutated CFTR: The protein is missing or shaped incorrectly. Chloride ions aren't pumped out, so water stays inside the cells. The mucus becomes thick and sticky.

Impact on Health: Thick mucus blocks the airways (making it hard to breathe) and blocks tubes in the digestive system (making it hard to absorb nutrients).

6. Genetic Screening

Genetic screening allows us to check DNA for specific mutations. There are three main types mentioned in your syllabus:

A. Identification of Carriers

A simple blood or mouth swab test for adults to see if they carry a recessive allele (like the one for CF) before they decide to have children.

B. Pre-implantation Genetic Diagnosis (PGD)

Used during IVF (In-Vitro Fertilisation). Embryos are grown in a lab, and one cell is removed to check its genes. Only "healthy" embryos are implanted into the mother.

C. Prenatal Testing (Testing the Fetus)

  1. Amniocentesis: A needle is used to take a sample of amniotic fluid (which contains fetal cells) at around 15–20 weeks of pregnancy. Risk: \(~1\%\) chance of miscarriage.
  2. Chorionic Villus Sampling (CVS): A sample is taken from the placenta. This can be done earlier (10–14 weeks). Risk: Slightly higher chance of miscarriage (\(1–2\%\)) than amniocentesis.

7. Ethical and Social Issues

Genetic screening is a powerful tool, but it raises difficult questions. You may be asked to discuss or evaluate these in your exam:

  • The Right to Life: Some argue that screening leads to an increase in abortions, which they find morally unacceptable.
  • The Cost of Care: Is it better to test and prepare for a child with special needs, or is it better to avoid the suffering altogether?
  • Emotional Impact: Knowing you are a carrier or that your child has a condition can cause massive stress and anxiety.
  • False Positives/Negatives: No test is \(100\%\) accurate. A "false positive" might lead to the termination of a healthy fetus.
  • "Designer Babies": Some worry that PGD could eventually be used to choose traits like eye color or intelligence, rather than just preventing disease.

Quick Review:
1. Mutations (substitution, insertion, deletion) change DNA and proteins.
2. Recessive traits (like CF) need two alleles to show; X-linked traits affect males more.
3. Screening (CVS, Amniocentesis, PGD) helps identify risks but carries ethical dilemmas.

Common Mistake to Avoid: Don't confuse Amniocentesis with CVS. Remember: CVS can be done Closer to the start of pregnancy (10 weeks), but it has a slightly higher risk!