Welcome to Monohybrid Inheritance and Genetic Screening

In our previous chapters of Topic 2: Genes and Health, we looked at how DNA is built and how mutations can occur. Now, we are going to explore the "rules" of how these genes are passed from parents to children and how we can use modern science to screen for genetic conditions like Cystic Fibrosis (CF). Don't worry if the terminology seems like a lot at first—genetics is like learning a new language, and once you know the vocabulary, everything else clicks into place!

1. The Language of Genetics

Before we can calculate the odds of inheriting a trait, we need to master the key terms used in the Pearson Edexcel specification. These are the building blocks of your exam answers.

  • Gene: A sequence of bases on a DNA molecule that codes for a specific sequence of amino acids in a polypeptide chain. (See Topic 2.8 for the full definition).
  • Allele: An alternative form of a gene. For example, the gene for mucus production has a "normal" allele and a "mutated" allele.
  • Genotype: The genetic makeup of an organism; the specific combination of alleles an individual has (e.g., \(FF\), \(Ff\), or \(ff\)).
  • Phenotype: The observable characteristics of an organism, resulting from the interaction between the genotype and the environment (e.g., having Cystic Fibrosis or being healthy).
  • Dominant: An allele that is always expressed in the phenotype, even if only one copy is present (represented by a capital letter, like \(F\)).
  • Recessive: An allele that is only expressed in the phenotype if two copies are present (represented by a lowercase letter, like \(f\)).
  • Homozygote: An individual where both alleles for a gene are the same (e.g., \(FF\) or \(ff\)).
  • Heterozygote: An individual where the two alleles for a gene are different (e.g., \(Ff\)).
  • Incomplete Dominance: A situation where neither allele is completely dominant over the other, resulting in a phenotype that is a "blend" or intermediate between the two.

Quick Review: If someone is a "carrier" for a disease, they are a heterozygote. They have one healthy dominant allele and one "hidden" recessive disease allele.

2. Monohybrid Inheritance

Monohybrid inheritance is the study of how a single characteristic (controlled by one gene) is passed down. We use Punnett Squares to predict the probability of offspring inheriting certain traits.

How to draw a Punnett Square:

Imagine two parents who are both carriers for Cystic Fibrosis (\(Ff\)).

  1. Write the father's alleles along the top.
  2. Write the mother's alleles along the side.
  3. Fill in the boxes by combining the letters.

The result of an \(Ff \times Ff\) cross:

  • \(25\%\) chance of \(FF\) (Homozygous Dominant - Healthy)
  • \(50\%\) chance of \(Ff\) (Heterozygous - Healthy Carrier)
  • \(25\%\) chance of \(ff\) (Homozygous Recessive - Has Cystic Fibrosis)

Common Mistake to Avoid: Students often think that if a couple has four children, one must have the disease. This is wrong! Each pregnancy is an independent event with a \(25\%\) probability.

3. Pedigree Diagrams

A pedigree diagram is a "family tree" that shows how a trait is passed through generations. In the exam, you may be asked to deduce if a condition is dominant or recessive based on one of these diagrams.

  • Squares represent males; Circles represent females.
  • Shaded shapes represent individuals with the phenotype (the condition).
  • Horizontal lines between shapes represent a couple having children.

Top Tip: If two healthy parents have a child with the condition, the condition must be recessive. Both parents must be carriers (\(Ff\)) to pass the recessive allele (\(f\)) to the child (\(ff\)).

4. The Impact of Cystic Fibrosis (CF)

The Salters-Nuffield curriculum uses CF as the primary example of monohybrid inheritance. You need to know how this single-gene mutation affects the body:

  • Gaseous Exchange: Sticky mucus blocks the bronchioles, reducing the surface area available for gas exchange and increasing the risk of lung infections.
  • Digestion: Mucus blocks the pancreatic duct. This prevents digestive enzymes from reaching the small intestine, leading to poor nutrient absorption and weight loss.
  • Reproduction: In males, the sperm duct (vas deferens) can become blocked or fail to develop. In females, thick cervical mucus can prevent sperm from reaching the egg.

5. Genetic Screening

Genetic screening allows us to identify abnormal alleles in DNA. There are four main types you need to know:

A. Carrier Identification

Used for couples with a family history of a genetic disorder. A simple blood or saliva test determines if they carry a recessive allele that could be passed to their children.

B. Pre-implantation Genetic Diagnosis (PGD)

This happens during In Vitro Fertilisation (IVF). Embryos are grown in a lab, and one cell is removed to check its DNA. Only healthy embryos (those without the disease alleles) are implanted into the mother's uterus.

C. Amniocentesis

Performed at 15–20 weeks of pregnancy. A needle is inserted through the abdomen into the amniotic fluid to collect fetal cells. It has a miscarriage risk of about \(1\%\).

D. Chorionic Villus Sampling (CVS)

Performed earlier, at 11–14 weeks. A small sample of tissue is taken from the placenta (the chorionic villi). Because it is done earlier, a woman can make a decision about the pregnancy sooner, but it has a slightly higher miscarriage risk (about \(1–2\%\)).

6. Social and Ethical Issues

Genetic screening is a controversial topic. You should be able to discuss these viewpoints in an "evaluate" or "discuss" exam question:

  • Right to Life: Some believe that all embryos have a right to life and that screening leading to abortion is unethical.
  • Informed Choice: Screening allows parents to prepare for a child with special needs or decide whether to continue a pregnancy.
  • Anxiety and Stress: The procedure itself or waiting for results can cause significant emotional distress.
  • Risk of Miscarriage: Procedures like CVS and amniocentesis carry a risk of losing a healthy fetus.
  • False Positives/Negatives: No test is \(100\%\) accurate. A false positive might lead to the termination of a healthy fetus; a false negative might lead to a child being born with an unexpected condition.
  • Stigma: Increased screening might lead to a society that is less accepting of people with genetic disabilities.

Key Takeaway: When answering ethics questions, always try to provide a balanced argument by looking at both the benefits (e.g., reducing suffering) and the risks (e.g., miscarriage or moral concerns).

Quick Review Summary
  • Inheritance: Uses Punnett squares and pedigree charts to track alleles.
  • Cystic Fibrosis: A recessive disorder affecting the lungs, gut, and fertility.
  • Screening: Includes PGD (before pregnancy), and CVS or Amniocentesis (during pregnancy).
  • Ethics: Focuses on the balance between parental choice, the rights of the fetus, and the safety of the procedures.