Patterns of inheritance

Welcome to Patterns of Inheritance!

Ever wondered why you have your mother’s eyes but your father’s height? Or why some diseases seem to run in families? This chapter is all about the "instruction manual" of life. We are going to explore how traits are passed from one generation to the next. Don’t worry if this seems like a lot of jargon at first—we will break it down bit by bit. By the end of these notes, you’ll be a pro at predicting genetic outcomes!

1. The Language of Genetics

Before we dive into the crosses, we need to speak the language. Think of these as the "rules of the game."

• Gene: A length of DNA that codes for a specific polypeptide (protein). Think of it as a single "instruction" in a recipe book.
• Locus: The specific position of a gene on a chromosome. (Plural: loci).
• Allele: A variant version of a gene. For example, the gene for eye color has different alleles (blue, brown, green).
• Genotype: The genetic makeup of an organism (the actual letters, like BB or Bb).
• Phenotype: The observable physical characteristics (e.g., "Brown eyes"). This is a result of the genotype interacting with the environment.
• Dominant: An allele that is always expressed in the phenotype, even if only one copy is present (represented by a capital letter, e.g., B).
• Recessive: An allele that is only expressed if two copies are present (represented by a lowercase letter, e.g., b).
• Homozygous: Having two of the same alleles (e.g., BB or bb).
• Heterozygous: Having two different alleles (e.g., Bb).

Quick Review Box:
If a person has the genotype Aa, they are heterozygous. If A is the dominant allele for "Tall," their phenotype will be "Tall."

2. Monogenic (Monohybrid) Inheritance

This is the simplest form of inheritance, where we look at how one single gene is passed down. We use Punnett squares to predict the outcome of a cross.

Gene Mutations and Diseases

Sometimes, a "typo" in the DNA (a mutation) causes a disease. The syllabus requires you to know these four specific examples:

• Cystic Fibrosis (Recessive): Affects cell membranes, leading to thick mucus in the lungs. You need two copies of the faulty allele to have the disease.
• Huntington’s Disease (Dominant): A neurological disorder. Because it is dominant, you only need one faulty allele to develop the disease later in life.
• Sickle Cell Anaemia: Caused by a mutation in the haemoglobin gene. This shows codominance (see below), as people with one allele have "sickle cell trait" but not the full disease.
• Phenylketonuria (PKU) (Recessive): A deficiency in an enzyme that breaks down the amino acid phenylalanine.

Common Mistake to Avoid: Students often think "dominant" means "most common." It doesn't! It just means it is always expressed if present. Huntington's is dominant but very rare.

Key Takeaway: Monogenic inheritance involves one gene. Recessive traits need two copies to show; dominant traits only need one.

3. Codominance and Multiple Alleles

Life isn't always black and white. Sometimes, alleles "share" the spotlight.

Codominance

This happens when both alleles are expressed in the phenotype of a heterozygote. Neither one "hides" the other.

Multiple Alleles: The ABO Blood Group

While we usually have two alleles, a population can have more. In the ABO blood system, there are three alleles: I^A, I^B, and I^O.

• I^A and I^B are codominant. If you have both, your blood type is AB.
• I^O is recessive. You only have Type O blood if your genotype is I^OI^O.

HLA Antigens

The Human Leucocyte Antigens (HLA) are also examples of multiple alleles and codominance. They are proteins on the surface of your cells that help your immune system recognize "self" from "non-self." This is why matching blood types and tissues is vital for organ transplants!

4. Linkage: When Genes Travel Together

In your cell, genes are located on chromosomes. If two genes are on the same chromosome, they are "linked."

Sex Linkage (Haemophilia)

The X and Y chromosomes determine sex. The X chromosome is much larger and carries many genes that the Y doesn't. Haemophilia (a blood-clotting disorder) is caused by a recessive allele on the X chromosome.

Why does this matter? Males only have one X chromosome. If they inherit the faulty allele, they will have the disease. Females have two Xs, so a healthy allele on one can "mask" the faulty one on the other (making them a carrier).

Autosomal Linkage

This refers to genes located on the same autosome (any chromosome that isn't a sex chromosome). These genes tend to be inherited together because they are physically stuck to the same piece of DNA.

Example: The syllabus mentions the link between ABO blood groups and Nail Patella Syndrome (a condition affecting nails and kneecaps). Because these genes are close together on Chromosome 9, they are often passed down as a "package deal."

5. Dihybrid Inheritance and Model Organisms

Dihybrid inheritance looks at two different genes at the same time (e.g., pea color AND pea shape). This results in a larger Punnett square (usually 16 boxes).

Model Organisms: Drosophila melanogaster

Scientists love using the fruit fly (Drosophila) to study inheritance. Why?
1. They reproduce very quickly.
2. They are cheap to keep.
3. They have clear, observable traits (like wing shape or eye color).

Did you know? Using Drosophila allowed scientists to discover sex-linkage and map out where genes live on chromosomes!

6. The Chi-Squared (\(\chi^2\)) Test

In genetics, we often expect a certain ratio (like 3:1), but the actual numbers we count are slightly different due to chance. The Chi-squared test tells us if the difference between our Observed (O) results and Expected (E) results is just down to luck or if something else is going on (like linkage).

The formula is: \( \chi^2 = \sum \frac{(O - E)^2}{E} \)

Step-by-step:
1. Calculate the Expected values based on genetic ratios.
2. For each category, find the difference (O - E).
3. Square it, and divide by E.
4. Sum them all up to get your \(\chi^2\) value.
5. Compare this to a "critical value" in a table. If your value is smaller than the table value, the difference is just due to chance.

7. Chromosome Mutations

Sometimes the mistake isn't in a single gene, but in the whole chromosome.

Non-disjunction

This is when chromosomes fail to separate properly during meiosis (cell division). This results in gametes (eggs or sperm) having too many or too few chromosomes.

• Down’s Syndrome: Caused by having an extra copy of Chromosome 21 (Trisomy 21).
• Turner’s Syndrome: Females who only have one X chromosome (Genotype XO).
• Klinefelter’s Syndrome: Males who have an extra X chromosome (Genotype XXY).

Translocation

This is when a piece of one chromosome breaks off and attaches to a different, non-homologous chromosome.

8. Genetic Counseling and Ethics

When families have a history of genetic disease, they may see a genetic counsellor.

Pedigree Analysis

This involves drawing a family tree using specific symbols (squares for males, circles for females, shaded for affected). It helps predict the probability of a child inheriting a condition.

Ethical Issues

Genetic testing brings up tough questions:
- If you knew you had the Huntington's allele, would you want to know?
- Should insurance companies be allowed to see your genetic results?
- Is it right to test an unborn baby for disabilities?

Summary Takeaway: Genetics is a mix of predictable ratios and random chance. Tools like Punnett squares, Pedigree charts, and Chi-squared tests help us make sense of the "lottery of life."