Welcome to Inherited Conditions, Pedigrees, and Variation!
Have you ever wondered why some people have hitched-hikers' thumbs, while others don't? Or why certain medical conditions seem to "run in the family"? In this chapter, we explore how genetic information is passed from parents to children, how we can track these traits using family trees (pedigrees), and why every human being is unique. Don't worry if the terms seem a bit like a new language at first—we will break them down step-by-step!
Note: To understand these topics fully, it helps to be familiar with basic genetic terms like DNA and genes, which you can find in the "Genes, alleles and monohybrid inheritance" chapter.
1. Key Genetic Terms
Before we dive into specific conditions, let’s quickly refresh the "vocab" you’ll need for this chapter:
Genotype: The specific alleles (versions of a gene) an individual has, such as \(Bb\) or \(bb\).
Phenotype: The physical characteristic that shows up (e.g., having blue eyes).
Homozygous: Having two of the same alleles, like \(BB\) or \(bb\).
Heterozygous: Having two different alleles, like \(Bb\).
Dominant: An allele that always shows up in the phenotype, even if only one copy is present (written as a capital letter, e.g., \(B\)).
Recessive: An allele that only shows up if there are two copies present (written as a lowercase letter, e.g., \(b\)).
2. Sex Determination
In humans, one pair of chromosomes determines whether we are biologically male or female. These are called the sex chromosomes.
Females have two \(X\) chromosomes: \(XX\).
Males have one \(X\) and one \(Y\) chromosome: \(XY\).
Because eggs always contain an \(X\) chromosome, but sperm can carry either an \(X\) or a \(Y\), it is the father’s sperm that determines the sex of the baby at fertilisation. There is always a \(50\%\) chance of a baby being male and a \(50\%\) chance of being female.
3. Named Inherited Conditions
The Edexcel syllabus requires you to know four specific conditions. Let's look at how they are inherited.
A. Cystic Fibrosis (Recessive)
This is caused by a recessive allele. This means a person must have two copies of the faulty gene (\(ff\)) to have the condition. If a person has one normal allele and one faulty allele (\(Ff\)), they are called a carrier. They don't have the disease themselves but can pass it on to their children.
B. Polydactyly (Dominant)
This condition causes a person to be born with extra fingers or toes. It is caused by a dominant allele. This means you only need one copy of the faulty gene (e.g., \(Pp\) or \(PP\)) to have the condition. There are no "carriers" for polydactyly—if you have the gene, you have the extra digits!
C. Sex-Linked Conditions
Some conditions are "sex-linked" because the gene responsible is located on the \(X\) chromosome. Because males only have one \(X\) chromosome, if they inherit a faulty gene on it, they don't have a second \(X\) chromosome to "mask" it. This makes these conditions more common in males.
Haemophilia: A condition where blood does not clot properly.
Red-green colour blindness: Difficulty distinguishing between red and green colours.
Quick Review: Why are carriers usually female in sex-linked traits?
A female can be \(X^H X^h\) (where \(h\) is the faulty gene). She is a carrier because her normal \(X^H\) protects her. A male is \(X^h Y\); he only has one \(X\), so if it's faulty, he has the condition!
4. Family Pedigree Diagrams
A pedigree is just a scientific way of drawing a family tree to track a specific trait. Here is how to read them:
Squares usually represent males.
Circles usually represent females.
Shaded shapes mean the person has the condition or trait being studied.
Horizontal lines between a circle and square represent a couple having children.
Vertical lines lead down to the next generation.
How to spot inheritance patterns in a pedigree:
1. If two unaffected parents have a child with the condition, the condition must be recessive. (The parents must be carriers).
2. If every affected person has at least one affected parent, the condition is likely dominant.
5. Gene Therapy
Gene therapy is a modern medical technique used to treat inherited disorders like Cystic Fibrosis. It involves trying to "fix" the underlying genetic cause rather than just treating the symptoms.
How it works for Cystic Fibrosis:
1. Scientists identify the healthy version of the gene.
2. This healthy gene is inserted into a virus (which has been modified so it isn't dangerous).
3. The virus acts as a "vector" (a carrier) to deliver the healthy gene into the patient's lung cells.
4. The goal is for the cells to start using the healthy gene to produce the correct proteins.
Note: This is difficult because the effects are often temporary, as the body naturally replaces lung cells over time.
6. Variation
Variation refers to the differences between individuals of the same species. There are two main causes:
A. Genetic Variation
This is caused by the genes we inherit. Examples include eye colour, blood group, and the ability to roll your tongue. Genetic variation is increased by:
- Meiosis: The process of making gametes (sperm and eggs) which ensures they are all genetically different.
- Random Fertilisation: Any sperm can fuse with any egg, creating a unique combination of DNA.
B. Environmental Variation
This is caused by the conditions in which we live. For example, a person might have the genetic potential to be tall, but if they have a poor diet (environment), they may not grow to their full height. Other examples include scars, language spoken, or dyed hair colour.
C. Combined Variation
Most traits, like height and body mass, are a result of both genetic and environmental factors.
Summary Checklist
Key Takeaways:
- Males are \(XY\), females are \(XX\).
- Cystic Fibrosis is recessive; Polydactyly is dominant.
- Haemophilia and colour blindness are sex-linked (on the \(X\) chromosome).
- Pedigrees help us track how traits pass through generations.
- Gene therapy uses viruses to deliver healthy genes to treat disorders.
- Variation can be genetic, environmental, or a mix of both.
Common Mistake to Avoid: Don't confuse "dominant" with "common." A dominant trait (like polydactyly) can actually be very rare in the population!