Welcome to the Blueprint of Life!
Have you ever wondered why you have your mother’s eyes but your father’s hair color? Or why two brown-eyed parents can sometimes have a blue-eyed baby? The answer lies in Genetics. In this chapter, we are going to explore how traits are passed from parents to children through genes and alleles. Don't worry if it seems like a lot of new words at first—we'll break them down step-by-step!
1. The Genetic Vocabulary
To understand inheritance, we first need to speak the language. Think of your DNA as a massive library of cookbooks. Each "book" is a chromosome, and each "recipe" is a gene.
Gene: A short length of DNA that codes for a specific protein (and therefore a specific characteristic).
Allele: Different versions of the same gene. For example, the gene for eye color might have a "blue" allele and a "brown" allele.
Phenotype: The physical characteristic you see (e.g., blue eyes).
Genotype: The actual combination of alleles an individual has (the "genetic code"). We use letters to represent these, like BB or Bb.
Dominant vs. Recessive
Some alleles are "stronger" than others:
Dominant Allele: This allele always shows up in the phenotype if it is present. We represent it with a Capital Letter (e.g., \(B\)).
Recessive Allele: This allele is "hidden" if a dominant allele is present. It only shows up in the phenotype if there is no dominant allele. We use a lower-case letter (e.g., \(b\)).
Homozygous vs. Heterozygous
Homozygous: Having two of the same alleles (e.g., \(BB\) or \(bb\)).
Heterozygous: Having two different alleles (e.g., \(Bb\)).
Haploid vs. Diploid
Diploid: Cells that have two sets of chromosomes (one from each parent). Most body cells are diploid.
Haploid: Cells that have only one set of chromosomes. These are gametes (sperm and egg cells), produced by meiosis.
Key Takeaway:
Your Genotype (the letters) determines your Phenotype (how you look). If you have at least one dominant allele, that is the trait you will see!
2. Monohybrid Inheritance
Monohybrid inheritance is the study of how one single gene is passed from parents to offspring. We use a genetic diagram called a Punnett Square to predict the outcome of a cross.
How to Draw a Punnett Square (Step-by-Step)
Imagine crossing two plants that are both Heterozygous for height. Tall (\(T\)) is dominant and short (\(t\)) is recessive.
1. Identify parent genotypes: \(Tt \times Tt\).
2. Separate the alleles into gametes: Each parent can give either a \(T\) or a \(t\).
3. Draw a \(2 \times 2\) grid and place one parent's alleles on top and the other on the side.
4. Fill in the boxes by combining the letters.
The Resulting Grid:
Top Row: \(T\), \(t\)
Side Column: \(T\), \(t\)
Box 1 (Top Left): \(TT\) (Homozygous Dominant - Tall)
Box 2 (Top Right): \(Tt\) (Heterozygous - Tall)
Box 3 (Bottom Left): \(Tt\) (Heterozygous - Tall)
Box 4 (Bottom Right): \(tt\) (Homozygous Recessive - Short)
Probabilities and Ratios
From the cross above, we can predict the probability for the offspring:
- Probability of being Tall: \(75\%\) (or \(3/4\))
- Probability of being Short: \(25\%\) (or \(1/4\))
- The phenotype ratio is \(3:1\).
- The genotype ratio is \(1 \text{ (TT)} : 2 \text{ (Tt)} : 1 \text{ (tt)}\).
3. Sex Determination
In humans, your biological sex is determined by the 23rd pair of chromosomes.
Females have two X chromosomes: \(XX\).
Males have an X and a Y chromosome: \(XY\).
When a sperm and egg meet (random fertilisation):
- All eggs carry an \(X\) chromosome.
- Half of the sperm carry an \(X\), and the other half carry a \(Y\).
If you draw a Punnett square for \(XX \times XY\), you will see that there is always a 50% chance (\(1:1\) ratio) of having a boy or a girl.
4. Multiple Alleles and Co-dominance
Sometimes, genetics isn't just "dominant or recessive."
Co-dominance: This happens when both alleles are expressed in the phenotype. Neither one "hides" the other.
ABO Blood Groups
This is a perfect example because it uses multiple alleles (\(I^A\), \(I^B\), and \(I^O\)):
- \(I^A\) and \(I^B\) are co-dominant.
- \(I^O\) is recessive.
The Genotypes:
- \(I^A I^A\) or \(I^A I^O\) = Blood Group A
- \(I^B I^B\) or \(I^B I^O\) = Blood Group B
- \(I^A I^B\) = Blood Group AB (Example of co-dominance!)
- \(I^O I^O\) = Blood Group O
5. Family Pedigrees
A family pedigree is like a family tree that shows how a specific trait is passed down through generations.
Reading the Symbols:
- Squares represent Males.
- Circles represent Females.
- Shaded shapes usually represent individuals who have the trait or condition.
- A horizontal line between two shapes represents a "marriage" or mating.
Quick Tip:
If two parents without a trait have a child with the trait, the trait must be recessive. The parents must be heterozygous "carriers."
6. Variation
Why are we all different? Variation is caused by two main factors:
1. Genetic Variation: Resulting from random fertilisation (which sperm meets which egg) and meiosis.
2. Environmental Variation: Changes caused by your surroundings (e.g., scars, language, or dyed hair).
3. Both: Many traits, like height or skin color, are influenced by both your genes and your environment (like nutrition).
Summary Checklist
Before moving on, make sure you can:
- Define key terms like allele, homozygous, and phenotype.
- Complete a Punnett square for a monohybrid cross.
- Explain why there is a \(50\%\) chance of a child being male or female.
- Predict blood groups using the concepts of multiple alleles and co-dominance.
- Interpret a family pedigree diagram.
Note: For more information on specific inherited conditions like Cystic Fibrosis or Polydactyly, see the chapter on "Inherited conditions, pedigrees and variation."