Basic Genetics: Your Ultimate Study Guide
Hello there! Welcome to the fascinating world of genetics. Ever wonder why you have your mum's eyes or your dad's smile? Or why some traits seem to run in families? This chapter is all about answering those questions. We're going to unravel the secrets of heredity – the passing of traits from parents to children.
Don't worry if this sounds complicated. We'll break it down step-by-step with simple explanations and real-life examples. Let's get started on your journey to becoming a genetics whiz!
1. Mendel's Laws: The Foundation of Genetics
Who was Gregor Mendel?
Gregor Mendel was a monk who lived in the 1800s. He's often called the "Father of Genetics" because his experiments with pea plants laid down the fundamental rules of inheritance. By carefully studying traits like plant height and seed colour, he figured out how characteristics are passed on, long before we even knew about DNA!
Key Genetic Terms You MUST Know
Before we dive into Mendel's laws, let's get our vocabulary straight. Understanding these terms is crucial!
Gene: A segment of DNA that codes for a specific trait. For example, the gene for eye colour.
Allele: Different versions of the same gene. For example, for the eye colour gene, you might have a 'blue eyes' allele and a 'brown eyes' allele.
Genotype: The genetic makeup of an organism, represented by letters. For example, TT, Tt, or tt.
Phenotype: The observable physical characteristic of an organism. For example, 'tall' or 'short'. The phenotype is determined by the genotype.
Homozygous: Having two identical alleles for a trait. For example, TT (homozygous dominant) or tt (homozygous recessive). Memory aid: 'Homo' means 'same'.
Heterozygous: Having two different alleles for a trait. For example, Tt. Memory aid: 'Hetero' means 'different'.
Dominant Allele: An allele that is always expressed in the phenotype, even if only one copy is present. It masks the recessive allele. We use a capital letter to represent it (e.g., T for tall).
Recessive Allele: An allele that is only expressed in the phenotype if two copies are present (homozygous). It is masked by the dominant allele. We use a small letter to represent it (e.g., t for short).
Quick Review Box
Genotype is the 'genes' (e.g., Tt).
Phenotype is the 'physical' appearance (e.g., Tall).
Mendel's First Law: The Law of Segregation
This law states that for any trait, the pair of alleles from each parent separate (or segregate) during the formation of gametes (sperm and egg cells). This means each gamete only carries one allele for each gene.
Analogy: Imagine you have a pair of socks, one blue and one white. When you pack for a trip, you put one sock in one suitcase and the other sock in another. The pair 'segregates'.
How to solve a genetic cross (Monohybrid Cross)
A monohybrid cross involves looking at one trait. We use a tool called a Punnett Square to predict the outcomes.
Step-by-step example: Let's cross a heterozygous tall pea plant (Tt) with another heterozygous tall pea plant (Tt).
Identify the parents' genotypes: Parent 1 = Tt, Parent 2 = Tt.
Determine the possible gametes from each parent: Parent 1 can produce 'T' or 't' gametes. Parent 2 can also produce 'T' or 't' gametes.
Draw the Punnett Square: Draw a 2x2 grid. Put one parent's gametes along the top and the other parent's gametes along the side.
Fill in the boxes: Combine the letters from the top and side for each box. This shows the possible genotypes of the offspring.
Determine the ratios:
- Genotypic ratio: 1 TT : 2 Tt : 1 tt
- Phenotypic ratio: 3 Tall : 1 short (because TT and Tt both result in a tall plant)
Parent 1 Gametes -> T t
Parent 2 Gametes
|-------|-------|
T | TT | Tt |
|-------|-------|
t | Tt | tt |
|-------|-------|
Mendel's Second Law: The Law of Independent Assortment
This law states that the alleles for one gene segregate independently of the alleles for another gene. This only applies to genes located on different chromosomes.
Analogy: Imagine you are getting dressed. The choice of your shirt (gene 1) does not affect your choice of trousers (gene 2). They are independent choices. The allele for seed shape (e.g., Round or Wrinkled) does not affect the allele for seed colour (e.g., Yellow or Green).
This law explains why we see so much variation. A dihybrid cross (involving two traits) between two heterozygotes (e.g., RrYy x RrYy) typically results in a 9:3:3:1 phenotypic ratio.
Key Takeaway for Mendel's Laws
Law of Segregation: Allele pairs separate during gamete formation.
Law of Independent Assortment: Genes for different traits sort independently of one another.
2. Inheritance in Humans
Sex Determination
Humans have 23 pairs of chromosomes. Pair 23 are the sex chromosomes, which determine if a person is biologically male or female.
Females have two X chromosomes: XX.
Males have one X and one Y chromosome: XY.
During meiosis, a female can only pass on an X chromosome in her eggs. A male can pass on either an X or a Y in his sperm. Therefore, it is the sperm that determines the sex of the child.
A Punnett square shows this clearly, resulting in a 50% chance of a boy and a 50% chance of a girl.
Sex Linkage
Sex linkage refers to traits that are controlled by genes located on the sex chromosomes (usually the X chromosome).
Famous examples are haemophilia and red-green colour blindness. The alleles for these conditions are recessive and found on the X chromosome.
Why are sex-linked traits more common in males?
Males have only one X chromosome (XY). If they inherit one recessive allele on their single X chromosome, they will have the trait. Females have two X chromosomes (XX). They must inherit two recessive alleles (one on each X) to have the trait. If they have only one, they are a carrier but don't show the trait themselves.
For example, let's use the alleles for colour blindness: XB (normal vision) and Xb (colour blind).
- A male with XbY is colour blind.
- A female needs XbXb to be colour blind. A female with XBXb has normal vision but is a carrier.
Multiple Alleles: ABO Blood Groups
Usually, a gene has two alleles. But sometimes, there are multiple alleles (more than two) for a single gene within a population. A classic example is the human ABO blood group system.
There are three alleles for blood type: $$I^A$$, $$I^B$$, and $$i$$.
Alleles $$I^A$$ and $$I^B$$ are codominant. This means if both are present, both are expressed equally (resulting in blood type AB).
Alleles $$I^A$$ and $$I^B$$ are both dominant over the allele $$i$$.
Allele $$i$$ is recessive.
Blood Group Genotypes and Phenotypes
Phenotype (Blood Type) Possible Genotypes
----------------------------------------------------------
A $$I^A I^A$$ or $$I^A i$$
B $$I^B I^B$$ or $$I^B i$$
AB $$I^A I^B$$
O $$i i$$
Key Takeaway for Human Inheritance
Sex determination: XX = female, XY = male.
Sex linkage: Traits on the X-chromosome, more common in males.
Multiple alleles: More than two allele options for a gene, like in ABO blood types.
3. Pedigree Analysis: Reading the Family Story
What is a Pedigree Chart?
A pedigree is like a family tree that shows the inheritance of a particular genetic trait or disease through several generations. Biologists use pedigrees to study inheritance patterns.
Decoding the Symbols
Square: Male
Circle: Female
Shaded shape: Affected individual (has the trait)
Unshaded shape: Unaffected individual
Horizontal line between a square and circle: Mating
Vertical line coming down from a mating line: Offspring
How to Analyse a Pedigree
By looking at the patterns, you can make educated guesses about the trait.
If two unaffected parents have an affected child, the trait is likely recessive. The parents must both be heterozygous carriers.
If the trait appears in every generation, it is likely dominant.
If a trait appears far more often in males than females, it is likely X-linked recessive.
Key Takeaway for Pedigree Analysis
Pedigrees use standard symbols to visually track a trait through a family's generations, helping us understand how it's inherited.
4. Variation: The Spice of Life
Variation refers to the differences between individuals of the same species. There are two main types.
Discontinuous Variation
This is where individuals fall into a few distinct, clear-cut categories with no intermediates. You either have the trait or you don't.
Examples: ABO blood group (you are A, B, AB, or O, nothing in between), ability to roll your tongue.
Cause: Usually controlled by a single gene.
Graph: Represented by a bar chart with separate bars.
Continuous Variation
This is where a characteristic shows a gradual range of values from one extreme to another.
Examples: Human height, weight, skin colour, intelligence.
Cause: Controlled by multiple genes (polygenic) and is often influenced by environmental factors.
Graph: Represented by a histogram, which forms a bell-shaped curve (a normal distribution curve).
The Causes of Variation
So, where does all this amazing variety come from? There are three main sources.
Hereditary Information (Genetic Recombination): This happens during sexual reproduction.
During meiosis, crossing over and independent assortment shuffle the alleles into new combinations in the gametes.
The random fusion of gametes at fertilisation creates a unique combination of genes in the offspring.
Environmental Factors: The environment can influence the expression of genes.
For example, your genes might give you the potential to be tall, but poor nutrition during childhood could stunt your growth.
Plants grown in the shade might be smaller and paler than genetically identical plants grown in the sun.
Mutation:
A mutation is a random change in the structure of a gene or chromosome.
Most mutations are harmful, but some can be neutral or even beneficial. Mutation is the ultimate source of all new alleles in a population.
Key Takeaway for Variation
Discontinuous: Distinct categories (e.g., blood type). Caused by one gene.
Continuous: A full range of values (e.g., height). Caused by many genes + environment.
Variation arises from genetic shuffling (meiosis, fertilisation), environmental influences, and mutation.