Biology | Inheritance

IB Biology Study Notes: Inheritance (Continuity and Change)

Welcome to the fascinating world of genetics! This chapter, Inheritance, sits firmly within the theme of Continuity and change. We are moving from studying how DNA is replicated and translated (continuity) to understanding how variations in that DNA are passed down (change) and expressed in the next generation.

Understanding inheritance is crucial because it explains not only why you look similar to your family but also how populations evolve and change over time. Don't worry if the vocabulary seems dense at first; we will break down the key terms used to describe how traits are passed on!

Key Takeaway from the Introduction

Genetics is the science of heredity and variation. It explains the mechanism behind the continuity of life and the source of biological change.

Section 1: The Foundation of Genetics – Terminology

Before we can calculate probabilities, we need a precise vocabulary.

1.1 Genes, Alleles, and Loci

• Gene: A segment of DNA that codes for a specific polypeptide or protein, resulting in a specific trait. (Think of a gene as the whole recipe for a cake.)
• Allele: A specific version of a gene. We inherit one allele from each parent for every trait.
• Locus (Plural: Loci): The specific fixed position of a gene on a chromosome.

Analogy: Imagine DNA is a massive cookbook. A gene is the "Chocolate Chip Cookie" recipe. The alleles are the different versions of that recipe—one might call for white sugar (version A), and another might call for brown sugar (version B). The locus is the page number where the recipe is found.

1.2 Genotype vs. Phenotype

• Genotype: The actual combination of alleles an organism possesses. This is the genetic code (e.g., AA, Aa, or aa).
• Phenotype: The observable physical or biochemical characteristics expressed by the organism. This is the result of the genotype (e.g., tall, short, brown eyes).

1.3 Homozygous vs. Heterozygous

Since diploid organisms have two copies of each chromosome (one from each parent), they have two alleles for every gene:

• Homozygous: Having two identical alleles for a gene (e.g., BB or bb).
• Heterozygous: Having two different alleles for a gene (e.g., Bb).

Section 2: Mendelian Genetics and Simple Crosses

Gregor Mendel, often called the "Father of Genetics," established the fundamental rules of inheritance using pea plants.

2.1 Dominant and Recessive Alleles

• Dominant Allele: An allele that is always expressed in the phenotype, even if only one copy is present (i.e., in a heterozygous individual). Represented by a capital letter (e.g., A).
• Recessive Allele: An allele that is only expressed in the phenotype if two copies are present (i.e., in a homozygous recessive individual). Represented by a lower-case letter (e.g., a).

Mendel's Law of Segregation:
This states that when gametes (sperm/egg) are formed, the two alleles for each trait separate (segregate) from one another, so that each gamete carries only one allele for that trait.

2.2 Monohybrid Crosses and Punnett Squares

A monohybrid cross studies the inheritance of a single trait. We use a Punnett square to predict the possible genotypes and phenotypes of the offspring from a cross between two parents.

Step-by-step: Constructing a Punnett Square

1. Determine Parental Genotypes: (e.g., Parent 1 is Tt, Parent 2 is Tt).
2. Determine Gametes: Use the Law of Segregation. Parent 1 can produce T or t gametes. Parent 2 can produce T or t gametes.
3. Set up the Square: Put one parent's gametes along the top and the other parent's gametes down the side.
4. Fill the Square: Combine the alleles in each box to show the possible offspring genotypes.

Example: Crossing two heterozygous tall pea plants (Tt x Tt, where T=Tall, t=short).

	T	t
T	TT	Tt
t	Tt	tt

Predicted Ratios:

• Genotypic Ratio: 1 TT : 2 Tt : 1 tt
• Phenotypic Ratio: 3 Tall : 1 Short

We express the phenotypic ratio as \(3:1\) for a standard monohybrid cross of two heterozygotes.

Common Mistake Alert!

Students often mix up the parental cross (P generation), the first generation (F1), and the second generation (F2). Remember, the F2 generation results from crossing two F1 individuals (like the Tt x Tt cross above).

Section 3: Beyond Simple Dominance

Not all traits follow the simple dominant/recessive pattern described by Mendel. Here we look at two variations.

3.1 Co-dominance (SL & HL)

In co-dominance, both alleles are fully expressed in the heterozygote, resulting in a phenotype that shows characteristics of both alleles simultaneously.

• Notation: We typically use a base letter (e.g., C for colour) and superscripts to denote the specific allele (e.g., \(C^W\) for white, \(C^R\) for red).
• Example: Roan Cattle - A cross between a red cow (\(C^R C^R\)) and a white bull (\(C^W C^W\)) produces offspring (\(C^R C^W\)) that have both red hairs and white hairs, giving them a spotted appearance (roan). Neither allele is hidden.

3.2 Multiple Alleles: ABO Blood Groups (SL & HL)

A gene has multiple alleles if there are three or more possible alleles for that trait in the population (though an individual can still only possess two of them).

The best example is the human ABO blood group system, controlled by three alleles: \(I^A\), \(I^B\), and \(i\).

• \(I^A\) and \(I^B\) are co-dominant with respect to each other.
• \(I^A\) and \(I^B\) are both dominant over \(i\) (the recessive allele).

Genotype(s)	Phenotype (Blood Type)
\(I^A I^A\) or \(I^A i\)	Type A
\(I^B I^B\) or \(I^B i\)	Type B
\(I^A I^B\)	Type AB (Co-dominance!)
\(i i\)	Type O (Recessive)

Did you know? Type O is the universal donor because the 'i' allele produces no surface antigen, meaning it won't trigger an immune response in recipients with A, B, or AB blood.

Section 4: Sex Linkage and Pedigree Analysis

4.1 Sex Linkage

Sex chromosomes determine the sex of an individual (XX = female, XY = male in humans). A sex-linked gene is one whose locus is on a sex chromosome (usually the X chromosome).

Why is X-Linkage important?

• Females (XX) have two copies of the X chromosome, meaning they can be homozygous or heterozygous for X-linked traits.
• Males (XY) only have one X chromosome. They inherit the X from their mother and the Y from their father.
• If a male inherits a single recessive allele on the X chromosome (e.g., for colour blindness), it will be expressed in the phenotype because there is no corresponding allele on the Y chromosome to mask it. This is why X-linked recessive disorders are much more common in males.

Notation Tip: When working with sex linkage, always use superscripts on the X chromosome (e.g., \(X^R X^r\) or \(X^r Y\)). Never put alleles on the Y chromosome unless specified.

Example: Red-Green Colour Blindness (Recessive, X-linked)

• Female Carrier: \(X^C X^c\) (Normal vision, but carries the recessive allele)
• Affected Male: \(X^c Y\) (Colour blind)

4.2 Analyzing Pedigree Charts

A pedigree chart is a family tree that tracks the inheritance of a specific trait, often a genetic disease.

• Squares represent males; Circles represent females.
• Shaded shapes indicate affected individuals (expressing the phenotype).
• A horizontal line connects parents; a vertical line leads to the offspring.

How to determine if a trait is Recessive or Dominant:

1. If a trait skips a generation (unaffected parents have an affected child), it is RECESSSIVE. (The unaffected parents must be carriers/heterozygotes).
2. If a trait appears in every generation, it is usually DOMINANT. (Affected parents often have affected children).

How to determine if a trait is X-Linked or Autosomal:

If X-linked Recessive:

• The trait is much more common in males.
• An affected mother (e.g., \(X^r X^r\)) must pass the trait to ALL her sons. If an affected mother has an unaffected son, the trait cannot be X-linked recessive.

Section 5: Polygenic Inheritance (HL Focus)

While Mendel studied traits controlled by a single gene, most human characteristics are far more complex.

5.1 The Concept of Continuous Variation

Polygenic inheritance occurs when one characteristic is controlled by two or more genes acting together. These traits show continuous variation, meaning the phenotypes exist on a spectrum rather than in distinct categories.

• Single-gene traits: Discrete (e.g., you are either Type A blood or not).
• Polygenic traits: Continuous (e.g., height, weight, skin colour, intelligence).

Example: Skin Colour
Skin colour is controlled by at least four genes, each with two or more alleles. The intensity of pigmentation is determined by the cumulative effect of the dominant alleles from all these genes. The more 'pigment-producing' dominant alleles an individual has, the darker their skin tone will be.

5.2 The Role of Environment

For most polygenic traits, the final phenotype is also heavily influenced by environmental factors. This is known as the interaction between genotype and environment.

• Example: Height – Height is polygenic, but an individual’s final height is also affected by diet and nutrition during childhood.
• Example: Skin colour – Genotype determines the potential range of melanin production, but exposure to sunlight (environment) greatly increases the expression of melanin.