🧬 IGCSE Biology (0610) Study Notes: Monohybrid Inheritance

Welcome to the world of inheritance! This is the chapter where we learn why you look like your parents and grandparents—it's all down to the microscopic rules of genetics. Understanding these rules (especially how to predict crosses) is vital for your exam and for understanding how life works! Don't worry if the vocabulary seems tricky at first; we will break down every term.

This section focuses on Monohybrid Inheritance, which means we are only studying one characteristic at a time (like flower colour or hair texture).

Key Takeaway from Introduction:

We are focusing on how a single gene determines a specific trait.

1. The Essential Vocabulary of Inheritance (Core)

To talk about genetics, we need a common language. Here are the must-know definitions:

1.1 What is Inheritance? (17.4.1)

Inheritance is simply the transmission (passing on) of genetic information from one generation (parents) to the next (offspring).

1.2 Genes, Alleles, and Chromosomes
  • Chromosome: These are thread-like structures found in the nucleus, made of DNA, containing the genetic information.
  • Gene (17.4.2): A gene is a specific length of DNA that codes for a specific protein. This protein usually determines a specific characteristic or trait (e.g., the gene for eye colour).
  • Allele (17.4.3): An allele is an alternative form or version of a gene. Think of a gene as the characteristic (flavour of ice cream), and the alleles are the specific versions (chocolate or vanilla).
1.3 Genotype vs. Phenotype

This is a super important distinction!

Genotype (17.4.2):

  • This is the genetic make-up of an organism, specifically referring to the pair of alleles present for a particular gene.
  • Analogy: It is the "recipe" hidden inside the cell.
  • Example: If a plant has alleles for tallness (T) and shortness (t), its genotype is Tt.

Phenotype (17.4.3):

  • This is the observable feature or physical characteristic of an organism that results from the genotype.
  • Analogy: It is the "finished cake" you can see.
  • Example: If a plant has the genotype Tt, its phenotype might be Tall.

Quick Review: Genotype (G) = Genetic Code; Phenotype (P) = Physical Appearance.

2. Describing Allele Combinations and Expression (Core)

Because we inherit one allele from each parent, we always have a pair of alleles for every gene. We use capital letters for dominant alleles and lowercase letters for recessive alleles (e.g., A or a).

2.1 Dominant and Recessive Alleles

Dominant Allele (17.4.8):

  • An allele that is always expressed in the phenotype, even if only one copy is present in the genotype.
  • Represented by a capital letter (e.g., A).
  • Trick: The dominant allele is the 'bossy' one—it only needs to show up once to take control.

Recessive Allele (17.4.9):

  • An allele that is only expressed in the phenotype when two copies are present (when there is no dominant allele present).
  • Represented by a lowercase letter (e.g., a).
  • If the dominant allele (A) is present, the recessive allele (a) remains hidden.
2.2 Homozygous and Heterozygous

We classify an organism based on the pair of alleles it carries:

Homozygous (17.4.4):

  • The organism has two identical alleles for a particular gene.
  • Examples: AA (Homozygous Dominant) or aa (Homozygous Recessive).
  • A homozygous individual is also called pure-breeding (17.4.5) because if two identical homozygous individuals breed together, their offspring will all have the same genotype (e.g., AA x AA only produces AA).

Heterozygous (17.4.6):

  • The organism has two different alleles for a particular gene.
  • Example: Aa.
  • In a heterozygous organism, the dominant allele (A) determines the phenotype.
  • A heterozygous individual will not be pure-breeding (17.4.7) because they can pass on two different alleles (A or a) to their offspring.

Key Takeaway:

To express a recessive trait, you need two recessive alleles (aa). If you have at least one dominant allele (AA or Aa), you express the dominant trait.

3. Predicting Results: Monohybrid Crosses (Core)

We use genetic diagrams and Punnett squares to predict the possible genotypes and phenotypes of offspring. This is essential for calculating the common 1:1 and 3:1 phenotypic ratios (17.4.11).

3.1 The Step-by-Step Genetic Diagram (17.4.11)

We use standard convention (P generation, F1 generation) when drawing crosses. Let's look at the classic cross: crossing two heterozygous individuals (Aa x Aa).


Example: Pea Plant Height (T = Tall, t = Short)

Step 1: Parental Phenotypes (P)

Tall Plant x Tall Plant (Both are heterozygous)

Step 2: Parental Genotypes (P)

Tt x Tt

Step 3: Gametes (Sex Cells)

Gametes only carry one allele. Separate the letters.

Parent 1 produces gametes: T and t
Parent 2 produces gametes: T and t

Step 4: Fertilisation (F1 Offspring) - Using a Punnett Square (17.4.12)

The Punnett square helps show all possible combinations when the gametes fuse:

Gametes ↓ T t
T TT Tt
t Tt tt

Step 5: Genotype and Phenotype Ratios (F1)

Genotypes:

  • TT: 1
  • Tt: 2
  • tt: 1
  • Genotype Ratio: 1 : 2 : 1

Phenotypes:

  • Tall (TT and Tt): 3
  • Short (tt): 1
  • Phenotypic Ratio: 3 : 1

Did you know? Gregor Mendel, the "Father of Genetics," established these rules by studying pea plants in the 1800s!


3.2 Predicting a 1:1 Phenotypic Ratio

A 1:1 ratio is produced when a heterozygous organism is crossed with a homozygous recessive organism (e.g., Tt x tt).

  • Genotypes: Tt (50%) and tt (50%)
  • Phenotypes: Tall (50%) and Short (50%)

Key Takeaway:

You must know the steps of a genetic diagram: P Genotypes → Gametes → F1 Offspring. The most common ratios are 3:1 and 1:1.

4. Tracking Traits: Pedigree Diagrams (Core)

Pedigree diagrams (17.4.10) are family trees used to show how genetic characteristics are passed through generations. You must be able to interpret them.

Symbols Used:
  • Square = Male
  • Circle = Female
  • Shaded Shape = Affected individual (shows the trait/disease)
  • Unshaded Shape = Unaffected individual (does not show the trait/disease)
  • Horizontal line between shapes = Mating/Parents
  • Vertical lines leading down = Offspring
Interpreting Recessive Traits:

The best way to spot a recessive characteristic is if:

Two unaffected parents have an affected child.
If Parent 1 (unaffected) and Parent 2 (unaffected) produce a Shaded Child (affected), the affected child must be homozygous recessive (aa). Since the parents are unaffected but carry the 'a' allele, they must both be heterozygous carriers (Aa). This pattern identifies a recessive trait.

Key Takeaway:

Pedigree charts are visual tools. Look for cases where a trait 'skips' a generation—this is a strong indicator of a recessive allele.

5. Extended Concepts in Monohybrid Inheritance (Supplement)

If you are studying the Extended syllabus (aiming for grades A*-C), you need to master these complex inheritance patterns.

5.1 The Test Cross (17.4.13)

Sometimes you observe a dominant phenotype (e.g., a tall pea plant), but you don't know the genotype. It could be TT or Tt.

A test cross is used to determine an unknown genotype by crossing the organism with a homozygous recessive individual (the 'known loser').

  • If the unknown organism is Homozygous Dominant (TT):
    TT x tt → All offspring are Tt (100% show the dominant phenotype).
  • If the unknown organism is Heterozygous (Tt):
    Tt x tt → Offspring are 50% Tt and 50% tt.

If even one offspring shows the recessive phenotype (e.g., is short), the unknown parent must have been heterozygous (Tt).

5.2 Codominance (17.4.14)

In simple dominance, one allele completely masks the other. In Codominance, both alleles contribute equally to the phenotype when they are both present in the heterozygous state.

  • Neither allele is truly dominant or recessive.
  • The result is a mixed or blended phenotype.
  • We use capital letters with superscripts to denote codominance (e.g., CR and CW).

Example: Flower Colour (CR = Red, CW = White)

  • Genotype CRCR: Phenotype is Red
  • Genotype CWCW: Phenotype is White
  • Genotype CRCW: Phenotype is Pink (both traits show up).
5.3 Inheritance of ABO Blood Groups (17.4.15)

The human ABO blood group system is a crucial example involving both multiple alleles (more than two allele types exist in the population) and codominance.

There are three alleles for blood type: IA, IB, and IO (sometimes written as i).

  • IA and IB are codominant.
  • IO is recessive to both IA and IB.

The four possible phenotypes (blood types) and their genotypes are:

Phenotype (Blood Type) Possible Genotypes
A IAIA or IAIO
B IBIB or IBIO
AB IAIB (Codominance)
O IOIO (Recessive)
5.4 Sex Linkage (17.4.16)

Sex chromosomes: Females have two X chromosomes (XX); Males have one X and one Y chromosome (XY).

A sex-linked characteristic is a trait determined by a gene located on one of the sex chromosomes (usually the X chromosome, as the Y chromosome is much smaller and carries fewer genes).

Why is it more common in males? (17.4.16)

If a gene for a disorder is recessive and located on the X chromosome (X-linked recessive), a male (XY) only needs to inherit one copy of the recessive allele (on his single X chromosome) to show the trait. Females (XX) need two copies, making it less common.

Example: Red-Green Colour Blindness (17.4.17)

This disorder is controlled by a recessive allele on the X chromosome.

  • We use X and Y to represent chromosomes, and superscripts for the allele (17.4.18).
  • XN = Normal vision allele (Dominant)
  • Xn = Colour blind allele (Recessive)
Genotype Phenotype
XNXN Unaffected Female
XNXn Carrier Female (Unaffected)
XnXn Affected Female
XNY Unaffected Male
XnY Affected Male

Note the difference: Males are either affected or unaffected. Females can be unaffected, affected, or carriers.

Key Takeaway for Extended:

Codominance results in a mixed phenotype. Blood groups involve codominance (A and B) and recessiveness (O). Sex linkage means the gene is on the X chromosome, explaining why males get X-linked recessive disorders more easily.