17.4 Monohybrid Inheritance Revision Notes

1. Overview

Monohybrid inheritance is the study of how a single characteristic is passed from parents to offspring through the transmission of genes. By understanding the relationship between alleles, we can use genetic diagrams to predict the probability of specific physical traits appearing in the next generation, which is essential for fields ranging from medicine to agriculture.

2. Key Definitions

• Inheritance: The transmission of genetic information from generation to generation.
• Genotype: The genetic make-up of an organism in terms of the alleles present (e.g., $Tt$ or $GG$).
• Phenotype: The observable features of an organism (e.g., tall height, green seeds).
• Allele: An alternative form of a gene.
• Homozygous: Having two identical alleles of a particular gene (e.g., $TT$ or $tt$).
• Heterozygous: Having two different alleles of a particular gene (e.g., $Tt$).
• Dominant Allele: An allele that is expressed if it is present in the genotype (represented by a capital letter).
• Recessive Allele: An allele that is only expressed when there is no dominant allele of the gene present in the genotype (represented by a lowercase letter).
• Pure-breeding: When two identical homozygous individuals breed together, all offspring will have the same phenotype as the parents.

3. Core Content

Pure-breeding vs. Heterozygous

• Homozygous (Pure-breeding): Individuals with genotypes like $AA$ or $aa$ are pure-breeding because they can only pass on one type of allele to their gametes.
• Heterozygous: Individuals with the genotype $Aa$ are not pure-breeding because they can pass on either the dominant ($A$) or the recessive ($a$) allele to their offspring.

Predicting Inheritance: Genetic Diagrams & Punnett Squares

To predict the outcome of a cross, follow these steps:

1. Define the symbols: e.g., $B$ = Brown eyes (dominant), $b$ = Blue eyes (recessive).
2. State the parental phenotypes: e.g., Brown eyes x Blue eyes.
3. State the parental genotypes: e.g., $Bb$ x $bb$.
4. Identify the gametes: Separate the alleles and circle them.
5. Use a Punnett Square: Place the gametes of one parent on the top and the other on the side.

Common Phenotypic Ratios

• 3:1 Ratio: Occurs when two heterozygous parents ($Aa$ x $Aa$) are crossed. 75% show the dominant trait, 25% show the recessive trait.
• 1:1 Ratio: Occurs when a heterozygous parent ($Aa$) is crossed with a homozygous recessive parent ($aa$). 50% dominant trait, 50% recessive trait.

Pedigree Diagrams

Pedigree charts show the inheritance of a trait over several generations.

• Squares represent males; Circles represent females.
• Shaded shapes represent individuals who express the trait being studied.
• Horizontal lines between a square and circle represent a mating pair.
• Vertical lines leading down to a bracket represent the offspring.

4. Supplement Content (Extended Only)

The Test Cross

A test cross is used to identify the unknown genotype of an organism with a dominant phenotype (is it $AA$ or $Aa$?).

• Process: Cross the unknown individual with a homozygous recessive ($aa$) individual.
• Result A: If any offspring show the recessive phenotype, the unknown parent must be heterozygous ($Aa$).
• Result B: If 100% of the offspring show the dominant phenotype, the unknown parent is likely homozygous dominant ($AA$).

Codominance

Codominance occurs when both alleles in a heterozygous organism contribute to the phenotype. Neither allele is dominant over the other.

• ABO Blood Groups: Controlled by three alleles: $I^A$, $I^B$, and $I^o$.
  • $I^A$ and $I^B$ are codominant.
  • $I^o$ is recessive to both $I^A$ and $I^B$.
• Phenotypes and Genotypes:
  • Group A: $I^AI^A$ or $I^AI^o$
  • Group B: $I^BI^B$ or $I^BI^o$
  • Group AB: $I^AI^B$ (Both A and B antigens are expressed)
  • Group O: $I^oI^o$

Sex-linked Characteristics

A sex-linked characteristic is a feature where the gene responsible is located on a sex chromosome (usually the X chromosome).

• Red-green colour blindness: The gene is carried on the X chromosome.
• Mechanism: Because males are $XY$, they only have one X chromosome. If they inherit the faulty allele, they will be colour blind. Females ($XX$) have two X chromosomes; if they have one faulty allele, they are "carriers" but have normal vision. They only express the trait if they have two faulty alleles.
• Genetic Notation: Use $X^R$ (Normal) and $X^r$ (Colour blind). A carrier female is $X^RX^r$. A colour blind male is $X^rY$.

5. Key Equations

• Phenotypic Ratio: $\text{Number of offspring with phenotype A} : \text{Number of offspring with phenotype B}$
• Probability: $\frac{\text{Number of specific outcomes}}{\text{Total number of possible outcomes}} \times 100%$
• Note: In biology, ratios should always be simplified (e.g., 3:1 rather than 75:25).

6. Common Mistakes to Avoid

• ❌ Wrong: Thinking the dominant allele is the "strongest" or "most common" in a population.
• ✅ Right: The dominant allele is simply the one expressed in the phenotype when the genotype is heterozygous.
• ❌ Wrong: Using different letters for the same gene (e.g., $B$ for brown and $g$ for green).
• ✅ Right: Always use the same letter in upper and lower case (e.g., $B$ and $b$).
• ❌ Wrong: Forgetting to use the $I^A / I^B / I^o$ notation for blood groups.
• ✅ Right: Always write the superscript to show the specific alleles.

7. Exam Tips

• Command Words: If asked to "Define" a term like genotype, use the exact syllabus wording: "the genetic make-up of an organism in terms of the alleles present."
• Show Your Work: In Punnett square questions, always write out the gametes and the final ratio. Even if your final answer is wrong, you can earn marks for the correct process.
• Contexts: Expect questions about plant height, seed colour, or human diseases like cystic fibrosis (recessive) or Huntington's (dominant).
• Numerical Values: Be prepared to calculate percentages. For example, in a 3:1 ratio, the probability of the recessive phenotype is 0.25 or 25%.