Biology (9610) | Inheritance

Inheritance: Unlocking the Secrets of Genetics (Syllabus 3.3.6)

Welcome to the chapter on Inheritance! This is where we uncover how traits are passed down from one generation to the next. Understanding genetics is fundamental to biology—it explains why you look like your parents, why organisms within a species vary, and ultimately, how populations evolve. Don't worry if the crosses seem tricky at first; we will break down the essential terminology and methods step-by-step!

Remember: This chapter links closely with your understanding of DNA structure (3.1.7) and Meiosis (3.1.9), which is the process that creates the gametes carrying these inherited traits.

3.3.6.1 Principles of Inheritance: Key Terminology

Before diving into genetic crosses, we must master the language of genetics.

Genotype and Phenotype

• Genotype: This is the specific genetic constitution of an organism—it refers to the actual combination of alleles an organism possesses (e.g., TT, Tt, tt).
• Phenotype: This is the observable expression of the genetic constitution, often described in words (e.g., Tall, Short, Blue eyes).

• The phenotype is the result of the genotype interacting with the environment.
  Example: A plant may have the genotype for being tall, but if it is grown in poor soil (environmental factor), its height (phenotype) will be stunted.

Genes and Alleles

• Gene: A segment of DNA that codes for a specific polypeptide, determining a particular characteristic.
• Locus (plural: Loci): The fixed position on a specific chromosome where a gene is found.
• Allele: A different version or variant of a gene. A diploid organism (like humans) inherits two alleles for every gene (one from each parent).

  Analogy: If 'Eye Colour' is the gene, then 'Blue,' 'Brown,' and 'Green' are the alleles.

Allele Interactions

Alleles are categorized based on how they express themselves in the phenotype:

• Dominant Allele: An allele that is always expressed in the phenotype, even if only one copy is present (in a heterozygous individual). Represented by a capital letter (e.g., T for Tall).
• Recessive Allele: An allele that is only expressed in the phenotype if two copies are present (in a homozygous individual). Represented by a lowercase letter (e.g., t for short).
• Codominant Alleles: Alleles that are both expressed equally in the phenotype of a heterozygous organism.

  Example: The inheritance of human ABO blood groups, where alleles \(I^A\) and \(I^B\) are codominant, resulting in the AB blood type.

Zygosity

• Homozygous: When an organism has two identical alleles for a specific gene.
  • Homozygous Dominant (e.g., TT)
  • Homozygous Recessive (e.g., tt)
• Heterozygous: When an organism has two different alleles for a specific gene (e.g., Tt).

3.3.6.2 Patterns of Inheritance: Genetic Crosses

We use fully labelled genetic diagrams (often incorporating Punnett squares) to predict or interpret the results of crosses. These diagrams must clearly show all stages.

Monohybrid Crosses (One Trait)

A monohybrid cross investigates the inheritance of a single characteristic.

Step-by-Step for a Monohybrid Cross (Tasting ability T vs non-tasting t)

1. Parental (P) Phenotypes: Taster (Homozygous Dominant) \(\times\) Non-Taster (Homozygous Recessive)
2. Parental (P) Genotypes: TT \(\times\) tt
3. Gametes: T, T \(\times\) t, t
4. Fertilisation (F1 Generation): Use a Punnett Square to combine gametes.
5. F1 Genotypes: All Tt
6. F1 Phenotypes: All Taster

If two F1 individuals (Tt \(\times\) Tt) cross, the expected ratio in the F2 generation is usually 3:1 (Phenotype) and 1:2:1 (Genotype).

Codominance Crosses

Since neither allele is truly dominant, we use different notation, often using a common letter for the gene and superscript letters for the alleles. Example: Flower colour in snapdragons, where R=Red and W=White.

• Pure Red: \(C^R C^R\)
• Pure White: \(C^W C^W\)
• Heterozygous (Codominant): \(C^R C^W\) (This produces a Pink phenotype, as both pigments are expressed).

Dihybrid Crosses (Two Independent Traits)

A dihybrid cross tracks the inheritance of two characteristics at the same time. This usually assumes that the two genes are located on different chromosomes, allowing for independent assortment during meiosis.

• If two heterozygous individuals cross (e.g., AaBb \(\times\) AaBb), the classic Mendelian phenotypic ratio is always 9:3:3:1.
• 9: Dominant for both traits (A_B_)
• 3: Dominant for A, Recessive for B (A_bb)
• 3: Recessive for A, Dominant for B (aaB_)
• 1: Recessive for both traits (aabb)

Tip: When determining gametes for a dihybrid cross, remember to include every possible combination of one allele from each gene (e.g., for AaBb, the gametes are AB, Ab, aB, ab).

Advanced Inheritance Patterns

Not all inheritance follows simple Mendelian ratios. The syllabus requires you to understand four specific variations.

1. Sex-Linkage

This occurs when the gene (locus) is located on one of the sex chromosomes (X or Y). In mammals, most sex-linked characteristics are carried on the X chromosome.

• Females (XX) have two copies of the gene, so they can be homozygous or heterozygous.
• Males (XY) only have one copy of the gene (they are hemizygous).

Why this matters: If a male inherits a single recessive allele on his X chromosome (e.g., X^rY), he will express the recessive phenotype because there is no dominant allele on the Y chromosome to mask it.

Example: Red-green colour blindness (r) is sex-linked. A genotype X^rY results in a colour-blind male, while a heterozygous female (X^RX^r) is a carrier but not colour-blind.

2. Autosomal Linkage

• Autosomal Linkage: This occurs when two genes are located on the same non-sex chromosome (autosome).
• When genes are linked, they tend to be inherited together because the chromosome is passed on as a unit during meiosis.
• Result: Autosomal linkage leads to reduced variation in offspring and phenotypic ratios that deviate significantly from the expected 9:3:3:1 Mendelian ratio.
• Exception: If a crossover event happens between the two gene loci during meiosis, new combinations (recombinants) can be produced, but these occur at a much lower frequency.

3. Multiple Alleles

Multiple alleles exist when there are more than two possible alleles for a single gene within a population. Remember, any single diploid organism can only carry two of those alleles.

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

• \(I^A\) and \(I^B\) are codominant.
• \(i\) is recessive to both \(I^A\) and \(I^B\).

4. Epistasis

Epistasis occurs when the allele of one gene affects or masks the expression of the alleles of another gene at a different locus.

• This is essentially gene interaction where the product of one gene (often an enzyme) influences the pathway of another gene.
• Analogy: Imagine two genes control hair colour (Gene A for pigment production, Gene B for brown/blonde shade). If Gene A is recessive homozygous (aa), it might fail to produce any pigment at all (albino), entirely masking whatever alleles are present at Gene B.
• Epistasis results in non-standard dihybrid phenotypic ratios, such as 9:3:4 or 12:3:1.

Analyzing Results: The Chi-Squared (\(\chi^2\)) Test

When you carry out a genetic cross in a laboratory, the observed results rarely match the perfectly predicted ratios (like 3:1 or 9:3:3:1). This is due to chance.

The Chi-Squared (\(\chi^2\)) Test is a statistical tool used to determine if the difference between the Observed (O) results and the Expected (E) results is due to chance alone, or if some other factor (like linkage or epistasis) is at play.

The Formula

$$\chi^2 = \sum \frac{(O - E)^2}{E}$$

Where \(\sum\) means 'the sum of', O is the Observed number, and E is the Expected number.

Step-by-Step Guide to Using the \(\chi^2\) Test

1. Establish the Null Hypothesis (\(H_0\)): This is the hypothesis that there is no significant difference between the observed results and the expected results (i.e., any deviation is due entirely to chance).
2. Calculate Expected (E) Frequencies: Determine the number of offspring expected for each phenotype category based on the theoretical genetic ratio (e.g., 3:1).
3. Calculate \(\chi^2\) Value: Use the formula provided above. You calculate the term $\frac{(O - E)^2}{E}$ for each phenotype category, and then sum these values together to get the total \(\chi^2\).
4. Determine Degrees of Freedom (df): This is calculated as: $df = (\text{Number of categories}) - 1$.

   Example: If you have 4 phenotypes (9:3:3:1), df = 4 - 1 = 3.

5. Compare to Critical Value: Look up the critical value in a statistical table using your calculated df and a chosen significance level (usually 0.05 or 5%).
6. Draw a Conclusion:
   • If the calculated \(\chi^2\) value is LESS THAN the critical value: We accept the Null Hypothesis (\(H_0\)). The deviation is due to chance, and the inheritance pattern matches the expected ratio (e.g., 9:3:3:1).
   • If the calculated \(\chi^2\) value is GREATER THAN the critical value: We reject the Null Hypothesis. The deviation is statistically significant and likely due to a factor other than chance (e.g., autosomal linkage, epistasis, or differential survival).