Mendelian Genetics Review and Extensions Overview

TL;DR

Mendelian genetics explains how traits pass from parents to offspring through discrete units called genes. We'll revisit dominant/recessive inheritance and then explore exceptions like incomplete dominance and epistasis. Understanding these patterns helps you predict offspring characteristics and disease risks.

1. The Mental Model

Imagine genes as instructions for building traits, like "tall" or "short." You get two copies of each instruction, one from each parent. How these two copies interact determines the final trait you show.

2. The Core Material

You've likely covered Mendelian genetics, which focuses on simple inheritance patterns where one allele (gene variant) is completely dominant over another recessive allele. For example, if 'T' makes you tall and 't' makes you short, then 'TT' and 'Tt' genotypes both lead to a tall phenotype, while only 'tt' leads to a short phenotype. This is the foundation.

Here's a quick recap of key terms:
* Allele: A specific version of a gene (e.g., the allele for tallness, 'T', or for shortness, 't').
* Genotype: The actual genetic makeup of an individual (e.g., TT, Tt, tt).
* Phenotype: The observable trait resulting from the genotype (e.g., tall, short).
* Homozygous: Having two identical alleles for a gene (e.g., TT or tt).
* Heterozygous: Having two different alleles for a gene (e.g., Tt).
* Punnett Square: A diagram used to predict the genotypes and phenotypes of offspring from a genetic cross.

2.1 Beyond Simple Dominance: Extensions to Mendelian Rules

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Not all genes follow the strict dominant/recessive pattern. Here are some important extensions:

Incomplete Dominance

In incomplete dominance, neither allele is fully dominant. The heterozygous phenotype is an intermediate blend of the two homozygous phenotypes.

• Example: A red flower (RR) crossed with a white flower (WW) produces pink flowers (RW).

Codominance

In codominance, both alleles are expressed equally and distinctly in the heterozygote. You don't get a blend; you see both traits.

• Example: Human ABO blood groups. Allele A and Allele B are codominant. A person with genotype AB has both A and B antigens on their red blood cells.

Multiple Alleles

Some genes have more than two possible alleles in a population. While an individual can only have two alleles, there might be several variations present across the population.

• Example: Human ABO blood groups again. There are three main alleles: $I^A$, $I^B$, and $i$. $I^A$ and $I^B$ are codominant to each other, and both are dominant over $i$.

Polygenic Inheritance

Many traits are influenced by multiple genes acting together, often with environmental factors. These traits usually show a continuous range of phenotypes, not distinct categories.

• Example: Human height, skin color, and intelligence are influenced by many genes.

Epistasis

Epistasis occurs when one gene's expression masks, modifies, or interferes with the expression of another gene at a different locus. The epistatic gene effectively "controls" or "silences" the hypostatic gene.

• Example: In Labrador retrievers, the "B/b" gene determines coat color (black dominant over chocolate), but a separate "E/e" gene determines whether any pigment is deposited in the fur at all. If a dog has 'ee' genotype, it will be yellow regardless of its 'B/b' genotype.

graph TD
    A["Mendelian Genetics Concepts"] --> B["Simple Dominance/Recessiveness"]
    A --> C["Extensions to Mendel"]

    C --> C1["Incomplete Dominance"]
    C1 -- "Heterozygote is blend" --> D["Example: Red + White = Pink Flower"]

    C --> C2["Codominance"]
    C2 -- "Both alleles expressed" --> E["Example: AB Blood Type"]

    C --> C3["Multiple Alleles"]
    C3 -- "More than 2 alleles in pop." --> F["Example: ABO Blood Groups"]

    C --> C4["Polygenic Inheritance"]
    C4 -- "Many genes + environment" --> G["Example: Human Height"]

    C --> C5["Epistasis"]
    C5 -- "One gene masks another" --> H["Example: Lab Coat Color (E/e gene masks B/b)"]

3. Worked Example

Let's look at a classic epistasis example: coat color in Labrador Retrievers.

Two main genes are involved:
* Gene B/b: Controls pigment color. 'B' (black) is dominant over 'b' (chocolate).
* Gene E/e: Controls pigment deposition. 'E' (pigment deposited) is dominant over 'e' (no pigment deposited). If a dog has 'ee', it's yellow, regardless of the B/b genotype.

Imagine you cross two Labrador retrievers that are both heterozygous for both genes: $BbEe \times BbEe$.

Let's predict the phenotypic ratios using a Punnett square for a dihybrid cross.
The gametes each parent can produce are BE, Be, bE, be.

Now, let's determine the phenotype for each genotype:

• Black Labs (B_E_): Needs at least one B and at least one E.

  • BBEE, BBEe, BbEE, BbEe (all the ones with B and E present)
  • Count: 9/16

• Chocolate Labs (bbE_): Needs two 'b' alleles and at least one 'E' allele.

  • bbEE, bbEe
  • Count: 3/16

• Yellow Labs (_ _ee): Needs two 'e' alleles, regardless of B/b.

  • BBee, Bbee, bbee
  • Count: 4/16

So, the phenotypic ratio is 9 Black : 3 Chocolate : 4 Yellow. This 9:3:4 ratio is characteristic of recessive epistasis, where the recessive homozygous genotype of one gene (ee) masks the expression of another gene.

4. Key Takeaways

• Mendelian inheritance involves discrete genes with dominant/recessive alleles determining traits.
• Incomplete dominance results in a blended heterozygous phenotype (e.g., pink flowers).
• Codominance shows both alleles fully expressed in the heterozygote (e.g., AB blood type).
• Multiple alleles mean a gene has more than two variants in the population, increasing phenotypic diversity.
• Polygenic inheritance describes traits controlled by many genes, often with continuous variation.
• Epistasis occurs when one gene's alleles mask or modify the expression of another gene.
• Punnett squares are vital tools for predicting offspring genotypes and phenotypes for all these inheritance patterns.

Common Mistakes to Avoid

Photo by KATRIN BOLOVTSOVA on Pexels

• Confusing incomplete dominance (blending) with codominance (both expressed).
• Assuming all traits follow simple dominant/recessive patterns.
• Forgetting to consider all possible alleles when dealing with multiple alleles.
• Not recognizing that epistasis changes the expected Mendelian dihybrid ratios (e.g., 9:3:3:1 becomes 9:3:4).

5. Now Try It

You're studying snapdragons. Red flowers (RR) crossed with white flowers (WW) produce pink flowers (RW). If you cross two pink snapdragons (RW x RW), what are the expected genotypic and phenotypic ratios of their offspring? Draw a Punnett square to show your work.

Success looks like: Accurately identifying the genotypic ratio (1:2:1) and phenotypic ratio (1 Red : 2 Pink : 1 White) based on the principles of incomplete dominance.