New Video Posted: Understanding Mendelian Genetics: Dominance, Codominance, and Incomplete Dominance

In this video, I explain some of the key terms in Mendelian genetics:

• dominant
• recessive
• codominance
• incomplete (partial) dominance

 

Blog Bonus: Free information sheet summarising the video and defining the key terms - download.

 

Introduction to Mendelian Genetics

Mendelian genetics is named after Gregor Mendel, who studied inheritance patterns in pea plants and published his findings in 1866. His experiments with pea plants, including traits like seed shape and flower colour, laid the foundation for understanding how traits are inherited. By cross-breeding plants with different traits, Mendel observed how traits reappear in subsequent generations, leading to the formulation of key genetic principles.

Dominant and Recessive Traits

Mendel discovered that certain traits, like round seeds, are dominant, while others, like wrinkled seeds, are recessive. Through cross-breeding experiments, he noted that in the first generation (F1) of true-bred plants that produced round or wrinkled seeds, the offspring all exhibited the dominant trait. However, when these F1 plants were self-pollinated, the second generation (F2) showed a 3:1 ratio of dominant to recessive traits (see Figure 1).

Using modern terminology, this can be explained with uppercase and lowercase letters representing dominant and recessive alleles, respectively. For instance, the round seed trait is represented by "R" and the wrinkled seed by "r." The F1 generation consists of heterozygous plants (Rr), which exhibit the dominant trait. The F2 generation reveals a combination of homozygous dominant (RR), heterozygous (Rr), and homozygous recessive (rr) plants, demonstrating Mendel's observed ratios (see Figure 1).

Figure 1: Crossing round and wrinkled pea plants

Codominance and Incomplete Dominance

Moving beyond simple dominance, we come to codominance and incomplete (partial) dominance.

In codominance, neither allele masks the other; both traits are fully expressed (see Figure 2). For example, in a hypothetical scenario with cows, crossing a blue cow (BB) and a yellow cow (bb) would result in offspring with both blue and yellow spots (Bb).

Figure 2: Codominance

In incomplete dominance (sometimes called partial dominance), the traits blend rather than mask one another (see Figure 3). Using the same example of cows, crossing a blue cow and a yellow cow would produce green offspring (Bb), illustrating a blend of the two parent traits.

Figure 3: Incomplete Dominance - also called partial dominance

Key Takeaways

1. Dominant and Recessive Traits: Dominant alleles mask recessive ones in heterozygous pairings.
2. Codominance: Both alleles are fully expressed in the phenotype.
3. Incomplete (Partial) Dominance: The traits blend, creating an intermediate phenotype.

Additional Resources

For further assistance, a Bioscience Glossary with over 2000 terms, chemical structures, and supporting videos is available. This glossary can help clarify additional terms and concepts in biosciences.

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Blog Bonus: Free information sheet summarising the video and defining the key terms - download.