110 Extensions to Mendel

Summary

This video explores extensions to Mendelian genetics, focusing on incomplete dominance, codominance, multiple alleles, epistasis, and incomplete penetrance. Examples such as snapdragon flower color, human blood types, and Labrador retriever coat colors illustrate these concepts. It explains how traits may not follow simple dominant-recessive patterns, with some genes interacting or expressing in unexpected ways. These extensions broaden our understanding of genetic inheritance, demonstrating the complexity of gene interactions and phenotypic expression beyond Mendel's initial findings.

Takeaways

• 😀 Mendel’s work on genetics was largely ignored for 40 years before being rediscovered in the early 20th century, confirming his conclusions and leading to the development of modern genetics.
• 😀 Incomplete dominance occurs when neither allele is fully dominant, as seen in snapdragons, where a red and white cross produces an intermediate pink phenotype.
• 😀 The molecular explanation of incomplete dominance in snapdragons is that heterozygotes produce less pigment than homozygotes, resulting in the pink color.
• 😀 Codominance is when both alleles are fully expressed, as seen in human blood types, where the IA and IB alleles express both A and B antigens in type AB blood.
• 😀 Blood types A, B, AB, and O are determined by three alleles of the I gene: IA, IB, and i, with IA and IB being dominant over i.
• 😀 Epistasis occurs when one gene masks the expression of another, as seen in Labrador retrievers where the E gene can mask the B gene, leading to golden labs regardless of the B allele.
• 😀 The B gene in Labrador retrievers determines the color of the coat, with the dominant allele producing black pigment and the recessive allele producing brown pigment.
• 😀 Golden labs result from a homozygous recessive ee genotype that prevents pigment deposition in the fur, regardless of the B gene's alleles.
• 😀 Mendel’s dihybrid cross would predict a 9:3:3:1 ratio in a cross of black labs, but due to epistasis, the actual ratio is 9:3:4, with golden labs accounting for the extra 4/16.
• 😀 Incomplete penetrance occurs when a genotype does not fully express its expected phenotype, such as in polydactyly, where the dominant allele may result in partial or no extra digits in some individuals.

Q & A

Why was Mendel's work largely ignored after it was published?

Mendel's work was ignored for about 40 years because scientists at the time did not recognize its significance. It wasn't until the early 20th century that other scientists independently rediscovered his work and confirmed his conclusions, giving rise to the field of genetics.

What is incomplete dominance, and how does it differ from Mendel's original predictions?

Incomplete dominance occurs when neither allele is completely dominant, and the heterozygote shows an intermediate phenotype. In Mendel's original work, he expected dominant traits to mask recessive traits, but incomplete dominance results in a third phenotype that is a blend of the two homozygous traits.

How does the molecular explanation of incomplete dominance work in snapdragons?

In snapdragons, the red allele encodes a functional enzyme that produces red pigment, while the white allele encodes a non-functional enzyme. In heterozygotes, the reduced enzyme activity produces half the pigment, resulting in a pink flower that is an intermediate between red and white.

Can you explain codominance using human blood types as an example?

In codominance, both alleles are fully expressed in the heterozygote. For human blood types, the IA allele and the IB allele are codominant. Individuals with both alleles (IAIB) will express both A and B antigens on their red blood cells, resulting in type AB blood.

What is the significance of multiple alleles in genetics, and how does it apply to human blood types?

Multiple alleles refer to a situation where more than two allele options exist for a gene. In the case of human blood types, the I gene has three alleles: IA, IB, and i. These alleles combine in different ways to determine blood type (A, B, AB, or O).

How does epistasis work, and can you provide an example from Labrador retrievers?

Epistasis occurs when one gene can mask or modify the expression of another gene. In Labrador retrievers, coat color is influenced by two genes: the B gene (determining black or brown pigment) and the E gene (determining pigment deposition). The homozygous recessive ee genotype results in golden labs, regardless of the B gene.

What is the expected phenotypic ratio in a Mendelian dihybrid cross for coat color in Labradors, and how does epistasis alter this?

In a typical Mendelian dihybrid cross between two heterozygous black labs (BbEe), the expected phenotypic ratio is 9:3:3:1. However, due to epistasis, the actual ratio is 9 black labs, 3 chocolate labs, and 4 golden labs, because the ee genotype masks the effects of the B gene.

What is incomplete penetrance, and how does it apply to polydactyly?

Incomplete penetrance occurs when a genotype does not fully express its expected phenotype. In the case of polydactyly, a dominant allele typically causes extra fingers, but not everyone with the allele shows full expression. Some may only have a small nub or partial finger, or no extra digit at all.

Why is it important to understand extensions to Mendel's rules, such as incomplete dominance, codominance, and epistasis?

Understanding extensions to Mendel's rules helps us grasp the complexity of genetic inheritance in real-world scenarios. These extensions, such as incomplete dominance, codominance, and epistasis, demonstrate that inheritance patterns can be more nuanced than Mendel's original predictions, allowing for a broader understanding of genetic traits.

What is the key difference between incomplete dominance and codominance?

In incomplete dominance, the heterozygote displays an intermediate phenotype, a blend of the two homozygous traits. In contrast, codominance results in both alleles being fully expressed simultaneously, where both phenotypes appear in the heterozygote.