3. Extensions to Mendelian Inheritance

Overview of interacting Genes

3. Extensions to Mendelian Inheritance

Overview of interacting Genes: Videos & Practice Problems

Topic summary

Genetics often involves complex interactions beyond simple dominant and recessive traits. Key concepts include polygenic inheritance, where multiple genes influence a trait, leading to continuous variation, such as height. Pleiotropy describes a single gene affecting multiple traits, while epistasis examines how different genes interact to shape phenotypes. Penetrance measures how many individuals with a specific allele express the associated phenotype, and expressivity assesses the degree of expression. Understanding these interactions is crucial for grasping the complexities of genetic traits.

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Interacting Genes Overview

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Interacting Genes Overview Video Summary

Genetics often presents a complex picture rather than clear-cut dominant and recessive traits. Many characteristics, such as height, weight, and hair color, exist on a continuum, influenced by multiple genetic factors. This complexity is exemplified by the concept of polygenic inheritance, where a trait is controlled by multiple genes, leading to a range of phenotypic expressions. For instance, in a plant exhibiting purple and white colors, two genes (P1 and P2) determine the color outcome. The combination of dominant and recessive alleles from both parents results in varying shades of purple or white, depending on the number of dominant alleles present. The more dominant alleles, the darker the color, while the absence of dominant alleles results in a recessive phenotype, such as white.

Another important concept is pleiotropy, where a single gene influences multiple traits or has various effects on an organism's phenotype. The expression of a pleiotropic gene can change over time or in response to environmental factors, demonstrating that the same gene can have different impacts at different life stages or in different tissues.

Variations in dominance, such as incomplete dominance and codominance, further illustrate how dominant alleles can affect phenotypes. For example, in blood types, the presence of both A and B alleles results in a codominant expression, leading to blood type AB.

Epistasis describes the interaction between two or more genes and how these interactions can influence phenotypic outcomes. This area of genetics is intricate and requires careful study to understand the various ways genes can interact.

Additionally, the concepts of penetrance and expressivity are crucial in understanding genetic expression. Penetrance refers to the proportion of individuals with a specific genotype that actually express the associated phenotype, while expressivity measures the degree to which a phenotype is expressed among individuals with the same genotype. These concepts highlight that not all alleles are expressed uniformly, leading to variability in phenotypic outcomes.

Each of these genetic principles contributes to the complexity of inheritance and phenotype expression, emphasizing the need for a deeper understanding of how genes interact to produce diverse traits.

Problem

Which of the following terms describes an interaction between two genes?

Penetrance

Pleiotropy

Incomplete dominance

Epistasis

Problem

Polygenic traits are controlled through which of the following ways.

Multiple alleles for one gene

Multiple genes

Epistasis

Penetrance

Problem

Polygenic traits are usually continuous traits?

True

False

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Here’s what students ask on this topic:

What is polygenic inheritance and how does it affect traits like height?

Polygenic inheritance refers to a trait being controlled by multiple genes, each contributing to the phenotype. This results in continuous variation rather than discrete categories. For example, height is influenced by several genes, each adding a small amount to the overall height. The combined effect of these genes produces a range of heights in the population, rather than just 'tall' or 'short'. This complexity makes it difficult to predict the exact height of an individual based solely on their parents' heights.

What is pleiotropy and how does it influence an organism's phenotype?

Pleiotropy occurs when a single gene affects multiple traits in an organism. This means that one gene can have various effects on different parts of the body or at different stages of life. For instance, a gene might influence both skin pigmentation and bone density. The effects of pleiotropic genes can change over time due to factors like age, environment, or other genetic interactions, making the study of these genes complex but crucial for understanding multifaceted traits.

How does epistasis differ from polygenic inheritance?

Epistasis involves the interaction between two or more genes to influence a single trait, whereas polygenic inheritance involves multiple genes independently contributing to a trait. In epistasis, one gene can mask or modify the effect of another gene, leading to unique phenotypes. For example, in coat color of certain animals, one gene might determine pigment production while another gene controls pigment deposition, and their interaction results in the final coat color.

What is the difference between penetrance and expressivity in genetics?

Penetrance refers to the proportion of individuals with a specific genotype who actually express the associated phenotype. For example, if 80 out of 100 people with a particular allele show the trait, the penetrance is 80%. Expressivity, on the other hand, measures the extent or degree to which a trait is expressed among individuals with the same genotype. Some individuals may show a strong expression of the trait, while others may show a milder form, even though they have the same genetic makeup.

What are some examples of variations in dominance, such as incomplete dominance and codominance?

Variations in dominance include incomplete dominance and codominance. In incomplete dominance, the heterozygous phenotype is a blend of the two homozygous phenotypes. For example, crossing red and white snapdragons results in pink offspring. In codominance, both alleles in the heterozygote are fully expressed. A classic example is the ABO blood group system, where individuals with both A and B alleles express both A and B antigens on their red blood cells, resulting in AB blood type.

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