Explains quantitative traits as characteristics that vary continuously across a population, contrasting them with qualitative traits and describing their polygenic, multifactorial basis and the role of environment. Covers the bell-shaped normal distribution, standard deviation, and examples such as height, weight, skin colour and heart disease risk. Includes a detailed look at skin colour and three mnemonics for recalling features and examples.
Topic 27 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 10 sections, about 643 words (5 min of reading), with 7 flashcards.
In this note
Quantitative traits.
What Are Quantitative Traits?
How They Differ From Qualitative Traits
The Genetic Basis
Role of the Environment
Pattern of Inheritance
Phenotypic Distribution in the Population
Examples of Quantitative Traits
Detailed Look at Human Skin Color
MNEMONICS
What Are Quantitative Traits?
Quantitative traits refer to characteristics that vary continuously across a population rather than appearing in distinct categories.
These traits form a spectrum of phenotypes—for example, height or skin color, where individuals differ by degrees, not by sharply separated types.
How They Differ From Qualitative Traits
Qualitative traits follow simple patterns of inheritance and typically have clearly defined categories—like flower color being red or white, controlled by one gene.
In contrast, quantitative traits do not follow simple Mendelian rules and are instead influenced by multiple genes.
An example: Height is a quantitative trait with a wide range of normal variation, but conditions like dwarfism or gigantism represent qualitative extremes of that trait.
The Genetic Basis
Quantitative traits are polygenic, meaning they are controlled by several genes working together.
The combined effect of many genes, each contributing a small amount, leads to the continuous variation seen in the population.
Because of this, they are also described as multifactorial traits.
Role of the Environment
These traits are influenced not only by genes but also by environmental factors, such as nutrition, climate, and lifestyle.
This means that even with the same genetic makeup, two individuals might show different phenotypes based on their surroundings.
The non-genetic component of quantitative traits includes environmental conditions and random variations during development.
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