12.2 Variation in Humans
Variation in humans is closely linked to inheritance. Human traits are controlled by pairs of alleles that can be dominant or recessive. The interaction between these alleles results in observable variations among individuals, ranging from distinct physical features like dimples and earlobe types to measurable physical differences like height and body weight.
Learning Objectives
- Relate human variation to the principles of inheritance involving dominant and recessive alleles.
- Identify examples of dominant and recessive traits for specific human characteristics.
- Conduct experiments to investigate continuous variation (height and body weight) and discontinuous variation (fingerprint patterns) in humans.
- Study the distribution of human tongue sensitivity towards phenylthiocarbamide (PTC) paper as a form of variation.
- State the meanings of inheritance and characteristic, and provide examples of traits.
Human Inheritance of Dominant and Recessive Traits
Compare dominant and recessive physical features in humans side by side.
Type of earlobes
Dominant: Free earlobes. Recessive: Attached earlobes.
Ability to roll tongue
Dominant: Can roll tongue. Recessive: Cannot roll tongue.
Type of hair
Dominant: Curly hair. Recessive: Straight hair.
Presence of dimples
Dominant: Presence of dimples. Recessive: Absence of dimples.
Short Explanation
How are human traits inherited?
In humans, characteristics are determined by pairs of alleles. A dominant trait is displayed whenever at least one dominant allele is present (homozygous dominant or heterozygous). In contrast, a recessive trait is only expressed when an individual inherits two recessive alleles (homozygous recessive).
Continuous variation
Traits such as height and body weight have a continuous spectrum of values without distinct categories, forming a bell-shaped distribution graph.
Discontinuous variation
Traits such as fingerprint patterns (loop, arch, whorl) and PTC taste sensitivity fall into distinct, separate categories and can be displayed using discrete bar charts.
Inheritance
The transmission of genetic information and characteristics from parents to their offspring.
Characteristic and trait
A characteristic is a heritable feature of an organism. A trait is a specific variant of a characteristic.
Try to Answer First
Answer in your mind, then press “Check Answer”.
Which earlobe type is considered a dominant trait in humans?
Under what genetic condition is a recessive trait expressed in an individual?
What type of graph or chart is constructed to represent discontinuous traits like thumbprint patterns or PTC taste sensitivity?
Common Mistakes
- Confusing a characteristic with a trait: Students often write “curly hair” as a characteristic, when “type of hair” is the characteristic and “curly” is the specific trait.
- Assuming that dominant traits are always the most common traits in a classroom, whereas allele frequencies depend on population genetics, not merely dominance.
- Plotting continuous data (height and body weight) as separate, unconnected discrete bars instead of a continuous distribution histogram/curve.
Concept Misunderstandings
A person with one dominant and one recessive allele expresses an intermediate or mixed trait.
For simple Mendelian inheritance shown in this topic, the presence of just one dominant allele fully masks the recessive allele, expressing the dominant trait completely.
Recessive traits disappear permanently if an offspring receives a dominant allele.
The recessive allele remains unaltered in the genotype and can be passed on to the next generation.
Fingerprint patterns change with age like body weight.
Fingerprint patterns are discontinuous traits determined genetically and remain distinct throughout life, unlike body weight which is continuous and influenced by environment.
Summary
- Human traits are inherited through alleles that are either dominant or recessive.
- Dominant traits appear in homozygous dominant or heterozygous states; recessive traits appear only in homozygous recessive states.
- Common dominant traits include tall height, curly hair, right-handedness, presence of dimples, ability to roll tongue, and free earlobes.
- Common recessive traits include short height, straight hair, left-handedness, absence of dimples, inability to roll tongue, and attached earlobes.
- Height and body weight represent continuous variation, whereas thumbprint patterns and PTC tasting represent discontinuous variation.
Short Activity
Objective: identify dominant and recessive traits, continuous and discontinuous variation, and the meaning of inheritance.
A. Objective Quiz
Based on Table 12.1 in the textbook, which of the following is a recessive trait in humans?
What type of graph is obtained when plotting the distribution of continuous characteristics such as height and body weight?
B. Fill in the Blanks
A recessive trait is only shown when both alleles are present.
Phenylthiocarbamide (PTC) is used to study human tongue sensitivity, where individuals are grouped using a chart based on their taste perception.
C. Matching / Drag and Drop
Drag each human characteristic to its correct dominant trait. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Human Characteristic to Its Dominant Trait
Write the correct dominant trait next to each human characteristic.
| Human Characteristic | Dominant Trait |
|---|---|
| 1. Type of earlobes | |
| 2. Type of hair | |
| 3. Presence of dimples | |
| 4. Tendency to use hands |
Answer choices: Free earlobes • Curly hair • Presence of dimples • Right-handed
