12.2 - Variation in Humans

Relation of Variation to Human Inheritance

• A human trait is controlled by a pair of alleles; an allele can be dominant or recessive.
• A dominant trait is observed when both dominant alleles are present or one dominant allele is paired with a recessive allele.
• A recessive trait is observed only when both recessive alleles are present.
• These genetic combinations cause variation in humans.

Dominant and Recessive Characteristics in Humans

Characteristic	Dominant trait	Recessive trait
Height	Tall	Short
Type of hair	Curly hair	Straight hair
Tendency to use hands	Right-handed	Left-handed
Presence of dimples	Presence	Absence
Ability to roll tongue	Can roll tongue	Cannot roll tongue
Type of earlobes	Free earlobes	Attached earlobes

Continuous and Discontinuous Variation in Humans

Height, Body Weight and Fingerprint Experiment

• Problem statement: Are the height, body weight and fingerprint of each pupil different?
• Aim: To study continuous and discontinuous variation in humans.
• Hypothesis: The height, body weight and fingerprint of each pupil are different.
• Materials: Graph paper and white paper.
• Apparatus: Height-measuring equipment, weighing machine and ink pad.
• Procedure:
  1. Measure and record each pupil’s height; plot number of pupils against height range.
  2. Weigh and record each pupil’s body weight; plot number of pupils against body-weight range.
  3. Press each pupil’s inked thumb on white paper; classify the pattern as loop, arch or whorl.
  4. Record the number of each thumbprint pattern and construct a bar chart.
• Discussion: Identify the graph shapes for height and body weight; infer the distributions of height, body weight and thumbprint patterns.
• Conclusion: State whether the hypothesis is accepted based on the results.

Phenylthiocarbamide Taste Experiment

• Aim: To study human tongue sensitivity towards phenylthiocarbamide (PTC) solution.
• Hypothesis: The PTC taste trait can be classified into specific groups.
• Materials: Drinking water and PTC paper.
• Procedure:
  1. Rinse the mouth with clean water.
  2. Place PTC paper on the tongue for a few seconds.
  3. Record the taste as bitter, salty, sour, sweet, no taste or another taste; remove the paper.
  4. Repeat with other pupils and construct a bar chart.
• Discussion: Determine the type of variation from the graph and whether the PTC taste trait is dominant or recessive.
• Conclusion: State whether the hypothesis is accepted based on the results.
• PTC is non-toxic when tasted in a small amount.