Objective: Illustrate how color blindness is inherited using a Punnett Square. Materials: ballpen, notebook, pencil Procedure: 1. Read the given problem. Color blindness is a recessive, sex-linked disorder in humans. A color blind man has a child with a woman who is a carrier of the disorder. KEY:
= normal female,
= normal female (carrier),
= red-green color blind female (homozygous),
= normal male,
= red-green color blind male 2. Using a Punnett square, illustrate the probability of having children who will have normal vision and children who will have color blindness. 3. Refer to the sample Punnett Square given in Activity 2: The Great Predictor. Critical Thinking Questions: 1. What is the genotype of the male children? 2. What is the genotype of the female children? 3. What is the percentage of color blind children? 4. What is the percentage that a son will be color blind? 5. What is the percentage of having a color blind daughter
The Correct Answer and Explanation is:
Punnett Square for Color Blindness Inheritance
Given:
- The trait for red-green color blindness is X-linked recessive.
- Male genotype: XᵇY (color blind)
- Female genotype: XᴮXᵇ (carrier)
KEY:
- Xᴮ = X chromosome with normal vision allele
- Xᵇ = X chromosome with color blindness allele
- Y = Y chromosome (no color vision gene)
Step 1: Set up the Punnett Square
| Xᴮ (mother) | Xᵇ (mother) | |
|---|---|---|
| Xᵇ (father) | XᴮXᵇ | XᵇXᵇ |
| Y (father) | XᴮY | XᵇY |
Step 2: Analyze the Offspring Genotypes
From the square:
- XᴮXᵇ – Female, carrier (normal vision)
- XᵇXᵇ – Female, color blind
- XᴮY – Male, normal vision
- XᵇY – Male, color blind
Step 3: Answer Critical Thinking Questions
- What is the genotype of the male children?
- XᴮY (normal) and XᵇY (color blind)
- What is the genotype of the female children?
- XᴮXᵇ (carrier) and XᵇXᵇ (color blind)
- What is the percentage of color blind children?
- XᵇXᵇ and XᵇY = 2 out of 4 = 50 percent
- What is the percentage that a son will be color blind?
- 1 out of 2 sons = 50 percent
- What is the percentage of having a color blind daughter?
- 1 out of 2 daughters = 50 percent
Explanation
Color blindness is inherited as a sex-linked recessive trait, meaning it is carried on the X chromosome and only shows up when no normal vision gene is present to mask it. Males have one X and one Y chromosome, while females have two X chromosomes. In this case, the father is color blind (XᵇY), and the mother is a carrier (XᴮXᵇ).
When creating a Punnett Square, we combine one sex chromosome from the father with one from the mother. This results in four possible combinations for their children:
- XᴮXᵇ: a daughter who is a carrier, just like the mother
- XᵇXᵇ: a daughter who is color blind
- XᴮY: a son with normal vision
- XᵇY: a son who is color blind
This shows that there is a 50 percent chance their children will have color blindness. More specifically, sons have a 50 percent chance of being color blind, and daughters also have a 50 percent chance, but only if they inherit the Xᵇ from both parents.
The Punnett Square is a powerful visual tool for predicting genetic outcomes. It simplifies the complex process of genetic inheritance and helps in understanding how traits, especially sex-linked disorders like color blindness, are passed from parents to offspring.
