What phenotypic ratio do you expect from the following crosses with reference to colour blindness in human beings? (IFS 2019, 15 Marks)

Introduction

Color blindness is a genetic disorder that affects the ability to perceive certain colors. It is caused by a mutation in the genes responsible for producing photopigments in the cones of the retina. The most common form of color blindness is red-green color blindness, which is inherited in an X-linked recessive manner. 

Color Blindness and Its Genetic Basis

• Genetic Nature: Color blindness is an X-linked recessive trait. This means the gene responsible for color blindness is located on the X chromosome.
• Inheritance Pattern: Since men (XY) have only one X chromosome, a single copy of the mutated gene will cause color blindness. Women (XX) have two X chromosomes, so they need two copies of the mutated gene to be colorblind. Otherwise, they are carriers.

Common Crosses and Expected Phenotypic Ratios

1. Cross between a Colorblind Male (XᴄY) and a Normal Vision Female (XX)

• Genotype of Parents: Male: XᴄY (colorblind), Female: XX (normal)
• Punnett Square:
  
  • Male (XᴄY) contributes Xᴄ or Y
  • Female (XX) contributes X or X
• Offspring Genotypes:
  
  • Sons (XY): All sons inherit the X from their mother and Y from their father, so they will be normal (XY).
  • Daughters (XᴄX): All daughters inherit Xᴄ from the father and X from the mother, making them carriers (XᴄX) but not colorblind.
• Expected Phenotypic Ratio:
  
  • Normal Sons: 50%
  • Carrier Daughters (Normal vision, but carry the gene): 50%

2. Cross between a Carrier Female (XᴄX) and a Normal Vision Male (XY)

• Genotype of Parents: Female: XᴄX (carrier), Male: XY (normal)
• Punnett Square:
  
  • Carrier female contributes Xᴄ or X
  • Normal male contributes X or Y
• Offspring Genotypes:
  
  • Sons (XᴄY): 50% chance of inheriting Xᴄ from the mother and Y from the father, making them colorblind.
  • Sons (XY): 50% chance of inheriting X from the mother and Y from the father, making them normal.
  • Daughters (XᴄX): 50% chance of inheriting Xᴄ from the mother and X from the father, making them carriers (normal vision).
  • Daughters (XX): 50% chance of inheriting X from both parents, making them normal.
• Expected Phenotypic Ratio:
  
  • Normal Sons: 25%
  • Colorblind Sons: 25%
  • Carrier Daughters: 25%
  • Normal Daughters: 25%

3. Cross between a Carrier Female (XᴄX) and a Colorblind Male (XᴄY)

• Genotype of Parents: Female: XᴄX (carrier), Male: XᴄY (colorblind)
• Punnett Square:
  
  • Carrier female contributes Xᴄ or X
  • Colorblind male contributes Xᴄ or Y
• Offspring Genotypes:
  
  • Sons (XᴄY): 50% chance of inheriting Xᴄ from the mother and Y from the father, making them colorblind.
  • Sons (XY): 50% chance of inheriting X from the mother and Y from the father, making them normal.
  • Daughters (XᴄXᴄ): 25% chance of inheriting Xᴄ from both parents, making them colorblind.
  • Daughters (XᴄX): 25% chance of inheriting Xᴄ from the father and X from the mother, making them carriers.
• Expected Phenotypic Ratio:
  
  • Colorblind Sons: 25%
  • Normal Sons: 25%
  • Colorblind Daughters: 25%
  • Carrier Daughters: 25%

Conclusion

The phenotypic ratio of color blindness in human beings can be predicted based on the principles of Mendelian genetics. In crosses involving individuals who are carriers of the color blindness gene, the expected phenotypic ratio is 1:1 for males and females.