Interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of monohybrid crosses and dihybrid crosses that involve dominance, codominance, multiple alleles and sex linkage

Sourav PanNovember 1, 2024

Answered step-by-step

Genetic diagrams, including Punnett squares, are useful tools for predicting the outcomes of genetic crosses. Below, I will explain how to interpret and construct these diagrams for monohybrid crosses and dihybrid crosses, including cases involving dominance, codominance, multiple alleles, and sex linkage.

Monohybrid Crosses

A monohybrid cross examines the inheritance of a single trait controlled by one gene with two alleles (one dominant and one recessive).

Example of Dominance

Trait: Flower color (Purple is dominant, White is recessive)

• Alleles:
  • Dominant allele (P) = Purple
  • Recessive allele (p) = White

Parental Generation (P):

• One parent (homozygous dominant): PP (Purple)
• Other parent (homozygous recessive): pp (White)

Punnett Square Construction:

1. Set up a 2×2 grid.
2. Write one parent’s alleles across the top and the other parent’s alleles along the side.

Results:

• F1 Generation: All offspring (100%) will be Pp (Purple phenotype).

Predicting F2 Generation

If you cross two F1 individuals (Pp x Pp):

Results:

• F2 Generation:
  • 1 PP (Purple)
  • 2 Pp (Purple)
  • 1 pp (White)

Phenotypic Ratio: 3 Purple : 1 White

Dihybrid Crosses

A dihybrid cross examines the inheritance of two traits, each controlled by different genes.

Example of Independent Assortment

Traits: Seed shape (Round is dominant, Wrinkled is recessive) and Seed color (Yellow is dominant, Green is recessive)

• Alleles:
  • Round (R) = Dominant
  • Wrinkled (r) = Recessive
  • Yellow (Y) = Dominant
  • Green (y) = Recessive

Parental Generation (P):

• One parent (homozygous for both traits): RRYY (Round Yellow)
• Other parent (homozygous for both traits): rryy (Wrinkled Green)

Punnett Square Construction:

1. Determine the gametes for each parent:
   • RRYY can produce: RY
   • rryy can produce: ry

Results:

• F1 Generation: All offspring are RrYy (Round Yellow).

Predicting F2 Generation

If you cross two F1 individuals (RrYy x RrYy), you create a 4×4 Punnett square to track both traits:

Results:

• F2 Generation:
  • 9 Round Yellow (RRYY, RRYy, RrYY)
  • 3 Round Green (RRyy, RrYy)
  • 3 Wrinkled Yellow (rrYY, rrYy)
  • 1 Wrinkled Green (rryy)

Phenotypic Ratio: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green

Codominance Example

In codominance, both alleles in a heterozygote are fully expressed.

Example: ABO Blood Groups

• Alleles:
  • A (IA) = A antigen
  • B (IB) = B antigen
  • O (i) = no antigens

If you cross an IAIB (AB blood type) individual with an ii (O blood type):

Results:

• F1 Generation: 50% A (IAi), 50% B (IBi)

Sex Linkage Example

Sex-linked traits are associated with genes found on the sex chromosomes (typically the X chromosome).

Example: Color blindness in humans is a recessive trait linked to the X chromosome.

• Alleles:
  • Normal vision: X^N
  • Color blindness: X^n

Crossing a Color-blind Male (X^nY) with a Normal Female (X^NX^N):

Results:

• F1 Generation:
  • 100% Normal vision females (X^NX^n)
  • 100% Normal vision males (X^NY)

If you cross X^NX^n females with X^nY males, you can predict:

Results:

• 50% Normal vision females (X^NX^n)
• 50% Color-blind females (X^nX^n)
• 50% Normal vision males (X^NY)
• 50% Color-blind males (X^nY)

Summary

• Monohybrid Crosses analyze one trait with dominant and recessive alleles, using a 2×2 Punnett square.
• Dihybrid Crosses analyze two traits, using a 4×4 Punnett square to account for independent assortment.
• Codominance shows both alleles in the phenotype, while sex linkage involves traits associated with the sex chromosomes.

By constructing and interpreting these Punnett squares, you can predict the genetic outcomes of crosses involving various inheritance patterns.