MONOHYBRID AND DIHYBRID CROSS

Monohybrid Cross 

• is a method of determining the inheritance pattern of a trait between two single organisms.

• a cross between parents who are true-breeding for a trait; i.e., both are homozygous for one allele of the gene, for example AA x aa, in which A is the dominant allele for a trait and a is the recessive allele for that same trait.

Sample Problem

In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds?

Analysis

• The figure above represents a monohybrid cross of F1-hybrid plants. 

• Both parent plants are heterozygous (Ss) for an allele that determines seed shape. 

• Presence of the dominant allele (S) in homozygous (SS) or heterozygous (Ss) plants results in spherical seeds. 

• Homozygous recessive (ss) plants have dented seeds.

• To solve the sample problem, youll need to set up a Punnett square.

• Punnett square - a diagram that is used to predict an outcome of a particular cross or breeding experiment

Steps to Solve the Sample Problem

• Set up a 2 by 2 Punnett square.

• Write the alleles for parent 1 on the left side of the Punnett square.

• Each gamete will have one of the two alleles of S the parent. In this particular cross, half of the gametes will have the dominant (S) allele, s and half will have the recessive (s) allele. We will use blue and brown to keep track of the alleles of each parent.

• Write the alleles for parent 1 on the left side of the Punnett square.

• Each gamete will have one of the two alleles of S the parent. In this particular cross, half of the gametes will have the dominant (S) allele, s and half will have the recessive (s) allele. We will use blue and brown to keep track of the alleles of each parent.

• Write the alleles from parent 2 above S s the Punnett square.

• For this heterozygous parent (Ss), half of the S gametes will have the dominant (S) allele, and half will have the recessive (s) allele.

• Fill the squares for parent 1.     S s 

• Fill each square with the allele from Parent 1 that lines up with the row.    SS S s

• Fill the squares for parent 2.     Ss ss

• Fill each square with the allele from Parent 2 that lines up with the column.

• Interpreting the results of a Punnett S s square 

• Genotypes that resulted from this monohybrid cross  (Ss x Ss) 

                                 25% homozygous dominant
                                 50% heterozygous 
                                 25% homozygous recessive

• Phenotypes that resulted from this monohybrid cross  (Ss x Ss) 

                                75% Spherical 
                                25% Dented

Dihybrid cross  

• is a cross between F1 offspring (first generation offspring) of two individuals that differ in two traits of particular interest.

• used to test for dominant and recessive genes in two separate characteristics

• The rules of meiosis, as they apply to the dihybrid, are codified in Mendels First Law and Mendels Second Law, which are also called the Law of Segregation and the Law of Independent Assortment, respectively

Example Problem

In summer squash, white fruit color (W) is dominant over yellow fruit color (w) and disk- shaped fruit (D) is dominant over sphere- shaped fruit (d).. If a squash plant true- breeding for white, disk-shaped fruit is crossed with a plant true-breeding for yellow, sphere-shaped fruit, what will the phenotypic and genotypic ratios be for: 

                     a. the F1 generation?
                     b. the F2 generation?

Steps to Solve the SampleProblem

• Write down the cross in terms of the parental (P1) genotypes and phenotypes: 

              WWDD (white, disk-shaped fruit) X wwdd (yellow, sphere- shaped fruit)

• Determine the P1 gametes, place them in a Punnett Square and fill in the resulting genotypes:   WWDD  X wwdd   

• Determine the genotypic and phenotypic ratios for the F1 generation: 

All F1 progeny will be heterozygous for both characters (WwDd) and will have white, disk- shaped fruit .

• Write down the cross between F1 progeny: 

                 WwDd (white, disk-shaped fruit) X WwDd (white, disk-shaped fruit)

• Determine the F1 gametes, place them in a Punnett Square and fill in the resulting genotypes:

• —Determine the genotypic and phenotypic ratios for the F2 generation:

—      Genotypic ratios:
 1/16 will be homozygous dominant for both traits (WWDD)
 2/16 will be homozygous dominant for color and heterozygous for shape (WWDd)
 2/16 will be heterozygous for color and homozygous dominant for shape (WwDD)
 1/16 will be homozygous dominant for color and homozygous recessive for shape (WWdd)

       4/16 will be heterozygous for both traits  (WwDd)

              2/16 will be heteozygous for color and homozygous recessive for shape (Wwdd)

       1/16 will be homozygous recessive for color and homozygous dominant for shape (wwDD)
  2/16 will be homozygous recessive for color and heterozygous for shape (wwDd)      1/16 will be homozygous recessive for both traits (wwdd)

—     This is a 1:2:2:1:4:2:1:2:1 genotypic ratio

• Determine the genotypic and phenotypic ratios for the F2 generation:

—     Phenotypic ratios:

9/16 will have white, disk-shaped fruit
3/16 will have white, sphere-shaped fruit
3/16 will have yellow, disk-shaped fruit
1/16 will have yellow, sphere-shaped fruit

—     This is a 9:3:3:1 phenotypic ratio