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

PUNNETT SQUARE

A Punnett square is a chart which shows/predicts all possible gene combinations in a cross of parents (whose genes are known). Punnett squares are named for an English geneticist, Reginald Punnett. He discovered some basic principles of genetics, including sex linkage and sex determination. He worked with the feather color traits of chickens in order to quickly separate male and female chickens. 

Genetic Problem using Punnett Squares - Example and Steps

Sample Problem

*In pea plants (which Gregor Mendel studied), tall pea plants are dominant over short pea plants. Using Punnett Squares, you can predict the genotypes and phenotypes of the offspring of a cross between a homozygous (purebred) tall pea plant and a homozygous (purebred) short pea plant. 

Step 1.

Designate letters which will represent the genes/traits. Capital letters represent dominant traits, and lowercase letters represent recessive traits. 

             T = tall t = short 

Step 2.

Write down the genotypes (genes) of each parent. These are often given to you or are possible to determine.

genotype = the genes of an organism; for one specific trait we use two letters to represent the genotype.

              TT X tt

                      (tall) (short) - both homozygous (same) or purebred 

Step 3.

List the genes that each parent can contribute.

                                                           Parent 1 Parent 2

Step 4.

Draw a Punnett square - 4 small squares in the shape of a window. Write the possible gene(s) of one parent across the top and the gene(s) of the other parent along the side of the Punnett square.

Step 5.

Fill in each box of the Punnett square by transferring the letter above and in front of each box into each appropriate box. As a general rule, the capital letter goes first and a lowercase letter follows.

Step 6.

List the possible genotypes and phenotypes of the offspring for this cross.

The letters inside the boxes indicate probable genotypes (genetic makeup) of offspring resulting from the cross of these particular parents. There are 4 boxes, and the genotypic results can be written either as fractions or percents. In this case, all 4 boxes out of the 4 are showing the Tt genotype. Therefore, each of the offspring has a 4/4 or 100% chance of showing the Tt genotype.

We have also written the phenotype (physical appearance) in each box under the genotype. Remember, T = tall and t = short (see step #1 above). Since a capital letter indicates a dominant gene, T (tall) is dominant over t (short). Therefore, each of the offspring has a 4/4 or 100% chance of being tall. 

What are the possible genotype(s) of a tall plant? 

What are the possible genotype(s) of a short plant? 

What would be the phenotype of TT? 

What would be the phenotype of tt? 

Why is the phenotype of Tt tall and not medium/average?

Practice

Using the steps of the Punnett Squares  do some Punnett squares on your own. Grab your paper and a pencil. For each problem, be sure to do the work for each step and don't forget the genotypes and phenotypes of the offspring. We'll get you started with the first problem.

Problem #1 - Use a Punnett square to predict the genotypic and phenotypic outcome (offspring) of a cross between two heterozygous/hybrid tall (Tt) pea plants. 

Step 1 - Designate letters which will represent the genes/traits. 

         T = tall t = short (and they do not always have to be the letter T!) 

Step 2 - Write down the genotypes of each parent. It is helpful if you write the phenotype in parenthesis under the genotypes. 

           Tt X Tt
        (tall) (tall) 

Step 3 - List the genes that each parent can contribute.

Step 4 - Write the possible gene(s) of one parent across the top and the gene(s) of the other parent along the side of the Punnett square.

*Now you're on your own to finish Steps 4 through 6.

Problem #2 - In pea plants, yellow peas are dominant over green peas.
Use a Punnett square to predict the phenotypic and genotypic outcome (offspring) of a cross between a plant heterozygous/hybrid for yellow (Yy) peas and a plant homozygous/purebred for green (yy) peas. 

Problem #3 - In pea plants, yellow peas are dominant over green peas.
Use a Punnett square to predict the phenotypic and genotypic outcome (offspring) of a cross between two plants heterozygous for yellow peas.