Monohybrid and Dihybrid Cross

Monohybrid and Dihybrid CrossProf Anju ChaudharyB.Ed , M.sc , Phd

Monohybrid cross

Monohybrid Cross 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 ProblemIn 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?

AnalysisThe 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.

AnalysisTo solve the sample problem, you'll 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 ProblemSet up a 2 by 2 Punnett square.

Steps to Solve the Sample ProblemWrite the alleles for parent 1 on the left side of the Punnett square.Each gamete will have one of the two alleles of the parent. In this particular cross, half of the gametes will have the dominant (S) allele, and half will have the recessive (s) allele. We will use blue and brown to keep track of the alleles of each parent.

Ss

Steps to Solve the Sample ProblemWrite the alleles from parent 2 above the Punnett square.For this heterozygous parent (Ss), half of the gametes will have the dominant (S) allele, and half will have the recessive (s) allele.

SSss

Steps to Solve the Sample ProblemFill the squares for parent 1. Fill each square with the allele from Parent 1 that lines up with the row.

Fill the squares for parent 2. Fill each square with the allele from Parent 2 that lines up with the column.

SSssSSssSSss

Steps to Solve the Sample ProblemInterpreting the results of a Punnett square Genotypes that resulted from this monohybrid cross (Ss x Ss)25% homozygous dominant 50% heterozygous25% homozygous recessive

SSssSSssSSss

Steps to Solve the Sample ProblemInterpreting the results of a Punnett squarePhenotypes that resulted from this monohybrid cross (Ss x Ss)75% Spherical25% Dented

SSssSSssSSss

Dihybrid cross

Dihybrid Crossa 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 characteristicsThe rules of meiosis, as they apply to the dihybrid, are codified in Mendel's First Law and Mendel's Second Law, which are also called the Law of Segregation and the Law of Independent Assortment, respectively

Example ProblemIn 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 Sample ProblemWrite down the cross in terms of the parental (P1) genotypes and phenotypes:

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

Steps to Solve the Sample ProblemDetermine the P1 gametes, place them in a Punnett Square and fill in the resulting genotypes:

Steps to Solve the Sample ProblemDetermine 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 .

Steps to Solve the Sample ProblemWrite down the cross between F1 progeny:

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

Steps to Solve the Sample ProblemDetermine the F1 gametes, place them in a Punnett Square and fill in the resulting genotypes:

Steps to Solve the Sample ProblemDetermine 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)

Steps to Solve the Sample Problem2/16 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

Steps to Solve the Sample ProblemDetermine the genotypic and phenotypic ratios for the F2 generation:Phenotypic ratios:9/16 will have white, disk-shaped fruit3/16 will have white, sphere-shaped fruit3/16 will have yellow, disk-shaped fruit1/16 will have yellow, sphere-shaped fruitThis is a 9:3:3:1 phenotypic ratio.

Thank You!