What are some reasons why an experimental monohybrid cross d

What are some reasons why an experimental monohybrid cross does not result in a 3:1 ratio? Also, what are some reasons an experimental dihybrid cross does not result in a 9:3:3:1 ratio?

Solution

Monohybrid Cross: A monohybrid cross is a mating between two individuals with different alleles at one genetic locus of interest. The characters being studied in a monohybrid cross are governed by two or multiple alleles for a single locus.
Assumptions:
1. The gene in question shows a dominant/recessive relationship
2. Cross heterozygotes (Aa x Aa)
If these two assumptions are meet, then what is the expected monohybrid phenotypic ratio 3 dominant : 1 recessive

Exceptions to the 3 : 1 ratio:

Incomplete Dominance:

Gene: Flower color

Alleles: R = red (pigment produced)

r = white (no pigment produced)

Cross heterozygotes:
            Rr x   Rr
Progeny:
            1/4 RR = red
            2/4 Rr = pink
            1/4 rr = white

Test Cross of the Pink Phenotype

            Rr x rr
Progeny:
            ½ Rr = pink
            ½ rr = white
Ratio: 1 : 1

Codominance

Gene: Hair color  

Alleles: r^o = red hair
r¹ = white hair
3. Cross heterozygotes:
            r⁰r¹ x r⁰r¹
             (Roan)
    1/4 r⁰r⁰ + 2/4 r⁰r¹ + 1/4 r¹r¹
        Red         Roan        White
        1       :       2       :       1
4. Roan = Rust Color
5. Results from a mixture of red hairs and white hairs.

2. Inviability of Progeny :

Lethal Genes: 3 : 0

Example: Chlorophyll production in corn (with photograph of albino corn)
Gene: chlorophyll production
Alleles:
            C = chlorophyll produced
            c = chlorophyll not produced (Albino Corn)
Cross heterozygotes:
                Cc x Cc
                 (green)
Progeny:
        1/4 CC + 2/4 Cc + 1/4 cc
           green       green      white (die)
Ratio: 3 green : 0 white

Dihybrid Cross: Dihybrid cross is a cross between two different lines (varieties, strains) that differ in two observed traits. In the Mendelian sense, between the alleles of both these loci there is a relationship of complete dominance - recessive.

Assumptions:

Exceptions to the 9 : 3 : 3 : 1 dihybrid phenotypic ratio:

1. Intermediate Dominance

Inviability of Progeny - Lethal genes

Genes are Linked

Epistasis

Epistasis: Example: Deaf-Mutism in Humans

Gene 1: Cochlea development
                D = normal development
                d = non-functional development
Gene 2: Auditory nerve development
                E = normal development
                e = non-development
Cross two heterozygotes: DdEe x DdEe
What is the phenotype of each heterozygote? Why? (Normal hearing)

DdEe x DdEe
What gametes will each parent of this cross produce?
(1/4 DE + 1/4 De + 1/4 dE + 1/4 de)

Set up a Punnett square and determine the results.
                    DdEe x DdEe
Progeny:
                9 D_E_         Normal
                3 D_ee          Deaf
                3 ddE_          Deaf
                1 ddee           Deaf
Ratio:  9 : 7 (instead of a 9:3:3:1)