3. Extensions to Mendelian Inheritance

Blue flower color is produced in a species of morning glories when dominant alleles are present at two gene loci, A and B. (Plants with the genotype have blue flowers.) Purple flowers result when a dominant allele is present at only one of the two gene loci, A or B. (Plants with the genotypes and are purple.) Flowers are red when the plant is homozygous recessive for each gene (i.e., aabb).

If an F₁ plant is backcrossed to one of the pure-breeding parental plants, what is the expected ratio of phenotypes among progeny? Why is the phenotype ratio the same regardless of which parental strain is selected for the backcross?

The crosses shown are performed between morning glories whose flower color is determined as described in Problem 24. Use the segregation data to determine the genotype of each parental plant.

Two pure-breeding strains of summer squash producing yellow fruit, Y₁ and Y₂, are each crossed to a pure-breeding strain of summer squash producing green fruit, G₁, and to one another. The following results are obtained:

Examine the results of each cross and predict how many genes are responsible for fruit-color determination in summer squash. Justify your answer. 

Two pure-breeding strains of summer squash producing yellow fruit, Y₁ and Y₂, are each crossed to a pure-breeding strain of summer squash producing green fruit, G₁, and to one another. The following results are obtained:

Using clearly defined symbols of your choice, give the genotypes of parental, F₁, and F₂ plants in each cross. 

Two pure-breeding strains of summer squash producing yellow fruit, Y₁ and Y₂, are each crossed to a pure-breeding strain of summer squash producing green fruit, G₁, and to one another. The following results are obtained:

If the F₁ of Crosses I and II are mated, predict the phenotype ratio of the progeny. 

Human ABO blood type is determined by three alleles, two of which (I^A and I^B) produce gene products that modify the H antigen produced by protein activity of an independently assorting H gene. A rare abnormality known as the 'Bombay phenotype' is the result of epistatic interaction between the gene for the ABO blood group and the H gene. Individuals with the Bombay phenotype appear to have blood type O based on the inability of both anti-A antibody and anti-B antibody to detect an antigen. The apparent blood type O in Bombay phenotype is due to the absence of H antigen as a result of homozygous recessive mutations of the H gene. Individuals with the Bombay phenotype have the hh genotype. Use the information above to make predictions about the outcome of the cross shown below.

IᴬIᴮHh×IᴬIᴮHh

Epistatic gene interaction results in a modification of the F₂ dihybrid ratio.

Give two examples of modified F₂ ratios produced by epistatic gene interactions and describe how gene interaction results in the ratios.

Epistatic gene interaction results in a modification of the F₂ dihybrid ratio.

What genetic principle is the basis of this expected F₂ ratio?

Epistatic gene interaction results in a modification of the F₂ dihybrid ratio.

What is the expected F₂ ratio?

Feather color in parakeets is produced by the blending of pigments from two biosynthetic pathways shown below. Four independently assorting genes (A, B, C, and D) produce enzymes that catalyze separate steps of the pathways. For the questions below, use an uppercase letter to indicate a dominant allele producing full enzymatic activity and a lowercase letter to indicate a recessive allele producing no functional enzyme. Feather colors produced by mixing pigments are green (yellow + blue) and purple (red + blue). Red, yellow, and blue feathers result from the production of one colored pigment, and white results from the absence of pigment production.

If a pure-breeding blue strain of parakeet (aa BB CC DD) is crossed to one that is pure-breeding purple, predict the genotype(s) and phenotype(s) of the F₁. Show your work