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Genetics Problem Set 3A
Genetics Problem Set 3C
Last Updated 3/15/06

Check out the Mendelian Genetics Site at The Biology Project, developed at The University of Arizona.

Genetics Problem Set 3B -- Incomplete Dominance, Codominance, Lethal Alleles, etc.

True/False Type Questions
1) Which of the following statements is true?
A) Expressivity is a measure of the dominance of a trait. B) When a genotype fails to produce its expected phenotype the allele causing the expected phenotype is nonpenetrant. C) The probability of any of two or more mutually exclusive events occurring is the sum of the separate probabilities of each event occurring. D) Alleles are alternate forms of a gene at a gene locus. E) Lethal genes can produce 2:1 phenotypic ratios in a monohybrid cross or all one phenotype. F) Most complementary loci are genes which code for enzymes in different metabolic pathways. G) Penetrance is a measure of the dominance of an allele. H) The probability of two independent events occurring together is the sum of separate probabilities. I) Incompletely dominant alleles produce the traits of both alleles when heterozygous. J) Codominance and incomplete dominance are the same. K) ABO blood groups are an example of multiple allelism. L) Lethal genes are carried in the heterozygous state.

Genetics Problems (Crosses)
1) In snapdragons red flower color (R) is incompletely dominant to white flower color (W) and the heterozygote has pink flowers. Green leaves (G) are dominant to purple leaves (g). A pink flowered, purple leafed snapdragon is crossed with a pink flowered snapdragon heterozygous for leaf color. If the two loci are not linked, what proportion of the progeny should be pink flowered, purple leaved plants?

2) In tomatoes the gene (R) for red color is dominant to the gene (W) for white color fruit while the gene for dark green leaves (D) is incompletely dominant to the gene for light green leaves (L). The fruit and leaf color loci are not linked. A red tomato with medium green leaves, named T2001, was testcrossed to a white tomato and out of a progeny of 100 plants two were white. T2001 is crossed to a dihybrid plant.

A) What fraction of the progeny are expected to have white fruits and leaves which are not dark green?
B) What fraction of the progeny are expected to have red fruits and/or medium green leaves?

3) Some ferrets have been known to sing (especially after imbibing alcoholic beverages). The recessive allele g causes ferrets to sing while G causes ferrets to snort. Two incompletely dominant alleles for dark brown (Br) and for blond (bl) fur color are also found in this species. The two loci segregate independently of each other. Singing light brown ferrets mate with snorting dark brown ferrets which had mothers that could sing. What is the expected phenotypic ratio produced by this cross? Write your answer in a blue book and circle the answer.

4) A gene yel- in Drosophila is a recessive lethal gene which gives flies a yellow body color in the heterozygous condition with yel+. At another unlinked locus the allele pnt- for pointy head is recessive to the dominant pnt+ and at a third unlinked locus the allele L for long bristles is incompletely dominant to the allele S for short bristles. Predict the phenotypic ratios for the progeny of the following crosses.

A) yellow body, short bristles X yellow body, medium bristles
B) yellow body, heterozygous round head X brown body heterozygous round head
C) pnt+pnt- yel-yel+ X pnt-pnt- yel+yel-
D) pnt-pnt- LL X pnt+pnt- LS
E) yellow body, medium bristles, heterozygous round head X yellow body, medium bristles, heterozygous round head
F) short bristle, round headed flies from a maternal line with pointy heads X long bristle, pointy headed flies.

5) Dr. Lucy Lepidoptera Is interested in the traits of butterfly species X. vulgaris. One strain of X. vulgaris has gray wings and gray bodies. Another strain is homozygous for the allele blk+ which is needed to color wings black. The allele min for small body size is incompletely dominant to the allele max for large body size, and the allele rip is a recessive lethal allele to +. What is the expected phenotypic ratio of a cross between the X. vulgaris genotypes blk+blk- minmax +rip X blk-blk- minmax +rip if the alleles segregate independently.

Genetics Problems (Mode of Inheritance)
1) Dr. Daisy Inflorescence is crossing varieties of show flowers inbred for many years. One variety of the show flowers has blue flowers with large leaves while the other has red flowers with small leaves. The F1 between these two varieties are all plants with both red and blue petals and medium size leaves. The F2 seedlings have the phenotypes in the table below. The alleles segregate independently.

Phenotype [Number of plants]
Red flowers and small leaves [59], Red flowers and medium leaves [120], Red flowers and large leaves [65], Red and blue flowers and small leaves [110], Red and blue flowers and medium leaves [264], Red and blue flowers and large leaves [123], Blue flowers and small leaves [67], Blue flowers and medium leaves [135], Blue flowers and large leaves [57]

Propose a mode of inheritance (hypothesis) for the results shown above?

===================== ANSWERS =======================

True/False Type Questions
1) The true statements are below.
B) When a genotype fails to produce its expected phenotype the allele causing the expected phenotype is nonpenetrant. C) The probability of any of two or more mutually exclusive events occurring is the sum of the separate probabilities of each event occurring. D) Alleles are alternate forms of a gene at a gene locus. E) Lethal genes can produce 2:1 phenotypic ratios in a monohybrid cross or all one phenotype. K) ABO blood groups are an example of multiple allelism. L) Lethal genes are carried in the heterozygous state.

Genetics Problems (Crosses)
1) Cross: RW gg X RW Gg, Pink/Purple phenotype = RW gg genotype = 1/2 RW X 1/2 gg = 1/4

2) T2001 genotype = RW DL, Cross: RW DL X RW DL
A) White fruits and leaves which are not dark green
White fruits & light green OR medium green leaves = WW LL & WW DL genotypes = (1/4 WW X 1/4 LL) + (1/4 WW X 1/2 DL) = 1/16 + 1/8 = 3/16
B) Red fruits and/or medium green leaves?
Everything but plants with white fruits AND light leaves OR white fruits AND dark leaves = 1 - [(1/4 WW X 1/4 LL) + (1/4 WW X 1/4 DD)] = 1 - [1/16 + 1/16] = 7/8

3) gg Singing G_ Snorting
BrBr Dark brown Brbr Light brown brbr Blond
Cross: Singing light brown X Snorting dark brown (with singing moms)
gg Brbr X G_ BrBr (with singing moms)
gg Brbr X Gg BrBr (with singing moms)

Singing dark brown = gg BrBr = 1/2 X 1/2 = 1/4
Singing light brown = gg Brbr = 1/2 X 1/2 = 1/4
Singing blond = gg BrBr = none
Snorting dark brown = G_ BrBr = 1/2 X 1/2 = 1/4
Snorting light brown = G_ Brbr = 1/2 X 1/2 = 1/4
Snorting blond = G_ brbr = none

1/4 Singing dark brown: 1/4 Singing light brown: 1/4 Snorting dark brown: 1/4 Snorting light brown

4) A) 1/6 brown medium: 1/6 brown short: 1/3 yellow medium: 1/3 yellow short
B) 3/8 brown round: 1/8 brown pointy: 3/8 yellow round: 1/8 yellow pointy
C) 1/6 brown round: 1/6 brown pointy: 1/3 yellow round: 1/3 yellow pointy
D) 1/4 round medium: 1/4 round large: 1/4 pointy medium: 1/4 pointy large
E) 1/16 brown long round: 1/48 brown long pointy: 1/8 brown medium round: 1/24 brown medium pointy: 1/16 brown short round: 1/48 brown short pointy: 1/8 yellow long round: 1/24 yellow long pointy: 1/4 yellow medium round: 1/12 yellow medium pointy: 1/8 yellow short round: 1/24 yellow short pointy
F) 1/2 medium round: 1/2 medium pointy

5) Note: Since the genotypes +rip and ++ do not differ in phenotype, and are not linked, you may disregard this locus (you will produce less progeny but the ratios will be the same).
blk- blk- = Graywings and bodies blk+_ = Black wings
minmin = Small body minmax = Medium body maxmax = Large body
Cross: blk+ blk- minmax X blk- blk- minmax

1/2 black: 1/2 gray AND 1/4 small: 1/2 medium: 1/4 large

1/8 black small: 1/4 black medium: 1/8 black large: 1/8 gray small: 1/4 gray medium: 1/8 gray large

Genetics Problems (Mode of Inheritance)
1) The simplest explanation is that the petal color and leaf size loci are not linked. The red petal allele is dominant to the blue petal allele and the small leaf allele is incompletely dominant to the large leaf allele.