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EXERCISE 11 – MENDELIAN GENETICS PROBLEMS
These problems are divided into subdivisions composed of problems that require application of a
specific genetic principle. These problems are intended to complement the lecture portion of this
course; specifically, the material described in lecture is to be applied to solve these problems. The
answers are provided in Appendix A. You are strongly advised against `consulting this appendix
before you have made a serious attempt to answer a problem.
A. Monohybrid Crosses
- In Coleus , some plants have shallowly crenated edges and others have deeply incised
leaves. A cross is made between homozygous deep and shallow individuals. The
shallow trait is dominant.
a. Using S and s to symbolize the genes for this trait, give the phenotypic and
genotypic ratios for the F 1
generation.
b. If self pollination is allowed, what is the phenotypic ratio for the F 2
generation?
- a. In a pea plant that breeds true for tall, what possible gametes can be produced? Use
the symbol D for tall, d for dwarf.
b. In a pea plant that breeds true for dwarf, what possible gametes will be produced?
c. What will be the genotype of F 1
offspring from a cross between these two types?
d. Assuming that the allele for tall is dominant, what will be the phenotype of F 1
offspring from a cross between these two types?
e. What will be the probable distribution of traits in the F 2
generation? (Illustrate with a
Punnett square).
- The ability to taste a bitter chemical, phenylthiocarbamide (PTC), is due to a dominant
gene. Use T and t to symbolize the two alleles of this gene.
a. What is the genotype of a nontaster? What are the possible genotypes of a
taster?
b. Could a person with two tasters as parents be a non-taster? How?
- A woman heterozygous for polydactyly (extra fingers and toes), a dominant trait, is
married to a normal man. What is the probability of producing an offspring that has
extra fingers or toes?
- Parents who do not have Tay Sachs disease produce a child who has this terrible
affliction. What are the chances that each child born of this union will be affected?
- In human beings, ability to curl the tongue into a U-shaped trough is a heritable trait.
"Curlers" always have at least one curler parent, but "noncurlers" may occur in
families where one or both parents are curlers. Using C and c to symbolize this trait,
what is the genotype of a noncurler?
- Albinism, the total lack of pigment, is due to a recessive gene. A man and woman plan
to marry and wish to know the probability of their having any albino children. What are
the probabilities if:
a. both are normally pigmented, but each has one albino parent.
b. the man is an albino, the girl is normal, but her father is an albino.
c. the man is an albino and the girl's family includes no albinos for at least three
generations.
- In a certain plant, both purple x purple and purple x blue yield purple and blue colored
progeny, but blue x blue gives rise only to blue.
a. What does this tell you about the genotypes of blue- and purple-flowered plants?
b. Which gene is dominant?
- Two short-haired female cats are mated to the same long-haired male. Several litters are
produced. Female No. 1 produced eight short-haired and six long-haired kittens.
Female No. 2 produced 24 short-haired ones and no long-haired. From these
observations, what deductions can be made concerning hair-length inheritance in these
animals? Assuming the allelic pair S and s, give the likely genotypes of the two female
cats and the male.
- In human beings, a downward pointed frontal hairline ("widow's peak") is a heritable
trait. A person with a widow's peak always has at least one parent who also has this trait,
whereas persons with a straight frontal hairline may occur in families in which one or
even both parents have widow's peak. When both parents have a straight frontal hairline,
all children also have a straight hairline. Using W and w to symbolize genes for this
trait, what is the genotype of an individual without widow's peak?
- Rh negative children (those not producing rhesus antigen D) may be born to either Rh
positive or Rh negative parents, but Rh positive children always have at least one Rh
positive parent. Which phenotype is due to a dominant gene?
B. Dihybrid Crosses
- In the fruit fly Drosophila melanogaster , vestigial wings and hairy body are produced by
two recessive genes located on different chromosomes. The normal alleles, long wings
and hairless body, are dominant. Give the genotype and phenotype of F 1
progeny
obtained from a cross between a vestigial-winged, hairy male and a normal, homozygous
female. If the F 1
from this cross are permitted to mate randomly among themselves,
what phenotypic ratio would be expected in the F 2
generation?
- In peas, a gene for tall plants ( T ) is dominant over its allele for short plants ( t ). The gene
for smooth peas ( S ) is dominant over its allele for wrinkled peas ( s ). The genes are not
linked. Calculate both phenotypic and genotypic ratios for the results of each of the
following crosses:
a. TtSs x TtSs b. Ttss x ttss c. ttSs x Ttss d. TtsSS x ss
- In a particular species of flower, tall is dominant to short, and orange petals are dominant
to the recessive white color. Use T and t to symbolize the alleles for height, and F and f
- If pale colored horses are crossed with chestnut-colored horses to produce "palomino",
an intermediate coat color:
a. What type of expression is suggested?
b. A number of matings between palominos produced 19 pale, 21 chestnut, and 44
palominos. Does this evidence support or contradict your answer to (a)? Why?
Codominance
1.For each of the following pairs of parental genotypes, calculate the phenotypic
ratios for the F 1
generation.
a. I
A
I
A
x ii b. I
A
I
A
x I
A
I
B
c. I
A
I
A
x I
B
i
d. I
A
I
A
x I
A
i e. I
A
i x I
A
i f. I
A
i x I
A
I
B
g. I
A
i x ii
- If Mr. and Mrs. Fecundity, both having blood type B, have 12 children, 3/4 of whom are
type B and 1/4 of whom are O, what are the genotypes of the parents?
- A family of six includes four children, each of whom has a different blood type: A, B,
AB and O. What are the genotypes of parents for this trait?
- A man with blood type B, with one parent of blood type O, marries a woman with blood
type AB. What will be the theoretical percentage of their children with blood type B?
- Mrs. Smith and Mrs. Doe were roommates at Harris Hospital and both had daughters at
about the same time. After Mrs. Smith took Susie home, she became convinced that the
babies had been switched. Blood tests were performed with the following results:
Mr. and Mrs. Smith were both type AB;
Mr. and Mrs. Doe were both type A;
Susie Smith was type A and Debbie Doe was type O.
Had a switch occurred?
- Mortimer has type B blood. His wife Murgatroyd is unsure of her blood type. If their
first child, Magnifica, is type B, their second offspring, Maximum, is AB and the twins,
Maud and Lyn, are A, can you determine the genotypes of Mort and Murg?
- In a well-publicized paternity case, the following facts were unearthed; the mother, a
strikingly beautiful, twice-convicted axe murderess, is blood type A, her child, Lizzie, is
type O, and the alleged father, a mild-mannered felon, is type B. Could he be the father?
Explain. Is there any chance that little Lizzie, the "Bad Seed", will grow up to be a
missionary lady?
D. Lethal Alleles
- Cystic fibrosis is caused by a recessive lethal gene and can be detected by an excess
concentration of chloride in sweat. If a sweat test reveals a man to be heterozygous and
his wife to be homozygous normal, what are the chances that their children will have the
disease? Could any of their grandchildren have the disease?
- In humans, sickle-cell anemia is caused by a recessive lethal allele Hb
s ; individuals who
are Hb
a Hb
s have sickle-cell trait, but are healthy.
a. What is the probability of two heterozygous individuals giving birth to a child with
sickle-cell trait?
b. What is the probability of two heterozygous individuals giving birth to a child with
sickle-cell anemia?
c. If a normal Hb
a Hb
a individual receives a blood transfusion from a Hb
a Hb
s individual
(heterozygous for sickle cell) what are the chances that the Hb
a Hb
a man and his
Hb
a Hb
a wife will have Hb
a Hb
s children?
- Albinism in corn plants is caused by a recessive lethal gene that results in death before
maturity. What will the adult phenotypic ratio be for the F 1
generation of heterozygous
parents?
- Huntington's chorea is a dominant lethal in humans. The disease does not appear until
later in life, so that afflicted individuals may already have produced children. What are
the F 1
genotypic and phenotypic ratios of parents who are homozygous dominant and
heterozygous?
E. Sex-linked Genes
- Red-green color blindness is inherited as a sex-linked recessive. If a color-blind woman
marries a man who has normal vision, what would be the expected phenotypes of their
children with reference to this character?
- Suppose that gene b is sex-linked, recessive, and embryonic lethal. A man marries a
woman who is heterozygous for this gene. If this couple had many normal children,
what would be the predicted sex ratio of these children?
- A man and his wife both have normal color vision, but a daughter has red-green color
blindness, a sex-linked recessive trait. The man sues his wife for divorce on grounds of
infidelity. Can genetics provide evidence supporting his case?
- In the mouse, the dominant sex-linked gene B results in a short, crooked tail. Its
recessive allele b produces a normal tail. If a normal-tailed female is mated with a bent-
tailed male, what phenotypic ratio should occur in the F 1
generation?
- In cats, a gene for coat color is sex-linked. Cats homozygous for allele A have yellow
coats; those homozygous for allele a have black coats; and heterozygotes have tortoise-
shell coats. What type(s) of offspring would result from a mating of a black male and a
tortoise-shell female? Is it possible to obtain a tortoise-shell male?
- On the X chromosome of Drosophila there may occur a recessive gene l, which is lethal
in the larval stage. A heterozygous female is crossed to a normal male; what F 1
adult
sex phenotypic ratio results?
- The diagram shows three generations of the pedigree of deafness in a family. Black circles
and squares indicate deaf persons. and indicate a female; and indicate a male.
Is the condition of deafness in this pedigree inherited as (1) a dominant autosomal
characteristic, (2) a recessive autosomal characteristic, (3) a sex-linked dominant
characteristic, or (4) a sex-linked recessive characteristic?
P 0
F 1
F 2
- In the following pedigree charts,
= afflicted female
= afflicted male
= normal female
= normal male
Indicate the pattern of inheritance.
A.
P 0
F 1
F 2
B.
P 0
F 1
F 2
C.
P 0
F 1
F 2
D.
P 0
F 1
F 2
The genotypes of the parents are: Male vvhh, Female VVHH.
One hundred percent of the F 1
will be VvHh (normal long wing, hairless body).
The F 2
phenotypic ratio is 9 normal long wing, hairless body; 3 vestigial wing, hairless body; 3
normal long wing, hairy body; 1 vestigial wing, hairy body. This can be obtained by multiplying
the two phenotypic ratios times one another.
The phenotypic ratio for wings is 3 normal long : 1 vestigial. The phenotypic ratio for hair is 3
hairless : 1 hairy. Since the two traits segregate independently -
- The two ratios can again be multiplied, since the genes (S+T) are not linked.
Genotypic Ratio Phenotypic Ratio
a) 1TT : 2Tt : 1tt 3 tall : 1 short
1SS : 2Ss : 1ss 3 smooth : 1 wrinkled
1TTSS : 2TtSS : 1ttSS 9 tall, smooth : 3 short, smooth: 3 tall,
2TTSs : 4TtSs : 2 ttSs short, smooth wrinkled : 1 short, wrinkled
1TTss : 2Ttss : 1 ttss
b) 1Tt : 1tt 1 tall : 1 short
1ss all wrinkled
1Ttss : 1ttss 1 tall, wrinkled : 1 short, wrinkled
c) 1tt : 1Tt 1 tall : 1 short
1Ss : 1ss 1 smooth : 1 wrinkled
1ttSs : 1TtSs 1 tall, smooth : 1 short,
1ttss : 1Ttss smooth : 1 tall, wrinkled:1 short, wrinkled
d) 1Tt All tall
1Ss All smooth
1TtSs All tall, smooth
- The parental genotypes are: TTff x TtFf
The F 1
genotypic ratio is: 1 : 1 : 1 : 1
The F 1
phenotypic ratio is: 1 : 1
- Four. For example: CcBb x ccbb ---Cc Bb
Cc bb
cc Bb
cc bb
- a) The parental genotypes are: ww FF x WW ff
b) All the F 1
's are WwFf (white belted with fused hoofs).
c) The genotypic ratio is: 1:2:1:2:4:2:1:2:
The phenotypic ratio is: 9:3:3:
- 1 green short: 1 green long: 1 striped short: 1 striped long
C. Modifications of Complete Dominance
Incomplete Dominance
- The genotypes of the black and white may be represented as BB and bb, respectively. A "blue"
chicken has the genotype Bb. Crossing two blues would produce 1/4 black, 1/2 blue, and 1/
white. Crossing a blue and a black would produce 1/2 blue and 1/2 black.
- a. If the parental genotypes are RR and rr, the offspring would be Rr.
b. 1 RR : 2 Rr : 1 rr
c. 1 RR : 1 Rr
- The genotypes may be symbolized as follows: AA = straight hair, aa = curly hair, Aa = wavy
hair.
a. 50%
b. 25%
c. 25%
- a. incomplete dominance
b. We suspect palominos to be heterozygous. If they are crossed, a 1:2:1 ratio should result. It
does, which supports our answer.
Codominance
- a. 100% A
b. 50% A, 50% AB
c. 50% AB, 50% A
d. 100% A
e. 75% A, 25% O
f. 50% A, 25% AB, 25% B
g. 50% A, 50% O
2. I
B
i x I
B
i
- Parents must be I
A
i and I
B
i
- 50%. The parental genotypes are: I
B
i x I
A
I
B
- Mr. and Mrs. Smith must both have genotypes I
A
I
B
. Mr. and Mrs. Smith could not have given
birth to Debbie Doe - Type O. A switch had not occurred.
- Mortimer is I
B
i. Murgatroyd is either I
A
i or I
A
I
B
The two smallest F 2
phenotypic classes are recombinants. Since map distance is defined as
#Recombinants/total X 100, 111/600 x 100 = 18%.
- a) A B C b) not with this information alone c) furthest away from A ( i.e., A B C D)
- The genes are linked; otherwise the phenotypic ratio would be 1:1:1:1. Dpy and Unc are the
smallest, and hence recombinant, classes. Therefore, 200/1000 x 100 = 20% or 20 map units.
- The genes are not linked.
G. Pedigree Analyses
- This condition is not autosomal dominant or sex-linked dominant because F 1
#7 is afflicted and
her parents are normal.
This condition is not sex-linked recessive because a cross between parents 1 and 2 would give all
normal heterozygous daughters and all afflicted sons. (Note: F 1 #2 is not deaf).
Deafness appears to be passed on in accordance with expectations for inheritance of a recessive
autosomal allele.
- a. dominant
b. recessive, probably sex-linked (male afflicted, female carrier)
c. recessive
d. dominant
