Download Chapter 25: Mendelian Genetics STUDY GUIDE Name

HONORS BIOLOGY – GENETICS
MONOHYBRID CROSSES
To solve genetics problems, REMEMBER:
1) Make an allele key: upper case letter = dominant, lower case = recessive
2) Determine the parent genotypes.
3) Separate the parent alleles and write one outside each box of a Punnett square.
4) Inside the boxes of a Punnett square write the new allele combinations.
5) Determine offspring genotype and phenotype ratios.
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1. Parent genotypes: Rr x Rr
R = ________ r = _________
Parent gametes: ___________________
Offspring: Genotype ratio ___________________
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Phenotype ratio ___________________
2. In flies, long wings are dominant over short. If two heterozygous
flies mate, what genotype and phenotype ratios are expected?
Allele Key: Long = _______ short = _______
Parent genotypes: ___________ X _____________
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Parent gametes: ____________________
Offspring: Genotype ratio ___________________
Phenotype ratio ___________________
3. In pea seeds, green (G) is dominant over yellow (g). Cross a
homozygous green plant with a heterozygous green plant.
Allele key: G = ____________ g = _______________
Parent genotypes: __________ X _____________
Parent gametes: _________________________
Offspring: Genotype ratio _____________
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Phenotype ratio _____________
4. Cross an albino rat with a heterozygous normal rat. Normal is dominant.
Allele key: long= _____ short = _______
Parent genotypes: ______ X ________
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Parent gametes: ____________________________
Offspring: Genotype ratio ___________________
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Phenotype ratio ___________________
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DIHYBRID CROSSES
Remember: each trait has two alleles in the genotype. Gametes need one letter from each pair
1) In mice, gray fur is dominant to white, furry is dominant to hairless. Cross two
heterozygous gray, heterozygous furry mice. Find phenotype ratio.
Allele Key: gray = ______ white = _______
furry = ______ hairless = ________
Parent genotypes: ________________________
Parent gametes: __________________________
(Consider ALL possible gamete combinations!)
Offspring Phenotypes:
Number of each
gray and furry ___________________
Gray and hairless _________________
White and furry __________________
White and hairless ________________
2) About 70% of Americans perceive a bitter taste from the chemical phenylthiocarbamide (PTC). The
ability to taste this chemical results from a dominant allele (T) and not being able to taste PTC is the
result of having two recessive alleles (t). Albinism is also a single locus trait with normal pigment being
dominant (A) and the lack of pigment being recessive (a). A normally pigmented woman who cannot
taste PTC has a father who is an albino taster. She marries a homozygous, normally pigmented man
who is a taster but who has a mother that does not taste PTC. What are the phenotypes of the possible
children (choose all that apply)?
Allele Key: taster = ______ nontaster = _______
Normal pigment = ______ albino = ________
Parent genotypes: ________________________
Parent gametes: __________________________
(Consider ALL possible gamete combinations!)
# Offspring Phenotypes:
Taster, color _______________
Taster, albino ______________
Non-taster, color____________
Non-taster, albino ___________
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TEST CROSS Determine an unknown genotype by crossing with recessive phenotype
Gray is dominant over white. Find the genotype for gray mouse when:
Crossed with homozygous gray
Crossed with heterozygous gray
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If all F1 show dominant phenotype, unknown parent was _________________
If any F1 show recessive phenotype, unknown parent was _____________________
INCOMPLETE DOMINANCE
Neither allele is completely dominant  “blending” of phenotypes in heterozygote
In the Japanese Four-O-Clock flower, the gene controlling flower color has alleles that are neither
dominant nor recessive. Plants that have two red alleles (RR) have red flowers. Plants with two
white alleles (WW) are white. Plants with one red allele and one white allele (RW) are pink
1. Cross a red flowered Japanese Four-O-Clock with a pink plant.
Alleles: red = _____ white = _____ pink = ____
Parent genotypes _______ and _________
F1 genotype ratio: _____________________
F1 phenotype ratio: _____________________
2. Cross two pink flowering plants.
Parent genotypes _______ and _________
F1 genotypes: _______________________
F1 phenotype ratio: _____________________
In some cats, the gene for tail length shows incomplete dominance.
Cats can have no tails (N), long tails (L), or short tails (NL).
Cross a short tail cat and a cat with no tail.
Parent genotypes _______ and _________
F1 genotypes: _____________________
F1 phenotype ratio: _____________________
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CODOMINANCE
Alleles are both dominant. In the heterozygote, both traits are expressed fully.
In some horses, the allele for red hair (CR) and the allele for white hair ( CW) are codominant. Hybrids
have a mixture of red hairs and white hairs and the horses are called roan.
1. Cross a red horse with a white horse. What are the genotype and phenotype ratios of the offspring?
Alleles: red = ______ white = _____ roan = _____
Parent genotypes _______ and _________
F1 genotypes: _____________________
F1 phenotype ratio: _____________________
2. Cross a roan with another roan. What is the likelihood of producing a white offspring from this
match? A red? A roan?
Parent genotypes _______ and _________
F1 genotypes: _____________________
F1 phenotype ratio: _____________________
Percent chance a foal (baby horse) will be white? _____
Percent chance for red? ______ Percent chance for roan? _____
3. In Erminette chickens, the gene for feather color has two codominant
alleles – one for black feathers (FB) and one for white feathers (FW).
Heterozygous chickens (FBFW) have BOTH black and white feathers,
resulting in a distinctive speckled feather
3. Cross a speckled hen with a white rooster.
Parent genotypes ________ and _______
F 1 genotypes: ___________________________
F 1 phenotype ratio: _______________________
If I breed these parents each year and get 15 chicks each year, after three years, how many speckled
chicks will I have?
Percent of speckled each year _______ x _____ # years = ___________ speckled in my farmyard.
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MULTIPLE ALLELES
For some genes, more than two different alleles exist in a population. Any organism still gets only two
(one from each parent), but there are more possible combinations than for simple dominant/recessive
genes. The best known example of multiple alleles involves the gene for blood type in humans. This
gene has three different alleles and is located on chromosome 9.
Allele IA (or A) - codes for type A blood
Allele IB (or B) - codes for type B blood. Alleles A and B are codominant.
Allele i (or O) - codes for type O blood. Allele O is recessive
1. The father of a child has type AB blood. The mother has type A.
Which blood types can their children have?
Parent genotypes ________ and _______
F 1 genotypes: ___________________________
Possible blood types in F 1 _______________________
2. A woman heterozygous for type O blood married a man
with type A blood. What will be the possible genotypes
and phenotypes of their children?
Parent genotypes ________ and _______
F 1 genotypes: ___________________________
F 1 blood types: _______________________
3. The mother has type A blood. Her husband has type B blood.
Their child has type O blood. The father claims the child
can’t be his. Is he right? Explain.
Parent genotypes ________ and _______
F 1 genotypes: ___________________________
Possible blood types in F 1 _______________________
4. Mrs. Smith has blood type AB and Mr. Smith has blood type O.
Mrs. Brown has blood type A and Mr. Brown has blood type B.
A baby is born with type O blood. Which couple does the
baby belong to and how do you know?
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SEX-LINKED INHERITANCE
Genes on the X and Y chromosome show a different pattern than autosomal genes. The X chromosome
is large and carries many more genes than the Y. Recessive disorders are more common in the male,
because he has only one X, and if it carries a recessive allele, the recessive trait will appear.
1. A boy, whose parents and grandparents had normal vision, is color-blind.
What are the genotypes for his mother and his maternal grandparents.
Use XB for the dominant normal condition and Xb for the recessive,
color-blind phenotype.
The boy’s genotype ___________________
Mother’s genotype _____________
Grandparents genotype _____________ and ______________
2. A woman with red-green color-blindness has a mother with normal
vision. Knowing that color-blindness is a sex-linked recessive gene,
can you determine what her father's phenotype is?
Woman’s genotype _____________________
Her mother’s genotype ______________________
Her father’s genotype _______________________
3. Imagine that you are a genetic counselor, and a couple that is planning to have children comes to
you for advice. Diane’s brother has Duchesne muscular dystrophy, a degenerative disease of the
muscles which is carried on the X chromosome. There is no history of this disease in Craig’s
family. What is the probability that Diane is a carrier for Muscular Dystrophy? What is the
probability that their child will have it?
Probability that Diane is a carrier? _________
Probability that any child will have MD? ______
4. Clouded leopards are a medium sized, endangered species of cat,
living in the very wet cloud forests of Central America. Assume that
the normal spots (XN, pictured here) are a dominant, sex-linked
trait and that dark spots are the recessive counterpart. Suppose a
male with dark spots mates with a female with normal spots. She
has four cubs and, conveniently, two are male and two female.
One male and one female cub have normal spots and one of each
has dark spots. What is the genotype of the mother?
Normal spotted female genotype _________________
Dark spotted male genotype ___________________
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PEDIGREES
I
II
III
1. Which shape indicates a male? ____________
Female? _______________
2. Is the trait shown dominant or recessive? ________________ How do you know?
___________________________________________________________________
3. Is this trait sex-linked or autosomal? ____________________ How do you know?
__________________________________________________________________
4. What is the genotype for individuals A: _______________ B ______________
C _________________ and D ___________________
5. Identify any other genotypes that you can. Write the alleles on the figure.
6. If individual D marries a person who is heterozygote for this condition, what is the probability
that any offspring will have the trait? _______________
7. If individual C marries a person who does not have this trait, what is the probability that any of
their offspring will have the trait? _______________
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The bison herd on Konza Prairie has begun to show a genetic defect. Some of the males have a condition
known as "rabbit hock" in which the knee of the back leg is malformed slightly. We do not yet know the
genes controlling this trait but for the sake of our question, we shall assume it is a sex-linked gene and
that it is recessive. Now, suppose that the herd bull (the dominant one which does most of the breeding)
who is normal (XN) mates with a cow that is a carrier for
rabbit hock. What are his chances of producing a
normal son?
Blood Type
Genotypes
A
B
AB
O
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Thomas Hunt Morgan –
Drosophila
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