6F - Practice Problems
For all problems, create a Punnett Square and give the phenotypic AND genotypic ratios.
Solve all problems in your IAN
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genotype - different combination of alleles (letters)
phenotype - physical appearance of the combination
PART A - Monohybrid Cross (Complete Dominance)
1. In pea plants, the allele for tallness is dominant (T) and shortness (t) is recessive. A homozygous dominant (TT)
is crossed with a homozygous recessive (tt).
Genotypic ratio: _________
Phenotypic ratio: ________
2. A homozygous cream-colored mouse (dd) is crossed with a heterozygous (Dd) dark mouse.
a. What is the probability that these parents will produce a cream-colored offspring?
b. What is the probability of a dark mouse?
3. In tomatoes, red fruit is dominant over yellow fruit. If a homozygous red fruit plant is crossed with a yellow
fruit plant:
a. What are the expected genotypic and phenotypic ratios of the F1 generation?
b. If two plants from the F1 generation were crossed what would be the resultant genotypic and phenotypic
ratios for this cross?
4. A widow’s peak hairline is dominant to straight hairline. Cross a heterozygous widow’s peak hairline person to
a straight hairline person.
5. In guinea pigs, short hair is dominant over long hair. A female guinea pig with short hair but whose father had
long hair is mated with a male whose parents were both short haired, but who has long hair. Using a Punnett
square, predict the genotypic and phenotypic ratios of their offspring.
6. You are working as a genetics specialist in a hospital. A couple comes to you saying they want to have a baby,
but they want the baby to have freckles and neither of the potential parents has this trait. You take out your
trusty, always useful, Biology textbook and find out that freckles are inherited as a dominant trait. What news
can you now give the anxious couple? Support this wise knowledge you have imparted by setting up a Punnett
square to prove your point.
COMPLEX PATTERNS OF INHERITANCE
Monohybrid Cross (Incomplete dominance)
7. In horses, some of the genes for hair color are incompletely dominant. Genotypes are as follows: brown
horses are BB, white horses are bb and a Bb genotype creates a yellow-tannish colored horse with a white
mane and tail, is called “palomino”. Show the genetic crosses between the following horses and record the
genotypic and phenotypic percentages:
a. brown x white
b. brown x palomino
c. palomino x palomino
Genotypic %: _________
Phenotypic %: ________
%: ________________
%: ________________
%: ________________
%: ________________
8. SpongeBob loves growing flowers for his pal Sandy! Her favorite flowers, carnations, are found in red (R), white
(W), and pink (RW) varieties.
 What would happen if SpongeBob crossed a carnation with red flowers with a carnation with white
flowers?
 What would be the result of a cross between a pink and a white carnation?
 What are the possible parent genotypes of a red offspring?
 Could red offspring be made from a cross of a White with a Pink? Explain.
9. A curly haired person and a straight haired person mate and all their offspring have wavy hair (note that wavy
hair is a phenotype in between that of the curly and straight haired individuals).
What would be the result of a cross between two wavy haired individuals? (Give phenotype %’s)
Monohybrid Cross (Codominance & Multiple alleles)
10. In cattle, red coat color (R) is not completely dominant to white coat color (W). Heterozygous individuals
(RW) have coats that
are roan colored (reddish, but with spots of white hairs).
What are the possible
offspring of the cross between a roan bull and a white cow?
 Could Red offspring be born to a White cow mated with a Roan bull?
11. Show how it is possible to get a red four o’clock plant (RR) and a white four o’clock plant (WW) from two pink
parents (RW).
12. Human Blood type is determined by three alleles --- A, B, and O. Both A and B alleles are dominant, while O is
recessive. A woman with a heterozygous type A blood and a man with type O blood want to know the
probability of having a child with type O blood.
13. Mom has type O blood. Dad has type AB blood. What percentage of their kids will inherit type B blood?
14. A wealthy elderly couple dies together in an accident. Soon a man shows up to claim their fortune, saying that
he is their only son who ran away from home as a boy. Other relatives dispute his claim. Hospital records show
that the deceased couple was Type AB and O. The claimant to the fortune is Type O. Could he be their son?
Explain.
Monohybrid cross (X chromosome inactivation)
15. Coat color in cats is a codominant trait and is also located on the X chromosome. Cats can be black, orange or
calico. A calico cat has black and orange splotches. In order to be calico, the cat must have an allele for the
black color and an allele for the orange color. Use a Punnet square to show why there are no male calico cats.
A female calico cat is crossed with a male black cat. What are the phenotypes of the offspring and in what
proportion?
Monohybrid Cross (Sex-Linked Traits)
Sex-linked traits are features that are associated with the genes on the sex chromosomes; usually X. Examples
of those are recessive genes for color-blindness and hemophilia.
16. In humans, normal vision (Xc) is dominant to colorblindness (Xc) and sex-linked. A normal-versioned man,
whose father was colorblind, marries a colorblind woman. What are the chances that a son will be colorblind?
A daughter? Explain.
17. A woman with red-green color-blindness (XcXc) has a mother with normal vision (XCXc). Knowing that colorblindness is a sex-linked recessive gene, what is her father's phenotype is?
18. In humans, normal blood clotting ability (XH) is dominant to hemophilia (Xh). Hemophilia is a sex-linked
recessive disorder. A woman whose father is hemophiliac marries a man with normal clotting ability. What is
the probability that her first child will have hemophilia? Assume that the woman's mother is homozygous
dominant.
19. Cross a woman carrier for hemophilia to a hemophiliac man.
a) What fraction of the offspring will be carrier females?
b) What fraction will be normal males?
c) What fraction will be normal females--those who do not have the disease?
d) What fraction will be hemophiliac females?
e) What is the genotype of the carrier female?
f) How many different genotypes are possible among the offspring?
20. Eye color in fruit flies is sex linked, with the recessive allele causing white eyes. Show the cross for a white
eyed female and red-eyed male. How many offspring will have white eyes and what is their sex?
21. Suppose a young lady comes to you for advice in your capacity as a marriage counselor. She tells you that her
brother has hemophilia, but both her parents are normal. She wishes to marry a man who has no history of
hemophilia in his family and wants you to tell her the probability of her children having this disease. What
would you tell her, and how would you explain your conclusions?
Sex determination
22. In humans, the male and female share 22 of the 23 pairs of chromosomes in each body cell. The 23rd pair is
known as the sex chromosomes because it determines the sex of the individual. In the male, the sex
chromosome consists of an X and a Y chromosome (XY) while the pair in females consists of two X
chromosomes (XX). The male is the one who determines the sex of the child and the female gives an X to all
eggs while the male randomly produces about 50% X sperm and 50% Y sperm.
If a male mates with a female, what are the chances of the child being a male? What about a female?
Part 2 – Dihybrid Cross
23. In garden peas, purple flowers (P) are dominant to white (p) flowers, and tall plants (T) are dominant to short
plants (t). If a purple tall plant (PpTt) is crossed with a white short plant (pptt), what are the resulting genotypic
and phenotypic ratios?
24. In rabbits straight ears (SS) are dominant to floppy ears (ss) and brown fur is dominant (BB) to white fur (bb).
Show the results of crossing:
 Homozygous Straight-eared & Homozygous Brown Fur X Pure white, floppy eared rabbit
 Two heterozygous Straight-eared brown rabbits
25. Carrion beetles lay their eggs in dead animals and then bury them in the ground until they hatch. Assume that
the preference for fresh meat (F) is dominant to the preference for rotted meat and that the tendency to bury
the meat shallow (S) is dominant to the tendency to bury the meat deep. Suppose a female carrion beetle
homozygous dominant for both traits mates with a male homozygous recessive for both traits. What will be
the genotype of the F1 generation?
26. Labrador fur colors:
Black (P) is dominant to brown or chocolate (b). There is a second gene where the dominant (E)
produces color, and recessive (e) turns off the color, producing yellow fur (albino).
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The chart below shows the cross of the following parents: BbEe x BbEe.
What are the phenotypes of each parent? Both have black fur
Describe the phenotype of each genotype in the Punnet square
below. What is the phenotypic ratio? 9 Black : 3 Chocolate: 4 Albino
What genotypes are the yellow (albino) lab? BBee, Bbee, bbee
What percentage of offspring are albino? 25%
BBEE
BbEE
BBEe
BbEe
BbEE
bbEE
BbEe
bbEe
BBEe
BbEe
BBee
Bbee
BbEe
bbEe
Bbee
bbee
KEY
Cystic Fibrosis: It’s in your genes
The traits of an organism are determined by proteins that are built according to instructions that are carried
in DNA in the nucleus of cells. An error or change in the sequence of DNA is called a mutation. The mutations
for cystic fibrosis are often found on chromosome 7. There are over 500 types of mutations for cystic fibrosis.
The mutations often affect the cystic fibrosis transmembrane conductance regulator (CFTR) gene that codes
for a critical channel protein. The protein controls the concentration of ions, which ultimately controls the
water movement in cells that produce mucous, sweat, saliva, and digestive enzymes. A faulty CFTR protein
results in mucous and digestive enzymes that are thick and sticky.
5 min animation - https://www.youtube.com/watch?v=FMAOEOmLoUE
https://www.youtube.com/watch?v=EuLVCYrurok
Genetic Counseling for Jordan (Monohybrid Crosses)
27. Cystic fibrosis is autosomal recessive disorder. Both Jordan’s mother and father are heterozygous for
cystic fibrosis. Perform a test-cross and write a conclusion based on the expected genotypes and
phenotypes of the offspring.
28. Deafness is autosomal recessive disorder. Both Jordan’s mother and father are homozygous
dominant for normal hearing. Perform a test-cross and write a conclusion based on the expected
genotypes and phenotypes of the offspring.
29. Osteoporosis is autosomal recessive disorder. Both Jordan’s mother and father are homozygous
recessive for osteoporosis. Perform a test-cross and write a conclusion based on the expected
genotypes and phenotypes of the offspring.
30. Based on the results above. Perform a dihybrid cross to evaluate the probability of producing a child
that has cystic fibrosis and osteoporosis.
31. Based on the results above. Perform a dihybrid cross to evaluate the probability of producing a child
that had cystic fibrosis and is deaf.
Pedigree
32. The trait represented by the colored circles and squares below is inherited as a dominant allele. This
is not a sex – linked trait. Shaded individuals show the dominant trait. What is the probably genotype
of each individual?
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Are there any homozygous dominant individuals in the pedigree above? How do know?
What is the probability of the trait appearing in offspring if 7 should marry 9?