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Medelian Inheritance
Level 1
1. From the cross Bb X bb , where B is dominant to b , how many different phenotypes are
produced among the offspring and in what proportions are they expected?
2. From the cross Bb X Bb , where B is dominant to b , how many different genotypes are
produced among the offspring and in what proportions are they expected?
3. In humans, freckles F are dominant over no freckles f . In a cross of two heterozygous
parents, what is the expected proportion of non-freckled children among their offspring?
4. The dominant allele A causes seeds of corn to be colored, whereas the recessive allele a
causes seeds to be colorless. From the cross Aa X Aa, what fraction of the seeds are
colorless?
5. Cystic fibrosis in humans is caused by a recessive allele. Two normal parents have a child
with cystic fibrosis. What is the probability that this couple’s next child will be normal?
6. A homozygous garden pea having round seeds (genotype RR ) is crossed with a
homozygous plant having wrinkled seeds (genotype rr ). The F1 are then crossed with RR
plants. What are the resulting phenotypes and in what proportions are they expected?
7. A homozygous garden pea having yellow seeds (genotype YY ) is crossed with a
homozygous plant having green seeds (genotype yy ). The F1 are allowed to self-fertilize,
producing F2 seeds. What are the phenotypes of the F2 seeds and in what proportions are they
expected?
Mendelian Genetics
level 1.2
8. With rare exception, human eye color is inherited as if a dominant allele (B ) causes eyes to
be brown and the corresponding recessive allele (b ) causes eyes to be blue. A blue-eyed man
marries a brown-eyed woman whose mother had blue eyes (and a bad temper, but that’s beside
the point). What proportion of their children are expected to have blue eyes?
9. In corn, colored seeds R are dominant over colorless r . Two plants having colored seed are
crossed, and the resulting (F1) seed are collected. All F1 plants have colored seed, however,
the F2 revealed 6 colorless seed and 97 colored ones. Give the genotypes of the parent plants.
10. Back to corn seed. A plant with colorless seed is crossed with one with colored seed, and
all of the F1 seeds are colored. Assuming the F1 plants are self-fertile, what is the expected
proportion of colored seed in F2 plants?
11. Mom and dad are both heterozygous for a given trait. What proportion of their children are
expected to be homogous dominant for the same trait?
12. Polydactyly (more than 5 fingers/toes) is dominant P over the normal 5 finger/toe situation
p . Joe has six fingers on each hand, yet his sister and both of his parents have only 5.
However, Joe noticed some surgical scars on his dad’s hands. Dad finally confessed he had a
few extra fingers taken off as a kid. What is the genotype of Joe, his mom, his dad, and his
sister with regard to this gene?
Mom
Mendelian Genetics
Dad
Joe
Sis
level 1.5
13. Peas are dwarf if they are homozygous for the recessive allele t , but tall if they have the
dominant allele T . Also, purple flowers P are dominant over white flowers p If a dwarf plant
with white flowers is crossed with a plant heterozygous for both traits, what is the genotype of
the parents, and what is the genotype and phenotype of the F1, and in what ratios?
14. Sickle cell anemia is a condition caused by a recessive allele s . It comes from Africa, and
so is mostly found in people of African descent. Interestingly, being heterozygous for the trait
provides and individual with a degree of tolerance to malaria, which kills more people than sickle
cell anemia. If two individuals without sickle cell anemia, and with tolerance to malaria have a
child, what is the probability the child will be normal with respect to sickle cell anemia, and
tolerant to malaria?
15. In humans (mostly those of Northern European descent), cystic fibrosis is a recessive trait c
. The normal allele is dominant C .
a. If both parents are carriers, but normal, what is the probability that their next child will
have the disease?
b. If one of the parents is a carrier, and the other is normal and not a carrier, what is the
probability that their next child will be a carrier?
16. Hemophilia is carried on the X chromosome. It is recessive, but it is expressed in males
carrying only one recessive allele. A normal man and a normal woman (who is a symptomless
carrier) have a child.
a. If the child is a boy, what is the chance he will have hemophilia?
b. If the child is a girl, what is the chance she will have hemophilia?
Mendelian Genetics
level 2
17. ABO blood types in humans are determined by a single gene having three alleles; A, B, and
O. A and B are codominant to O.
a. A mother with blood type A has a child with blood type O. What is the mother’s
genotype.
b. A mother with blood type B has a child with blood type O. What are all possible
genotypes of the father?
18. The inheritance of coat color in Labrador Retrievers is complicated business involving a
type of gene interaction called epistasis (the expression of one gene influences the
expression of another). There are 2 primary genes for coat color in Labs, and 3
phenotypes (black, chocolate, and yellow). The first gene is for the black pigment (B,
black, is dominant to b). The second is for the deposition of the pigment (E, deposition,
is dominant to e). Genotypes and phenotypes are listed below.
Black labs:
Chocolate:
Yellow
B_, E_
bb, E_
B_, ee (Yellow, with attractive eyes and nose.)
bb, ee (Yellow, but with pale facial features and not show quality, but still
nice, and you can get them cheaper from a careless breeder).
a. A homozygous show quality yellow lab is crossed with a chocolate lab. Given the
uncertainty of the genotype of the deposition gene in the chocolate, give all possible
genotypic/phenotypic ratios.
b. In the above cross, is it possible for a black lab to show up in the litter? Explain.
c. Two black labs that are heterozygous at both loci are bred. What are the possible
genotypic and phenotypic outcomes, and in what proportions are they expected?
Mendelian Genetics
level 3
19. Genes carried on the X or Y chromosome are sex-linked. The majority are on the X
chromosome (it's much bigger) and are often referred to as X-linked. Red/Green color
blindness and hemophilia are two such traits.
a. Red/green color blindness is a sex-linked recessive trait carried on the X
chromosome. A R/G color blind woman has a child with a normal male (human).
What is the probability that their male children will be R/G color blind?
b. What is the probability that their female children will be R/G color blind?
20. If the male is color blind and the female is a normal-sighted carrier, what is the
probability that their child will be a color blind male?
How about the probability that it will be a color blind female?
If a color blind male has a child with a normal sighted woman whose father was color
blind, what is the chance that their first child will be a color blind female?
21. A woman is a carrier of the hemophilia gene, and she has a child with a normal male.
What is the probability that their first child will be a male hemophiliac?
What is the probability of a male child being hemophiliac?
What is the probability of a female child being hemophiliac?
ABO blood type is controlled by a single gene with 3 alleles. A and B are codominate to O
(recessive). The 3 alleles are designated IA, IB, and i. Type A blood (phenotype) results from
two genotypes (IA, IA or IA, i); Type B blood results from 2 genotypes (IB, IB or IB, i), Type AB
blood results from one genotype ( IA, IB), and type O blood results from one genotype (I, i).
22. If a type O woman has a child with a type A blood, what are the possible genotypes of
daddy?
23. Without genetic testing, and only knowing the blood type, what are the possible blood
types of a child born to a type A woman and a type B man?
24. A type A child is born to a mother with type B blood. What is/are the genotype(s) of the
mother and father?