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Genetics
Genetics Terms
Before genetic problems may be adequately understood and solved, you must first understand the process of
meiosis and also know how to apply the following.
Homologous Chromosomes: chromosomes that are similar in size, shape and carry similar genetic
information. One comes from the mother and one from the father. They are paired during meiosis and
move to different sex cells.
P1 Generation: Original parents of the cross
F1 Generation: First successive generation; the offspring of the P1 generation.
F2 Generation: Second generation of offspring.
Gene: portion of a chromosome that codes for a specific protein; a trait. There are two genes for each
trait, one copy comes from mom and one comes from dad
Allele: an alternative form of a gene. For example, a gene that codes for height may have two possible
forms, tall or short.
Genotype: a description of the genes using letters. Ex: Tall (T) and short (t) genes could have the
following combinations: TT, Tt or tt.
Homozygous: two of the same genes in the genotype. Ex: TT or tt.
Heterozygous: two different genes in the genotype. Ex: Tt
Phenotype: a description of the physical characteristics resulting from the genes. Ex: Tall, Short
Dominant: Some genes are more “dominant” than others. In a heterozygous genotype, the dominant
gene appears in the phenotype. The dominant gene is written with a capital letter. Ex: Tt is a
heterozygous genotype. T is written with a capital letter and is therefore dominant. The organism will
be tall.
Recessive: a gene that does not appear in the phenotype when paired with a dominant gene. Recessive
genes are usually indicated with a lower case letter that is the same letter as the dominant gene. In our
example, short is recessive, so it is indicated with a lower case “t” (not an “s”).
Monohybrid Cross: a cross between two organisms that looks at a single pair of genes. Ex: TT
crossed with Tt. Each gamete will posses one copy of the gene- T or t.
Dihybrid Cross: a cross between two organisms that looks at two pairs of genes. Ex: BbTt crossed
with BBTT. Each gamete will possess one copy of EACH gene- BT, Bt, bT, or bt.
Incomplete Dominance: Alleles are equally dominant. In a heterozygous genotype, the alleles blend.
Since there is no recessive gene, both genes are written with capital letters. Ex: Red and White alleles
would be written as R and W. A genotype of RR would be red, WW would be white, and RW would be
pink.
Codominance: Again, both alleles are equally dominant. In a heterozygous genotype, the alleles
BOTH SHOW UP. Both alleles are indicated with capital letters. Ex: Red and White alleles would be
written as R and W. A genotype of RR would be red, WW would be white, and RW might be red and
white striped.
Multiple Allele: Traits that have more than two forms of the gene (more than two alleles). Ex: Blood
type alleles include A, B and o. A and B are codominant and o is recessive.
Sex-Linked: Traits that are coded for on the X chromosome of the 23rd pair in humans. Remember that
the 23rd pair determines the gender of the individual. Females are XX and males are XY. Sex-linked
traits are indicated by use of superscripts on the X chromosome only (since the Y chromosome does not
carry the trait). Ex: Colorblindness is a sex-linked trait (carried on the X chromosome). The possible
N N
N n
n n
N
n
genotype would be written: X X , X X , X X , X Y, or X Y.
Pedigree: a tool to evaluate patterns of inheritance within a family. Females are indicated with circles
and males with squares. A blackened circle or square indicates that the individual showed symptoms of
the disease or trait. For example, a colorblind woman would be shown as a darkened circle, but a
woman who was a carrier of colorblindness (only had one copy of the allele) and a woman who had two
healthy genes would both be indicated by an empty circle.
MONOHYBRID PROBLEMS
1.
If in pea plants, G=green pods and g = yellow pods, what would be the genotypic and
phenotypic results of this cross:
Gg X GG
offspring genotype probabilities
offspring phenotype probabilities
2.
If in pea plants T=tall and t=short, what would be the genotypic and phenotypic results of this
cross:
Tt X Tt
3.
offspring genotype probabilities
offspring phenotype probabilities
Cross a white seed plant with a heterozygous brown seed plant.
offspring genotype probabilities
offspring phenotype probabilities
4.
In watermelons, green-skinned fruit is due to the dominant gene "G", while striped
skin is
due to its recessive allele, "g". If you cross a homozygous green-skinned watermelon plant with a
homozygous striped one, what F1 results (genotype and phenotype) would you expect? Crossing
two F1 individuals would give the F2 generation. What is the F2 genotype results (genotype and
phenotype)?
5.
In fruit flies, long wing is dominant over vestigial (short) wing. If two flies heterozygous for this
trait are crossed, what is the probability that their offspring will be heterozygous?
6.
In squash, the allele for white fruits (W) is dominant over the allele (gene) for yellow (w). If a
white-fruited plant is crossed with a yellow-fruited plant and all the offspring are yellow, what are the
possible
genotypes of the parents and the offspring?
a.
b.
parents genotype
____________
offspring genotype ____________
7.
The genetic disease called cystic fibrosis is inherited through a recessive gene. If both parents
are
heterozygous for this trait, what is the probability that they will have a child who suffers from
this disease?
8.
In cats, the gene for short hair is dominant over the gene for long hair (angora). A short-haired
tom cat is mated with an angora female. She bears eight kittens, six are short-haired and two are
long-haired. How do these numbers compare with the expected ratio? If you mated these cats four
more times and obtained a total of forty offspring, would the results be a closer approximation of the
expected ratio? Explain. Give the genotype of the tom cat, the female, and the kittens.
9.
Some dogs bark while they are trailing an animal. Others are silent. The barking trait is
inherited from a dominant gene. A barker, who had a silent mother is mated to a silent trailer. The
female bears four puppies. What are the chances that there will be silent puppies in this group?
Explain and show the cross?
10.
Brown hair color is dominant to blond hair color. If two brown-haired parents have one blondhaired child, what is the probability that their second child will have brown hair?
REVIEW
11.
If in pea plants, round is a dominant trait and wrinkled is a recessive trait, what would be the
results (phenotype and genotype) of this cross: Rr X Rr
12.
In poultry, black is due to a dominant gene and red is due to a recessive. Cross a
homozygous black chicken with a homozygous red. What will be the appearance of the F1? Cross
two of the F1 individuals to get an F2? State the genotype and phenotype probabilities of the
offspring.
13.
A tall plant crossed with a dwarf one produces offspring of which about one half are
What are the genotypes of the parents. (Tall is dominant.)
a.
tall genotype___________
b.
dwarf genotype__________
dwarf.
14.
Pea plants can have either yellow pods or green pods. Cross 2 heterozygous green pea
plants. State the offspring probabilities.
genotype
_____________
phenotype _____________
15.
Cross a homozygous green plant with a heterozygous green plant. (see problem above)
State the offspring probabilities.
genotype
_____________
phenotype _____________
16.
In humans, blue eye color is recessive to brown eye color. (It is actually a bit more
complicated than that) A couple who both have brown eyes have twelve children, eleven of whom
are brown eyed. One is blue-eyed. What are the likely genotypes of the parents?
17.
In peas, yellow seed color is dominant over green seed color. If a heterozygous yellow plant is
crossed with a green plant, what is the probability that the offspring will be green?
18.
Brown eyes are dominant over blue eyes. Could two people with brown eyes have three
children with blue eyes? Explain and show how.
INCOMPLETE OR CODOMINANT PROBLEMS
1.
In a certain type of flower, color is incompletely dominant. Cross a pure red flower with a pure
white flower. What will be the phenotype of the offspring?
2.
In flowers, color is an incomplete dominance trait. (see above problem) When the genotype is
heterozygous, the phenotype is pink. Cross a pink flower with a pink flower. What will be the
genotype and the phenotype of the offspring?
3.
When a pure red cow is bred with a pure white bull, the F1 offspring is a roan (this means that
it has both red and white hairs). What will be the results of a cross between a red bull and a roan
cow? Could a farmer raise a head of roan cattle that would only produce roan offspring? Explain.
4.
In Guppy vultures, pure long legs when crossed with pure short legs cause medium legs.
Cross a long Guppy vulture male with: 1. a medium hen and 2. a short hen and give the phenotype
of the offspring.
MULTIPLE ALLELE PROBLEMS
1.
If a man with blood type AB marries a woman heterozygous for type A, what is the probability
that their child will be type B?
2.
A mother has type A blood and a father has type B blood. If their baby has type O blood, what
is the genotype of the parents?
3.
Use a punnett square to show the possible genotypes and phenotypes for blood type of the
offspring of two parents, one with blood type O and one with blood type AB.
4.
Two women gave birth to girls in the same hospital at the same time. The nurses think they
may have accidentally switched the babies' name tags and given the babies to the wrong parents.
One baby, Jane, is type O. The other baby, Mary, is blood type A. The father in one set of parents,
the Reds, is blood type A, and the mother is type B. The father in the other set of parents, the
Greens, is blood type AB, and the mother is type O. Figure out which baby belongs to which parents.
Show your work and reasoning.
DIHYBRID PROBLEMS
1.
In horses, black is dependent on the dominant gene (B) and chestnut on its recessive allele
(b). The trotting trait (T) is dominant over the pacing trait (t). Cross a homozygous black trotter with a
chestnut
pacer. What will be the appearance (genotype and phenotype) of the F1? If two of the
F1 individuals were mated, what kinds of offspring could they have (genotype and phenotype) and in
what proportions?
2.
In poultry, feathered legs (F) are dominant over clean (f) legs and pea-comb (P) are dominant
over single comb (p). What gametes are produced by a chicken with the genotype FfPp? (This
problem does
NOT require a punnett square)
3.
W = Normal wings
E = Normal eyes
w = underdeveloped wings e = small eyes
Cross a fruit fly heterozygous for both traits with one that is homozygous recessive
traits. Give the phenotype probabilities of the F1 generation.
for both
4.
A pea plant heterozygous for both smooth seeds and red flowers is crossed with a wrinkled
seeded,
homozygous red flowered plant. What is the probability that the offspring will be
heterozygous smooth, homozygous red? (you are not given information on the recessive gene for
color, this is not actually needed to answer the question)
5.
Green is dominant over yellow and rough seeds are dominant over smooth. What will be the
phenotype and genotype probabilities of a cross of a heterozygous green smooth with a
heterozygous green heterozygous rough?
6.
Rough is coded by a dominant gene. Black is coded a by a dominant gene. A rough black
guinea pig bred with a rough white one gives 28 rough black; 31 rough white; 11 smooth black; and
10 smooth white. What are the probable genotypes of the parents?
7.
Suppose hair color is incompletely dominant. Hair can be either pure black or pure blond. If
the genotype
is heterozygous, hair would appear brown. Cross a heterozygous curly brown
haired person with a straight brown haired person. What would be the expected phenotypes (and
percents) of the offspring? (This problem is a DIHYBRID and a INCOMPLETE DOMINANT
PROBLEM)
REVIEW
8.
Brown eyes are dominant over blue eyes. Curly hair is dominant to straight hair. A male
parent homozygous for brown eyes and straight hair and a female parent with blue eyes and
homozygous curly hair have a child. Determine the F1 generation. Express the genotype and the
phenotype. Determine the F2 generation. Express the possible genotype and phenotypes along with
genotypic and phenotypic
probabilities.
9.
In peas, tall is dominant over short and red flower color is dominant over white. If two
heterozygous tall, heterozygous red plants are crossed, what is the probability that the offspring will
be tall and white?
10.
In pea plants, green pods and tall stems are dominant traits, while yellow pods and dwarf stem
length are recessive traits. Give the phenotype and genotype probabilities for a cross between a
heterozygous green, dwarf plant and a heterozygous green, heterozygous tall plant.
11.
In guinea pigs, rough coats (R) are dominant over smooth coats (r) and black coats (B) are
dominant over
white (b). Cross a homozygous rough black guinea pig with a smooth white one.
What will be the
phenotypes of the F1? the F2?
12.
In cattle, roan (red and white hairs) results from a heterozygous genotype. Cattle can also be
either pure red or pure white. Polled (no horns) cattle are dominant over horned. A roan
heterozygous polled bull is mated with a roan horned cow. What will be the phenotype of this cross?
(This problem is a DIHYBRID and a CODOMINANT PROBLEM)
SEX-LINKED PROBLEMS
1.
Show, using a Punnett Square, how the sex of a child is determined. Does the egg from the
mother or the sperm from the father determine the sex of the offspring?
2.
Color blindness is a sex-linked recessive trait. What is the probability that a color blind woman
and a color-blind man will have: a. a normal sighted son b. a color blind daughter
3.
What is the probability that two parents with normal color vision will have color blind sons or
daughters if the mother's father is color -blind?
4.
Hemophilia is a sex-lined blood disorder. People with this disease can bleed to death from a
small cut because their blood does not clot. It is caused by a recessive allele. You are a normal
(homozygous) female who is about to marry a hemophiliac male. What are your chances of having
a child with this blood disorder?
5.
In Drosophila, the gene for red eye color, R, is dominant to the gene for white eye color, r. The
trait is sex-linked. A red-eyed male was bred to a heterozygous red-eyed female. Mark true/false.
The genes for eye color are carried on the X chromosome.
The male's genotype is XRYr.
The female's genotype is XRXr.
Half of the gametes produced by the female should contain Xr.
All the gametes produced by the male should contain Yr.
Of the offspring produced, all females are expected to have red eyes.
Of the offspring produced, all males are expected to have white eyes.
The offspring of a white-eyed male must have the genotype XrY.
Female offspring produced from this cross could have the genotype XrXr.
PEDIGREE PROBLEMS
1.
Determine the genotypes of each of the following 12 individuals (color blindness is sexlinked)?
2.
The following is a pedigree of myopia (near-sightedness). This condition is caused by a
recessive gene, but is NOT sex linked. Determine whether the 8 individuals are A. Heterozygous
B. Homozygous C. Not enough sufficient data to determine genotype.
3.
Huntington’s Disease is caused by a dominant gene. (not sex linked) Based on the laws of
probability, what are the likely genotypes of the numbered individuals?
4.
This pedigree below shows the blood types of some individuals. List all the possible
genotypes of each numbered individual.
CHALLENGE PROBLEMS
1.
What kind of gametes (sperm) could a man who was heterozygous for long eye lashes, free
earlobes, and brown eyes produce? If he were to marry a woman of exactly the same genotype for
these 3 traits, what size Punnett square would be needed? DO NOT ACTUALLY MAKE THE
CROSS!!!!!! Just the kinds of gametes he would produce and the number of squares in the Punnett
diagram.
2.
In cocker spaniels, white spotting is recessive to solid color. You want to know if your red
solid-colored cocker is homozygous or heterozygous for spotting. How could you find out? If yours is
homozygous, what results would you expect? If your is heterozygous, what results would you
expect from the cross?
3.
In poultry, feathered legs (F) are dominant over clean legs (f) and pea comb (P) are dominant
over single comb (p). Two cocks, A and B, are bred to two hens, C and D. All four birds are feather
legged and pea combed. Cock A with both hens produced offspring that are feathered and pea
combed. Cock B with hen C produced both feathered and clean, but all pea combed, but with hen D
he produced all feathered, but part
pea combed and single. What are the genotypes of all four
birds?