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.
1. Parent genotypes: Rr x Rr R = ______r = ______
Parent gametes: ______
Offspring: Genotype ratio ______
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 ______
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 ______
Phenotype ratio ______
4. Cross an albino rat with a heterozygous normal rat. Normal is dominant.
Allele key: long= _____ short = ______
Parent genotypes: ______X ______
Parent gametes: ______
Offspring: Genotype ratio ______
Phenotype ratio ______
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 ______
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
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 à “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: ______
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.
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?
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 ______
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? ______
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?
Thomas Hunt Morgan –
Drosophila
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