Name: _______________________ Hour: ________ HOW TO WORK A GENETIC PROBLEM 1. Assign each trait a LETTER—Make a Key Dominant trait = Capital letter (...
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Name: _______________________ Hour: ________

HOW TO WORK A GENETIC PROBLEM 1. Assign each trait a LETTER—Make a Key Dominant trait = Capital letter (D) Recessive trait = small letter (d) 2. Determine the Genotypes of the Parents Pure — the same (DD) or (dd)

Hybrid — mixed


Homozygous Dominant Homozygous recessive Heterozygous

3. Complete the Possibilities on the PUNNETT SQUARE Male traits — top side of square Female traits — left side of square 4. Answer the question(s).

Monohybrid Cross………………………………………………………….2-5 Problems 1-11 Incomplete Dominance…………………………………………………5-8 Problems 1-7 Dihybrid Cross……………………………………………………………….8-11 Problems 1-6 Sex-Linked……………………………………………………………………..11-14 Problems 1-8 Blood Type………………………………………………………………………14-16 Problems 1-5 Pedigree…………………………………………………………………………..16-20 Problems A-C


“Monohybrid Cross” Practice Problems (crosses dealing with one trait) 1. Brown eyes are dominant over blue eyes. If two hybrid brown-eyed people marry, what are the chances that they will have a blue-eyed child? ___________


_____ x _____


Probability: Genotype: BB = _______ Genotype: Bb = _______ Gentoype: bb = _______

Probability: Phenotype: Brown Eyes = ______ Phenotype: Blue Eyes = _______

2. A pea plant with purple flowers is crossed with a white one. Both are taken from pure strains. (Color is usually dominant over white.) Show this cross.


_____ x _____

p= Probability: Genotype: PP = _______ Genotype: Pp = _______ Genotype: pp = _______

Probability: Phenotype: Purple = ______ Phenotype: White = ______

Cross two of their offspring. What are the chances of getting white flowers in this generation? ______________________________________________ P=

_____ x _____

p= Probability: Genotype: PP = _______ Genotype: Pp = _______ Genotype: pp = _______

Probability: Phenotype: Purple = ______ Phenotype: White = ______


3. Black is dominant to brown coat color in mice. Cross a hybrid black mouse with a pure brown one. Write the gene types of the offspring, and tell whether they are black or brown. _________________________________________ B=

_____ x _____

b= Probability: Genotype: BB = _______

Probability: Phenotype: Black mice = ______

Genotype: Bb = _______ Genotype: bb = _______

Phenotype: brown mice = _______

4. Tall is dominant to short in tomato plants. Cross a pure tall plant with a pure short one. What do the offspring look like? ____________________________


_____ x _____


Probability: Genotype: TT = _______ Genotype: Tt = _______ Genotype: tt = _______

Probability: Phenotype: Tall plants = ______ Phenotype: short plants = _______

5. Hitchhiker’s thumb is dominant to straight thumb in humans. If two hybrid hitchhikers marry, can they have a child with a straight thumb? _____________ H=

_____ x _____


Probability: Genotype: HH = _______ Genotype: Hh = _______ Genotype: hh = _______

Probability: Phenotype: Hitchhikers thumb = ______ Phenotype: straight thumb = _______


6. Smooth seed coats are recessive to wrinkled seed coats in beans. Cross two smooth-coated plants and see what the chances are for getting wrinkled-coated plants. ________________________________________________________ W =

_____ x _____

w =

Probability: Genotype: WW = _______ Genotype: Ww = _______ Genotype: ww = _______

Probability: Phenotype: Wrinkled = ______ Phenotype: smooth = _______

7. The chromosomes for male (father) are written XY. The female (mother) is written XX. Show this mating on a Punnett Square. If a family has eight children, how many would you expect to be boys? _______________________________ _____ x _____

8. In humans, attached earlobes are recessive to unattached. A man with attached earlobes, whose father had unattached earlobes, marries a woman with unattached earlobes, whose father had attached earlobes. A=

_____ x _____

a= AA = Aa = aa = A. B. C. D. E.

What is the man’s genotype? ________ What is the woman’s genotype? ________ What is the genotype of the woman’s mother? ________ What is the genotype of the man’s father? _________ If the man and woman have eight children, how many would be expected to be heterozygous for attached earlobes? _________ F. How many of the children would be expected to have unattached earlobes? _________ G. How many would have a homozygous dominant genotype? __________



Round tomatoes are dominant to oval ones. Cross the following: (A) oval and homozygous round: phenotype = _____________________ (B) both heterozygous: % of oval tomatoes = _____________________ (C) heterozygous round and oval: % of round = ____________________


Horned cattle are recessive to hornless. Cross the following: (A) heterozygous hornless and horned: % of pure = ________________ (B) both heterozygous hornless: % of hornless = __________________ (C) horned and homozygous hornless: phenotype = _________________


Trotting in horses is dominant to the pacing trait. Cross the following: (A) homozygous trotter and pacer: % of pacers = _________________ (B) pacer and heterozygous trotter: % of pacers = _________________ (C) two heterozygous trotters: % of pacers = ____________________


“Incomplete Dominance” Practice Problems 1. When red petunias are crossed with white ones, the resulting offspring are pink! Write the genotypes for the three colors of petunias: _______ = Red

________ = white

________ = pink

Show a cross between two pink flowers. What are the chances of getting red flowers from such a cross? _____________ R= r= Rr=

_______ x _______

2. In guinea pigs, a brown male mated with a cream-colored female produces cream-colored offspring with brown spots. If a spotted guinea pig is crossed with a cream-colored one, determine the phenotype of their offspring. Write each of the possible genotypes and tell what the phenotype would be. What are the chances of finding a brown guinea pig in the litter? _______________________

B= b= Bb=

_______ x _______

Probability: Genotype: BB = _______ Genotype: bb = _______ Genotype: Bb = _______

Probability: Phenotype: Brown = ______ Phenotype: Cream = ______ Phenotype: Spots = ______


3. A black cat crossed with an orange one results in a striped condition known as tortoise-shell. If two tortoise-shell cats should mate, what is the phenotypic ratio that could be found in the offspring? _____________________________

B= b= Bb=

_______ x _______

Probability: Genotype: BB = _______ Genotype: bb = _______ Genotype: Bb = _______

Probability: Phenotype: Black = ______ Phenotype: Orange = ______ Phenotype: Tortoise-shell = ______

If there were a dozen kittens in the first litter, how many would resemble their parents? __________

4. Work a problem showing a cross between two heterozygous parents, using “A” for (any) dominant allele and “a” for (any) recessive allele. Then, determine: (1) (2) (3) (4)

the phenotypic ratio if dominance is complete: ___________________ the genotypic ratio if dominance is complete: ___________________ the phenotypic ratio if dominance is incomplete: _________________ the genotypic ratio if dominance is incomplete: __________________

What generalization can be drawn from this data?

5. Red coat color in cattle is incompletely dominant to white. The hybrid is called “roan.” Complete these crosses: R= r= Rr =

A) red x white: phenotype _________________________ B) two roan: % of roan _____________________________ C) roan x white: % of white _________________________


6. Long-eared rabbits crossed with short-eared mates have medium-eared offspring. Show these crosses: E= e= Ee =

A) long x medium: % of medium ______________________ B) short x medium: % of short _______________________ C) two medium: phenotypic ration _____________________

7. Orange flowers pollinated by white flowers result in yellow ones! Show these crosses: F= f= Ff =

A) two yellow: % of white __________________________ B) yellow x white: % of white _______________________ C) orange x white: phenotype ________________________


“Dihybrid Cross” Practice Problems (crosses dealing with two traits) 1. In horses, trotters are dominant over pacers, and black is dominant over chestnut. What are the chances of getting a chestnut pacer from a cross between two hybrid black trotters? _____________________

T= t= B= b= ________ x ________ What is the phenotypic ratio for this cross? _________ black trotters _________ black pacers _________ chestnut trotters _________ chestnut pacers

2. A black, smooth-haired terrier is crossed with a tan, wire-haired one. All the puppies are black and wiry. Show a cross between two of these hybrid puppies. What are the chances of getting a smooth, tan puppy? ____________________

B= b= W= w= ________ x ________ What is the phenotypic ratio for this cross? _________ Black Wiry-haired _________ Black smooth-haired _________ tan Wiry-haired _________ tan smooth-haired


3. In a particular flower, axial flower position is dominant and terminal flower position is recessive. Long stems are dominant over short ones. In a cross between a heterozygous plant and a homozygous recessive one, what are the chances of getting the desirable long-stemmed, terminal-budded offspring?_______________________________

A= a= L= l= ________ x ________ What is the phenotypic ratio for this cross? _________ Axial Long _________ Axial short _________ Terminal Long _________ Terminal short

4. A woman carrying the gene for hemophilia marries a man with no hemophilia in his family. They both have brown eyes, but the man is homozygous, while the woman is known to be a hybrid.

N= n= B= b= ________ x ________

What are the chances of having a daughter with hemophilia? ________________ What are the chances of having a son with hemophilia? _____________________ Of the sons with hemophilia, what percentage will have blue eyes? ____________


5. In cows, the hornless trait is dominant to the horned trait. Red cows bred to white ones result in a blended color called roan. Cross a horned, roan cow with a white bull known to be hybrid for horns.

H= h= R= r= Rr = ________ x ________ List the genotype and phenotype for each type of offspring below. Genotype Phenotype ____________ ____________ ____________ ____________

___________________________________________ ___________________________________________ ___________________________________________ ___________________________________________

What are the chances of getting a roan cow? ___________________________ What are the chances of getting a roan cow with horns? __________________

6. A blue-eyed curly haired man whose mother was colorblind marries a browneyed woman whose parents both had straight hair. Her father had blue eyes, and her family seems normal (N) with no history of color blindness (n). Most of his relatives have straight hair. Write the genotypes of this man and woman. B=

_______________ x ________________

b= C= c= N= n=


Sex-Linked Problems 1. In humans, red-green colorblindness is carried on the X chromosome. A colorblind man marries a normal woman. From this marriage one colorblind daughter is born. Give the genotypes of these three people. Man = _____________ Woman = _____________ Daughter = ______________

C= c=

______ x ______

2. Hemophilia is also sex-linked. If a normal woman marries a normal man, and two of their three sons are hemophiliacs, what is her genotype?______________

Of the daughters born to this couple, what percentage would probably be carriers? _____________________ H= h=

_______ x ________

3. There are three children in a family. Henry is a hemophiliac, George and Mary are not. George and Mary, knowing that hemophilia is hereditary and not wishing to pass the gene to their children, come to you for genetic counseling. What would your advice be to each? (Use Punnett Squares to help if you need them to illustrate your answer.) Henry’s genotype = _________ George’s genotype = ________ Mary’s genotype = __________

Advice: ___________________________ Advice: ___________________________ Advice: ___________________________


4. A normal man marries a normal woman and they have a colorblind son. No one in the immediate family history has been known to be colorblind. From which of the boy’s four grandparents did he receive his colorblind trait? Illustrate this with a pedigree chart labeled with any known genotypes.

5. A gene (g) is known in fruit flies to be sex-linked, recessive, and lethal. If a normal male is mated with a heterozygous female, what will be the sex-ratio of their offspring? _________________________________________________ ________ x _________

6. In cats there is a gene for coat color that is both co-dominant and sexlinked. The two alleles are yellow and black, but the heterozygous condition is a combination known as tortoise-shell. Show the offspring expected from a cross of tortoise-shell female and a yellow male. ________ x _________

Is it possible to produce a tortoise-shell male through any combination of parents? Why or why not? _________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ 13

7. Under what circumstances is it possible for both a father and a son to have hemophilia?

8. Pattern baldness is a sex-influences trait. If the recessive gene is present, it is expressed mainly in the presence of the male hormone testosterone. A normal man marries a woman whose father had pattern baldness. Show this cross.

________ x _________

Why is it said that sons inherit their baldness from their mothers and not from their fathers?


Blood Typing Problems 1. A couple preparing for marriage have their blood types done along with other blood tests. Both are AB. What possible blood types could the children of this marriage possess? If they had six children, how many would you predict would have the same blood type as their parents? _____________________________

________ x _________

2. A woman sues a man for the support of her child. She has type A blood, her child has type O, and the man is type B. Show how this man could be the father of her child.

________ x _________

An investigation of the man’s family history reveals that his parents were both AB blood types. Does this support or weaken the woman’s paternity claim?_________________________________________________________

________ x _________


3. A man with type A blood marries a woman with type B blood. They have four children. Each of them has a different blood type? (Each one has one of the four different blood types.) Show this cross. What are the genotypes of the parents?

________ x _________ Genotypes of Parents: Man = ____________ Woman = __________ 4. A wealthy, elderly couple die together in an accident. A man shows up to claim their fortune saying he is their only son who ran away from home as a boy. Other relatives say this isn’t true. Hospital records show that the couple were blood types AB and O. The man is also type O. Do you think this man is their son? Why? ________________________________________________________.

________ x _________

5. Mrs. Smith and Mrs. Jones gave birth to baby boys (named John and Tom, respectively) on the same day in a large city hospital. After Mrs. Smith took her baby home, she began to suspect that it was Mrs. Jones’ baby and that the hospital had somehow mixed the infants up. Blood tests revealed that Mrs. Smith has blood type B, while Mr. Smith’s type is O. John Smith has type B blood. Mr. Jones is type A and Mrs. Jones is type AB. The infant Tom is blood type O. Had a mix-up occurred? _______________ Smith Parents _________ x _________

Jones Parents _________ x _________


Pedigrees: Family Trees Rules for Reading a Pedigree: 1.

Females are represented by a O (circle) and males are represented by a (square).


A horizontal (


Children of parents are shown at the bottom of a line extending from the marriage line.


Generations are represented by Roman Numerals (I, II, III…) and individuals by Arabic (1, 2, 3…)


A shaded O (circle) or (square), indicates a person who has the trait. A half shaded O (circle) or (square) indicates a carrier not showing the trait. Some Pedigrees don’t half shade.

) line between O and ♂ represents a marriage (mating).

** Autosomal Dominant (chromosomes 1-22) ** Autosomal Recessive (chromosomes 1-22) ** Sex-link (chromosome X for females, chromosome Y for males)

1. 2.

In generation II, how many children have attached earlobes? What sex is the child with attached earlobes? Are his earlobe genes “pure” or “hybrid” ? 3. What kind of earlobes do that child’s parents have? 4. What are the gene types of the parents? Use the Punnett Square below to show how such a child is produced from these parents.


Pedigree A: Deaf Mutism One form of deafness in humans is a type in which the individual inherits not only deafness, but also the inability to talk. Use symbols D for dominant, d for recessive.

1. 2. 3. 4.

What do the shaded symbols represent? _____________________________________ How many children did the first generation have? _______________________________ What is the gene type of the parents in I? ____________________________________ Write the gene types of the following using the letter D. II-7 ______ III-2 ______ II-4 ______ IV-9 ______ 5. How many marriages are represented in generation III? __________________________ 6. If a woman who is hybrid for deafness wants to marry IV-13, show them the possibility of having a deaf child by using a Punnett Square. The chances are ___________________________

7. In pedigree A, why do parents 13 and 14 in generation III have four deaf children? __________________________________________________________________________ 8. How can I-1 and I-2 have deaf children when they are not deaf themselves? __________________________________________________________________________ 9. List by number the persons in Generation II for which you cannot determine the pedigree: __________________________________________________________________________


Pedigree B: Hemophilia Hemophilia is a disease of the circulatory system. It is sometimes called bleeder’s disease because persons with the disease have a very long clotting time when injured. An affected person could bleed to death from minor wounds or internal injuries. Use symbols H or dominant, h for recessive.

1. Why is hemophilia treated “specially” when considering genetic traits? ___________________________________________________________________ 2. From whom did II-1 inherit his disease?________________________________ How do you know? ________________________________________________ 3. Write the gene types of the following people: (Hint: don’t forget to note which ones are male!) I-2 _______ II-8 ________ III-9 _______ II-7 ________ 4. What is meant by the term “carrier”? _________________________________ ___________________________________________________________________ 5. Use a colored pencil to indicate all those who are known to be carriers. 6. If a woman with no hemophilia in her family history marries a hemophiliac, what are the chances they will have a son who suffers from this disease? _________________ Prove this with a Punnett Square.

What does this mean for their daughters? __________________________________________


Pedigree C: Brachydactyly Brachydactyly is a condition in which fingers are abnormally short. The last two joints of the middle finger are shortened. In the other four fingers, the last two joints are fused into one and are also shortened. Use symbols B for dominant, b for recessive.


Is the brachydactyly trait dominant or recessive? ________________________ Use a specific example from the pedigree that supports your answer: __________ ______________________________________________________________


What are the most likely genotypes for II-9 and II-10? ____________________ Explain your answer: ______________________________________________ ______________________________________________________________


How are II-2 and II-3 related? ______________________________________


How many children do parents II-6 and II-7 have? _______________________


Which of his grandchildren most resembles II-10?________________________