Practice Problems

Now let’s try some of these Punnett squares!

1.    An albino man and a normally pigmented woman who is heterozygous for the trait, have children.  What is the probability that they will have an albino child?

50%

2.    Polydactyly is a condition where those affected have more than five fingers and/or toes.  It is a dominant disorder.  If a man without polydactyly marries a woman who is heterozygous for the condition, what is the proportion of their children that will have polydactlyly?

50%

3. In chile pepper plants, an allele for an ultra hot taste (H) is dominant over the allele for more mild flavor (h).  Give the genotypic and phenotypic ratios for the results of each of the following crosses:

                                                         (a) Hh    X    hh 1:1 and 1:1

                                                         (b) Hh    X    Hh 1:2:1 and 3:1

                                                         (c) Hh    X    HH 1:1 and 100%

4.     In human beings, the allele for brown eyes is dominant (B) over blue eyes (b).  Suppose a blue-eyed man marries a brown-eyed woman whose father was blue-eyed.  What proportion of their children would you predict will have blue eyes?

50%

5. A brown-eyed man (Bb) whose father was brown-eyed (BB or Bb) and whose mother was blue-eyed (bb)marries a blue-eyed woman (bb) whose father (Bb) and mother (Bb) were both brown-eyed.  The couple has a blue-eyed son (bb).  For which of the individuals mentioned can you be sure of the genotypes?  What are their genotypes?  What genotypes are possible for the others?

6.   In velociraptors, assume that 4- inch razor-sharp claws (R) are dominant over dull claws (r).  Suppose a female velociraptor with 4- inch razor-sharp claws who is heterozygous (Rr) mates with a dull-clawed male raptor (rr) who's mother (Rr) and father (Rr) had 4- inch razor-sharp claws.  What proportion of the young would you predict will have 4- inch razor-sharp claws (50%)?  What is your most precise estimate of the number of young from a 80 egg nest resulting from this mating that would have 4-inch razor-sharp claws (40)?  What would be the father’s parent’s genotypes?

7. A man is accused in a paternity suit of being the father of a child.  He has type B (IB IB or IBi) blood, while the woman has type AB blood (IAIB).  The child's blood is tested and found to be type O (ii).  The medical technician who is conducting the test, checks and double checks the results, and announces to the court, that there is a major problem.  Could the accused man potentially be the father of the baby? (Yes, if IBi)  What's the problem that the med-tech has detected? Since the mother is AB she can not form any eggs with a (i), therefore she is not the mother.

Practice Problems- dihybrid crosses

( Rember, foil to determine the possible gametes!)

8.   In cats, white spots are caused by a dominant allele called W, while ww individuals are solid-colored.  Also short hair(S) is dominant to long hair(s).  What would the probability be of getting a white-spotted, long-haired cat from this cross: WWSs x Wwss? (50%) What are the phenotypes of the parents? (white spotted short hair and white spotted long hair) (Rember, foil to determine the possible gametes!)

9.  What type of plants (phenotypes) will be produced by two parent plants heterozygous for both seed color AND for seed shape? (Round is dominant over wrinkled and Yellow dominant over green) (9:3:3:1) 

10. Superman is homozygous dominant for super strength and heterozygous for x-ray vision (SSXx).  He falls in love with Wonder Woman who is homozygous recessive for both traits.  What is the probability that a child will inherit both super strength and x-ray vision? (50%)

11. In dogs, short hair is dominant over long; dark hair is dominant over light.  One dog is heterozygous for hair length and shows the recessive trait for hair color.  Another dog is homozygous dominant for hair length and heterozygous for hair color.  Construct a Punnett square showing a cross between these two animals.  Identify possible phenotypes

Short dark and short light (1:1)

12. A tossed coin heads six times straight.  What is the probability of it being tails on the 7th toss or for that matter any toss?

 C.  50%  

13. Two coins are tossed together 40 times.  Approximately how many times should both land tails?

A.  10  

14. Tay-Sachs disease is a fatal, recessive disorder.  People born with this disorder generally die before they are five years old.  A couple has had three children all of whom have died of Tay-Sachs disease.  What is the probability that their fourth or any child will be healthy with regard to Tay-Sachs disease?

B.  3/4               

15. What is the probability that the above couple will have a child who has Tay-Sachs disease?

C.  1/4    

16. What is the probability that the above couple would have three children all with Tay-Sachs disease?

A.  1/64 

17. A blood test can be performed on individuals to see if they are homozygous or heterozygous for Tay-Sachs disease.  One member of a particular couple is homozygous and the other is heterozygous.  What is the probability that this couple will have a child with Tay-Sachs disease?

A.  0%   

18. What is the probability that the couple’s children will be carriers (heterozygous)?

C.  50%   

19. What is the probability that this couple will have two children who are both carriers (heterozygous)?

B.  25%        

20. In fruit flies, gray body (G) is dominant over black body (g).  Two gray flies were mated and produced 150 gray flies and 50 black flies.  The parent were probably these genotypes:

B.  Gg and Gg  

21. What will be the result if a Gg fruit fly mates with a gg fruit fly?

C.   About 50% of the offspring will show the recessive trait.

22. What will be the result if a GG fruit fly mates with a gg fruit fly?

A.    All offspring will show the dominant trait.

23. What will be the result if a gg fruit fly mates with a gg fruit fly?

B.    All offspring will show the recessive trait.

24.  What will be the result if a Gg fruit fly mates with a Gg fruit fly?

D.   About 75% of the offspring will show the dominant trait. 

25. What will be the result if a Gg fruit fly mates with a GG fruit fly?

A.    All offspring will show the dominant trait.

26. A pair of animals both with the genotypes Aa had one offspring.  Its genotype was aa.  What is the probability that the next offspring the animals produce will have the genotype aa?

C.  1/4  

27.   If two offspring result from the cross Aa x Aa, what is the probability that both offspring will have the genotype aa?

A.  1/16  

28. Which kind of phenotype (dominant or recessive) can have more than one genotype? Give one example. dominant (HH or Hh)

29. Is it possible to be heterozygous for a characteristic and show the recessive phenotype? Explain your answer. no if it is heterozygous it will show the dominant phenotype

30. In a hybrid cross between homozygous dominant and homozygous recessive parents (this is the Parental cross), there are 32 offspring in the F2 generation.  How many of the 32 offspring would be expected to show both recessive traits? First determine the F1.

F1 would all be heterozygous for both traits and you should recognize that the F2 phenotype ratio from such a cross would be 9:3:3:1. Therefore 1/16 of 32 = 2 offspring should be recessive for both traits.

31. Straight wings are dominant over curly wings in fruit flies. How would you determine whether a straight-winged fly is heterozygous or homozygous?

 Perform a test cross. Mate the dominant phenotype to a recessive phenotype if any offspring are the recessive phenotype then the mystery parent was hetero, if all (large # of offspring, were ony the dominant phenotype then we can safely assume, the mystery was homozygous dominant.

32. The houseplant Coleus has curves on the edge of its leaves.  Deep curves (D) are dominant over shallow curves (d).  Draw a Punnett square showing the possible offspring of a cross between a homozygous plant with deep-curved leaves and one with shallow-curved leaves.  What are the possible phenotypes and genotypes of the offspring? 

 DD x dd = 100% deep curves and 100% Dd

33. In guinea pigs, a black coat is dominant over a white coat.  Can two white-coated parents produce an offspring with a black coat?  Why or why not?

 No, both white coat parents would be bb, therefore no dominant alleles are present for their gametes.

34. In certain cattle, the polled (hornless) trait is dominant over horns. A polled bull is mated to a cow with horns.  Their calf is born and develops horns.  The bull is then mated to another cow, which is polled.  Their calf also develops horns.  Identify the genotypes of all individuals.

cow #1 pp calf#1 pp, therefore bull Pp

calf #2 pp therefore cow #2 was also Pp

35.  Sickle cell anemia (SCA) is a human genetic disorder caused by a recessive allele. A couple plan to marry and want to know the probability that they will have an affected child.  With your knowledge of Mendelian inheritance, what can you tell the parents if:

a)    Both are normal, but each has one affected parent and the other parent has no family history of SCA.

b)   The man is affected by the disorder, but the woman has no family history of SCA.

Scenario A) affected grandparents are ss, therefore both parents are carriers Ss. Ss x Ss = You could tell these parents that the probablility of their children having SSA is 25% and that 50% of their children would be carriers, just like them and 25% should be SS.

Scenario B) Dad is ss and we can assume that mom is SS (not sure but a safe assumption) therefore their children should be all normal but all carriers (Ss).