1. Record your answers to the monohybrid practice problem here.

1. What colored offspring will two parents with genotype “BB” produce? Explain your answer.

What if both rabbits were heterozygous “Bb”? What color will the offspring be?

2. What is the probability that a brown offspring will be produced?

3. What is the probability that a white offspring will be produced?

4. What is the phenotypic ratio of brown to white offspring?

5. What is the probability that a homozygous dominant offspring will be produced?

6. What is the probability that a heterozygous offspring will be produced?

7. What is the probability that a homozygous recessive offspring will be produced?

8. What is the genotypic ratio of this monohybrid cross?

9. What are the phenotypic and genotypic ratio for a cross between a homozygous dominant individual and a heterozygous individual?   Construct a Punnett Square.


A.  Your body cells contain 46 chromosomes within the nucleus. As you probably know, half of this genetic information (23 chromosomes) is inherited from your mother and half (the other 23 chromosomes) from your father.

B.  The genetic information carried by the gametes (the sperm and egg), when incorporated into a fertilized egg, will determine all the physical and physiological traits of the offspring.

C.  There is a special type of cell division that changes the chromosome number from the normal diploid number (46) to the haploid number (23) found in sperm and eggs.

D.  This special type of cell division is called meiosis. 

E.  The stages of meiosis, in most respects, are similar to those of mitosis.

F.  The important differences include the following:

     1.  The chromosome number is reduced from diploid to haploid to form gametes

     2.  An exchange of genetic material takes place in a process known as crossing over; gametes produced by meiosis are not genetically alike.

     3.  Meiosis involves two cell divisions (meiosis I and meiosis II); one occurs immediately after the other.  There is no DNA replication between the two divisions.

G.  The genes on a chromosome may exist in more than one form, called alleles.

     1.  Individuals inherit two alleles for each trait, one received from the mother in the egg and the other from the father in the sperm.

     2.  If the two inherited letters represent different forms of the gene (different alleles), an individual is called heterozygous for that trait. The allele that is expressed in heterozygous individuals is referred to as dominant and is represented by a capital letter (B).

     3.  The allele not expressed in heterozygous individuals is called recessive. Recessive alleles are represented with lowercase letters (b).

     4.  If individuals inherit two identical alleles for a trait (BB or bb), they are said to be homozygous for that trait.

     5.  The combination of alleles for a trait is an individual’s genotype (such as BB or Bb).

     6.  The physical description of a specific trait is called the phenotype (such as brown eyes, type AB blood, or freckles).

1.  In this exercise, you will work to identify some of the more common phenotypic differences in humans.  Shade in or circle the trait that best fits your alleles.  From your allelic information, type in what you predict your parent’s alleles would be.  Make sure that the genotype you highlight for yourself is possible based off of your parents’ genotypes.

Follow the examples for face shape below.  My face shape is triangular which is a recessive trait.   My mother and father have round face shapes.  The only way I could have inherited their recessive alleles is for both parents to be heterozygous for face shape.

  Maternal Alleles  Paternal Alleles  Trait  Alleles (Genotype)
  a.  Ff    Ff  face shape    FF or Ff – round ff – triangular
  b.        hair    HH – curly Hh – wavy hh – straight
  c.        eye size    EE – large Ee – medium ee – small
  d.            eye distance  DD – close together Dd – medium spacing dd – far apart
  e.        eyebrow shape  BB or Bb – thick bb – thin
  f.        eyelash length  LL or Ll – long ll – short
  g.        nose size    NN – big Nn- medium nn – small
  h.        lips    GG or Gg – thick gg – thin
  i.        ear lobes    RR or Rr – free rr – attached
  j.        cleft in chin    TT or Tt – present tt – absent
  k.        freckles    QQ or Qq – freckles qq – no freckles


2.  Dimples are a dominant trait.

     A man heterozygous for dimples marries a woman with no dimples.

Determine the following:

a.  Genotype of the man:


b.  Genotype of the woman:


c.  Possible genotypes of children:


d.  Probability of dimples in their children:

3.  PKU disease is recessive.

     A man and a woman are both heterozygous carriers for PKU disease.

Determine the following:

a.  Genotype of the man:


b.  Genotype of the woman:


c.  Possible genotypes of children:


d.  Probability of PKU disease in their children:

4.   A man is heterozygous for dimples and a heterozygous carrier for PKU disease. He marries a woman who is also heterozygous for dimples and a heterozygous carrier for PKU disease.

Determine the following:

a.  Genotype of man:

b.  Genotype of woman:

c.  Possible gametes of the man

d.  Possible gametes of the woman



Now that you are beginning to be an expert in solving genetics problems, let us try something a little more challenging. Up to this point, we have dealt with traits that have only two alleles. Human blood types (A, B, AB and O)are inherited by multiple alleles. There are three possible alleles for blood type, and two of those alleles are codominant.

Codominance is a special type of inheritance in which two alleles are equally dominant. Both alleles are expressed independently of each other, resulting in a heterozygous individual that shows both homozygous phenotypes.  For example, red blood cells with both A and B proteins in their cell membranes.

Multiple alleles mean that there are more than two possible alleles for that trait, although each person will inherit only two of those alleles (one from the mother and one from the father).

In human blood types, the alleles IA and IB are codominant. Both IA and IB are dominant over the recessive allele i.

(IA, IA) or (IA, i)type A blood
(IB, IB) or (IB, i)type B blood
(IA, IB)type AB blood
(ii)type O blood

5.  When Howard Hughes (the reclusive multimillionaire) died, he left no legitimate heirs. Soon, however, a long succession of people claiming to be his children began to appear.  A young man claiming to be Howard Hughes’s child sued for a share of the estate. The judge ordered blood tests to determine the validity of the claim.  Howard Hughes had blood type B, the mother of the young man had blood type A, and the young man himself had type O blood.

a.  If you were the judge, how would you rule?  Explain your answer.

b.  Use the Punnett Square below to explain probability of question 5.  


6. If the mother is type AB and the father is type O, what possible blood type(s) might their children be?

            a.  ANSWER 

            b.  Use the Punnett Square below to explain probability of question 6. 


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