Human Inheritance

Part 1: Human Traits Governed by Mendelian Genetics

Breaking news! These are all myths! Only ear wax is inherited in Mendelian ratios!


Small piece of PTC paper


  1. Record your phenotype and genotype for the following Mendelian traits:

    Trait   Your Phenotype   Your Genotype
    Tongue roller (R/r)

    Dominant allele allows you to roll your tongue

    Free ear lobes (E/e)

    Dominant allele leads to free ear lobes

    Mid-digital hair (M/m)

    Dominant allele results in hair on middle joint of finger

    Widows peak (W/w)

    Dominant allele results in widows peak

    Hitch-hikers thumb (H/h)

    Recessive allele results in ability to bend thumb more than 50°

    Bent little finger (B/b)

    Dominant allele causes first joint to point inwards

    Dimples (D/d)

    Dominant allele results in dimples

    Taste PTC (T/t)

    Dominant allele allows you to taste PTC

    Polydactyly (P/p)

    Dominant allele results in more than five fingers

  2. Two people who are heterozygous for tasting the chemical PTC marry. List the genotypes possible for their children regarding the tasting of PTC. Show your work.
  3. Alfred is heterozygous for tongue rolling and has five fingers. Alfreda, his wife, cannot roll her tongue and is heterozygous for polydactyly.
    1. What is Alfred’s genotype?
    2. What is Alfreda’s genotype?
  4. What are Alfred’s possible sperm with regard to these two traits?
  5. What are Alfreda’s possible eggs with regard to these two traits?
  6. What is the probability that their first child will not roll its tongue and will have the normal number of fingers?

Part 2: Human Traits not Governed by Mendelian Genetics

There are many exceptions to Mendel’s Rules. For example, blood types in humans exhibit two exceptions: codominance and multiple alleles.

  1. Codominance: both alleles are expressed simultaneously (A and B alleles)
  2. Multiple Alleles: more than two alleles exist in a population (ABO)

Background Information

Monoclonal antibodies are used to detect the blood surface markers governed by the ABO and rh factor blood determinants. A sample of blood is combined with each specific antibody: anti-A, anti-B, anti-O or anti-rh+ antibodies. A positive reaction, usually indicated by blood coagulation, indicates the presence of that particular blood surface marker. Antigens are molecules that trigger an immune response, causing the production of antibodies specific to that antigen.

Blood types A, B, AB, and O. Students are meant to indicate the antibodies an antigens for each blood type.


  • Synthetic blood
  • Synthetic antibodies
  • Blood typing tray
  • Toothpicks


  1. Record your unknown blood type.
  2. Place one drop of the unknown blood into each of the three wells on the plastic plate.
  3. Apply one drop of anti-A antibody to one of the three wells and mix with a toothpick. If clumping occurs, the red blood cells have the A antigen embedded in their surface.
  4. Repeat the procedure using anti-B antibody in the second well.
  5. Finally repeat the procedure for the third well using anti-Rh factor antibody.


What is your unknown blood type?

Is it Rh+ or Rh-?

Think about It

  1. Can two people with type A blood have a baby with type O blood? Explain your answer using a Punnett Square.
  2. A mother is Rh negative and her husband is Rh positive. If she becomes pregnant, what are the implications from an immunological standpoint for both fetus and mother, as well as subsequent fetuses?

Part 3: Human Genetics Problems

Genetic counselors are trained to detect inheritance patterns of genetic diseases based on information they obtain from the family. Imagine that you are a genetic counselor and you must solve the following cases based on the information provided. Use the following steps to solve each problem:

  1. Create a legend that indicates the gene pairs (alleles) involved. Use a capital letter to denote the dominant allele and lowercase letter to denote the recessive allele.
    Example: D= dimples d= no dimples
  2. Write the genotype and phenotype of the parents. Example: DD → dimples
  3. Use a Punnet Square to cross the potential gametes of the parents.
  4. Determine the probability based on the Punnet Square.


  1. Autosomal Recessive Inheritance (trait only expressed if homozygous recessive)
    An albino man (nn) marries a normally pigmented woman (N_) who has an albino mother. What is the chance that their children will be albino?
  2. Autosomal Dominant Inheritance (trait expressed as long as one dominant allele is present)
    A daughter wants to know what the chances are that she will develop Huntington’s disease, a degenerative disorder of the nervous system that appears during the ages of 30s to 40s. Her mother has Huntington’s while her father does not have the disease. Try to determine the possibilities from the information you have at hand. What further information do you need in order to more accurately determine the probability?
  3. X-Linked or Sex-Linked Recessive Inheritance (males more likely to express trait than females)
    Hemophilia is a blood disorder that is sex-linked. A woman carrier has children with a normal man. Determine the chances for girls and boys with hemophilia. [Remember that females have the XX genotype and malesn have the XY genotype. Do not place an allele on the Y chromosome. Example: XNXn for female; XY for male]
  4. Multiple Alleles (more than two alleles in the gene pool of the population)
    In the population as a whole there are three alleles (A, B and O) determining blood type, although any one individual can have at most only two of the three alleles. A woman and her son are both blood type O. The woman claims that a man with blood type A is the father of the boy. Is this possible? Explain your answer.
  5. Sex Determination (female genotype XX and male genotype XY)
    1. What fraction of human offspring receive an X chromosome from the mother?
    2. What fraction receive a Y chromosome from the father?
    3. Mr. and Mrs. Jackson have seven boys and one girl. What is the probability that their next child will be a girl?

Part 4: Pedigree Analysis

We will trace the inheritance pattern of the autosomal recessive trait albinism through four generations. The legend is as follows:

Screen Shot 2015-07-13 at 11.30.50 AM

In the pedigree chart below determine the genotypes of each individual. Use a Punnet Square analysis to help you. Remember that the genotype of affected individuals is nn. If you cannot determine both gene pairs of a normal individual, indicate the genotype as N_. Put the genotype next to each symbol.

Screen Shot 2015-07-13 at 11.31.00 AM

Part 5: Practice Problems

  1. A certain disease is linked to the Y chromosome. What is the chance that a female could get this disease?
  2. Polydactyly is an autosomal dominant condition. Assuming two non-polydactyl individuals make a baby, what is the chance of them having a polydactyl child?
  3. If one of the parents described in question 2 is heterozygous for polydactyly, what is the chance of polydactyl offspring?
  4. Two plants, which are heterozygous for a gene that describes plant height, are crossed. What percentage of their offspring will have the same phenotype as the parents if the trait is inherited via complete dominance? Incomplete dominance?
  5. In dogs, wire hair (S) is dominant to smooth (s). In a cross of a homozygous wire-haired dog with a smooth-haired dog, what will be the phenotype of the F1 generation? Genotypes?
  6. A new disease was found to be sex-linked recessive. A man and a woman, wishing to have children together would like to know which percentage of their offspring could possibly have the disease. Neither parent has this disease. What is the highest possible percentage of their offspring that will have this disease? What proportion of their female offspring could get it? Males?
  7. A boy, whose parents and grandparents have normal vision, is color-blind. Color-blindness is an X-linked recessive condition. What are the genotypes for his mother and his maternal grandparents?
  8. Which of the following genotypes is not possible for the offspring of the following cross? AABb x aaBB
    1. AaBB
    2. AaBb
    3. AABB
    4. AABb
  9. In a recent case in Spokane, Washington, a woman claimed a man was the father of her child. The man denied it. The man’s lawyer demanded that blood types be taken to prove the innocence of his client. The following results were obtained: Alleged father, Type O. Mother, Type A. Child, Type AB.
    1. What are the possible genotypes for these three people?
    2. Do you agree with the court’s decision? Why or why not?
  10. It was suspected that two babies had been exchanged in a hospital. Mr. and Mrs. Jones received baby #1 and Mr. and Mrs. Simon received baby #2. Blood typing tests on the parents and the babies showed the following:
    1. Mr. Jones: Type A
    2. Mrs. Jones: Type O
    3. Baby #1: Type A
    1. Mr. Simon: Type AB
    2. Mrs. Simons: Type O
    3. Baby #2 Type O

    Were the babies switched? Do you know this for sure?