Pedigrees

Sections

Full-Length Text

PEDIGREES

• Allow for the tracking of traits through multiple generations
• Allows for the prediction of the probability that a trait will appear in an offspring

• Squares represent male
• Circles represent female
• Shaded shapes indicate the individual is affected with the trait being studied

Carrier

Individuals that are phenotypically normal but carry the recessive trait (which can be passed on to offspring)

Cystic Fibrosis

• Disease due to recessive allele

Huntington's Disease

• Disease due to dominant allele

Full-Length Text

• Here we will learn about pedigrees, which are tools used to study the inheritance pattern of traits throughout multiple generations.

• Start a table to understand some key concepts of pedigrees.

• Denote that they allow us to trace traits through multiple generations.

• Denote that they also allow us to predict the probability that a trait will appear in an offspring.

Now, let's use a pedigree to trace a recessive trait through three generations.

• Use an open square to represent an unaffected male.

• Use an open circle to represent an unaffected female.

• Show that shaded shapes indicate that those individuals are affected with the trait being studied.

Now, let's use a pedigree to answer a common clinical question: what's the chance my child will be affected with a heritable recessive trait?

• For this scenario:
  • Write that upper case D is the dominant allele.
  • Write that lower case d is the recessive allele.

• Draw an open box with the letter: A in it, to signify that here we are asked to determine the probability that male boy, "A" is going to be affected with a heritable trait.

Let's show what we know about A's family tree.

Start with family 1. First, the parents:

• Draw an open square and a circle next to one another.

• Use a connecting line to indicate mating between the two individuals.

• Show three offspring connected to their parents by vertical lines.

• Shade two of the offspring to indicate they are affected with the recessive trait.

Next, family 2.

• Shade the male to indicate he has the recessive trait but leave the female open.

• Draw their two offspring as unaffected.

• Connect the unaffected male from family 1 with one of the daughters from family 2.

• Then, draw the two offspring of this union:
  • Draw a girl next to the boy we drew earlier.
  • Shade the girl to indicate she has the recessive trait.

Now, let's try to answer the question: what's the chance that her brother ("A") also has the recessive trait?

Step back through the tree:

• Show that each of the shaded individuals has a genotype: little d, little d.

Now for the open individuals. Let's tackle the parents in family 1, first:

• Show that they must both be heterozygotes: Big d, little d because they have children that have the recessive trait (despite the fact that neither of the parents have the trait).

• Write that these two are carriers – individuals that are phenotypically normal but carry the recessive trait (and can pass it on to their offspring).

• Write that the male parent of individual A must be a heterozygote because he is unaffected but does have an affected child.

Now, family 2:

• The father is affected.

• The mother must have at least 1 dominant allele because neither child is affected.

• We can't conclude whether the other allele is dominant or recessive (could be either) so place a "?" in its position.

• Write that both offspring are heterozygotes because their father is affected.

Now, we're ready to calculate the probability of Individual A having the recessive trait.

• Recall that when two heterozygotes mate, their offspring has a 1 in 4 chance of being homozygous for the recessive allele.

• Write that Individual A has a 1 in 4 chance of having the recessive trait: being little d, little d.

Now, with an understanding of how pedigrees work, let's explore their use when tracing two relatively common genetic diseases – Huntington's Disease and Cystic Fibrosis.

We'll start with Cystic Fibrosis, a disease due to a recessive allele.

• Here, we're trying to determine the probability that 3rd generation child C will be affected with the recessively inherited disease.

• Write that upper case F is the dominant allele while lower case f is the recessive allele.

• Draw two mating couples, family 1 and family 2.

• Draw the two children of family 1.

• Shade the male child to indicate he has cystic fibrosis and label this Individual A.

• Draw the three children of family 2.

• Shade one of the male children to indicate he has cystic fibrosis.

• Label the female Individual B.

• Indicate that we are trying to determine the chance that the child of Individual A and Individual B will have cystic fibrosis.

• Underneath the shaded individuals, write that their genotypes are little f, little f.
  • This means Individual A is homozygous for the recessive allele.

• Because the child in family 2 is homozygous recessive, the parents have to be heterozygotes (carriers).

• This means that Individual B can be either homozygous for the dominant (non-cystic fibrosis) allele, or be a carrier.
  • Show that if Individual B is homozygous dominant, then the child has 0% chance of having cystic fibrosis (though she would be a carrier).
  • If Individual B is a heterozygous carrier, then the child has a 50% chance of having cystic fibrosis.

Finally, we'll look into Huntington's disease, which is due to a dominant allele.

• Again, we're trying to determine the probability that 3rd generation child C will be affected with a disease, but now the disease is dominantly inherited.

• Write that upper case H is the dominant allele and lower case h is the recessive allele.

• Draw the two parents in family 1.

• Shade the male.

• Draw the children and shade them all.

• Label a male Individual A.

• Draw the two parents in family 2.

• Shade the male.

• Draw children but only shade one and label her Individual B.

• Draw the two children of Individual A and Individual B, shading one and writing that his genotype has at least one dominant allele.

• Label the other child Individual C and indicate that we want to determine the chance that she will have Huntington's disease.

To do so, we need to first determine the genotypes of A and B.

• We know that they are both heterozygotes because they were both born of an affected individual (having at least one dominant allele) and an unaffected individual (two recessive alleles).

• Recall that: when two heterozygotes mate, their offspring has a 25% chance of being homozygous for the recessive allele....

• So, C has a 75% chance of being either Hh or HH (either of which are affected).

This ends our tutorial on the use of pedigrees to track and study the transmission of traits through the generations.

UNIT CITATIONS:

1. Campbell, N. A. & Reece, J. B. Biology, 7th ed. (Pearson Benjamin Cummings, 2005).

2. Alberts, B., Bray, D., Hopkin, K., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. Essential Cell Biology, 3rd ed. (Garland Science, 2010).

3. Griffiths, A. J. F., Gelbart, W. M., Lewontin, R. C. & Miller, J. H. Modern Genetic Analysis: Integrating Genes and Genomes, 2nd ed. (W.H. Freeman and Company, 2002).

4. Cummings, M. Human Heredity: Principles and Issues. (Cengage Learning, 2012).

5. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. Molecular Biology of the Cell, 5th ed. (Garland Science, 2008).

6. Strachan, T. & Read, A. Human Molecular Genetics. (Garland Science, 2010).

7. Griffiths, A. J. F. Introduction to Genetic Analysis, Volume 10. (Macmillan, 2008).

8. Okada, S. Biochemical Basis of Inherited Human Disease. (Ardent Media, 1973).

9. AP Biology, 2nd ed. (Barron's Educational Series).

10. Chiras, D. D. Human Biology. (Jones & Bartlett Publishers, 2013).

11. Callihan, L. Biology. (Research & Education Assoc., 2011).

12. Kent, M. Advanced Biology. (OUP Oxford, 2000).

13. Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., Ploegh, H. & Matsudaira, P. Molecular Cell Biology, 6th ed. (W. H. Freeman and Company, 2008).

14. Schaaf, C. P., Zschocke, J. & Potocki, L. Human Genetics: From Molecules to Medicine. (Lippincott Williams & Wilkins, 2011).

15. Pierce, B. A. Transmission and Population Genetics. (Macmillan, 2006).