High school biology worksheet

Biology 30—Lab III.2 Page 6

Pedigree Studies Page 5

PEDIGREE STUDIES

Pedigrees are not reserved for show dogs and race horses. All living things, including humans, have pedigrees. A pedigree is a diagram that shows the occurrence and appearance, or phenotype, of a particular genetic trait from one generation to the next in a family. Genotypes for individuals in a pedigree usually can be determined with an understanding of inheritance and probability.

In this investigation, you will:

a) learn the meaning of all symbols and lines that are used in a pedigree,

b) calculate expected genotypes for all individuals shown in pedigrees.

Part A: Background Information

The pedigree in the diagram to the right shows the pattern of inheritance in a family for a specific trait. The trait being shown is ear lobe shape. Geneticists recognize two general ear lobe shapes, free lobes and attached lobes. The gene responsible for free lobes (E) is dominant over the gene for attached lobes (e).

Figure 1

In a pedigree, each generation is represented by a Roman numeral. Each person in a generation is numbered. Thus each person can be identified by a generation number and individual number. Males are represented by squares whereas females are represented by circles.

Part B: Reading A Pedigree

In Figure 1, persons I1 and I2 are the parents. The line which connects them is called a marriage line. Persons II1, 2, and 3 are their children. The line which extends down from the marriage line is the children line. The children are placed left to right in order of their births. That is, the oldest child is always on the left.

1. What sex is the oldest child?

2. What sex is the youngest child?

Using a different pedigree of the same family at a later time shows three generations. Figure 2 shows a soninlaw as well as a grandchild. Generation I may now be called grandparents.

3. Which person is the soninlaw?

4. To whom is he married?

5. What sex is their child?

Figure 2	Figure 3

Part C: Determining Genotypes from a Pedigree

The value of a pedigree is that it can help predict the genes (genotype) of each person for a certain trait.

All shaded symbols on a pedigree represent individuals who are homozygous recessive for the trait being studied. Therefore, persons I1 and II2 have ee genotypes. They are the only two individuals who are homozygous recessive and show the recessive trait. They have attached ear lobes.

All unshaded symbols represent individuals who have at least one dominant gene. These persons show the dominant trait.

To predict the genotypes for each person in the pedigree, there are two rules you must follow.

Rule 1: Assign two recessive genes to any person on a pedigree whose symbol is shaded. (These persons show the recessive trait being studied.) Small letters are written below the person’s symbol.

Rule 2: Assign one dominant gene to any person on a pedigree whose symbol is unshaded. (These persons show the dominant trait being studied.) A capital letter is written below the person’s symbol.

These two rules allow one to predict some of the genes for the persons in a pedigree. Figure 3 shows the genes predicted by using these two rules.

To determine the second gene for persons who show the dominant trait, a Punnett square is used. In Figure 3, we already know that the grandfather (I1) is ee. If the grandmother (I2) were EE, could any ee children (like II2) be produced? A Punnett Square shows this combination to be impossible. Thus, the grandmother must be heterozygous or Ee.

6. Can an Ee parent and an ee parent have the results shown in generation II? Prove your answer by showing the results in a Punnett square.

	e	e
E
e

7. Predict the second gene for person II3 and II4 (Read them from the Punnett square).

8. Could child II3 or II4 be EE ? Explain.

To predict the second gene for person II1, a different method must be used, since he could be either EE or Ee.

9. Can an EE person married to an ee person (II2) have children with free ear lobes?

10. Can an Ee person married to an ee person have children with free ear lobes?

11. Prove your answers by showing the results of these crosses in the Punnett squares below.

	e	e		e	e
E			E
E			e

In this case, the second gene from person II1 cannot be predicted using Punnett squares. Either genotype Ee or EE may be correct. When this situation occurs, both genotypes are written under the symbol (Figure 4). Predicting the second gene for III1 results in her being heterozygous. Although her mother must provide her with one recessive gene, she has free lobes, so the second gene must be dominant (Figure 4).

Figure 4

At some time in the future, if II1 and II2 have many more children, one might be able to predict the father’s second gene. For example, if they have ten children and all show the dominant free lobes, one could safely conclude that he is EE. If, however, they have some children with attached ear lobes (ee), then he must be Ee.

12. Examine the pedigree on the right. Which Punnett square would best fit this family? Explain. B, Both parents must pass on a recessive gene.
a)	E	e	b)	E	e	c)	E	E
E			E			E
E			e			E

Analysis

1. Using the pedigrees below, predict the genotypes for these families.

a)

b)

c)

2. Examine the pedigree below.

a) How many generations are shown?

b) Identify by generation and number those persons with attached ear lobes.

c) Give the genotype for all persons have attached ear lobes.

3. Predict the genotypes for all persons in question 2.

Pedigree Studies

Pedigree Studies Page 1 of 6