Name: Puzzling Pedigrees Essential Question: How can pedigrees be used to study the inheritance of human traits? Studying inheritance in humans is more difficult than studying inheritance in fruit flies or pea plants. For obvious reasons, geneticists studying humans cannot set up breeding experiments to study the resulting offspring! Clearly, other approaches must be used when studying human genetics. Family trees (also called "pedigrees") provide a useful way to gather evidence about inherited traits in humans. One approach to studying genes in humans is to construct a pedigree from data on a single trait within a family. These data can then be used for genetic analyses. Researchers use pedigrees to look for inheritance patterns as traits are passed from one generation to the next. These patterns can provide clues to the way the trait is passed from parents to their offspring. The genotypes of most of the individuals in a pedigree can be determined by examining the pattern in which the trait is inherited from one generation to the next. By applying Mendel's principles, you can figure out whether the trait being examined is dominant or recessive. For some individuals in a pedigree, however, there may not be enough information to determine the full genotype. Part A Learning How to nterpret Pedigrees You have used Punnett squares and your knowledge of which trait is dominant to predict what fraction of the offspring of a particular set of parents are likely to have each trait. But how do scientists find out which trait is dominant when they cannot do breeding experiments? They analyze data provided by pedigrees. The figure below is a pedigree, or a diagram of a family s pattern of inheritance for a specific trait. Notice that in a pedigree, each person is represented by a number and each generation is represented by a Roman numeral. n this way, each person can be identified by a generation numeral and an individual number (i.e. person -6 is the daughter-in-law in this family). Males are represented by squares and females by circles. n this pedigree, unshaded symbols (squares or circles) indicate people who display the dominant trait. These individuals are either homozygous dominant or heterozygous for the trait. Shaded symbols indicate people who are homozygous for the recessive trait. n this pedigree, -1 and -2 are the grandparents. The horizontal line that connects them is called a marriage line. The vertical line that extends down from the marriage line connects the children to the parents. Children are listed in order of their births from left to right. n other words, the oldest child is always placed on the extreme left. n this pedigree, persons -1, -2, -3, -4, and -5 are the children of persons -1 and -2.
The trait being analyzed in this pedigree is ear-lobe shape. There are two general ear-lobe shapes, free lobes and attached lobes. The gene responsible for free ear lobes, represented by the capital letter E, is dominant over the gene for attached ear lobes, represented by the lowercase letter e. People with attached ear lobes are homozygous for the recessive trait and are represented as ee. Persons -1 and -5 are homozygous recessive (ee) and have attached ear lobes. The people represented by the unshaded symbols have two possible genotypes: EE or Ee. Demonstrate your ability to interpret a pedigree by answering the questions below. Free Ear Lobes (left) vs Attached Ear Lobes (right) mages from Wikimedia Commons 1. How many generations are represented in this pedigree? 2. What is the genotype of -2? How do you know? Explain your answer. 3. What are the genotypes of -1, -2, -3, and -4? How do you know? Explain your answer. 4. What are the two possible genotypes for -6? Why is the genotype unknown? Explain your answer. 5. f -6 is EE, what is the genotype of her child (-1)? 6. What sex is the oldest child in generation? 7. Who is the daughter-in-law in this family?
Part B Recognizing Patterns in Pedigrees Although most human traits are the result of interactions between multiple genes and/or environmental factors, some hereditary disorders in humans are caused by a single gene. Pedigrees from several generations of an affected family enable scientists to determine whether such conditions are dominant or recessive. Knowing this, scientists can predict how likely it is that a child of particular parents will have the condition. Phenylketonuria, or PKU, is an example of such a genetic condition. ndividuals with PKU lack the ability to break down proteins normally. This causes the build-up of a chemical in the brain that can lead to mental disabilities. f PKU is diagnosed shortly after birth, the child can be given a special diet. Typically, children with PKU who are placed on this special diet for the first 10 years of life do not develop symptoms of the condition. n most of the United States, newborns are routinely tested for PKU within a few days after birth. Analysis of pedigrees indicates that PKU almost always appears in children of people who do not have the condition. The figure below shows a family in which two grandchildren inherited PKU and five did not. These numbers vary from family to family. Family with PKU 1. s PKU likely to be a dominant or a recessive trait? Use evidence from the pedigree to support your answer. 2. What are the most likely genotypes of persons -1 and -2? Explain your answer.
n the study of genetic diseases, a person who is heterozygous for a recessive genetic condition is called a carrier. Such a person does not have the condition, but can pass on an allele for it to his or her children. The recessive allele is hidden, or masked until it shows up in a homozygous individual who has the condition. A person who has a recessive condition is not called a carrier. n some pedigrees, symbols with a dot in the middle are used to represent carriers. Draw dots inside the shapes of the known carriers in the Family with PKU pedigree above. We have just observed that recessive conditions can skip a generation, even though the allele is still present in carriers. s this pattern the same for all other hereditary diseases? Consider polydactyly, which causes individuals to have an extra finger on their hands or extra toes on their feet. This condition is not life threatening, but it does have a tendency to run in families. The pedigree below shows a typical family history of a family with polydactyly. As you can see, one grandchild in this family has polydactyly, but her siblings do not. Proportions of offspring with polydactly vary from one family to the next. Family with Polydactyly 1. s polydactyly likely to be a dominant or a recessive trait? Use evidence from the pedigree to support your answer. 2. What are the most likely genotypes of persons -1 and -2? Support your answer with evidence from the pedigree. 3. There is a small village in a mountain valley in Spain where a large number of people are polydactyl. Why do you think this trait is so prevalent in this village? Explain your answer.
Another inheritance pattern that occurs in humans involves recessive alleles that are sex-linked. As you learned earlier, sex-linked alleles are those located on one sex chromosome but not on the other. Remember that, in humans, females are XX and males are XY. Most human sex-linked alleles are located on the X chromosome. Since the Y chromosome is always inherited from the father, males only receive X-linked alleles from their mothers. A male therefore needs only one copy of a sex-linked recessive allele to exhibit the recessive trait. n contrast, a female must inherit two such recessive alleles one from each parent to exhibit the trait. Red-green colorblindness is a common sex-linked disorder that involves a malfunction of light-sensitive cells in the eyes. Affected individuals often have difficulty distinguishing reds from greens. The pedigree below shows the occurrence of red-green colorblindness in four generations of a family. The circles with a dot represent carriers. Circles representing females whose genotypes are unknown are marked with a question mark. Family with Red-Green Colorblindness V 1. Why is the genotype of person -2 unknown? Could the genotype ever be determined? How? 2. t is rare but not impossible for females to exhibit sex-linked (X-linked) recessive traits. Describe a scenario that could produce a female offspring with an X-linked recessive trait.
3. Draw a pedigree representing the following family history. Be sure to indicate the gender, condition, and genotype of each individual. Use shaded symbols to represent individuals with the recessive trait. Use symbols with a dot to represent carriers. Below your pedigree, write a brief paragraph explaining how you were able to determine the gender, condition, and genotype of the children and grandchild. Family History A man and woman marry. The woman is a carrier of hemophilia, a recessive X-linked disorder (X H X h ). The man does not have hemophilia (X H Y). They have 2 boys and 1 girl. The mother passed the gene on to the two boys who died in childhood. The daughter grows up and marries a man without hemophilia. Their first child has the disorder.