2. Mendel's Laws of Inheritance

Two mothers give birth to sons at the same time at a busy urban hospital. The son of mother 1 is afflicted with hemophilia, a disease caused by an X-linked recessive allele. Neither parent has the disease. Mother 2 has a normal son, despite the fact that the father has hemophilia. Several years later, couple 1 sues the hospital, claiming that these two newborns were swapped in the nursery following their birth. As a genetic counselor, you are called to testify. What information can you provide the jury concerning the allegation?

The accompanying pedigree below shows a family in which an autosomal recessive disorder is present. Family members I-2 and II-2 are affected by the disorder and have the genotype dd. A pregnancy involving II-4 has just undergone genetic testing for a VNTR that is linked to the disease gene. The VNTR has a recombination frequency of r = 20 with the disease gene. The VNTR has two alleles, V₁ and V₂. The gel electrophoresis patterns for each family member are shown, including the VNTR genotype for II-4. Based on the information given, answer the following questions about the family.

Excluding II-4, what is the genotype of each family member for the disease gene?

Galactosemia is an autosomal recessive disorder caused by the inability to metabolize galactose, a component of the lactose found in mammalian milk. Galactosemia can be partially managed by eliminating dietary intake of lactose and galactose. Amanda is healthy, as are her parents, but her brother Alonzo has galactosemia. Brice has a similar family history. He and his parents are healthy, but his sister Brianna has galactosemia. Amanda and Brice are planning a family and seek genetic counseling. Based on the information provided, complete the following activities and answer the questions.

Draw a pedigree that includes Amanda, Brice, and their siblings and parents. Identify the genotype of each person, using G and g to represent the dominant and recessive alleles, respectively.

Draw a pedigree containing two parents and four children. Both of the parents have AB blood type. The first child is type A, the second child is type AB, and the third child is type B.

What is the name of the genetic phenomenon producing this observation?

The following pedigree is characteristic of an inherited condition known as male precocious puberty, where affected males show signs of puberty by age 4. Propose a genetic explanation of this phenotype.

Alkaptonuria is an infrequent autosomal recessive condition. It is first noticed in newborns when the urine in their diapers turns black upon exposure to air. The condition is caused by the defective transport of the amino acid phenylalanine through the intestinal walls during digestion. About 4 people per 1000 are carriers of alkaptonuria.

Sara and James had never heard of alkaptonuria and were shocked to discover that their first child had the condition. Sara's sister Mary and her husband, Frank, are planning to have a family and are concerned about the possibility of alkaptonuria in one of their children.

The four adults (Sara, James, Mary, and Frank) seek information from a neighbor who is a retired physician. After discussing their family histories, the neighbor says, 'I never took genetics, but I know from my many years in practice that Sara and James are both carriers of this recessive condition. Since their first child had the condition, there is a very low chance that the next child will also have it, because the odds of having two children with a recessive condition are very low. Mary and Frank have no chance of having a child with alkaptonuria because Frank has no family history of the condition.' The two couples each have babies and both babies have alkaptonuria.

What was incorrect about the information given to Sara and James? What is incorrect about the information given to Mary and Frank?

Alkaptonuria is an infrequent autosomal recessive condition. It is first noticed in newborns when the urine in their diapers turns black upon exposure to air. The condition is caused by the defective transport of the amino acid phenylalanine through the intestinal walls during digestion. About 4 people per 1000 are carriers of alkaptonuria.

Sara and James had never heard of alkaptonuria and were shocked to discover that their first child had the condition. Sara's sister Mary and her husband, Frank, are planning to have a family and are concerned about the possibility of alkaptonuria in one of their children.

The four adults (Sara, James, Mary, and Frank) seek information from a neighbor who is a retired physician. After discussing their family histories, the neighbor says, "I never took genetics, but I know from my many years in practice that Sara and James are both carriers of this recessive condition. Since their first child had the condition, there is a very low chance that the next child will also have it, because the odds of having two children with a recessive condition are very low. Mary and Frank have no chance of having a child with alkaptonuria because Frank has no family history of the condition." The two couples each have babies and both babies have alkaptonuria.

What are the genotypes of the four adults?

The accompanying pedigree shows a family in which one child (II-1) has an autosomal recessive condition. On the basis of this fact alone, provide the following information.

What is the chance that among the three children in generation II who have the dominant phenotype, one of them is AA and two of them are Aa? (Hint: Consider all possible orders of genotypes.)

The accompanying pedigree shows a family in which one child (II-1) has an autosomal recessive condition. On the basis of this fact alone, provide the following information.

What is the probability that all three of the children in generation II who have the dominant phenotype are Aa?

The accompanying pedigree shows a family in which one child (II-1) has an autosomal recessive condition. On the basis of this fact alone, provide the following information.

What are the probabilities for each of the possible genotypes for II-2, II-3, and II-4?

The accompanying pedigree shows a family in which one child (II-1) has an autosomal recessive condition. On the basis of this fact alone, provide the following information.

Using the same alleles, give the possible genotypes for II-2, II-3, and II-4.

The accompanying pedigree shows a family in which one child (II-1) has an autosomal recessive condition. On the basis of this fact alone, provide the following information.

Using A for the dominant allele and a for the recessive allele, give the genotypes for I-1, I-2, and II-1.

Select a human hereditary disease or condition you would like to know more about. Using the OMIM website (http://www.ncbi.nlm.nih.gov/omim), search for the disease and prepare a short synopsis of your findings. Include the following information:

Any available information about the population(s) in which the disease is most common.

For a number of human hereditary conditions, genetic testing is available to identify heterozygous carriers. Some heterozygous carrier testing programs are community-based, often as part of an organized effort targeting specific populations in which a disease and carriers of a disease are relatively frequent. For example, carrier genetic testing programs for Tay–Sachs disease target Ashkenazi Jewish populations and sickle cell disease carrier testing programs target African American populations. The testing is usually free or available at minimal cost, the wait time for results is short, and the results are confidential and unavailable to third parties such as insurance companies. Neither the Tay–Sachs nor the sickle cell allele produces serious consequences for heterozygous carriers.

From a genetic perspective, what is the value of the information obtained by genetic testing of the type described?

For a number of human hereditary conditions, genetic testing is available to identify heterozygous carriers. Some heterozygous carrier testing programs are community-based, often as part of an organized effort targeting specific populations in which a disease and carriers of a disease are relatively frequent. For example, carrier genetic testing programs for Tay–Sachs disease target Ashkenazi Jewish populations and sickle cell disease carrier testing programs target African American populations. The testing is usually free or available at minimal cost, the wait time for results is short, and the results are confidential and unavailable to third parties such as insurance companies. Neither the Tay–Sachs nor the sickle cell allele produces serious consequences for heterozygous carriers.

Do you personally think you would participate in the kind of carrier genetic testing described if you were a member of a population targeted for such testing?