2. Mendel's Laws of Inheritance

Pedigrees

2. Mendel's Laws of Inheritance

Pedigrees

3:28 minutes

Problem 30

Textbook Question

Consider the three pedigrees below, all involving a single human trait.Which combination of conditions, if any, can be excluded?dominant and X-linkeddominant and autosomalrecessive and X-linkedrecessive and autosomal

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Analyze the inheritance patterns in each pedigree to determine if the trait is dominant or recessive. A dominant trait typically appears in every generation, while a recessive trait may skip generations.

Examine the sex distribution of affected individuals in each pedigree. If the trait is X-linked, it will often show a distinct pattern, such as males being more frequently affected in the case of recessive X-linked traits.

For each pedigree, check if the trait can be explained by autosomal inheritance. Autosomal traits affect both sexes equally, while X-linked traits often show a sex bias.

Compare the observed inheritance patterns in the pedigrees with the expected patterns for each of the four conditions (dominant and X-linked, dominant and autosomal, recessive and X-linked, recessive and autosomal). Eliminate any condition that does not match the observed data.

Summarize which combinations of conditions can be excluded based on the analysis of the pedigrees and the inheritance patterns observed.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Dominant vs. Recessive Traits

In genetics, traits can be classified as dominant or recessive based on their expression in offspring. A dominant trait only requires one copy of the allele to be expressed, while a recessive trait requires two copies. Understanding this distinction is crucial for analyzing pedigrees, as it influences how traits are inherited and displayed across generations.

Autosomal vs. X-linked Inheritance

Inheritance patterns can also be categorized as autosomal or X-linked. Autosomal traits are located on non-sex chromosomes and affect both sexes equally, while X-linked traits are associated with genes on the X chromosome and often exhibit different patterns of inheritance in males and females. This distinction is vital for interpreting pedigree charts and determining which combinations of conditions are plausible.

Pedigree Analysis

Pedigree analysis is a method used to trace the inheritance of traits through generations in a family. By examining the presence or absence of traits in individuals and their relatives, one can infer the mode of inheritance (dominant, recessive, autosomal, or X-linked). This analysis helps in identifying which combinations of inheritance patterns can be excluded based on observed data.

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Textbook Question

A 'wrongful birth' case was recently brought before a court in which a child with Smith–Lemli–Opitz syndrome was born to apparently healthy parents. This syndrome is characterized by a cluster of birth defects including cleft palate, and an array of problems with the reproductive and urinary organs. Originally considered by their physician as having a nongenetic basis, the parents decided to have another child, who was also born with Smith–Lemli–Opitz syndrome. In the role of a genetic counselor, instruct the court about what occurred, including the probability of the parents having two affected offspring, knowing that the disorder is inherited as a recessive trait.

Consider the three pedigrees below, all involving a single human trait.Given your conclusions in part (a), indicate the genotype of the following individuals:II-1, II-6, II-9If more than one possibility applies, list all possibilities. Use the symbols A and a for the genotypes.

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Textbook Question

Consider the three pedigrees below, all involving a single human trait.For each combination that you excluded, indicate the single individual in generation II (e.g., II-1, II-2) that was most instrumental in your decision to exclude it. If none were excluded, answer 'none apply.'

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Textbook Question

The gene controlling the Xg blood group alleles (Xg⁺ and Xg⁻) and the gene controlling a newly described form of inherited recessive muscle weakness called episodic muscle weakness (EMWX) (Ryan et al., 1999) are closely linked on the X chromosome in humans at position Xp22.3 (the tip of the short arm). A male with EMWX who is Xg⁻ marries a woman who is Xg⁺ and they have eight daughters and one son, all of whom are normal for muscle function, the male being Xg⁺ and all the daughters being heterozygous at both the EMWX and Xg loci. Following is a table that lists three of the daughters with the phenotypes of their husbands and children.Create a pedigree that represents all data stated above and in the following table.

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Textbook Question

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?

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Textbook Question

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?

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Textbook Question

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.

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