Draw all possible conclusions concerning the mode of inheritance of the trait portrayed in the following limited pedigree.
Table of contents
- 1. Introduction to Genetics51m
- 2. Mendel's Laws of Inheritance3h 37m
- 3. Extensions to Mendelian Inheritance2h 41m
- 4. Genetic Mapping and Linkage2h 28m
- 5. Genetics of Bacteria and Viruses1h 21m
- 6. Chromosomal Variation1h 48m
- 7. DNA and Chromosome Structure56m
- 8. DNA Replication1h 10m
- 9. Mitosis and Meiosis1h 34m
- 10. Transcription1h 0m
- 11. Translation58m
- 12. Gene Regulation in Prokaryotes1h 19m
- 13. Gene Regulation in Eukaryotes44m
- 14. Genetic Control of Development44m
- 15. Genomes and Genomics1h 50m
- 16. Transposable Elements47m
- 17. Mutation, Repair, and Recombination1h 6m
- 18. Molecular Genetic Tools19m
- 19. Cancer Genetics29m
- 20. Quantitative Genetics1h 26m
- 21. Population Genetics50m
- 22. Evolutionary Genetics29m
2. Mendel's Laws of Inheritance
Pedigrees
Problem 25a
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. Draw a pedigree of the families of this couple, showing the relevant individuals.

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Step 1: Understand the inheritance pattern. Smith–Lemli–Opitz syndrome is inherited as an autosomal recessive trait. This means that an individual must inherit two copies of the recessive allele (one from each parent) to express the disorder. Carriers (heterozygous individuals) have one recessive allele and one dominant allele but do not show symptoms.
Step 2: Analyze the pedigree. The pedigree shows two affected children (shaded symbols) in generation II, born to unaffected parents (unshaded symbols). This indicates that both parents are carriers of the recessive allele.
Step 3: Assign genotypes. Represent the dominant allele as 'A' and the recessive allele as 'a'. Since the parents are unaffected but have affected children, their genotypes must be heterozygous (Aa). The affected children must have the genotype 'aa'.
Step 4: Calculate the probability of having two affected offspring. Use a Punnett square to determine the probability of each genotype for the offspring of two heterozygous parents (Aa x Aa). The possible genotypes are: AA (25%), Aa (50%), and aa (25%). The probability of having an affected child (aa) is 25%. The probability of having two affected children is the product of the probabilities for each child: (0.25) x (0.25).
Step 5: Summarize findings for the court. Explain that the disorder is genetic and inherited in an autosomal recessive manner. Both parents are carriers, and the recurrence risk for each child to inherit the disorder is 25%. The probability of having two affected children is the product of individual probabilities. This information highlights the importance of genetic counseling for family planning.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Recessive Inheritance
Recessive inheritance occurs when a trait is expressed only when an individual has two copies of the recessive allele, one inherited from each parent. In the case of Smith–Lemli–Opitz syndrome, which is inherited as a recessive trait, both parents must be carriers of the mutated gene for their offspring to be affected. This means that even if the parents appear healthy, they can still pass on the recessive allele to their children.
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Punnett Square
A Punnett square is a tool used in genetics to predict the probability of an offspring inheriting particular traits from its parents. By mapping out the possible combinations of alleles from each parent, it helps to visualize the likelihood of different genotypes and phenotypes in the offspring. For Smith–Lemli–Opitz syndrome, a Punnett square can illustrate the chances of the parents having affected children based on their carrier status.
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Pedigree Analysis
Pedigree analysis is a method used to trace the inheritance patterns of traits through generations in a family. It involves creating a diagram that represents family relationships and the presence or absence of specific traits. In this case, the pedigree chart can help identify how Smith–Lemli–Opitz syndrome is passed down, revealing whether the parents are carriers and the probability of their children being affected.
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