Assignment 2.1 Genetics
Read the following genetic cases and questions. You will need to provide rationale for each answer that is provided utilizing scientific evidence. Include a Punnett square diagram of inheritance pattern for each genetic abnormality. Also discuss the bolded further thought statements. Submit a formal paper with in-text citations and reference page, following APA 7 guidelines.
Case 1
A young woman is referred for genetic counseling. She has a 3-year-old boy with developmental delay and small joint hyperextensibility. The pediatrician has diagnosed fragile X–associated mental retardation. She is currently pregnant with her second child at 14 weeks of gestation. The family history is unremarkable.
Question: What is the genetic mutation responsible for fragile X–associated mental retardation? How does it cause the clinical syndrome of developmental delay, joint hyperextensibility, large testes, and facial abnormalities?
Answer: Fragile X–associated mental retardation is a syndrome caused by a genetic mutation of the X chromosome. There is a fragile site on the long arm of the X chromosome. Fragile X is the second most common genetic cause of intellectual disability. Abnormal FMR1 expression would explain in part the symptoms of mental retardation and enlarged testes. It is not known why the absence of FMR1 expression leads to joint laxity and hyperextensibility and facial abnormalities.
Question: Which parent is the probable carrier of the genetic mutation?
Answer: The mother is most likely carrier of genetic mutation.
Further thought: Explain why this parent and the grandparents are phenotypically
unaffected.
Question: What is the likelihood that the unborn child will be affected?
Answer: This depends on the sex of the child. Boys have a higher chance of being
affected.
Case 2
A 25-year-old woman recently gave birth to a baby diagnosed with Down syndrome. Neither she nor the baby’s father have a personal or family history of Down syndrome. Karyotyping of the parents shows that the mother has a balanced robertsonian translocation: 45,XX,+t(14q;21q).
Question: What major genetic abnormalities are associated with Down syndrome?
Answer: Down syndrome is an example of autosomal aneuploidy. It is the result of trisomy of chromosome 21. It occurs in 1 in 800 to 1000 live births.
Further though: Explain Robertsonian translocation and how this genetic error is
inherited.
Question: How might this woman’s age contribute to her risk of having a child with Down syndrome?
Answer: risk of Down syndrome increases with age. After age 35 the risk rises from 3% to 5% after age 45. The risk is related to the age of the maternal eggs. There is no increased risk with paternal age. Uterine changes with age also increase risk for Down syndrome live births.
Question: What is the chance that this couple would have a subsequent child with Down syndrome?
Answer: For a female carrier of a balanced robertsonian translocation, 45,XX,+t(14q;21q), the chance of having a child with Down syndrome is 10%. Unlike with Down syndrome as a result of trisomy 21 (47,XX+21 or 47,XY+21), this risk not affected by maternal age.
Question: Why does the extra genetic material of chromosome 21 lead to Down syndrome?
Answer: Down syndrome illustrates the principle of gene dosage, which states that the amount of a gene product produced per cell is proportionate to the number of copies of that gene present. In other words, the amount of protein produced by all or nearly all genes that lie on chromosome 21 is 150% of normal in trisomy 21 cells. There may be a critical region of chromosome 21, which, when present in triplicate, is both necessary and sufficient to produce the phenotypic features of Down syndrome.
