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# Introduction to Genetics - Chapter 17 ## Figure 17.12: Phenylketonuria - **(A)** Gene function normal - **(B)** Abnormal gene ### Gene Function - **Normal gene:** - **Phenylalanine hydroxylase** - **Produces:** Tyrosine - **Abnormal gene:** - **Phenylalanine hydroxylase absent** - **Resu...

# Introduction to Genetics - Chapter 17 ## Figure 17.12: Phenylketonuria - **(A)** Gene function normal - **(B)** Abnormal gene ### Gene Function - **Normal gene:** - **Phenylalanine hydroxylase** - **Produces:** Tyrosine - **Abnormal gene:** - **Phenylalanine hydroxylase absent** - **Results in:** - **Phenylalanine accumulates in blood** - **Condition:** Phenylketonuria ## Chromosomal Abnormalities - Faults during meiosis can lead to: - Too many or too few chromosomes - Abnormal shapes or segments - Such aberrations can be lethal, impacting gamete formation. - **Non-lethal conditions include:** - Down's syndrome - Cri-du-chat syndrome ### Down's Syndrome - **Description:** Three copies of chromosome 21 (trisomy 21). - Caused by failure to separate normally during meiosis. - Characteristics: - Short stature - Pronounced eyelid folds - Round face - Protruding tongue - Learning disability present, varies in severity. - Life expectancy shorter than normal, higher incidence of: - Cardiovascular and respiratory diseases - Early dementia - Associated with increasing maternal age (especially over 35 years). ### Cri-du-chat Syndrome - **Description:** Characteristic mewing cry of an affected child. - Caused by the deletion of part of chromosome 5. - Associated with learning disabilities. ## Abnormalities of the Sex Chromosomes - If sex chromosomes fail to separate normally during meiosis, the offspring may have an incorrect number of chromosomes. - **Turner’s Syndrome:** - Occurs with one X chromosome (XO). - Affected individuals are female with undeveloped sexual characteristics. - Aorta coarctation present in 15% of cases. - Intelligence usually normal. - **Klinefelter’s Syndrome:** - Occurs in individuals with an XXY chromosome configuration. - Affected individuals are male, often taller than average with learning disability. - Genitalia underdeveloped; infertility common. - Development of feminine characteristics like enlarged breasts (gynecomastia). ## Spot Check 1. What is an oncogene? 2. Describe the genetic basis of phenylketonuria. ## Review and Revise Complete the following statement: 1. The disorder phenylketonuria is inherited as a ______________ disorder and is characterized by a lack of the enzyme ______________. This enzyme converts ______________ to ______________, and in its absence, the former substance accumulates in the blood and is toxic to the ______________ system.