Nondisjunction Mutations

1. Nondisjunction Mutations Chapter 12

2. Nondisjunction ("not coming apart") is the failure of chromosome pairs to separate properly during cell division. • This could arise from a failure of homologous chromosomes to separate in meiosis I, or the failure of sister chromatids to separate during meiosis II or mitosis. • The result of this error is a cell with an imbalance of chromosomes. Such a cell is said to be aneuploid. • Loss of a single chromosome (2n-1), in which the daughter cell(s) with the defect will have one chromosome missing from one of its pairs, is referred to as a monosomy. • Gaining a single chromosome, in which the daughter cell(s) with the defect will have one chromosome in addition to its pairs is referred to as a trisomy.

3. If these gametes are fertilized, it will result in an embryo in which all the cells have an abnormal chromosome # • Great video demo here • Narrated video here

4. Trisomy 21: Down syndrome

5. Down syndrome • The condition is characterized by a combination of major and minor differences in structure. • Often Down syndrome is associated with some impairment of cognitive ability and physical growth, and a particular set of facial characteristics. • Down syndrome in a fetus can be identified with amniocentesis during pregnancy, or in a baby at birth. • Individuals with Down syndrome tend to have a lower than average cognitive ability, often ranging from mild to moderate developmental disabilities. • A small number have severe to profound mental disability. • The incidence of Down syndrome is estimated at 1 per 800 to 1,000 births, although it is statistically much more common with older mothers. • http://en.wikipedia.org/wiki/Down_syndrome

6. Trisomy 18: Edwards syndrome

7. Edwards syndrome • The incidence of the syndrome is estimated as one in 3,000 live births[2]. • The incidence increases as the mother's age increases. • The syndrome has a very low rate of survival, resulting from heart abnormalities, kidney malformations, and other internal organ disorders. • Only 50% of liveborn infants live to 2 months, and only 5–10% survive their first year of life. • http://en.wikipedia.org/wiki/Edwards_syndrome

8. Trisomy 13: Patau syndrome

9. Patau syndrome • The risk of this syndrome in the offspring increases with maternal age at pregnancy, with about 31 years being the average.[1] • Patau syndrome affects approximately one in 10,000 live births. • A few symptoms are: • mental & motor challenged • polydactyly (extra digits) • microcephaly low-set ears • holoprosencephaly (failure of the forebrain to divide properly). • heart and kidney defects

10. XXY - Klinefelter’s syndrome

11. Klinefelte’s syndrome • The condition exists in roughly 1 out of every 1,000 males. • The principal effects are development of small testicles and reduced fertility. • Some degree of language learning impairment may be present,[7] and neuropsychological testing often reveals deficits in executive functions.[8] • In adults, possible characteristics vary widely and include little to no signs of affectedness, a lanky, youthful build and facial appearance, or a rounded body type with some degree of gynecomastia (increased breast tissue). • http://en.wikipedia.org/wiki/Klinefelter%27s_syndrome

12. Monosomy X: Turner’s syndrome

13. Turner’s syndrome • Occurring in 1 out of every 2500 girls, the syndrome manifests itself in a number of ways. • There are characteristic physical abnormalities, such as short stature, swelling, broad chest, low hairline, low-set ears, and webbed necks.[3] • Girls with Turner syndrome typically experience gonadal dysfunction (non-working ovaries), which results in amenorrhea (absence of menstrual cycle) and sterility. • Concurrent health concerns are also frequently present, including congenital heart disease, hypothyroidism (reduced hormone secretion by the thyroid), diabetes, vision problems, hearing concerns, and many autoimmune diseases.[4] • Finally, a specific pattern of cognitive deficits is often observed, with particular difficulties in visuospatial, mathematical, and memory areas.[5] • http://en.wikipedia.org/wiki/Turner%27s_syndrome

14. XYY – Jacobs syndrome

15. XYY • 47, XYY boys have an increased growth velocity during earliest childhood, with an average final height approximately 7 cm above expected final height.[3] • Severe acne was noted in a very few early case reports, but dermatologists specializing in acne now doubt the existence of a relationship with 47,XYY.[4] • Testosterone levels (prenatally and postnatally) are normal in 47,XYY males.[5] Most 47,XYY males have normal sexual development and usually have normal fertility. • Since XYY is not characterized by distinct physical features, the condition is usually detected only during genetic analysis for another reason. • XYY boys have an increased risk of learning…and delayed speech and language skills. • http://en.wikipedia.org/wiki/XYY

16. Prenatal detection • Amniocentesis: a small amount of amniotic fluid, which contains fetal tissues, is extracted from the amniotic sac surrounding a developing fetus, and the fetal cells (DNA) is examined for genetic abnormalities. • Can be performed between the 16th-20th week of pregnancy.

17. Prenatal detection • Chorionic Villus Sampling (CVS): It entails getting a sample of the chorionic villus (placental tissue) and testing it. CVS usually takes place 10-12 weeks of pregnancy (earlier than amniocentesis)