Chapter 9

1. Chapter 9 Patterns of Inheritance Part 4

2. Changes In Chromosome Number • When non-dysjunction happens during meiosis, a chromosome number different from the parents can result. This can affect the health and fertility of the offspring. • Non-dysjunction generally increases with increasing age of the mother. • Non-dysjunction may also occur in the father, but occurs far less frequently. • Non-dysjunction occurs when one or more pairs of chromosomes do not separate properly during meiosis. • The result is an uneven number of chromosomes after fertilization.

3. Changes In Chromosome Number • For example, if an egg cell received 24 chromosomes (rather than the normal 23) during meiosis, the zygote that results from fertilization of the egg (24 chromosomes) with the sperm (23 chromosomes) will have 47 chromosomes (instead of the normal 46 chromosomes). • This zygote will have three of one chromosome (called trisomy).

4. Changes In Chromosome Number • In another example, if the received 22 chromosomes (rather than the normal 23) during meiosis, the zygote that results from fertilization of the egg (22 chromosomes) with the sperm (23 chromosomes) will have 45 chromosomes (instead of the normal 46 chromosomes). • This zygote will have only one of one chromosome (called monosomy).

5. Non-dysjunction Leads to Trisomy and Monosomy

6. Changes In Chromosome Number • Both trisomy and monosomy are types of aneuploidy, a condition in which cells have too many of too few copies of a chromosome. • Autosomal aneuploidy is usually fatal in humans and results in many spontaneous abortions (miscarriages). • However, many plants and some animals such as some fish and insects are polyploid, meaning that they have three or more copies each chromosome.

7. Down Syndrome: Autosomal Chromosome Number Change • Though most autosomal aneuploidy individuals die before birth, trisomy 21 individuals can actually live into adulthood. • Trisomy 21 is also known as Down Syndrome. • Individuals born with Down Syndrome are born with three copies of chromosome 21. • This syndrome is the most common type of aneuploidy in humans. • It occurs in one out of every 800 to 1000 births and affects more than 350,000 individuals in the US alone.

8. Down Syndrome • Some outward signs of Down Syndrome include: • Upward slanting eyes • A fold of skin that starts at the inner corner of each eye • A deep crease across the sole of each palm and foot • One, instead of two, horizontal furrows on the fifth finger • Slightly flattened facial features • Main symptoms of Down Syndrome include: • Mental impairment • Heart defects • Abnormal growth and development of skeleton resulting in shortened body parts, loos joints, and misaligned bones of the fingers, toes, and hips • Weak muscles and reflexes • Slow to develop motor skills

9. Down Syndrome

10. Sex Chromosome Number Changes • Changes in the number of sex chromosomes occurs in about 1 out of every 400 live births. • Generally, these changes lead to difficulties in learning, impaired motor skills such as speech delay. • Problems due to an altered number of sex chromosomes may be so subtle that the underlying cause is never diagnosed.

11. Turner Syndrome: Female Sex Chromosome Number Changes • Individuals with Tuner Syndrome have only one X chromosome, denoted with the genotype XO. • About 1 in 2,500 to 10,000 newborn girls are XO. • About 98% of these embryos spontaneously abort during early pregnancy so there are fewer reported cases than other sex chromosome abnormalities. • These girls grow up with short stature, as well as nonfunctional ovaries, which results in a lack of sex hormones and, therefore, very little development of secondary sexual characteristics.

12. Turner Syndrome

13. Triple X Syndrome: Female Sex Chromosome Number Changes • Females who are born with three or more X chromosomes are said to have Triple X (XXX) Syndrome. • This occurs in about 1 out of every 1,000 births. • Since only one X chromosome is typically active in female (due to X chromosome inactivation-Dosage Compensation Theory), having extra copies of the X chromosome generally doesn’t result in physical or medical problems.

14. Klinefelter Syndrome: Male Sex Chromosome Number Changes • About 1 out of every 500 males has an extra X chromosome, making this male and XXY individual with Klinefelter Syndrome. • This syndrome develops at puberty. • These males tend to be overweight, tall, and within normal range of intelligence. • They do make more estrogen and less testosterone than normal males, resulting in feminizing effects such as small testes, low sperm count, sparse facial hair and body hair, high-pitched voice, and enlarged breasts. • Testosterone injections during puberty can reverse these effects.

15. Klinefelter Syndrome

16. XYY Syndrome: Male Sex Chromosome Number Changes • About 1 in 500 to 1000 males has an extra Y chromosome. • These males tend to about 7 inches taller than normal and may have mild mental impairment.

17. Prospects in Human Genetics • Parents-to-be have many options to answer the main question that is on most parents’ minds after the news that they are expecting. • Will my baby be healthy?

18. Genetic Counseling • Genetic counseling starts with diagnosis of parental genotypes, family pedigrees, and genetic testing for known disorders. • Using information gained from these things, genetic counselors can predict a couple’s probability of having a child with a genetic disorder. • Counselors must also take into account the age of the parents, since risks of problems increases with age of the parents, especially the mother. • They must also consider that there is a small risk (about 3%) that birth processes can affect any child (for example, cerebral palsy.

19. Prenatal Diagnosis • There are several methods of prenatal diagnosis that are commonly used to not only determine sex of the fetus, but to also screen for more than 100 known genetic problems. • All of these include some risk to the fetus including miscarriage or other abnormalities. • Some of these include: • Amniocentesis (can screen for Down Syndrome, cystic fibrosis, sickle cell anemia, etc.) • Chorionic villus sampling (CVS) similar to amniocentesis but can be performed earlier during the pregnancy • Fetoscopy

20. Amniocentesis

21. Pre-implantation Diagnosis • This is a procedure used in association with in vitro fertilization. • This involves removing a cell from the ball of cells formed after fertilization before the cells begin to differentiate. • It doesn’t harm the developing embryo in any way. • Doctors test this cell for genetic defects before the embryo is implanted into the mother’s uterus.

22. Phenotypic Treatments • Surgery, prescription medications, hormone replacement therapies and even dietary controls are just some of the ways that medical professionals can reduce and even eliminate the symptoms of many genetic disorders. • Treatments such as these can be used for many genetic disorders including PKU (phenylketonuria) and ALD.

23. Genetic Screening • This involves the widespread, routine testing for alleles associated with genetic disorders. • It can provide information on reproductive risks and help families that are already affected by a genetic disorders such as PKU. • Affected infants receive early treatment so they never develop the symptoms of the disorder. • This screening also helps scientists and doctors to more closely estimate the prevalence and distribution of harmful alleles in the population.

24. Prospects in Human Genetics • On one hand, it may be good to know if a child will be born with a genetic disorder or that you yourself carry a “bad” allele that you could pass on to your child. • But, what would you do with this information? • What if genetic testing revealed that you carry a “bad” allele, what would you do with this information? • What would your insurance company do with this information if made aware of it? • How would you feel if you had a child with a genetic disorder if you conceived it knowing that that was a distinct possibility? • There are no easy answers.

25. Prospects in Human Genetics • What do you think you would do with the information that you had passed a harmful allele to your child and you’re now about to have a baby with a life-threatening or fatal genetic disorder? • Would you want to know?