CHROMOSOME ABNORMALITIES

Atypical Chromosome Numbers Some developmental disabilities are caused by gene or chromosome abnormalities. Atypical chromosome numbers occur when too many, or too few, chromosomes appear in an embryo’s cells. Effects depend on which chromosome is affected. Usually atypical numbers result in miscarriage. Trisomy = 3 identical chromosomes, instead of pair Trisomy 21 = Down syndrome

Normal vs non-disjunction Non-disjunction occurs when sex cells are produced with an extra chromosome. homologous chromosomes don’t separate properly during anaphase I or II one gamete has an extra chromosome, and the other is missing a chromosome. If one of these joins normal gamete in fertilization, diploid zygote will have an atypical number of chromosomes. Mitosis then copies this error in all cells Down syndrome is the result of non-disjunction Meiosis I: Normal vs non-disjunction Meiosis II:

Incidence increases greatly with age of mother Eggs formed at birth but do not complete meiosis until released, 1 per month Delay in release can cause cell damage

Chromosome Number Condition Traits Trisomy 13 Patau syndrome 1 in 16 000 live births. severe physical and intellectual disability have heart defects, brain or spinal cord abnormalities, extra fingers and/or toes, a cleft lip, Only 5 –10% of babies survive past their first year. Trisomy 18 Edwards syndrome 1 in 5000 live births severe intellectual disability and low birth weight small, head, jaw and mouth; clenched fists; heart and organ defects Trisomy 21 Down syndrome 1 in 800 live births. mild to moderate intellectual disability, XXY Klinefelter syndrome 1 in 500 to 1 in 1000 males. affects male sexual development small testes that do not produce enough testosterone. Testosterone injections help treat the condition.

Patau syndrome Edwards syndrome Klinefelter syndrome Down syndrome Cracking the Code: Understanding Rare Chromosome Disorders - YouTube

Damage to Chromosome Structure A mutation is any damage that occurs to a chromosome. may be as small as a few base pairs or as large as the entire structure of the chromosome. 4 different ways chromosome structure can be damaged

1. DELETION part of the chromosome is actually lost. causes include viruses, radiation and chemicals may carry a specific gene which may have a large effect on the host.

2. DUPLICATION gene sequence is repeated one or more times within a chromosome. At some point, too many repeats can affect the function of the gene.

3. INVERSION gene segment momentarily becomes free from its chromosome and then reinserts in the opposite order. This can completely alter the gene’s activities.

4. TRANSLOCATION part of a chromosome changes place with another part of either the same or a non-homologous chromosome. Translocations can result in some cancers and leukemia.

Genetic Testing Examining genes can allow for the diagnosis, treatment, and prevention of genetic illnesses. several types of testing: Karyotype analysis - discovers chromosome abnormalities BiologySource Carrier testing - tests parental genes before conception Cystic fibrosis (CF) and Tay-Sachs caused by gene mutations passed on to offspring by both parents. Presymptomatic (predictive) testing - searches for specific genetic diseases that run in families. Usually done for disorders that appear later in life Diagnostic genetic testing - confirms a diagnosis This type of test can be done at any point in a person’s life.

Prenatal testing - detects chromosomal problems in a fetus. embryo becomes a fetus about 7 weeks. Down syndrome and spina bifida common tests Amniocentesis - genetically testing of fetus between weeks 14 to 20 of pregnancy. Chorionic villus sampling is also used. Tissue surrounding the fetus is removed and tested. These tests have risks and are used only if concerns arise Newborn screening - tests for some genetic disorders shortly after birth. Phenylketonuria (PKU) testing is done this way.

PATTERNS OF INHERITANCE Albinism is a genetic condition in humans, in which the skin, eyes, and hair have no pigment, due to varying amounts of melanin. It is very rare, and is not life-threatening. Other genetic disorders, though, cause severe medical problems. Mutations in chromosomes constantly create new alleles, both harmful and harmless.

TAY-SACHS DISEASE Tay-Sachs disease appears around the age of eight months. It is known by the deterioration of brain cells and spinal cords. By their first birthday, children are blind, mentally handicapped, and have little muscular activity. They will likely die before the age of 5.

TAY-SACHS DISEASE It is caused by the absence of a specific enzyme in the lysosomes of brain cells. The recessive allele does not code for the production of the enzyme that breaks down lipids. As the lipids build up, they will eventually destroy the brain cells that house them. There is no treatment for Tay-Sachs disease, though there is now a blood test to identify heterozygous carriers. Can you think of any implications of this?

PHENYLKETONURIA (PKU) Phenylalanine is an amino acid essential for human growth and development. Tyrosine is used to make body hormones and melanin. An enzyme in our bodies converts phenylalanine to tyrosine. Children with PKU will have phenylalanine broken down abnormally. They will appear normal at birth, but within a few months, they will be severely handicapped. Infants are tested for PKU now, and a positive test will result in a special diet that prevents harmful products from accumulating. They will lead healthy lives once the nervous system is fully developed.

SICKLE-CELL ANEMIA Sickle-cell anemia is the result of a defect in hemoglobin in red blood cells. It leads to blood clots and reduced blood flow to vital organs. As a result, people have reduced energy, suffer various illnesses, and are in constant pain.

SICKLE-CELL ANEMIA When oxygen is released from hemoglobin, sickle cells begin to clump and block blood vessels, preventing oxygen from getting to various parts of the body.

DUCHENNE MUSCULAR DYSTROPHY Duchenne muscular dystrophy results in individuals becoming engorged with fat deposits. It is the wasting away of muscle tissue. Unfortunately, there is no cure for this disease.

HUNTINGTON’S DISEASE Huntington’s disease is a lethal disorder where the brain deteriorates over a period of about 15 years. Symptoms include irritability and mild memory loss, followed by involuntary muscle movements. As the brain deteriorates, symptoms become more severe, leading to loss of coordination, memory, and speech. Most people die in their 40’s or 50’s, not knowing if their children have the mutant allele.