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4.4 Abnormal Meiosis and Fertilization KARYOTYPE: - A picture of the chromosomes of an individual arranged by ______, and type. - Karyotypes can be used to determine the gender of an unborn baby and to determine if there are any chromosomal abnormalities. Normal Male Karyotype Normal Female Karyotype This individual has an extra 18th chromosome, which results in a condition called Edward’s Syndrome. - A pregnant woman can have an amniocentesis performed in order to create a karyotype for her baby. In this procedure, some amniotic fluid is extracted. The fluid contains cells from the baby, from which a karyotype can be made. How does an individual get an extra chromosome??? ABNORMAL MEIOSIS In meiosis, special diploid cells undergo 2 divisions to produce 4 haploid gametes (sperm or egg cells). Diploid Cell Meiosis I Meiosis II 4 gametes (sperm or egg cells) NON-DISJUNCTION - when there is an error that occurs in meiosis. - The chromosomes fail to separate properly. - It can happen in either meiosis I or meiosis II - As a result, some gametes will be missing a chromosome, and some will have an extra chromosome. - - If one of these abnormal gametes participates in fertilization, a zygote with an abnormal chromosome number is produced. Since the zygote is the first cell of the new organism, when this cell undergoes cell division, all the new cells will be identical to it. Therefore, all the cells in the organism will have an abnormal chromosome number. If the error occurs in meiosis I, all 4 of the gametes will be abnormal If the error occurs in meiosis II, 2 of the gametes will be normal, and the other 2 gametes will be abnormal. - In the example below, the parent diploid cell has 4 chromosomes. The 4 haploid gametes should each have 2 chromosomes. Nondisjunction Disorders - In most cases, the zygote will die because there is either too much or not enough information in each cell. But in some cases the zygote will survive and a person with a nondisjunction disorder is born. DOWN SYNDROME (TRISOMY 21) - People with Down syndrome have an extra chromosome for the chromosome pair 21. - People with Down syndrome may have a mental disability – although a wide range of mental abilities is possible. TURNER SYNDROME (MONOSOMY X) - Normal females have two X chromosomes: XX - Females with Turner’s syndrome have only one X chromosome instead of two. - These women do not mature sexually (they can’t have babies) - This is the only nondisjunction disorder in which the individual is missing a chromosome and still lives! KLIENFELTER SYNDROME (XXY) - Normal males have one X chromosome and one Y chromosome: XY - Males with Klinefelter syndrome have two X chromosomes and one Y chromosome. - These males produce high levels of female sex hormones and are usually sterile (because of a low sperm count) EDWARDS SYNDROME (TRISOMY 18) - An extra chromosome 18 - Almost every organ system in the body is affected. - Most fetuses die in utero. - The ones that do make it to term usually do not live more than a few months. - About 1% live to the age of 10 PATAU SYNDROME (TRISOMY 13) - An extra chromosome 13 - Results in serious developmental problems including brain, kidney, heart, and eye defects. - Children rarely live more than a few months. XYY - These are males with an extra Y chromosome. - Individuals are somewhat taller than average and often have below normal intelligence. - At one time, it was thought that these men were likely to be criminally aggressive, but this hypothesis has been disproven over time. XXX (Trisomy X) - Females with an extra X chromosome - Individuals are health and fertile – usually cannot be distinguished from normal females except by karyotypes. Abnormal Chromosome Structure - Sometime errors occur during the crossing over procedure during synapsis in meiosis I. - This can result in duplication of information or the loss of information. Deletion: occurs if a chromosome fragment does not-reattach to the homologous chromosome Ex: The condition Crit du Chat results from deletion of part of chromosome 5 Individuals have: a cry like a kitten (due to problems with larynx and nervous system); unusual facial features; severe cognitive, speech and motor delays; behaviour issues; poor growth Duplication: occurs if a chromosome fragment attaches to an already complete chromosome. Inversion: occurs if a chromosome fragment attaches to the correct chromosome but in the reverse order. Translocation: Occurs if a chromosome fragment attaches to a non-homologous chromosome. (Information is on the wrong chromosome. Sometimes this disrupts the information on that chromosome).