Genetic Disorders - Errors in Meiosis and Screening PDF

Genetic Disorders Errors in Meiosis and Genetic Screening Trisomy 21 Trisomy 21 - Down Syndrome  Let’s Review! What are the two processes that produce genetic variation during meiosis? Why are they important?  Let’s Review! What are the two processes t...

Genetic Disorders Errors in Meiosis and Genetic Screening Trisomy 21 Trisomy 21 - Down Syndrome  Let’s Review! What are the two processes that produce genetic variation during meiosis? Why are they important?  Let’s Review! What are the two processes that produce genetic variation during meiosis? Crossing Over and Independent Assortment Why are they important? They increase genetic diversity between individuals of the same species → allows organisms to respond to changes in their environment more rapidly  BIOLOGY BIG IDEA Sexual Reproduction serves to increase genetic diversity in a much faster way than asexual reproduction This allows organisms to respond to changes in their environment more rapidly and also have greater diversity between individuals of the same species This comes at a cost however, as this complex process takes a lot of energy and errors can occur in the process In biology, everything has a “cost”  ERRORS IN MEIOSIS 2 of the processes that increase genetic variation during meiosis 1. Crossing Over 2. Independent Assortment While these do increase genetic variation they also can cause chromosomal abnormalities Many errors produce gametes that cannot survive If the gamete does survive and is involved in the process of fertilization, it will produce a zygote ⚬ Since every cell of an offspring is produced from the first zygote cell, every cell in that offspring will contain the error  ERRORS IN MEIOSIS There are 2 types of chromosomal errors that can occur during meiosis 1. Changes in Chromosome Structure 2. Changes in Chromosome Number Crossing Over DURING PROPHASE 1, PAIRS OF HOMOLOGOUS CHROMOSOMES MAY EXCHANGE PIECES OF CHROMOSOME THIS RESULTS IN THE EXCHANGE OF MATERNAL AND PATERNAL GENES  ERRORS IN CHROMOSOME STRUCTURE During crossing over, sometimes the chromosomes do not reform correctly Sometimes non-homologous pairs may cross over, this produces chromosomes that contain genes that are not normally on that chromosome  ERRORS IN CHROMOSOME STRUCTURE 4 types of errors in chromosome structure 1. Deletion - a piece of chromosome is removed 2. Duplication - a section of a chromosome appears 2 or more times in a row 3. Inversion - a section of a chromosome is reversed 4. Translocation - a segment of one chromosome becomes attached to another chromosome *Note* these errors are not exclusive to meiosis  1. DELETION ERRORS Deletion: a piece of Example of a Genetic Disorder chromosome is removed caused by a Deletion Error: Cri du Chat Deletion in chromosome 5 Children make high-pitched, cat-like cries Low birth weight, widely spaced eyes, recessed chin, and developmental and cognitive delays No cure  2. DUPLICATION ERRORS Duplication: a section of a chromosome appears 2 or more Example of a Genetic Disorder caused by a Duplication Error: times in a row Charcot-Marie-Tooth Disease A duplication of a gene on chromosome 17 Muscle weakness and loss of sensation in legs, feet, and hands A high foot arch with flexed toes No cure  3. INVERSION ERRORS Inversion: a section of a Example of a Genetic Disorder chromosome is inverted/reversed caused by an Inversion Error: FG Syndrome Inversion of a section in the X chromosome Occurs almost exclusively in males Intellectual disabilities, delayed motor development, and low muscle tone No cure  4. TRANSLOCATION ERRORS Translocation: a segment of one Example of a Genetic Disorder chromosome becomes attached to caused by a Translocation Error: another chromosome Chronic Myelogenous Leukemia A cancer of the white blood cells Translocation between chromosome 9 and chromosome 22 Forms an abnormal gene that must be treated via a drug that stops increased white blood cells production  ERRORS IN CHROMOSOME NUMBER During Anaphase 1 of meiosis, pairs of homologous chromosomes are pulled to opposite poles of the cell During Anaphase 2 of meiosis, sister chromatids are pulled apart and to opposite poles of the cell Nondisjunction - the failure of homologous chromosome pairs or sister chromatids to separate during meiosis Anaphase 1 Anaphase 2  ERRORS IN CHROMOSOME NUMBER Nondisjunction can occur in either Anaphase 1 or Anaphase 2 Nondisjunction results in the production of gametes that have too many or too few chromosomes Aneuploidy → occurrence of one or more extra or missing chromosomes  GENETIC DISORDERS ASSOCIATED WITH CHROMOSOME NUMBER Many genetic disorders are the result of an individual having an incorrect number of chromosomes Trisomy - the gain of an extra chromosome as the result of nondisjunction ⚬ Down Syndrome - an additional copy of chromosome 21 ⚬ Results in intellectual disabilities, flattened face, short stature, and more Monosomy - the loss of a chromosome as a result of nondisjunction ⚬ Turner Syndrome - involves a missing X chromosome ⚬ Results in female sexual characteristics that are underdeveloped GENETIC DISORDERS ASSOCIATED WITH CHROMOSOME NUMBER  OTHER FACTORS Other factors that may increase the risk of chromosome abnormalities include: Maternal Age: Since women are born with all the eggs they will ever have, some errors may crop up in the eggs' genetic material as they age. Older women are at higher risk of giving birth to babies with chromosome abnormalities than younger women. Environment: It is possible that the environment may play a role in the occurrence of genetic errors. ⚬ Learn more: https://www.genome.gov/about-genomics/fact-sheets/Chromosome-Abnormalities-Fact-Sheet  UNDERSTANDING GENETIC DISORDERS There are at least 5,000 currently identified genetic disorders They can be much more common than people think ⚬ Eg. Approx 1 in every 3000 babies is born with cystic fibrosis ⚬ Some genetic disorders can be more likely based on a variety of factors (eg. Down Syndrome, Sickle Cell Anemia) Learn more: https://www.genome.gov/For-Patients-and-Families/Genetic-Dis orders  GENETIC SCREENING Prenatal Genetic Testing - tests performed on a fetus that are used to check for genetic-based abnormalities The decision on whether to have prenatal testing performed and what to do with the information from these tests can result in difficult personal decisions These issues represent one of the largest current ethical debates in the field of biology and contribute to the scientific discrimination of groups of individuals who may have genetic disorders  TESTING PROCEDURES Genetic Screening can be performed prior to the decision to even have a baby by analyzing the genes of both parents ⚬ We will learn more about this and practice it ourselves! Non-invasive tests - no direct sampling of fetal cells ⚬ Maternal blood tests and ultrasounds Invasive tests - collecting DNA sample from the fetus ⚬ Amniocentesis - sample of amniotic fluid ⚬ Chorionic villus sampling - Sample of cells from part of the placenta (chorion)  Learning Check 1. I can identify sources of error in meiosis that change chromosome structure or number 2. I am able to describe how these errors occur 3. I can name and describe at least one type of genetic disorder based on either errors in chromosome structure or number Practice Questions: 1. In detail, explain why an error in chromosome structure could impact the development of an embryo. 2. Explain which type of error a karyotype is effective in identifying and why.