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INTRODUCTION TO LINKED GENES AND SEX LINKAGE H. Biology/ Ms. Kim Locating Genes on Chromosomes • Genes – Are located on chromosomes – Code for protein that determines specific traits. Figure Gene Linkage • Linked genes – Usually inherited together because located near each other on the SAME chromosome • Genes closer together on the same chromosome are more often inherited together • Each chromosome – Has 100’s or 1000’s of genes • Sex-linked genes exhibit unique patterns of inheritance; genes linked on the X or Y chromosome • Thomas Hunt Morgan (1910) noticed that eye color in fruit flies and wing size always seemed to be inherited together in offspring. – This seemed to go against Mendel’s Law of Independent Assortment • Morgan discovered chromosomes are a group of linked genes • Law of Independent Assortment was still true, but chromosomes assort independently, not genes. Gene Linkage • Genes located on the same chromosome will NOT sort independently • They will be inherited together – Gene Linkage: • Tendency of 2 alleles (for different traits) to be inherited together • The closer the genes are, the more likely they are to be inherited together Mendel’s Principle of Independent Assortment? • Remember: this refers to genes on separate chromosomes Gene 1 Gene 2 • What would cause genes on the same chromosome to be inherited separately? – Crossing over! • Location of Gene on a chromosome is it’s LOCUS KARYOTYPE AND GENETIC DISORDERS H. Biology Chromosomes Determine Your Gender • Humans have 23 pairs of chromosomes • 22 pairs are autosomes (chromosomes that are NOT involved in making gender) – Also called homologous chromosome pairs • 1 pair are sex chromosomes (NOT HOMOLOGOUS; different genes) – The sex chromosomes are “X” and “Y” X » “XX” is a girl Egg cells ALWAYS have “X” chromosome » “XY” is a boy Y X Y A karyotype – an ordered, visual representation of ALL chromosomes in a human cell – Arranged according to homologous pairs Pair of homologous chromosomes Centromere Sister chromatids 5 µm a REAL view… wow!!! What is a genetic disorder? What causes a genetic disorder? • Caused by an abnormality in an individual’s DNA How do these abnormalities occur? 1) Non-disjunction – Addition or subtraction of chromosome or set of chromosomes 2) Mutations – Change to DNA sequence (gene) Abnormal Chromosome Number • When nondisjunction occurs – Pairs of homologous chromosomes do NOT separate normally during meiosis – Gametes contain 2 copies or NO copies of a particular chromosome • What are some disorders caused by non-disjunction? • Downs Syndrome – Trisomy 21 • Klinefelter’s Syndrome – XXY • Turner Syndrome – Monosomy X What happens when meiosis goes wrong? NONDISJUNCTION!!! Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n+1 n+1 n1 n–1 n+1 n –1 n Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II n Aneuploidy Results from the fertilization of gametes in which nondisjunction occurred Offspring have an abnormal number of a particular chromosome What are some disorders caused by nondisjunction? • Downs Syndrome – Trisomy 21 • Kleinfelter’s Syndrome – XXY • Edwards Syndrome – Trisomy 18 • Patau Syndrome – Trisomy 13 • Turner Syndrome – Monosomy X 1.) Trisomy 18-Edwards Syndrome 2.) Trisomy-13: Usually Fatal 3.) Klinefelter - XXY • Klinefelter syndrome – a result of an extra chromosome in a male, producing XXY individuals – 1/2000 live births XYY individuals Triple X Syndrome XXX individuals • Females • Healthy • Same phenotype as XX individuals • 1/1000 live births 4.) Turner Syndrome • Turner syndrome – Is the result of monosomy X, producing an X0 karyotype – 1/5000 live births – Only known monosomy in humans – Normal intelligence – Sterile 5.) Trisomy 21 Human Disorders Due to Chromosomal Alterations • Down syndrome –Is usually the result of an extra chromosome 21 • trisomy 21 What are some disorders caused by mutations? • • • • • Sickle cell anemia (recessive) Cystic fibrosis (recessive) Maple syrup urine disorder (recessive) Congenital hypothyroidism (recessive) Huntington’s Disease (dominant) Cystic Fibrosis • Example of recessive disorder • Affect mostly people of European descent • Symptoms – Mucus buildup in the some internal organs – Abnormal absorption of nutrients in the small intestine Sickle Cell Anemia oAnother recessive disorder oAffects one out of 400 African-Americans oHemoglobin is the O2 carrying protein in red blood cells o SCA is an inherited blood disease where hemoglobin clumps together causes red blood cells to stiffen and curl into a sickle/crescent shape o Because of this, the RBC cannot transport O2 effectively Symptoms Physical weakness, pain, organ damage, and even paralysis Human Chromosomal Disorders Recessive Disorders • Cystic Fibrosis – Excess mucus in lungs, digestive tract, liver – More susceptible to pneumonia, infection – Early death ~ average age of 37 • Tay-Sachs – Lipid accumulation in brain cells, mental retardation – Blindness, death in childhood Dominant Disorder • Huntington’s disease – Deterioration of nervous system, uncontrollable movements – Develops in middle age, death soon after 32 Dominantly Inherited Disorders • Some human disorders – Are due to dominant alleles only need 1 dominant allele • Example is achondroplasia – Form of dwarfism lethal when homozygous for the dominant allele Another Dominant Disorder • Huntington’s disease (HD) – degenerative disease of nervous system – No obvious phenotypic effects until about 35 to 40 years of age HD Normal