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Download Chromosome Theory of Inheritance -States that genes or alleles
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Chromosome Theory of Inheritance -States that genes or alleles (form of a gene) are located at specific loci (locations) on chromosomes -Chromosomes segregate and show independent assortment during meiosis Linked Genes -Traits that are located near each other on the same chromosome and tend to be inherited together If Genes Are Linked -Independent assortment of trait fails -Linkage may be “strong” or “weak” Crossing-Over -Breaks up linkages and creates new ones -Recombinant offspring formed that doesn’t match the parental types Linkage Strength -Degree of strength related to how close the traits are on the chromosomes -Weak linkage: farther apart -Strong linkage: Closer together Genetic Linkage Maps -Constructed from crossing-over frequencies -1 map unit= 1% recombination frequency -Map units are used to express relative distances along the chromosomes -The less often a gene crosses over with another one, the closer it must be to it, so if the frequency is low, the distance between the two must be small Alteration of Chromosome Number or Structure Because of Mistakes During Meiosis Cause Some Genetic Disorders: -Nondisjunction occurs when members of a pair of homologous chromosomes do not separate properly during meiosis I, or sister chromatids don’t separate properly during meiosis II Result -One gamete receives two copies of the chromosome while the other gamete receives none -If the faulty gametes engaging in fertilization, the offspring will have an incorrect chromosome number, this condition is called aneuploidy -Nondisjunction: If chromosomes don’t separate properly during meiosis I or II, gametes will end up with too many or too few chromosomes, leading to chromosomal abnormalities in the zygotes they produce When Meiosis Goes Wrong Single Chromosome Alterations -Fertilized eggs that have received thee copies of the chromosome in question are said to be trisomic -Those that only receive one copy of a chromosome are monosomic Human Disorders Caused by Chromosome Alterations -Down Syndrome: Aneuploid condition also known as trisomy 21, three copies of chromosome 21 -Turner Syndrome: Monosomic condition where females have one sex chromosome x. Are females but sterile. Is the only monosomic condition in humans -Klinefelter Syndrome: Aneuploid condition in which a male posses the sex chromosomes XXYare male but sterile Polyploidy-Multiple Chromosome Alterations -Is the condition of having more than two complete sets of chromosomes, forming a 3n or 4n individual. Rare in animals this condition is fairy frequent in plants -This often results in sterility, but increased vigor of those polyploidy plant individuals Portions of a chromosome may also be lost or rearranged during meiosis, resulting in chromosomal structural mutations that can have large effects on the zygote (4 types) -Deletion: occurs when a chromosomal fragment is lost, resulting in missing genes -Duplication: occurs when the chromosome fragment that broke off during deletion becomes attached to its sister chromatid and the zygote gets a double dose of the gene added on the chromosome -Inversion: occurs when a chromosome fragment breaks off and reattached to its original positionbut backward -Translocation: occurs when the deleted chromosome fragment joins a non-homologous chromosome