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Chapter 13 Vocabulary: Sex chromosomes Female sex chromosomes Male sex chromosomes Autosomes Dosage compensation Genetic mosaics Linked genes Aneuploidy Monosomics Trisomics Outline 13.1 Sex Linkage and the Chromosomal Theory of Inheritance Chromosomal theory of inheritance – traits are carried on chromosomes We take this for granted, but it had to be discovered at some point! Sex chromosomes (X and Y) Autosomes (everything else) Sex linked genes appear on the X chromosome 13.2 Sex Chromosomes and Sex Determination Autosomes are homologous Sex chromosomes are not Female carries XX Male carries XY Y chromosome only carries 330 genes X chromosome carries 2,062 genes Genes that appear on the X chromosomes can be passed from mother to sons and daughters. Can be passed from fathers only to daughters because fathers give a Y to their sons Recessive allele on X chromosome Affects more males than females because males only have 1 X so trait is expressed, females have another X that can mask the expression of a recessive trait If mutated genes are on X chromosomes females have another X chromosome that can carry the dominant allele and mask the expression of the mutant recessive gene. Males only have 1 X chromosome and most of the traits are not on the Y chromosome so they have no opportunity to mask it. Ex. Color blindness and hemophilia A In females one X chromosome is randomly selected for modification. This chromosome will (with the exception of about 3 dozen genes) remain condensed and inactive. This happens very early on in development (about 200 cells big). It is random within all these early cells, but from then on all cells that are made from that cell will have the same chromosome condensed and inactive. So in an adult some regions of the body are expressing different x chromosomes. Mosaic. This means that if the alleles on the chromosomes are not alike then different tissues can be expressing different forms of a gene. This happens so that males and females express the same levels of certain genes found on the X chromosome. Dosage compensation 13.3 Exceptions to the Chromosomal Theory of Inheritance Mitochondrial and chloroplast DNA is inherited only from the egg cell. Egg cells have more cytoplasm and therefore organelles. This is maternal inheritance. Differs from x-linked because sons and daughters get this DNA from mom; nobody gets it from dad. 13.4 Genetic Mapping Frequency of recombination is related to relative gene loci. The closer 2 genes are the more likely they will stay together and be inherited together. (linked) If they are far enough apart multiple crossover events can lead to independent assortment. Crossing over disrupts linkages 13.5 Selected Human Genetic Disorders Nondisjunction leads to aneuploidy Monosomics Trisomics Nondisjunction of sex chromosomes Genomic imprinting – allele comes only from one parent Pre-natal diagnosis: Genetic counselors Pedigrees Amniocentesis Chorionic villi sampling Most important information to convey is how to work x-linked crosses. Mosaics, dosage compensation, linkage, non-disjunction and euploidy are other highlights. Again don’t ask them to memorize specific disorders or their genetics.