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Chapter 062. Principles of Human Genetics (Part 5)

Crossing-over and genetic recombination. During chiasma formation, either of the two sister chromatids on one chromosome pairs with one of the chromatids of the homologous chromosome. Genetic recombination occurs through crossing-over and results in recombinant and nonrecombinant chromosome segments in the gametes. Together with the random segregation of the maternal and paternal chromosomes, recombination contributes to genetic diversity and forms the basis of the concept of linkage. After the first meiotic division, which results in two daughter cells (2n), the two chromatids of each chromosome separate during a second meiotic division to yield four gametes with a haploid state (1n). When. | Chapter 062. Principles of Human Genetics Part 5 Figure 62-3 Homologous Chromosomes Chromai ids. No recombination rn gameles Recombination in gambles Recombination in gametes Sourc Fauci AS K p r DU Braunwaid E Hauttr SU UonflQ DU Jam on JL Ldïcafio Ji Wa.vjrL r r . .n-r cj ar Ct .r1 cd cn id. 17th Edition I tittp a ccajtmadion com Copvnfltit H1 McO tw-Hill Comparil Inc All rlflht rt.arPHd- Crossing-over and genetic recombination. During chiasma formation either of the two sister chromatids on one chromosome pairs with one of the chromatids of the homologous chromosome. Genetic recombination occurs through crossing-over and results in recombinant and nonrecombinant chromosome segments in the gametes. Together with the random segregation of the maternal and paternal chromosomes recombination contributes to genetic diversity and forms the basis of the concept of linkage. After the first meiotic division which results in two daughter cells 2n the two chromatids of each chromosome separate during a second meiotic division to yield four gametes with a haploid state 1 n . When the egg is fertilized by sperm the two haploid sets are combined thereby restoring the diploid state 2n in the zygote. Regulation of Gene Expression Mechanisms that regulate gene expression play a critical role in the function of genes. The transcription of genes is controlled primarily by transcription factors that bind to DNA sequences in the regulatory regions of genes. As described below mutations in transcription factors cause a significant number of genetic disorders. Gene expression is also influenced by epigenetic events such as X-inactivation and imprinting processes in which DNA methylation or histone modifications are associated with gene silencing. Several genetic disorders such as Prader-Willi syndrome neonatal hypotonia developmental delay obesity short stature and hypogonadism and Albright hereditary osteodystrophy resistance to parathyroid hormone short stature brachydactyly .