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C15 DNA C15 Genetics Continued Chromosome theory of inheritance took form ~1902. Thomas Morgan Hunt, at Columbia University,1st to associate a specific gene with a specific chromosome, early 1900’s. He studied Drosophila melanogaster and found the eye color of fruit flies is located on the X chromosome. Wild type – most common phenotype in nature, ex. red eyes in fruit fly. Mutant phenotype – alternatives to the wild type, ex. white eyes in fruit fly. Linked genes – genes located on the same chromosome that tend to be inherited together. Genetic recombination – the production of offspring with new combinations of traits inherited from two parents. 2 types of offspring can be produced: Parental types – offspring matching a parental phenotype. Recombinants – offspring that do not match either parent’s phenotype. The further apart linked genes are, the greater the chance for cross-overs. Maximum value =50% which is equal to the percentage of recombinants from genes on different chromosomes (non-linked). Linkage map – a genetic map based on recombination frequencies, portrays sequence but not exact locations. Cytological maps – locate genes with respect to chromosomal features, such as stained bands. Sex Chromosomes XX is female, XY is male. “If you have a Y, you’re a guy.” In humans, the anatomical signs of sex begin to emerge when the embryo is about 2 months old. 1990, British team identified SRY, sex1 C15 DNA determining region of Y. In the absence of SRY, the gonads develop into ovaries. SRY codes for a protein that regulates other genes. Other genes on the Y are required for normal testicular function. Sex-linked genes – genes located on a sex chromosome (usu. X). Hemizygous – male X-linked traits expressed as they stand alone. X-linked disorders (also recessive): Colorblindness – difficulty discriminating between red and green colors. Duchenne muscular dystrophy – progressive weakening of muscles due to lacking a muscle protein called dystrophin. Hemophilia – a clotting factor, Factor VIII, deficiency which leads to excessive bleeding when injured. Treated with Factor VIII replacement therapy. (Plagued royal families of Europe). The X chromosome becomes almost completely inactivated in female mammals during embryonic development. The inactive chromosome condenses into a Barr body. Female calico cat exemplifies mosaicism due to this. Nondisjunction – when chromosomes or chromatids fail to separate properly during meiosis. Aneuploidy – offspring with an abnormal chromosome number. Trisomic – having three chromosomes of one type, ex. Trisomy 21. Monosomic – missing a chromosome, ex. Turner’s syndrome. Polyploidy – having >2 complete chromosome sets. Triploidy (3n); tetraploidy (4n). Alterations of chromosome structure: Deletion – chromosomal fragment is lost during cell division. Duplication – fragment joins the homologous chromosome and you have extra nucleotides. 2 C15 DNA Inversion – fragment breaks off and reattaches to the proper chromosome, now upside down. Translocation – fragment attaches to a nonhomologous chromosome. Down syndrome – Trisomy 21, characterized by shorter lifespan, facial features, short stature, and mental retardation. Increased risk in older mothers. 1 out of every 700 children born in the US. XXY – Klinefelter’s syndrome, 1 in every 2000 live births, have male sex organs; testes are abnormally small and the man is sterile. XYY – taller than average. XXX – Trisomy X, one in every 1000 live births. XO – Turner syndrome, females of shorter stature and sterile without replacement hormones. 1 in every 5000 live births. Cri du chat – “Cry of the cat,” caused by deletion in chromosome 5. Mental retardation, small head, and a cry that sounds like a distressed cat’s mewing. Usu. die in infancy or early childhood. Chronic myelogenous leukemia (CML) – caused by a portion of chromosome 22 switching places with a tip of chromosome 9. Genomic imprinting – Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm via methylation (adding of methyl groups to nucleotides at specific loci which inactivates that allele). Ex, PraderWilli syndrome (from father) patients have mental retardation, obesity, short stature, and small hands and feet. Angelman syndrome (from mother) patients have uncontrollable laughter and jerky movements…Both caused by deletion of a segment of chromosome 15. At least 20 mammalian genes have been found that are subject to imprinting, and there may be a couple of hundred more. Fragile X syndrome – X chromosome hangs by a thread of DNA. Most common form of mental retardation. More severe when inherited from mother than father. More males have this disorder. 3 C15 DNA Extranuclear genes exhibit a non-Mendelian pattern of inheritance. Maternal inheritance is the rule for plastids in plants, and mitochondrial genes in plants and mammals 4