Human Sex Chromosomes PDF

Human Sex Chromosomes The small region of homology between the sex chromosomes allows them to line up and segregate from one another during anaphase of meiosis I, contributing to the pattern of sex-linked traits seen in pedigrees. Segregation of the Sex Chromosomes Random fertilization should...

Human Sex Chromosomes The small region of homology between the sex chromosomes allows them to line up and segregate from one another during anaphase of meiosis I, contributing to the pattern of sex-linked traits seen in pedigrees. Segregation of the Sex Chromosomes Random fertilization should result in a 1:1 ratio of XX and XY progeny. At birth, sex ratios show a slight excess of males.  Sex linked genes found only on sex chromosomes Sex determination differs among species Humans: Females XX and Males XY In humans, X chromosome is larger and carries more genes than the Y chromosome Genes found on the X but not the Y are X-linked genes Loading…  Sex-linked genes follow specific patterns of inheritance For a recessive sex-linked trait to be expressed A female needs two copies of the allele Homozygous A male needs only one copy of the allele hemizygous Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings  Loading… Sex-linked (X linked) recessive disorders much more common in males Color blindness Hemophilia X Inactivation in Females In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during development condenses into a Barr body If a female is heterozygous for a gene on the X chromosome, she will be a mosaic for that character All females are mosaic, but it is only clearly visible in calico cats Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings  Linked Genes More genes than chromosomes Each chromosome has hundreds or thousands of genes Genes located on the same chromosome are called linked genes tend to be inherited together Morgan used fruit flies to see how linkage affects inheritance of two characters Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings  Morgan crossed flies that differed in body color and wing size  Body color and wing size were inherited together in specific combinations parental phenotypes; genes did not assort independently reasoned they were on the same chromosome Non-parental phenotypes; Genetic recombination Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings  Mendel’s results: phenotype matching one of the parental phenotypes are parental types nonparental phenotypes (new combinations of traits) are called recombinant types Loading… A 50% frequency of recombination is observed for any two genes on different chromosomes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings  Morgan’s results: genes can be linked, but the linkage was incomplete; recombinant phenotypes Some process must sometimes break the physical connection between genes on the same chromosome crossing over Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings