Chromosomal Basis of Inheritance Chapter 15

Objectives Understand the concept of “Linked Genes” Understand how recombination of genes affect genetic variability Understand how frequency of recombination of linked genes is related to their loci distance from one another Be familiar with patterns of inheritance for genes on sex chromosomes Be familiar with errors that may occur in chromosomal inheritance due to problems associated with meiosis

Karyotype Using a microscope, it is possible to count and characterize the individual chromosomes during the time they are coiled and condensed. A photograph of the entire set of chromosomes can be made. Then the images of the individual chromosome can be cut out and arranged by shape and size in an orderly arrangement called a karyotype

Linked Genes Linked genes are those that reside on the same chromosome and tend to be inherited together –Autosomal genes reside on the autosomal chromosomes (pairs 1-22) –Sex-linked genes are found on the sex chromosomes (pair 23, usually on the X)

Recombination of Genes Genetic recombination is said to have occurred when offspring exhibit new combinations of traits not seen in the parent Independent assortment may recombine genes that are unlinked Linked genes can become unlinked through recombination events like crossover (during meiosis)

Gene Mapping Maps of genes on chromosomes can be constructed from recombination data Recombination data for linked genes reflects the “distance” of the 2 loci from one another The farther apart 2 loci are from one another the more frequent the observed recombination due to crossover Linkage map: genetic map based on recombination frequencies

Sex Linked Inheritance Males and females differ in their sex chromosome combination (females XX; males XY) Barr bodies Because the X contains genes and the Y “does not”, inheritance patterns of sex-linked genes vary between the sexes –recessive traits more prevalent in males

Errors in Inheritance Genetic disorders can occur due to: –changes in chromosome number nondisjunction –changes in gene sequence deletion duplication inversion –changes in gene location translocation

Nondisjunction Failure of a pair of chromosomes to separate during gamete formation Genetic disorders like: –Down’s syndrome (#21, 3n) –Turner syndrome (XO) –Metafemale (XXX) –Kleinfelter Male (XXY) –Jacob’s Male (XYY)

Changes in Gene Sequence Deletion: loss of a chromosomal segment Duplication: addition of a chromosomal segment Inversion: flipping of information on a chromosome

Translocation: the movement of information from one member of a homologous pair to a chromosome of a different (non homologous) pair Changes in Gene Location