The Chromosomal Basis of Inheritance Chapter 15 Biology – Campbell Reece

Chromosome Theory Chromosome Theory of Inheritance Mendelian genes have specific loci on chromosomes Mendelian genes have specific loci on chromosomes It is the chromosomes that undergo segregation and independent assortment It is the chromosomes that undergo segregation and independent assortment Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome

Linked Genes Genes located on the sex chromosome are called sex-linked genes Ex. Baldness, color-blindness, hemophilia Ex. Baldness, color-blindness, hemophilia Linked genes – genes located on the same chromosome tend to be inherited together This violates Mendel’s Law of Independent Assortment, because it is the chromosomes that separate, not the genes This violates Mendel’s Law of Independent Assortment, because it is the chromosomes that separate, not the genes

Colorblindness Test

Sex-Linked Genes Tend to be found more often on the X chromosome (because it is larger) Therefore, men tend to show sex-linked traits more because they only have one X If the trait is caused by a recessive gene, a male only requires the one X to have the trait, but a female must have the trait on both Xs

Gene Mapping Genetic map – an ordered list of the genetic loci along a particular chromosome The farther apart two genes are on a chromosome, the more likely it is that crossing over will occur between them The farther apart two genes are on a chromosome, the more likely it is that crossing over will occur between them Using the recombination frequencies, you can calculate the distance (in map units) between genes on a chromosome Using the recombination frequencies, you can calculate the distance (in map units) between genes on a chromosome

Gene Mapping

Alterations of Chromosome Number Nondisjunction – the homologous chromosomes fail to separate properly during meiosis Aneuploidy – an abnormal number of chromosomes Trisomic – have 3 copies of a chromosome Trisomic – have 3 copies of a chromosome Monosomic – have only 1 copy of a chromosome (the other is missing) Monosomic – have only 1 copy of a chromosome (the other is missing)

Alterations of Chromosome Structure Deletion – a piece of the chromosome is missing Duplication – an extra copy of a segment of the chromosome Inversion – a reverse segment within a chromosome Translocation – a piece from one chromosome attaches to a nonhomologous chromosome

Alterations of Chromosome Structure

Chromosomal Disorders Down Syndrome (Trisomy 21) – caused by an extra 21 st chromosome

Chromosomal Disorders Klinefelter syndrome – an extra X chromosome in a male (XXY) XYY – males that are somewhat taller than normal Turner syndrome – a missing sex chromosome (XO) cri du chat – caused by a deletion in chromosome 5 (causes mental retardation)