Chapter 15 The Chromosomal Basis of Inheritance

Theory of Inheritance Based on 1902 research by Walter S. Sutton, Theodor Boveri and others responsible for the Chromosome Theory of Inheritance. It is the idea that Mendelian genes were found at specific loci on chromosomes, and that it is the chromosomes that undergo segregation and independent assortment.

Morgan’s Experimental Evidence Morgan worked with a species of fruit fly, Drospohila melanogaster. Drosophila have 3 pairs of autosomes and 1 set of sex chromosomes and easy to mate. After breeding for a year, a white eyed mutant fly was produced. Determined the gene was sex linked.

Linked Genes Linked genes tend to be inherited together because they are located on the same chromosome. Linked genes tend to deviate from the typical inheritance patterns described by Mendel.

Recombination of Linked Genes For genes located on the same chromosome, most of the offspring have the parental phenotype. 83% of Morgan’s results suggested linkage in the case of body color and wing size. 17% suggested linkage to be incomplete.

Crossing over accounts for the recombination of linked genes. Alleles break at corresponding points, rejoin and end portions of sister trade places. Alfred H. Sturtevant, determined a method for creating an ordered list of genetic loci along a particular chromosome  genetic mapping.

Link

Genetic and Linkage Maps Alfred H. Sturtevant, determined a method for creating an ordered list of genetic loci along a particular chromosome  genetic mapping Recombination frequencies depend on the distances between genes on a chromosome.

Events are random, making the chance of crossing over approximately equal at all points along a chromosome. The further apart the genes are, the higher the probability of crossing over, the higher the recombination frequency.

A genetic map based on recombination frequencies is called a  Linkage map. Based on the assumption that the probability of crossover between 2 genetic loci is proportional to the distance separating the loci. Sturtevant expressed the distance between genes in map units: 1 map unit = 1% recombination frequency “Centimorgans”

Linkage Map

Sex Genes The female sex is determined by a homozygous dominant X, the male is heterozygous XY. The Y chromosome is much shorter than the X and only short segments located a the ends of the Y are homologous with genes on the X. These homologous regions allow the X and Y to pair and behave like homologous chromosomes during meiosis in the testes.

Y chromosome Sexual anatomy does not develop until the 2 nd month of life. The direction the gonads take depend on the hormonal conditions in the embryo. 1990, a British research team identified a SRY gene (sex determining region on Y) responsible for testis development on the Y chromosome. The SRY gene is a trigger, responsible for regulating other genes involved in the development of male characteristics.

Sex Linked Genes A gene located on a sex chromosome, Historically on the X. Sex linked genes follow the same inheritance patterns as seen in eye color locus of Drosophila. Fathers pass sex linked genes to all daughters, mothers pass to both sons and daughters.

Sex linked traits can be recessive in which case males inheriting the gene will always express. Female have a much lower percent chance of expressing a sex linked trait. Color-blindness, Duchene muscular dystrophy and hemophilia are examples of X linked disorders.

Genetic Disorders and chromosome number Nondisjunction where the members of a pair of chromosome fail to separate properly during meiosis I or sisters fail during meiosis II. If the resultant gamete unites with a normal gamete, the offspring will have an abnormal number of chromosomes  aneuploidy.

If a chromosome is present in triplicate in a fertilized egg, the cell is considered trisomic. If a chromosome is missing, the cell is considered monosomic. Mitosis will transmit the anomaly to all embryonic cells.

Estimated risk of Down’s Syndrome according to maternal age

Polyploidy Organisms may have more than 2 sets of chromosomes  polypoidy. This is fairly common in the plant kingdom, although much less in animals. In general, polyploids are more normal looking than aneuploids. One missing chromosome seems to disrupt the genetic balance more than an extra set.

Tympanoctomys barrerae

Alteration of chromosome structure Breakage of a chromosome can lead to 4 types of changes: 1.Deletion 2.Duplication 3.Inversion 4.translocation

Extranuclear Chromosomes Mitochondria, chloroplasts and other plastids contain small, circular DNA for proteins and RNA. These organelles reproduce on their own and transmit their genes to the daughter organelles.