Chapter 15 The Chromosomal Basis of Inheritance Theory

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