The Chromosomal Basis of Inheritance

Chromosomal Theory of Inheritance Principle that states that genes are located at specific positions (loci) on chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns.

Thomas Hunt Morgan Early 20th Century Worked with fruit flies – Drosophila melanogaster Prolific breeders, hundreds of offspring, 2 week generation period Only 4 pair of chromosomes, could see under light microscope Work confirmed chromosomal theory

Sex-linked Genes A gene located on with the X or the Y Y linked Very few genes on Y, inherited from father, very few disorders on Y X linked 1,100 genes on the X chromosome Have genes for many characters unrelated to sex Males inherited X linked Disorder more since they only have 1 X, females have 2, so recessive allele can be masked by other X. Hemophilia, color blindness and Duchenne Muscular Dystrophy

X inactivation XX One X becomes inactivated during early embryonic development in each female cell, random process Condensed into a compact object called a Barr Body Tortoiseshell Cate – shows mosaicism

Linkage Genes located near each other on the same chromosome tend to be inherited together in genetic crosses Phenotypic results deviate from those expected when linked Fig. 12.9 Linkage between two genes and inheritance Morgan found a higher proportion of the combinations of traits seen in the P generation flies than would be expected if the two genes assorted independently.

Genetic Recombination Production of offspring with combinations of traits that differ from those found in either P generation parent. Ex. If 50% of all offspring are recombinants, genetics say there is a 50% frequency of recombination – the appearance of new phenotypes. The occurrence of recombination of 50% or higher indicates that the genes are linked. This occurs through crossing over. You can find the recombinant frequency by dividing the recombinants by the total offspring and multiply by 100. (Fig. 12.10)

Genetic mapping Ordered list of genetic loci along a particular chromosome Alfred H. Sturtevant, student of Morgan, developed method of putting together a genetic map. Predicted the farther apart two genes are, the higher probability that a crossover will occur between them and therefor the higher the recombination frequency. Sturtevant expressed the distance between genes in map units, defining one map unit as equivalent to 1% recombination frequency.

Alteration of chromosome number Nondisjunction – occurs when chromosomes fail to separate correctly during meiosis (homologous chromosomes) Results in aneuploidy – abnormal number of a particular chromosome Ex. Monosomy, trisomy Polyploidy refers to more than 2 sets of chromosomes Triploidy, tetraploidy Common in plants, bananas are 3n, strawberries 8n

Chromosome alterations Deletion Duplication Inversion Translocation

Chromosomal Disorders – detect on Karyotype Autosomes Down syndrome - trisomy 21 Mosaicism syndrome – triomy 9 Edwards syndrome – trisomy 18 Patua syndrome – trisomy 13 Sex chromosomes XXX – triplo X XXY – Klinefelters XO – Turners – only known viable monosomy in humans XYY – Jacobs syndrome