Chromosomes & Inheritance

• The chromosomal basis for inheritance was described in 1902 independently by a group of scientists. It basically states that genes have specific loci on chromosomes and that during mitosis and meiosis the segregate and undergo independent assortment.

• Thomas Hunt Morgan proved theory with the use of fruit flies (Drosophila melanogaster).

Mutant vs Wildtype• Because of their fast reproductive cycle and their ease of use,

Drosophila melanogaster quickly became a model organism for the study genetics.

• most common trait is the wild type and less common trait the mutant type

• traits named by the mutant type (human genes named by the wild type) – white eyes w; red eyes w+ – determined red is dominant with white being expressed more often in males

(sex linked recessive)

• genes on located on the same chromosome are linked – demonstrated with body color (gray &

black (b & b+)) & wing shape (normal & vestigial (vg & vg+)

– F1 all gray with normal wings – test cross ( F1 with true breeding recessive

(b b+ vg vg+ x b+ b+ vg+ vg+) • if genes are located on separate genes the

recombinants should be numerically equal for all traits

• results are as follows: – cross yielded 965 gray-normal, 944 black vestigial,

206 gray-vestigial, & 185 black-normal – since 2 phenotypes are mostly present the genes

are linked – the small percentage of mutants are due to

crossing over » the greater the percentage of new

recombinants the greater the distance of the genes on the same chromosome

Chromosomal basis of inheritance• Humans & many animals

– X & Y – the presence of the Y determines male

• contains the region SRY (sex-determining region of Y)

– although the chromosomes are different in most aspects, there is a small homologous region that allows the 2 chromosomes to act as homologues during meiosis

– penetrance of sex linked disorders from a female carrier • 50% of males (fragile-x, Duchenne Muscular Dystrophy, hemophelia) • 0% females unless the father has the disease, then 50%

– random X-inactivation (lyonization) causes the creation of barr bodies • inactivation with methyl groups • condensed chromosome • reactivated when an ova is formed

Other Sex Determining Systems• X-O system (grasshopper)

– XX vs X – XX female, X male

• Z-W System (chicken) – ZW female, ZZ male

• Haploid/diploid system (bees) – diploid - female, haploid - male