The Chromosomal Theory of Inheritance:

• Chromosomes and genes are present as pairs in diploid cells

• homologous chromosomes separate during meiosis

• fertilization restores chromosome number to diploid condition

• The chromosomal theory of inheritance claims it is the chromosome that segregates and assorts independently.

Morgan and

Linked Traits

Sex Linked Traits

• Eye color is linked on the X Chromosome

• Females carry 2 copies of the gene while males have only one copy

• If mutant allele is recessive, white eyed female has the trait on both X’s

• White eyed male can not hide the trait since he has only one X.

• Who is responsible when a boy inherits a sex linked condition?– The mother.

• A carrier of a condition is heterozygous for the trait, so only females can carry sex linked traits.

Color Blindness

• Recessive condition that effect men more than women.

• Why?

• In flies gray body(b+) is dominant to black body (b), and long wings (vg+) are dominant to vestigial wings (vg).

• What is the expected phenotypic ratio from a cross between a heterozygous gray, heterozygous normal winged fly with a black vestigial fly?– 1:1:1:1 ratio

• But what he got was…..

• What explains the non-Mendelian results?– There was No independent assortment!

• The traits are linked on the same chromosome.

• If the traits are linked, then all of the offspring should resemble the parental types, black vestigial or gray normal.

• How did we get the recombinant traits of black, normal and gray vestigial?

Using Recombination Frequency to Map Traits

• Recombination frequency between black and vestigial traits is 17%

• Recombination frequency between cn and black is 9%

• Recombination between cn and vg is 9.5%

• Map these traits on the chromosome.

• Determine the order of genes along a chromosome based on the following recombination frequencies:

• A - B 8% A--B

• A - C 28% A-------C

• A - D 25% D-----A

• B - C 20% B---C

• B - D 33% D---------B

• DABC

X Inactivation in Females

• How does an organism compensate for the fact that some individuals have a double dosage of sex-linked genes, while others have only one?– X inactivation , the inactive X condenses

into a Barr body– The inactivation is random and

approximately half the X’s from Mom and half the X’s from Dad will inactivate

• The calico cat is the result of multiple alleles on the X chromosome combines with X inactivation.

• One allele causes orange fur and the other causes black fur.

• The male cat will be either orange or black.

• Only a heterozygous female will be calico with regions that are orange and regions that are black based on the X inactivation.

                                                            

Non-Disjunction Diseases

• http://cas.bellarmine.edu/tietjen/HumanBioogy/bills_developmental_abnormalities.htm