Chromosomal Theory of Inheritance Do Now: Below are pairs of homologous chromosomes. Assume that the color bands on the chromosomes in each illustration represent the same gene. Compare and contrast illustration 1 with 2. Chromosomal Theory of Inheritance #1 #2

Similarities Difference Both illustrations are of pairs of homologous chromosomes Both illustrations show chromosomes with genes at a given loci. Appears that the chromosomes in each illustration contain the same genes based upon the coloration of each band. One of the homologous chromosome pairs in illustration #2 is the same size as the pair in illustration #1. Similarities Difference The homologous chromosomes in illustration #1 have both genes on the one pair while illustration #2 shows the genes are on different pairs of homologous chromosomes.

Linked and Unlinked Genes Unlinked genes may be on different chromosomes, or so far apart on the same chromosome that they are often separated by recombination. If two genes are inherited together more than 50% of the time, this is evidence that they are linked on the same chromosome. ... "Genetic Linkage." ?????

What type of chromosome are the genes on? Autosomes – chromosomes which are considered ‘homologous’ Sex – chromosomes which are considered ‘nonhomologous’ Why???

X-Y System of Sex Determination Fruit Fly (D. melanogaster) Human (H.sapiens) Probability of being male is 50%. Probability of being female is 50%.

Sex-linked Genetic Crosses: 1 Sex-linked Genetic Crosses: 1. Crosses involving genes located on the sex chromosomes 2. Must write the X and Y in the genotype when solving the crosses 3. Must include male and female in the writing of possible phenotypes

Sample Sex-linked Cross Genotype: XHY Genotype: XHXh

Human Sex-linked Traits: Hemophilia Red-green colorblindness Duchene muscular dystrophy Fruit Fly Sex-linked Trait: Red and white eye color Cat Sex-linked Trait: Calico

Inheritance of a Sex-linked Trait