Sex Linkage and Polygenic Traits [11.2] SPI 4 Determine the probability of a particular trait in an offspring based on the genotype of the parents and the particular mode of inheritance (Sex-Linked Traits).

Sex Determination The X and Y chromosome are the sex chromosomes –All other chromosomes are called autosomes X & Y determine gender –XX= female –XY= male X XX Y

Females have 2 copies of the X chromosome. Both don’t need to be active at the same time. One X is inactivated and packaged up. It’s called a Barr Body.

Which X is turned off varies from cell to cell. Coat color of a calico cat is because of the random deactivation of one X chromosome, where the alleles for coat color are found.

Sex-Linked Traits Traits that are coded for by alleles on the X or Y chromosome are sex-linked traits. –X-linked –Y-linked Most sex-linked traits are X-linked X-linked disorders are a big deal for males because they only have 1 copy of the X chromosome Since females have 2 X, if only 1 X is a problem it can be silenced or deactivated.

X B Y X B X B X B X B Y X b X B X b X b Y An Example… Red-Green colorblindness is a recessive X-linked trait. Below is a cross of a woman who is heterozygous for red-green colorblindness and an unaffected male. Are their sons or are their daughters more likely to be affected? What would have to happen for a daughter to be affected?

X b Y X B X B X b X B Y Another Example… Male pattern baldness is a recessive X- linked trait. Below is a cross of a balding man and an unaffected woman. Are the sons affected? The daughters? From which of his parents did the dad inherit his faulty X chromosome?

Polygenic Traits Some phenotypes are determined by multiple genes. Skin color, eye color, and fingerprints are all determined by the mix of expressions from many different genes.