Sex Linkage and Polygenic Traits

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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

X X

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.

XB Y

XB XBXB XBY

Xb XBXb XbY

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?

Xb Y

XB XBXb XBY

XB XBXb XBY

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.