sex-linked traits. sex determination sex chromosomes – determines the sex of an individual yy xx ...
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Sex-linked Traits
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Sex Determination
Sex chromosomes – determines the sex of an individualY X
Males have X and YTwo kinds of gametes
Female have two X’sOnly one type of gamete
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Which parent determines the sex of the offspring?
XY
male
X Y
XX
Female
X
X
It is the male that determines the sex of the offspring.
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Sex Linked TraitsTraits controlled by genes located on sex
chromosomes (X or Y)Most are X-linked
The X chromosome is larger and has more genesA Y has very few genes
Show inheritance pattern that differs for autosomal traits.
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T.H. Morgan worked with fruit flies
• Fruit flies normally have red eyes.• One day, in one of his cultures of fruit flies,
Morgan found a fly with white eyes. • This white-eyed fly was male, so Morgan
mated it to red-eyed female fly to see if he could determine how this particular trait is inherited.
• When he mated it with a normal female, the offspring all had red eyes. This was expected in light of Mendel's experiments.
• The trait reappeared in the F2 generation. However, there was a twist because the trait only reappeared in males.
• There were no female flies with white eyes!
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Males only have one copy of the X chromosome (hemizygous), plus a Y
Female have two X chromosomes, and may be homozygous or heterozygous for a trait
X-linked genes are never passed from father to son. The Y chromosome is the only sex chromosome that passes from father to son.
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Females with one copy of the normal gene and one copy of the mutated gene are called carriers. They don’t show the trait.
Males are never carriers – if they have a mutated gene on the X chromosome, it will be expressed
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X-linked disorders
RecessiveRed-green color
blindnessHemophiliaDuchenne muscular
dystrophy Ichthyosis
DominantHypertrichosis
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Red-green color blindnessCan not distinguish red from green.
Normal red-green Red-green colorblind
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What a red-green colorblind person seesNormal Red-green colorblind
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Hemophilia
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X Chromosome InactivationWhile females have two X chromosomes,
only one is active.The other is turned off.
This is random, in some cells the maternal X is turned off and in others the paternal X is turned off.
All descents of that cell will have same X turned off
It is condensed and known as a Barr Body.
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