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Published byBriana Dickerson Modified over 8 years ago
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SEX-LINKED TRAITS
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Karyotype: a picture of chromosomes.
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… Autosomes: the first 22 homologous pairs of chromosomes. Autosomes are the same for both males and females.
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Sex chromosomes: determines the sex of the individual. The sex chromosomes are the 23rd pair of chromosomes.
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XX = female
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XY = male
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Which parent determines the sex of an offspring? DAD
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Why? All moms have the genotype XX. When egg cells are made, they will all carry a single X chromosome.
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All dads have the genotype XY. When sperm cells are made, 50% will have an X chromosome and 50% will have a Y chromosome. Therefore, males and females are born in roughly a 50:50 ratio.
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SEX-LINKED TRAITS: those traits that are controlled by genes on the X or Y chromosomes. NOTE: The Y chromosome is much smaller than the X chromosome and only contains a few genes. Most sex-linked traits are on the X chromosome.
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In humans, hemophilia is a sex-linked trait. Having hemophilia is recessive (X h ) to being normal (X H ). The heterozygous female is called a carrier. Cross a carrier female with a normal male. X H X h X _____
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In humans, hemophilia is a sex-linked trait. Having hemophilia is recessive (X h ) to being normal (X H ). The heterozygous female is called a carrier. Cross a carrier female with a normal male. X H X h X X H Y
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XHXH XhXh XHXH Y
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XHXH XhXh XHXH XHXHXHXH Y
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XHXH XhXh XHXH XHXHXHXH XHXhXHXh Y
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XHXH XhXh XHXH XHXHXHXH XHXhXHXh YXHYXHY
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XHXH XhXh XHXH XHXHXHXH XHXhXHXh YXHYXHYXhYXhY
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Genotypic ratio: 1 X H X H :1X H X h :1X H Y :1X h Y Phenotypic ratio: 2 normal females: 1normal male: 1 male with hemophilia
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Cross a carrier female with a male with hemophilia. X H X h X _____
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Cross a carrier female with a male with hemophilia. X H X h X X h Y
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XHXH XhXh XhXh Y
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XHXH XhXh XhXh XHXhXHXh Y
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XHXH XhXh XhXh XHXhXHXh XhXhXhXh Y
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XHXH XhXh XhXh XHXhXHXh XhXhXhXh YXHYXHY
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XHXH XhXh XhXh XHXhXHXh XhXhXhXh YXHYXHYXhYXhY
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Genotypic ratio: 1 X H X h :1X h X h :1X H Y :1X h Y Phenotypic ratio: 1 normal female: 1 female with hemophilia:1 normal male: 1 male with hemophilia
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In humans, red-green colorblindness is a sex- linked trait. People with red-green colorblindness can not tell the difference between red and green. Colorblindness is the result of a recessive allele. Cross a female with colorblindness with a male with normal vision. X n X n X _____
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In humans, red-green colorblindness is a sex- linked trait. People with red-green colorblindness can not tell the difference between red and green. Colorblindness is the result of a recessive allele. Cross a female with colorblindness with a male with normal vision. X n X n X X N Y
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XnXn XnXn XNXN Y
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XnXn XnXn XNXN XNXnXNXn Y
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XnXn XnXn XNXN XNXnXNXn XNXnXNXn Y
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XnXn XnXn XNXN XNXnXNXn XNXnXNXn YXnYXnY
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XnXn XnXn XNXN XNXnXNXn XNXnXNXn YXnYXnYXnYXnY
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Genotypic ratio: 2 X N X n : 2 X n Y Phenotypic ratio: 2 normal females: 2 males with colorblindness
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Why are sex-linked traits more common in males than in females? Because a male only has to inherit ONE recessive allele in order to get a sex-linked trait and a female has to inherit TWO recessive alleles in order to acquire the sex-linked trait.
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It is easier to inherit one recessive allele than two. If the female only inherits one recessive allele, then they are a carrier but have the normal phenotype.
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