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Genetics
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CHAPTER 14 NOTESSEX-LINKED TRAITS
Karyotype: a picture of chromosomes.
Autosomes:
the first 22 homologous pairs of chromosomes.
• Autosomes are the same for both males and females.
Sex chromosomes:
determines the sex of the individual.
• The sex chromosomes are the 23rd pair of chromosomes.
XX = female
XY = male
Which parent determines the sex of an offspring?
DAD
Why?
• All moms have the genotype XX. When egg cells are made, they will all carry a single X chromosome.
• 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.
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.
In humans, hemophilia is a sex-linked trait. Having hemophilia is recessive (Xh) to being normal (XH). The heterozygous female is called a carrier. Cross a carrier female with a normal male.
XHXh X _____
In humans, hemophilia is a sex-linked trait. Having hemophilia is recessive (Xh) to being normal (XH). The heterozygous female is called a carrier. Cross a carrier female with a normal male.
XHXh X XHY
XH Xh
XH
Y
XH Xh
XH XHXH
Y
XH Xh
XH XHXH XHXh
Y
XH Xh
XH XHXH XHXh
Y XHY
XH Xh
XH XHXH XHXh
Y XHY XhY
Genotypic ratio:
1 XHXH :1XHXh :1XHY :1XhY
Phenotypic ratio:
2 normal females: 1normal male: 1 male with hemophilia
Cross a carrier female with a male with hemophilia.
XHXh X _____
Cross a carrier female with a male with hemophilia.
XHXh X XhY
XH Xh
Xh
Y
XH Xh
Xh XHXh
Y
XH Xh
Xh XHXh XhXh
Y
XH Xh
Xh XHXh XhXh
Y XHY
XH Xh
Xh XHXh XhXh
Y XHY XhY
Genotypic ratio:
1 XHXh :1XhXh :1XHY :1XhY
Phenotypic ratio:
1 normal female: 1 female with hemophilia:1 normal male: 1 male with hemophilia
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.
XnXn X _____
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.
XnXn X XNY
Xn Xn
XN
Y
Xn Xn
XN XNXn
Y
Xn Xn
XN XNXn XNXn
Y
Xn Xn
XN XNXn XNXn
Y XnY
Xn Xn
XN XNXn XNXn
Y XnY XnY
Genotypic ratio:
2 XNXn : 2 XnY
Phenotypic ratio:
2 normal females: 2 males with colorblindness
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.
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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