CHAPTER 14 NOTES

SEX-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  __ XHY ___

 

 

 

XH

Xh

 

XH

XH XH

XH Xh

 

Y

XH Y

Xh Y

 

 

Genotypic ratio:

1 XH X :1XH Xh :1XH Y :1Xh Y

 

Phenotypic ratio:

2 normal females: 1normal male: 1 male with hemophilia

Cross a carrier female with a male with hemophilia.

 

__ XHXh __  X  __ XhY ___

 

 

 

XH

Xh

 

Xh

XH Xh

Xh Xh

 

Y

XH Y

Xh Y

 

 

Genotypic ratio:

1 XH X :1Xh Xh :1XH Y :1Xh Y

 

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  __ XNY ___

 

 

Xn

Xn

 

XN

XN Xn

XN Xn

 

Y

Xn Y

Xn Y

 

Genotypic ratio:

1 XN X :1XN Xn :1Xn Y :1XnY

 

Phenotypic ratio:

2 normal females: 2 males with color-blindness

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.