Below are the karyotypes of two Below are the karyotypes of two individualsindividualsWhat is the difference?What is the difference?

Below are two additional karyotypes.Below are two additional karyotypes.

What is the difference?What is the difference?

What We Have LearnedWhat We Have Learned

If you are female, the 23If you are female, the 23rdrd pair of pair of chromosomes are chromosomes are homozygous homozygous

XX XX

If you are If you are male, the 23male, the 23rdrd

pair of pair of chromosomes chromosomes

are are heterozygous heterozygous

XYXY

Based on what you know about the Based on what you know about the determination of sex in humans,determination of sex in humans,

Which parent determines the sex Which parent determines the sex of the offspring? of the offspring?

The offspring The offspring of any mating of any mating

between between humans will humans will have a 50:50 have a 50:50

chance of chance of having 2 X having 2 X

chromosomes, chromosomes, XX, XX, which iswhich is femalefemale, or , or

having one X having one X and one Y and one Y

chromosome, chromosome, XYXY, which is , which is

malemale..

DrosophilaDrosophila (fruit flies), inherit sex (fruit flies), inherit sex chromosomes in the same way as chromosomes in the same way as humans. humans.

Some traits are controlled by genes located Some traits are controlled by genes located on sex chromosomes and are called on sex chromosomes and are called sex-sex-linked traitslinked traits. . The alleles for sex linked traits are written The alleles for sex linked traits are written as superscripts of the Xas superscripts of the X?? and Y and Y

?? chromosomes.chromosomes.

Because the X and Y chromosomes are Because the X and Y chromosomes are not homologous, the Y chromosome has not homologous, the Y chromosome has no corresponding allele to the one on no corresponding allele to the one on the X chromosome.the X chromosome.

Remember that any recessive allele on Remember that any recessive allele on the X chromosome of a male will not be the X chromosome of a male will not be masked by a corresponding dominant masked by a corresponding dominant allele on the Y chromosome.allele on the Y chromosome.

Since the Y chromosome has no Since the Y chromosome has no corresponding allele to the one on the X corresponding allele to the one on the X chromosome any trait carried on the X chromosome any trait carried on the X chromosome will be expressed in males.chromosome will be expressed in males.

In fruit flies red eyes In fruit flies red eyes (R) are dominant to (R) are dominant to white eyes (r). white eyes (r).

XXRRXXRR XXRRXXrr

XXRRYY XXrrYY

XX?? XX??

Based on what we know about sex linked Based on what we know about sex linked inheritance of eye color in fruit flies, inheritance of eye color in fruit flies, what would be the genotype of the what would be the genotype of the female fruit fly below?female fruit fly below?

Create a Punnet Square that shows the Create a Punnet Square that shows the cross of a male fruit fly with white eyes with cross of a male fruit fly with white eyes with a homozygous dominant red eyed female. a homozygous dominant red eyed female.

XR Xr

XR

Xr Y

XR Xr

XR

XR Y

XR Y

Color blindness Color blindness in humans is also in humans is also a sex linked trait a sex linked trait that is carried on that is carried on

the X the X chromosome. chromosome.

XX XX

Normal Normal VisionVision

XXCC XX

Normal Vision Normal Vision but a carrier for color but a carrier for color

blindnessblindness

XXCC XXCC

Color BlindColor Blind

For Women there are three For Women there are three possibilitiespossibilities

Normal VisionNormal Vision

XX YY

. . . But for males . . . But for males there are only there are only

2 possibilities2 possibilities

XXCC YY

Color BlindColor Blind

Remember that Remember that since the Y since the Y

chromosome has chromosome has no corresponding no corresponding allele to the one allele to the one

on the X on the X chromosome any chromosome any trait carried on trait carried on

the X the X chromosome will chromosome will be expressed in be expressed in

males.males.

If a color blind If a color blind man marries a man marries a women with women with normal vision, normal vision, what are the what are the chances of one of chances of one of their children their children being color blind? being color blind?

XC X

X X

X

YY

XC XC X

X Y

Woman who is a Woman who is a carrier for color carrier for color blindness marries blindness marries a normal vision a normal vision man, what are the man, what are the chances of one of chances of one of their children their children being color blind? being color blind?

XC X

XC X

XC YY

X X X

X Y