Determining Sex Females are XX, producing only X gametes. Males are XY, producing both X & Y gametes.

Determining Sex

Females are XX, producing only X gametes. Males are XY, producing both X & Y gametes.

2

Sex-linked Traits

Traits (genes) located on the sex chromosomes

Sex chromosomes are X and Y

XX genotype for femalesXY genotype for malesMany sex-linked traits carried on X chromosome

The X-Chromosome contains over 2300 genes

Most of these genes are unrelated to sex

Genes on X-Chromosome are expressed in both sexes

X-Chromosome has a centromere in the center of the chromosome

X-Chromosome

The Y-Chromosome contains around 25 genes

It is short & stubbyThe centromere is near the endMost important function is to

determine sex

Y-Chromosome

5

Sex-linked Traits

Sex Chromosomes

XX chromosome - female Xy chromosome - male

fruit flyeye color

Example: Eye color in fruit flies

6

Sex-linked Trait Problem

Example: Eye color in fruit flies (red-eyed male) x (white-eyed

female) XRY x XrXr

Remember: the Y chromosome in males does not carry traits.

RR = red eyedRr = red eyedrr = white eyedXY = maleXX = female

XR

Xr Xr

Y

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Sex-linked Trait Solution:

XR Xr

Xr Y

XR Xr

Xr Y

50% red eyed female

50% white eyed male

XR

Xr Xr

Y

Red/Green ColorblindnessHemophiliaBaldnessMuscular Dystrophy

Some Sex-Linked Traits

Only females can be carriers

It is all or nothing for males

Notice the shape of Y-Chromosome

Sex-Linked Recessive Traits

http://www.dnai.org/d/index.html

Trait will show up in phenotype if in genotype

Sex-Linked Dominant Trait

Pedigrees

Another method used to study human heredity is a pedigree.

It is a a visual tool that is easier to follow.

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Female Carriers

A circle represents a female.

A square represents a male.

A horizontal line connecting a male and female represents a marriage.

A vertical line and a bracket connect the parents to their children.

A half-shaded circle or square indicates that a person is a carrier of the trait.

A completely shaded circle or square indicates that a person expresses the trait.

A circle or square that is not shaded indicates that a person does not express the trait but don’t know if they are carrier.

Section 14-1

Figure 14-3 A Pedigree

Tay-Sacs Pedigree

1. How many generations are there in this pedigree?

2. Who died in Generation 2?

3. How many kids were produced from generation 1?

Tay-Sacs Pedigree

4. Who got married in generation 3?

5. How many kids lived in generation 4?

What is Hemophilia?It is a blood clotting disorder.

Victims lack ONE of EIGHT proteins required for blood to clot.

Victims may die from cuts…or bruises!

It is a… …sex-linked trait!