Bio101 Sex Linked Inheritance

Humans have 23 pairs of chromosomes

22 pairs are autosomes

AUTOSOMES AND SEX CHROMOSOMES- One pair determines biological sex:

the sex chromosomes, X and Y

AUTOSOMES AND SEX CHROMOSOMES

Look at the size difference. The X is a large chromosome with over 1,000 genes. The Y is one of the smallest, with a few dozen.

AUTOSOMES AND SEX CHROMOSOMES- Individuals who inherit two X chromosomes usually develop as female

one X and one Y, as male

SEX IS DECIDED BY THE SPERM- eggs

An egg always carries an X — a female parent has only X chromosomes to give

SEX IS DECIDED BY THE SPERM- A sperm carries

either an X or a Y, in roughly equal numbers

SEX IS DECIDED BY THE SPERM

X sperm plus X egg gives XX. Y sperm plus X egg gives XY

SEX IS DECIDED BY THE SPERM- So the father’s gamete determines the sex of the

offspring

and the ratio is about 50:50 for the same reason any coin flip is

THE Y CHROMOSOME- Only the tips of the Y

are homologous with regions of the X. The rest of the Y is not shared with the X at all

THE Y CHROMOSOME

Those small homologous regions are what let X and Y pair and segregate properly during meiosis I

THE Y CHROMOSOME- The Y carries SRY-

the gene that triggers testis development. The testes then produce testosterone, which drives the rest.

THE Y CHROMOSOME- Sex determination is more complicated than one gene

and some people are born with intersex characteristics

SEX-LINKED GENES

A gene located on either sex chromosome is a sex-linked gene

SEX-LINKED GENES- Genes on the X are X-linked

genes on the Y are Y-linked

SEX-LINKED GENES- Most Y-linked genes are involved in male development

There are very few of them

SEX-LINKED GENES- The X carries hundreds of genes that have nothing to do with sex

blood clotting, color vision, muscle protein. Those are the ones that produce the inheritance pattern we care about today.

WHY MALES ARE AFFECTED MORE

A female has two X chromosomes, so she has two copies of every X-linked gene

WHY MALES ARE AFFECTED MORE- A male has one X and one Y

so he has only one copy. He is hemizygous

WHY MALES ARE AFFECTED MORE- For a recessive X-linked trait to show in a female,

she needs two copies of the allele

WHY MALES ARE AFFECTED MORE- A male needs only one

There is no second X to mask it — the Y carries no matching gene

WHY MALES ARE AFFECTED MORE- asymmetry

the whole reason X-linked recessive disorders are far more common in males

THREE CROSSES TO KNOW- (a) Affected father × homozygous normal mother

all daughters are carriers, no sons affected

THREE CROSSES TO KNOW- (b) Carrier mother × normal father

half the sons affected, half the daughters carriers

THREE CROSSES TO KNOW- (c) Carrier mother × affected father

half the daughters affected, half the sons affected

THREE CROSSES TO KNOW

Notice (a): an affected father gives his X to every daughter and his Y to every son. Fathers never pass an X-linked allele to a so

CARRIER MOTHERS- A female with one mutant and one normal allele is a carrier

usually with no symptoms

CARRIER MOTHERS- Follow the figure. Carrier mother × unaffected father gives four equally likely outcomes

25% unaffected daughter, 25% carrier daughter, 25% unaffected son, 25% affected son

CARRIER MOTHERS

So half her sons are affected, and half her daughters are carriers — the trait looks like it

skips generations and shows up in nephews

X-LINKED RECESSIVE DISORDERS- Red-green color blindness

the most common; about 1 in 12 males and 1 in 200 females

X-LINKED RECESSIVE DISORDERS- Duchenne muscular dystrophy

a missing muscle protein, dystrophin; about 1 in 3,500 male births

X-LINKED RECESSIVE DISORDERS- Hemophilia

a missing blood clotting factor, so bleeding does not stop normally

X-LINKED RECESSIVE DISORDERS

Hemophilia ran through the royal families of Europe from Queen Victoria, a carrier, which is

why it is sometimes called the royal disease

X INACTIVATION- Here is a problem

females have two copies of every X-linked gene and males have one. Why is that not a double dose?

X INACTIVATION- In mammalian females, one X in each cell is randomly inactivated

early in embryonic development

X INACTIVATION

It happens by chemical modification — methyl groups on the DNA and changes to histone proteins

X INACTIVATION- Which X gets silenced is random in each cell

and all that cell’s descendants keep the same choice

X INACTIVATION- So a female heterozygous for an X-linked gene is a mosaic

patches of cells expressing one allele, patches the other

BARR BODIES

The inactivated X condenses into a compact object called a Barr body, visible at the edge of the nucleus

BARR BODIES- You can count them

A typical female cell has one; a typical male cell has none

BARR BODIES- And the visible proof: a tortoiseshell or calico cat is always female

Orange and black fur alleles sit on the X, and each patch is a clone of cells with the same X

switched off.

22 pairs of autosomes plus one pair of sex chromosomes

The sperm determines sex

Males are hemizygous for X-linked genes

one copy, nothing to mask it. That is why X-linked recessive disorders are far more common in males

Fathers never pass an X-linked allele to a son

they pass it to every daughter

X inactivation

silences one X per cell at random, producing Barr bodies and mosaic females