Lecture #30Introduction to Sex Linkage

Honors BiologyMs. Gaynor

Gene Linkage Linked genes

Usually inherited together because located near each other on the SAME chromosome

Genes closer together on the same chromosome are more often inherited together

Each chromosomeHas 100’s or 1000’s of genes

Sex-linked genes exhibit unique patterns of inheritance; genes on the X or Y chromosome

Morgan’s Experimental Evidence

Thomas Hunt MorganProvided convincing evidence that chromosomes are the location of Mendel’s heritable alleles

Sex linkage explained Thomas Hunt Morgan

(Columbia University 1910) Fruit Flies (Drosophila)

melanogaster)

http://nobelprize.org/nobel_prizes/medicine/articles/lewis/index.html

© 2007 Paul Billiet ODWS

Morgan’s Choice of Experimental Organism

Morgan worked with fruit fliesLots of offspring A new generation can be bred every two weeks

They have only 5 pairs of chromosomes

Morgan and Fruit Flies Morgan first observed and noted

Wild type (most common) phenotypes that were common in the fly populations

Traits alternative to the wild type are called mutant phenotypes

WILDTYPE

MUTANT

w+ w

The case of the white-eyed mutant

Character TraitsEye color Red eye (wild type)

White eye (mutant)

P PhenotypesWild type (red-eyed) female x White-eyed male

F1 Phenotypes All red-eyed

Red eye is dominant to white eye

Hypothesis

A cross between the F1 flies should give us: 3 red eye : 1 white eye

F2 Phenotypes Red eye White eye

Numbers 3470

82%

782

18%

So far so good

An interesting observation

F2

Phenotypes Red-eyed males

Red-eyed

females

White-eyed males

White-eyed

females

Numbers 1011 2459 782 0

24% 58% 18% 0%

The F2 generation showed the 3:1 red: white eye ratio, but only males had white eyes

A reciprocal cross

Morgan tried the cross the other way round

white-eyed female x red-eyed maleResult

All red-eyed females and all white-eyed males

This confirmed what Morgan suspectedThe gene for eye color is linked to the X

chromosome

Morgan’s Discovery: Sex Linked Traits

Eye color is linked on X Chromosome

Females carry 2 copies of gene; males have only 1 copy

If mutant allele is recessive, white eyed female has the trait on both X’s

White eyed male can not hide the trait since he has only one X.

The Chromosomal Basis of Sex

An organism’s sexIs an inherited phenotype

determined by the presence or absence of certain chromosomes

XX = girlXY = boy

Inheritance of Sex-Linked Genes

The sex chromosomes Have genes for many

characters unrelated to sex (especially the X chromosome)

A gene located on either sex chromosome Is called a sex-linked gene

(Usually on X chromosome)(Usually on X chromosome)

What genes are on the X chromosome?

carries a couple thousand genes but few, if any, of these have anything to do directly with sex determination

Larger and more active than Y chromosome

What genes are on the Y chromosome?

Gene called SRY triggers testis development, which determines male sex characteristics

This gene is turned “on” ~6 weeks into the development of a male embryo

Y-Chromosome-linked diseases are rare

Sex-linked genes follow specific patterns of

inheritance

Fathers pass sex-linked alleles to ALL their daughters but NONE to their sonsXY (Father) XX (daughter)XY (Father) XY (son)

Mothers can pass sex-linked alleles to BOTH sons and daughters XX (Mother) XX (daughter)XX (Mother) XY (son)

Sex Linkage If sex-linked recessive on Xn

Females have to be Xn Xn to show sex-linked trait

Xn X Females do NOT show sex-linked trait

Males have to be Xn Y to show sex-linked trait

**Most sex-linked disorders affect males; sometimes females

Sex-Linked Disorders Some recessive alleles found on

the X chromosome in humans cause certain types of disorders Color blindness Duchenne muscular dystrophy Hemophilia Male pattern baldness

X-Linked Trait = Male Pattern Baldness

Baldness

Another X-Linked Trait = Hemophilia

About 85% of hemophiliacs suffer from classic hemophilia 1 male in 10 000 cannot produce factor VIII

The rest show Christmas disease where they can’t make factor IX

The genes for both forms of hemophilia are sex linked

Hemophiliacs have trouble clotting their blood

Another X-Linked Trait = Red-Green Colorblindness

Normal vision Color blind simulationhttp://www.onset.unsw.edu.au/issue1/colourblindness/colourblindness_print.htm

Another X-Linked Trait = Duchenne Muscular Dystrophy