Download Sex Linked Traits - Thomas Hunt Morgan Fruit Fly Experiment

Sex-Linked Traits
 Some terms to understand:
1. Parental types – phenotype of offspring match parent
phenotype.
2. Recombinant (types) – offspring have new combination of
phenotypes different than parents.
Ex. Heterozygous Yellow Round peas crossed with green
wrinkled peas can yield green round peas (recombinant).
 Sex Linked genes = located on the sex chromosomes (23rd
pair in humans; associated with the X chromosome
 Sex-linked genes are recessive.
 Examples:
1. Color blindness
2. Hemophilia – lacks protein(s) that create platelets to
clot blood
3. Muscular dystrophy – lacks protein, dystrophin, for
proper muscle structure
4. (Patterned) Baldness
- Females must inherit two recessive alleles to express a sex
linked trait (XX)
o One from her mom; one from her dad
XBXB = homozygous dominant = sees normal color
XBXb = heterozygous = sees normal color
XbXb = homozygous recessive = color blind
- Male only inherits one recessive allele to express a sex
linked trait (Xy)
o One from his mom
XBy = hemizygous = sees normal color
Xby = hemizygous = color blind
Example:
Two parents were shocked to find out they had a color blind
child when neither parent is color blind. What are the
genotypes of the parents and what is the sex of the child.
 So,…sex-linked traits are more commonly seen in males;
allele passed from mother to son.
 Thomas Hunt Morgan’s Experiment
- First evidence of associating a specific gene on a
specific chromosome.
- Worked with fruit flies = Drosophila melanogaster
o Only 4 pair of chromosomes – 3 pair of autosomes;
1 pair of sex chromosomes
XX = female; Xy = male (like humans)
 Sex–linked example in Drosophila :
*Note: notation is different in Drosophila - (many) multiple
alleles)
 The most common phenotype in population = wild types
- letter with superscripted plus = wild type [ex: v+ ]
 Alternatives to wild types = mutant phenotypes (assumed
to have mutated from wild type)
- lower case (same) letter = mutant allele (usually
recessive) [ex: v ]
 y = y chromosome
Example: Eye color ~ (wild type) red, (mutants) vermillion,
white, prune
 So what did Morgan do see to determine that eye
color is associated with sex chromosomes…
- Cross true breeders to get F1
- Then, monohybrid cross ~ F1 X F1…What should
this phenotypic ratio be?....
- You are correct! 3:1 …
But…This did NOT happen
v = vermilion eye color (mutant) v+ = wild type eye
color (red)
v v+
v
v+
vv
vv+
vv+
v+v+
This happened instead…
cross: vv+ (female) wild type & v+y (male) wild type
Still got a 3:1 ratio, but…
males: 50% vermilion, 50% wild
females: 100% wild (but 50% are heterozygous)
If eye color wasn’t sex-linked then one would expect to get
some vermillion (red) eyed females.
X-chromosome Inactivation (XCI)
 Human females inherit two copies of every gene on the X
chromosome, whereas males inherit only one.
 But for the hundreds of other genes on the X, are males at
a disadvantage in the amount of gene product their cells
produce?
- The answer is no, because females have only a single
active X chromosome in each cell.
 The Barr body is one of the X chromosomes.
- Compact appearance reflects its inactivity.
- So, the cells of females have only one functioning copy of
each X-linked gene — the same as males.
 This affects expression of traits. Since X-inactivation is
basically random, this means that the expression of traits
in different cells of a woman's body is actually different,
even if the cells are genotypically identical.