Linkage, Sex linkage,
Pedigrees
Linked genes
• We know two traits
should assort
independently. Why?
• So, offspring
phenotype ratios
should be:
– 9:3:3:1
– But they’re not
– Why?
Linked genes
• The genes reside on the
SAME chromosome
• What produces
recombinants (purple,
round & red, long)?
• Crossing over!
Linkage
• If genes reside on the SAME chromosome,
then how are recombinants generated?
• What happens in Meiosis I?
• Crossing over recombines (shuffles) alleles
Sex Linkage & Recombination
• T. H. Morgan and his students discovered
both phenomena in fruit flies
• Eye color – sex linkage
• Body color, wing type, other eye color –
autosomal linkage
Recombination
•
•
•
•
G = gray body (dom)
g = black body (rec)
L = long wings (dom)
l = short wings (rec)
• GgLl x ggll 
– GgLl ALL Gray long
– Few Ggll & ggLl
Recombination
•
•
•
•
•
G = gray body (dom)
g = black body (rec)
L = long wings (dom)
l = short wings (rec)
All on same chromosome
• GgLl x ggll 
– Few Ggll & ggLl
– Recombination frequency =
17%
Recombination freq & Genetic map
• Recombination frequency = 17%
• If possibility of crossing over is = at all
points along chromosome, then the farther
apart two genes are, the greater the
chance of recombination between them
Sex linkage
• Morgan & students
made true breeding
red-eyed female
and true-breeding
white-eyed males
Sex linkage
• Made true breeding
red-eyed female and
true-breeding whiteeyed males
• All offspring red-eyed
• Now let’s cross those
F1 offspring
Sex linkage
• Cross those F1
offspring
• ONLY males have white
eyes, and
approximately ½ of all
males are affected
• Why are females never
affected?
Sex linkage
• Red female with white
male
• We know male can only
contribute a white allele
• If mother is
heterozygous?
Sex linkage
• Red female with white
male
• We know male can only
contribute a white allele
• If mother is
heterozygous?
– 1:1 white-eyed : red-eyed,
irrespective of sex
Common sex-linked disorders
• Red-green color blindness
– Lack either red sensitive or green-sensitive
cone cells in retina
• Hemophilia
– Lack one or more blood-clotting proteins
• Duchenne muscular dystrophy
– Progressive muscle wasting
Conclusions
• Appearance of few recombinants signals
some degree of linkage
• For any X-linked gene, males with a
recessive allele will show the recessive
phenotype.
• When looking at pedigrees of disease
phenotypes, an excess of affected males
suggests that disease gene lies on Xchromosome