1. By the early 20th century…
Chromosome Theory of Inheritance
Haploid gametes
Mendel rediscovered
Morgans fly lab

2. Morgan’s Flies
• New generation every two weeks • Four pairs of chromosomes • Plenty of visible phenotypic variation

3. Experiment Wild-type red-eye female x mutant white-eye male
homozygous XWXW hemizygous XwY
Figure 15.3
Hypothesis: white allele is on the Y

4. Results XWXW XwY Homo. red x Hemi. white = 100% red Hetero x Hemi redP F1 F2
Homo. red x Hemi. white
= 100% red
Hetero x Hemi red
= F: 100% red
M: 50% white

5. Conclusions
• The white-eye allele must be on the X aka X-linked sex-linked • First solid evidence indicating that a specific gene is associated with a specific chromosome • Applies to humans too
X Y
X X’
carrier
X Y X X X X’ X‘Y

6. to have a 0.25 chance of an affected daughter
Affected x Carrier
Takes a heterozygous carrier mom…
…and an affected dad
to have a 0.25 chance of an affected daughter
carrier
healthy
affected
affected

7. Royal Hemophilia
Queen Victoria, carrier
Hemophilia is an x-linked recessive condition that results in too little blood clotting factor produced, leading to uncontrolled bleeding

8. Red vs Green
‘Colorblindness’ results from defects in a cluster of genes on the X chromosome.
As long as one normal copy of a red-absorbing gene and one normal green-absorbing gene are
present, vision is normal.

9. X Inactivation If males have only 1 allele of each
of the X chromosome, do they
produce less of the protein X-linked
genes produce?
Nope - one X is de-activated,
at random, in cells of females
The Lyon hypothesis states that in cells with multiple X chromosomes, all but one are inactivated during mammalian embryogenesis[3]. This happens early in embryonic development at random in mammals A normal human female has only one barr body per somatic cell, while a normal human male has none.
In mammals, males are heterogametic (XY) and females homogametic (XX). Dosage compensation is achieved by inactivation at random of one of the two X chromosomes. The heterochromatized X chromosome appears as a darkly-staining bodies attached to the nuclear membrane. The phenomenon was first described by Dr Murray L. Barr, a Canadian cytogeneticist, and heterochromatin bodies are now called Barr Bodies. [The other dark bodies within the nucleus are nucleoli, which represent repetitive rDNA genes].     Barr body testing was introduced in the 1966 Olympic games, in an effort to detect male atheletes trying to "pass" as females, to gain a competitive advantage. Teams from eastern Europe wer particularly suspect. Such allegations had been made for many years, and a number of athletes were stripped of their medals as a result of ambiguous genital sex.  Cytological testing apparently never detected deliberate fakery. It did however detect a number of cases of testicular feminization syndrome (TFS), a genetic condition in which an XY (male) zygote develops as a phenotypically female adult, due to failure of androgen receptors. Such individuals would test positive for the presence of a Barr body. It appears that in most if not all cases, the athletes were themselves unaware of their condition.
mosaicism

10. Observation #2
• Mendel concluded that pea traits sort independent of one another
• Morgan noticed that
some traits do not sort
independently
• He proposed these traits
are linked - located on the
same chromosome – and
that chromosomes
independently assort
a B c D
AB C d
BUT NOT ALWAYS!

11. Observation #3
“Chromosome pairs are observed to twist together…conceivable they could break and trade pieces”
Crossing over in M2
Recombinant offspring are those that show new combinations of the parental traits

12. Recombination Likelihood of recombination determined by distance
frequently recombined
infrequently recombined
linked
independent
Calculate % recombination to determine distance apart
1% recomb. = 1 map unit

13. Recombination
1% of crossover frequency = 1 unit of distance between two genes
Use to create chromosome ‘maps’ of relative locations of genes