An unusual observation by Morgan:
Demonstration that the gene for white eyes is on the X chromosome
Morgan saw that in most cases a cross between a white eyed female and a red eyed male produced offspring
where all females had red eyes and all males had white eyes. However, some unusual strains of white eyed females,
when crossed with WT red eyed males, produced about 5% of white eyed females and about 5% of red eyed males in
the F1 progeny. This was NOT due to mutations. What’s going on?
Nondisjunction
To understand what is happening, we need to know a bit about nondisjunction (unfortunately the textbook doesn’t
really cover this until chapter 8. The figures below are modified from figure 8.18 on pg. 231.)
In the figure below, we will assume the red and the yellow chromosomes both represent the X chromosome in a female
fruit fly and we are looking at how theses homologous chromosomes separate during meiosis. The blue chromosome
represents the sex chromosome from the sperm (it could be either an X or a Y chromosome – for this example it will be
useful to think of it as a Y chromosome). NOTE: we are only following the sex chromosomes. Assume that all of the
autosomes are dividing normally as well, and that the zygotes will have a diploid complement.
In this example, all of the fly zygotes are normal for the autosomes. What is/are the chromosomal complement(s) for
the sex chromosomes? What sex(es) are the flies?
Sometimes, during meiosis, the chromosomes do not separate during anaphase, and two chromosomes go to one pole
(one cell) and no chromosome goes to the other pole (the other cell). This is known as nondisjunction (nondisjunction
can occur with autosomes or sex chromosomes, but we will only consider the sex chromosomes for this problem). This
causes genetic imbalances in the gametes. The outcome is a little bit different if nondisjunction occurs in meiosis I or in
meiosis II as shown in the figure below. NOTE: assume all other chromosomes are segregating correctly- nondisjunction
usually only occurs with individual chromosomes/one homologous pair.
Back to the original cross:
With this understanding of nondisjunction, let’s examine the unusual situation of the white eye female fly in the original
cross.
Morgan’s student Calvin Bridges hypothesized the unusual female had the genetic complement XwXwY (remember, all
other chromosomes are normal, in other words the flies are diploid).
1. Why is the fly a female?
o The fly could be meta-female because of the imbalance between the sex vs autosomal chromosomes, or
the Y’’ could have shut off or have been suppressed.
2. What event(s) led to the unusual XwXwY female? Use the diagram above to help you, and consider these two
possibilities.
o Both ways that would have produced an X’’X’’Y’’ both included the splitting of the the chromosomes
going wrong and not being pulled apart into chromatids.
a. Explain how this happened if the female’s parents were the following genotypes: XwXw x X+Y
 The fly would have inherited a the wrongly split X’’X’’ from a parent and an Y from the other
parent
b. Explain how this happened if the female’s parents were the following genotypes: X+Xw x XwY
 The fly would have inherited a X’’ and a wrongly split X’’Y
3. When the unusual XwXwY female produces gametes, most of the time (90%), the two X chromosomes separate
normally in meiosis, and the Y chromosome will go to one half of the gametes. But in 10% of the cases the X
chromosomes go through nondisjunction. What will be the chromosomal complement(s) of the gametes that
result from nondisjunction in the XwXwY female?
o
The resulting chromosomal gametes would
have a X’’X’’ and then a Y.
The figure below diagrams the original cross that
Morgan pondered, and what should result if
Bridges’ hypothesis was correct.
Bridges examined the chromosomes of the F1 offspring under a microscope. He found that the phenotypes of the F1s
matched up with the expected combinations of the sex chromosomes he hypothesized. This provided the evidence that
the white gene was located on the X chromosome!