Page 104 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

96 Chapter 4 The Chromosome Theory of Inheritance

FAST FORWARD Sprinters: © Robert Michael/Corbis RF

Transgenic Mice Prove That SRY Is the Maleness Factor
Genes similar to human SRY have been identified on the Y chro- into the genome of chromosomally female (XX) mice. If SRY is
mosomes of nearly all mammalian species. In 1991, researchers the crucial determinant of maleness, then XX mice containing
used mouse transgenic technology to show definitively that the an SRY transgene would nevertheless be male.
SRY gene is the crucial determinant of maleness. A transgenic First, the scientists isolated the DNA of the mouse SRY gene
mouse is one whose genome contains copies of a gene that came using cloning technology to be discussed in later chapters. Next,
from another individual—or even from another species. Such using a method called pronuclear injection, transgenic mice were
genes are called transgenes. One focus of genetic engineering is generated that contained the SRY gene on one of their auto-
technology for the manipulation and insertion of transgenes. somes. To perform pronuclear injection, researchers collected
To determine if SRY is sufficient to determine maleness, many fertilized mouse eggs from mated females and injected the
researchers wanted to introduce copies of the mouse SRY gene sperm or egg nucleus (called a pronucleus when in the zygote)
with hundreds of copies of the SRY gene DNA (Fig. A). Enzymes
Figure A Using pronuclear injection to generate mice in the pronucleus integrated the DNA into random locations in
transgenic for the SRY gene. the genome (Fig. A).
(1): © Brigid Hogan, Howard Hughes Medical Institute, Vanderbilt University; After the injected zygotes matured into early embryos, they
(2): © Charles River Laboratories were implanted into surrogate mothers. When the mice were
SRY gene DNA born, cells were taken from their tails and tested for the presence
of the SRY transgene using molecular biology techniques.
Figure B shows at the right a transgenic mouse (transformed
with SRY) obtained in this study. Although it is chromosomally XX,
it is phenotypically male. This result demonstrates conclusively
that the SRY gene alone is sufficient to determine maleness.

Injection into pronucleus of
zygote (fertilized egg) Figure B An XX mouse transformed with SRY is
phenotypically male. Both the transformed XX mouse at the
right and its normal XY littermate at the left have normal male
genitalia. Arrows point to the penis.
SRY © Medical Research Council/Science Source
XY XX + SRY

Random integration of SRY gene
DNA into a chromosome

Tail cells tested for presence
of SRY transgene

The X chromosome contains about 1100 genes, most activates testes development in XY (or sex-reversed XX)
of which have nothing to do with sex; they specify proteins embryos. The embryonic testes secrete hormones that trig-
needed by both males and females. ger the development of male sex organs and prevent the
Why does having an SRY gene mean that you will be formation of female sex organs. In the absence of SRY
male and not having SRY mean that you will be female? protein, an ovary develops instead of a testis, and other
Approximately six weeks after fertilization, SRY protein female sex organs develop by default.