Whether we realize it or not, humans, like all organisms, are genetically programmed to reproduce, but today we have many more reproductive choices than our ancestors. Birth control, for example, prevents procreation by those who might otherwise have many children. On the other hand, advances in fertility treatments allow people to produce offspring even though, in the past, they might not have been able to have children. With these changes, we see more non-traditional family structures emerging, such as older couples or singles choosing to have children.
With a growing number of states legalizing gay marriage, and the recent U.S. Supreme Court rejection of the Defense of Marriage Act (DOMA) and Proposition 8 of California, gay and lesbian couples with children are becoming increasingly common family units. For years, gays and lesbians have found ways to make families, whether through adoption or the use of sperm donors and surrogate mothers. However, a homosexual couple could never biologically reproduce like a heterosexual couple, with a man and woman each donating 50% of his or her DNA to the child.
With new research in genetics and fertility, this may someday change. To understand how it could be possible for two same-sex partners to each contribute genetically to their offspring, one must first understand how the genetic makeup of organisms can be controlled through the use of cloning technology.
Cloning: How Dolly did it
Merely mentioning the word “cloning” typically evokes strong reactions for many people because of the legal and moral questions it raises. But what exactly is cloning? Put simply, cloning is the creation of an organism with the exactly the same genetic makeup as another organism . Cloning actually happens naturally all around us through a variety of methods employed by many different organisms. In bacteria and fungi, cloning occurs when a single cell copies its DNA and divides into two cells. Many plants and other multicellular organisms reproduce asexually (without sex) by fragmentation or vegetative propagation, producing genetically identical offspring. Fragmentation is when a piece of an organism breaks off and forms another organism. For example, when an earthworm is cut in half, each half grows into a new, full-size worm, that is genetically identical to the other half.. Vegetative propagation occurs when plants grow miniaturized plants from special leaves. Even insects and vertebrates like fish and lizards can produce clones through a process called parthenogenesis, which is when offspring are generated from unfertilized eggs . However, when we think of cloning, most of us think of the most famous clone: Dolly the Sheep. Dolly was created using a technique called somatic cell nuclear transplant (SCNT).
To better understand SCNT, let’s take a closer look at the individual terms. Somatic cells are any of the cells in your body that aren’t egg or sperm cells, like skin or kidney cells. Chromosomes are the structures in our cells that contain all of our DNA. While egg and sperm cells have only one set of chromosomes, somatic cells have two complete sets—one from Mom’s egg and one from Dad’s sperm. The chromosomes are housed in a cellular compartment called the nucleus. SCNT is essentially the transfer of a nucleus containing the two sets of chromosomes from a somatic cell into the nucleus of an egg from which the single set of chromosomes has been removed (a process called enucleation). When researchers made Dolly, they took a mammary cell from one female sheep, transplanted the nucleus from that cell into an enucleated egg cell from another sheep, stimulated growth of the egg and then transplanted the egg into the womb of a third sheep . Dolly was an exact replica of the sheep that donated the somatic cell (Figure 1).
Figure 1. Diagram of Somatic Cell Nuclear Transplant. The nucleus of a somatic cell, which contains a set of chromosomes from the mother (in red) and from the father (in blue), is removed and placed into an enucleated egg cell. The newly created egg has the exact genetic material as the original somatic cell and develops into a clone of the organism from which the somatic cell was removed.
Genomic Imprinting: Ligers, tigons and bears? Oh my.
If cloning, which was first accomplished in frogs in the 1950s, is that simple, it should be easy to take a set of chromosomes from one egg and a set of chromosomes from another and put them in an enucleated egg to create a baby from two females, right? Well, not quite. The biggest problem preventing researchers from doing this is a phenomenon called “genetic imprinting” . When we talk about inheriting genes from our parents, most often we are talking about dominant and recessive genes. For these genes, if you receive a dominant gene like curly hair from one parent, and a recessive gene like straight hair from the other, you will have curly hair. In this case, both the curly hair gene and the straight hair gene are turned on, but you only see the expression of the curly hair. In order for you to have straight hair, both your parents must have straight hair. In mammals, however, some traits, or characteristics, are controlled by genes, called imprinted genes, that work in a different way.
For imprinted genes, only one of the copies you inherit is actually turned on, depending on whether it came from your mother or your father. For example, let’s consider ligers and tigons. Ligers are the biggest of all big cats, frequently weighing in at twice their parents’ weight and reaching lengths greater than 12 feet. Ligers are produced when a female tiger mates with a male lion. Tigons, on the other hand, are typically the same size as their parents and are born when a female lion is crossed with a male tiger . The actual DNA of offspring from a lion and a tiger is the same, regardless of how they were paired. In other words, both ligers and tigons get half of their DNA from a tiger and half from a lion. However, ligers and tigons look quite different because certain genes are only turned on in the DNA passed down from the mother, while they are turned off in the DNA from the father and vice versa. The DNA in eggs and sperm of newborn animals is not yet imprinted. Rather, imprinting occurs as these cells grow during the life of the organism .
If something goes wrong with these imprinted genes in humans (like if a child is missing a copy of a gene normally expressed by Dad), it can result in serious problems, such as developmental and neurological disabilities . Scientists did successfully create Dolly using only DNA from a female, but they were able to avoid the imprinted gene problem by using DNA from an adult somatic cell. The two sets of DNA in the somatic cell had already been properly imprinted, meaning that the right genes were already turned off or on by the sperm and the egg that created the sheep that donated the somatic cell.
Next: Of mice and men
Will we be able to overcome this obstacle? As it turns out, scientists have already attempted to tackle this problem in mice. Researchers knew that mice created by combining the DNA from two mature eggs die before they are born. This is because their DNA only has female imprinting, so the resulting embryo is missing all the imprinted genes from the sperm. In Japan, a lab suspected that two of the most important imprinted genes were at the heart of the matter. In sperm, one of these genes, Insulin-like Growth Factor 2 (IGF2) is turned on, while the other gene, H19, is turned off. H19 is turned on in eggs, and when H19 is on, it turns off IGF2, resulting in no expression of IGF2 in eggs. This lab used genetic techniques to delete the H19 gene in an immature egg from a newborn mouse. The researchers used an immature egg because imprinting would not yet have occurred on the DNA. They then combined the DNA from the immature egg with DNA from a mature egg, placing both sets of chromosomes in an enucleated egg. With H19 missing, IGF2 from the immature egg turned on just like it does in sperm cells. Thus, the mature egg expressing H19 and the immature egg expressing IGF2 provided the embryo with the genetic programming it would normally receive from a male and a female mouse. Using this technique, the researchers successfully created mice that developed normally and were able to reproduce (Figure 2) .
Figure 2. Schematic of the Technique Used to Combine the DNA of Two Female Mice (A) The molecule H19 turns off the expression of the IGF2 gene. In normal mice, H19 is off in DNA from sperm, leading to the expression of IGF2, whereas in eggs, H19 is on, so IGF2 is not expressed. This balance between the two sex-specific imprinted sets of DNA is necessary for healthy offspring in mice. (B) If both sets of DNA have female imprinting, H19 will be on in both, so IGF2 will not be expressed. Embryos are not able to develop without IGF2. (C) Researchers deleted the H19 gene from the DNA of a mouse egg so IGF2 would be expressed. This provided the embryo with the genetic instructions normally supplied by a male and a female mouse. Thus, the embryo was able to develop normally.
The End of Men?
Lest all you men out there worry that you may become obsolete, it will be a very long time before this technique could ever be used in humans. Without even considering the ethical issues that arise when we experiment with human DNA and the creation of children, the science still has a long way to go. First of all, in the mouse experiment described above, only 2 out of 371 implanted embryos survived, highlighting the difficulty of controlling the imprinting process. In addition, imprinting in mice is different from imprinting in humans, so in order to perform this technique in humans, we need to know a lot more about what genes are important for human imprinting. So, for a while anyway, procreation still necessitates DNA from a man and from a woman.
Hannah Foster is a PhD candidate in the Molecules, Cells, and Organisms program at Harvard University.
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