Genetics – GOLLYGEE!

08.31.2018

the persistent myth of the super sperm

Fertilization occurs when a sperm cell fuses with an egg.

Over at Aeon, Robert D Martin debunks a persistent myth of biology. Many people, including scientists, believe that during fertilization, sperm are in competition, racing to be the first to fertilize an egg. Because sperm competition is observed in chimpanzees and other mammals closely related to humans, people think that the same holds true for men. But Robert points out all the evidence against this theory.

First, human sperm contains a higher proportion of deformed or abnormal sperm compared to chimpanzees. If sperm were in competition to fertilize an egg, you would expect for there to be fewer nonmotile sperm than are observed. Chimpanzees, have relatively few abnormalities in their sperm cells.

Secondly, much of the sperm’s transport to the ovary is achieved passively. Through the womb and oviducts, a wafting and pumping motions propel sperm through the female tract. And in fact much of the selection of intact sperm is happens because of environment in the woman’s vagina and cervix.

Many sperm do not even make it into the neck of the womb (cervix). Acid conditions in the vagina are hostile and sperm do not survive there for long. Passing through the cervix, many sperm that escape the vagina become ensnared in mucus. Any with physical deformities are trapped. Moreover, hundreds of thousands of sperm migrate into side-channels, called crypts, where they can be stored for several days. Relatively few sperm travel directly though the womb cavity, and numbers are further reduced during entry into the oviduct. Once in the oviduct, sperm are temporarily bound to the inner surface, and only some are released and allowed to approach the egg.

Robert continues from there, explaining what happens when too many sperm reach an egg and how cervical mucus can contain and release viable intact sperm for up to 5-10 day. The essay is quite interesting and well worth a read, so I won’t spoil it all. It’s an interesting counterargument to the manly notion that the best sperm gets the egg.

08.30.2018

the problem with cancer cells

HeLa cells, cancer cells originally isolated from Henerietta Lacks, are among the most widely used cell lines for scientific research

Cell lines are frequently used in cancer research studies. They are pretty easy to maintain and they grow fast. The cell lines give us insight into some of the cellular pathways involved in tumor biology. They are often used as early-stage screens for potential cancer therapeutics, even though scientists know that they do not exactly share the same biology as an actual tumor. Cancer cells grow rapidly and they generate many mutations in the process. In a few cycles, the cells that you have in culture are different genomically than the cells that you started with. But still, having some information on what cells maybe doing in a tumor is better than no information at all.

Now Derek Lowe calls attention to a new study in Nature, which points out a potential problem with these cell lines in culture. In this new paper, the researchers found that not only are cancer cells different from the tumor that they started from, but there can be many differences within a strains of any given cell line. When they observed 27 strains of the MCF7 breast cancer line, the discovered rapid genetic diversification. They then looked at 13 additional cell lines and saw similar results. The genetic differences changed activation of gene expression, cell morphology and cell proliferation.

Derek Lowe sums up what this means for compound screening in cancer cell lines:

At least 75% of the compounds that showed strong inhibition of one MCF7 line were totally inactive against others. That’s going to confound experiments big-time, and this paper is a loud warning for people to be aware of this problem and to do something about it.

11.23.2014

the gay gene

Science magazine reports on a study lending support to the existence of a gene for homosexuality. The study, led by J. Michael Bailey and Alan Sanders, was published this week at Psychological Medicine. The scientists analyzed the genomes of 409 pairs of gay brothers from 384 different families. A genome-wide linkage scan led to the identification of two potential markers, one on chromosome Xq28 (which had been reported in a previous smaller study) and another in a region of chromosome 8. Bailey, for one, was surprised to find any linkage:

Bailey says he went into the project skeptical, largely because Hamer had studied just 38 pairs of gay brothers. “I thought that Dean did a fine but small study, but if I had to bet, I would have bet against our being able to replicate it.”

Samantha Allen at The Daily Beast is unimpressed:

If you don’t see gay people celebrating this news in the streets, understand that we’ve been hearing news about a potential biological basis for homosexuality for a long time now. In 1991, neuroscientist Simon LeVay suggested that small differences in the size of certain cell clusters in the hypothalamus could influence sexual orientation in men. In 1993, geneticist Dean Hamer published a paper in Science that claimed that genetic markers on the X chromosome could influence the development of a same-sex orientation in men…

As the discipline of genetics changed, so too did the scientific approach to homosexuality. In 2012, scientists examined the possibility that variations in hormone levels in the womb could influence the expression of genes that affect sexual orientation, a line of inquiry that falls under the emerging sub-discipline of epigenetics. The popular media, once so easily convinced by LeVay that homosexuality resulted from brain size and by Hamer that homosexuality was genetic, promptly changed its tune to declare that homosexuality was now epigenetic. Hooray? If it’s hard to get excited about these studies, it’s because, at this point, biological explanations for homosexuality are like iPhones—a new one comes out every year.

Bailey and Sanders acknowledge their work has limitations. Even the strongest linkage on Xq28 wasn’t statistically significant, and genome wide association studies are better at homing in on genes for a trait than the linkage study they performed this time:

Sanders admits that the strongest linkage identified from an isolated genetic marker on Xq28 doesn’t clear the threshold for significance. But he contends that the case is bolstered by neighboring markers, which appear to be shared at higher rates between pairs of gay brothers. “The convergence of the evidence pointed towards” Xq28 and chromosome 8, he asserts.

Bailey and Sanders may soon have more data to back their claim—or refute it. They are now working on a GWA study that includes genetic data from the just-published work plus DNA samples from more than 1000 additional gay men. Based on the results published this week, “it looks promising for there being genes in both of these regions,” Bailey says, “but until somebody finds a gene, we don’t know.”

Meanwhile, the New Scientist worries about the implications of the research:

On the one hand, if sexual orientation is something people are born with, and cannot change even if they want to – akin to skin colour or handedness – this should overturn the notion that people choose to be gay and could equally well choose not to be. That knowledge would help rebut those who suggest that gayness is the result of a morally unacceptable decision, or a psychological disorder. It might also help people who struggle to understand or declare their own homosexuality.

On the other, some could try to redefine homosexuality as a biological abnormality. There is no way to change people’s sexuality, but if key genes are found, it might be possible to detect homosexuality before birth, or to “cure” people by altering those genes.

GOLLYGEE!

Category: Genetics (page 1 of 8)

the persistent myth of the super sperm

the problem with cancer cells

the gay gene