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Today, The Visible Embryo is linked to over 600 educational institutions and is viewed by more than 1 million visitors each month. The field of early embryology has grown to include the identification of the stem cell as not only critical to organogenesis in the embryo, but equally critical to organ function and repair in the adult human. The identification and understanding of genetic malfunction, inflammatory responses, and the progression in chronic disease, begins with a grounding in primary cellular and systemic functions manifested in the study of the early embryo.

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Pregnancy Timeline by SemestersLungs begin to produce surfactantImmune system beginningHead may position into pelvisFull TermPeriod of rapid brain growthWhite fat begins to be madeHead may position into pelvisWhite fat begins to be madeImmune system beginningBrain convolutions beginBrain convolutions beginFetal liver is producing blood cellsSensory brain waves begin to activateSensory brain waves begin to activateInner Ear Bones HardenBone marrow starts making blood cellsBone marrow starts making blood cellsBrown fat surrounds lymphatic systemFetal sexual organs visibleFinger and toe prints appearFinger and toe prints appearHeartbeat can be detectedHeartbeat can be detectedBasic Brain Structure in PlaceThe Appearance of SomitesFirst Detectable Brain WavesA Four Chambered HeartBeginning Cerebral HemispheresFemale Reproductive SystemEnd of Embryonic PeriodEnd of Embryonic PeriodFirst Thin Layer of Skin AppearsThird TrimesterSecond TrimesterFirst TrimesterFertilizationDevelopmental Timeline
CLICK ON weeks 0 - 40 and follow along every 2 weeks of fetal development
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Home | Pregnancy Timeline | News Alerts |News Archive May 1, 2015

In this graph, two unaffected parents each carry one copy of a gene mutation for
an autosomal recessive disorder. They have one affected child and
three unaffected children, two of which carry one copy of the gene mutation.
Image Credit: US National Library of Medicine





Most People Carry Recessive Disease Mutations

Humans carry an average of one or two mutations per person that can cause severe genetic disorders or prenatal death.

These new numbers were made possible by a long-term collaboration between medical research and a unique community which maintains detailed family histories.

"These records offered a fantastic opportunity to estimate disease mutation carrier rates in a new way that disentangles the effects of genetics from socioeconomic factors," said lead author Ziyue Gao, graduate student at the University of Chicago.

Estimate results are published in the journal GENETICS.

Most genetic disorders that result in sterility or childhood death are caused by recessive mutations, DNA sequence variations (variants) that are harmless when a person carries only one copy. But when the same mutation is inherited — meaning one mutation from each parent — recessive mutations can cause devastating diseases like cystic fibrosis.

Recessive disease mutations are much more common than those that are harmful only in a single copy. That is because single copy dominant mutations are more easily eliminated by natural selection. But exactly how common are recessive disease-causing mutations in humans? Previous efforts to estimate their number have relied on studies of disease in children born to related parents. With this method, increased rates of childhood mortality and disease are assumed to be due to recessive mutations. But this method mixes genetics with socioeconomics.

For example, in some places, marriage between close relatives correlates with poverty. In those cases, children with related parents can have higher disease and death rates simply because their families suffer from poor nutrition or lack of access to medical care. "There are many different — but non-genetic — factors that can bias this kind of approach," said Gao. However, the new method elegantly sidesteps this problem. It relies on facts collected by the Hutterite community.

The Hutterites are a religious group that settled in North America in the 1870s and keeps meticulous genealogical records. They live a communal life ensuring uniform access to healthcare and food.

Co-author Carole Ober of the University of Chicago has worked closely with a group of Hutterites from South Dakota for two decades, studying genetic contributions to disease using a large 13-generation family tree tracing the ancestry of more than 1,500 living people.

Molly Przeworski, a population geneticist at Columbia University, realized that this ancestry tree could be used to estimate the number of recessive disease mutations carried by the group’s founders in the 18th and 19th century. This was possible because Ober’s team along with other medical researchers had compiled comprehensive records on the frequency of disorders that cause sterility or childhood death in the Hutterites. Using this information, the team estimated that there were around three mutations of this type for every five people among the original founders. But that only accounted for mutations that allow children carrying two copies to survive at least until birth.

Based on estimates of the proportion of recessive mutations that cause death during fetal growth, the team concluded each founder carried approximately one to two recessive mutations causing sterility or death before adolescence.

Gao: "This number is probably lower than the real average for most populations, but is in the right ballpark. More importantly unlike previous estimates, it is unaffected by socioeconomics."

Gao cautions that the number of recessive disease mutations still will vary from person to person, and the study result number doesn’t necessarily help predict a specific couple’s risk for passing on a genetic disorder. She also points out that most infant mortality worldwide is caused by non-genetic factors like nutrition and infectious disease, rather than inherited disorders.

Surprisingly, the recessive disease mutation estimate for humans is similar to those of fruit fly and fish species, even though these organisms all have different genome sizes. Says Gao: “We don’t yet understand why the number of recessive lethal mutations might be relatively constant across distantly related organisms. It’s an interesting evolutionary question for further research.”

An Estimate of the Average Number of Recessive Lethal Mutations Carried by Humans
Ziyue Gao, Darrel Waggoner, Matthew Stephens, Carole Ober, and Molly Przeworski
Genetics April 2015 199: 1243–1254 doi: 10.1534/genetics.114.173351

This study was supported by grants from the National Institutes of Health. Z.G. was funded in part by the William Rainey Harper Fellowship from the University of Chicago.

About the Genetics Society of America (GSA)
Founded in 1931, the Genetics Society of America (GSA) is the professional scientific society for genetics researchers and educators. The Society’s more than 5,000 members worldwide work to deepen our understanding of the living world by advancing the field of genetics, from the molecular to the population level. GSA promotes research and fosters communication through a number of GSA-sponsored conferences including regular meetings that focus on particular model organisms. GSA publishes two peer-reviewed, peer-edited scholarly journals: GENETICS, which has published high quality original research across the breadth of the field since 1916, and G3: Genes|Genomes|Genetics, an open-access journal launched in 2011 to disseminate high quality foundational research in genetics and genomics. The Society also has a deep commitment to education and fostering the next generation of scholars in the field. For more information about GSA, please visit www.genetics-gsa.org.

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