Summary of She Has Her Mother’s Laugh by Carl Zimmer

BookSummaryClub Blog Summary of She Has Her Mother’s Laugh by Carl Zimmer

Do you know how genetic inheritance work? Sure, you have your mom’s nose and your dad’s smile but what about when it skips a few generations? How does that work? Even more interesting is that the mechanics of inheritance shifts over time and between cultures. This seems like it should not happen as genetics should be straightforward but there are diverse interpretations of genetic inheritance and they do actually have an effect.

Understanding genetic inheritance is taking a look at how it has been applied and studied in terms of science, medicine, culture and history.

In this book summary, readers will discover:

  • The monk and the emergence of genetics
  • How single cells create complex lifeforms
  • What contributes to inheritance
  • What determines our height
  • Chimeras
  • Innate versus acquired traits

Key lesson one: The monk and the emergence of genetics

In 1853, an Austrian monk named Gregor Mendel with an inquiring mind found himself experimenting with peas. Mendel had been fascinated with hybrid plants that were a product of cross-pollinated seeds of two different species. At the time, biologists knew that hybrid plants could exhibit traits from both parent plants but they did not know which traits would appear nor why.

So Mendel decided to investigate this further by planting 22 varieties of peas. He began crossing pea plants to see what would happen. He crossed yellow peas with green peas and found the first generation to be all yellow peas. The next generation, however, had both yellow and green peas. The same occurred when he crossed wrinkled peas with smooth peas and tall plants with short plants. 

From his experiments, Mendel came up with the three principles of genetics. The first was that hybrid plants inherit traits from both parent plants but only one of these is expressed at a time. The second principle was that the traits expressed in the first generation are the dominant traits. The third principle states that those traits which do not show in the first generation but are expressed in subsequent generations are recessive traits. 

Mendel was correct on all fronts. Through the years, we have discovered that these traits are coded in genes. Each gene consists of two alleles that can be either recessive or dominant. Only one dominant allele needs to be present for that trait to be expressed. In contrast, two recessive alleles need to be present for the expression of the trait. Mendel set the foundation for these discoveries with his humble pea plants and his thirst for knowledge.

Key lesson two: How single cells create complex lifeforms

Reproduction begins with a zygote which is a single cell containing DNA from both parents. This zygote splits into two and continues to multiply with each new cell having a copy of this newly made DNA. This mass of cells eventually becomes an embryo before growing into a foetus and eventually a baby. We are all, therefore, products of a single cell. This puzzled scientists for the longest time. How was it possible that such a complex lifeform can be built from a single cell?

Mary Lyon, a geneticist found an answer to that question while conducting experiments on mice in 1961. The mice she was experimenting with had a genetic mutation on their X chromosomes. Female mice that inherited the mutation had mottled fur with no other problems with their health. Male mice that inherited the mutation died. Lyon suggested that this occurred because females possessed two X chromosomes and if one had the mutation it could be ignored. Males, on the other hand, only have one X chromosome and it cannot be shut down. 

This is possible due to methylation. Methylation occurs when cells coat individual genes on our DNA sequences in a molecular shield. This process basically switches off those genes and is how the female mice in Lyon’s experiment were able to survive. Methylation can also explain how a complex lifeform can come from a single cell. One zygote multiples into the trillions of cells that make up the human body with each cell possessing a copy of our entire DNA sequence. These cells are called pluripotent as they have the ability to differentiate into many possible cell variations. However, methylation allows for only specific genes to be expressed within each cell. It thereby activates genes that will determine the cell’s function and as the cell divides from there, their function becomes fixed.

Key lesson three: What contributes to inheritance

Inheritance does not only refer to genetics. It can refer to wealth and status as well and this is why inheritance is not just biological, it also has cultural implications. Inheritance is a rather complex concept and is best explained when considering the Habsburgs. 

The Habsburgs were a powerful European family from the fifteenth to the eighteenth century. They ruled the Austro-Hungarian Empire and secured their power by passing the throne from father to son as was the cultural model of inheritance. Thus, long before the study of genetics, inheritance was the basis of the transfer of wealth, status and power. The Romans knew this and believed that the traits of good leaders were found in the blood and therefore passed from parent to offspring. Thus, to mingle the blood of superior nobles to those outside their class was seen as a disgrace resulting in inferior offspring. The Habsburgs took this thinking a little too seriously. They only married within a very specific gene pool to keep their blood untainted.

What the Habsburgs did not know is that this practice did not keep their bloodline pure but instead made them more susceptible to genetic disorders. After centuries of inbreeding, the Habsburgs were rife with genetic diseases and infertility which is another side effect of inbreeding. It was through these factors that the Habsburg dynasty died out. So, as much as genes were the driving force of inheritance, the cultural idea behind inheritance also influenced the way in which genes were passed on through the generations. 

Key lesson four: What determines our height

As a general rule, everyone is more or less the same height as their parents. However, this is not true all the time. Just consider individuals with dwarfism that have children of average height. This simple observation has fascinated scientists for centuries. Adolphe Quetelet tried to figure it out in 1823 by measuring a sample of the population. He mapped their heights and found that it formed a bell curve. This is what we now know as a normal distribution. Most of the heights were congregated near the average whilst there were very few individuals who were extremely tall or extremely short. Close to six decades later, a similar study was conducted by Francis Galton also yielded the same results – a bell curve. Galton concluded that the similar results can only mean that height is inherited. It is passed on and that is why the bell curve is maintained.

Galton was not exactly correct. Geneticists have now figured out that our height is only 86 per cent due to our genes. There are several different genes that are responsible for height and not just one. In addition, environmental factors can also play a role in determining your height. Early childhood nutrition is one of those environmental factors and in the 1970s Robert Fogel, an economist found a correlation between average heights and economic wellbeing. This proved that the wealthier the nation, the taller its citizens.

Key lesson five: Chimeras

When it comes to genetics there is nothing more fascinating than a chimera. Chimeras are those who have two distinct sets of DNA. The first example of a human chimera was Mrs McK who in 1953 baffled everyone when she went to donate blood. When her blood was analyzed, it was found that she had both Type A and Type O blood. Given that the four blood types that are known are O, A, B and AB; OA blood just did not fit in.

They then figured Mrs McK probably had received a blood transfusion which could result in the mixed blood type. She had not. This led to Mrs McK being the first human chimera reported. So scientists then tried to figure out how she ended up with two sets of DNA. The scientists, Robert Race and Ruth Sanger linked Mrs McK’s case with one they had seen in fraternal twin cows whereby one twin carried their twin’s blood type in addition to their own. They asked Mrs McK if she had a twin and she did, but he had passed away when he was a child. 

So, Mrs McK’s case as the first human chimaera had been solved. The other way chimeras can form is when twin embryos fuse together in the womb or when one twin is absorbed by the other. Recently, it was discovered that chimerism is sometimes a result of bone marrow transplants. However it occurs, it has important implications in genetic testing results from a blood or DNA test may not be as straightforward as we would expect. 

Key lesson six Innate versus acquired traits

Innate traits are those found in our DNA and passed on from one generation to the next. Acquired traits refer to conditions and behaviours that are acquired during our lives. However, scientists have discovered that acquired traits can be passed on like innate traits as well. 

Experiments on mice were the key to learning about these traits. Mice were exposed to a chemical called acetophenone which gives off a smell that is similar to almonds. After exposure, the mice were then given an electric shock. After three days, the mice froze when exposed to the chemical as they waited for the shock that was to surely follow. This was expected. What was totally unexpected was that the offspring of these mice also demonstrated the same behaviour when exposed to acetophenone for the first time. It was also passed on to the generation that followed. This proved that learned behaviours could also be passed on genetically. The same results were found when mice were subjected to stress as well. 

Now, the question seems to be if this is the same for human? Could intergenerational trauma be passed on in our genes? Research in this area is ongoing and the results could be life-changing. 

The key takeaway from She Has Her Mother’s Smile is:

Genetic inheritance is not as simple as it may seem. It has a cultural component as well which makes it all the more complex and fascinating. The more we learn about genetics, the more questions arise about how much we really understand. One thing is for certain though, it is remarkable to be a part of the generation that learns more about what the previous generation has given us and what we will contribute to future generations. 

How can I implement the lessons learned in She Has Her Mother’s Smile:

Well, if you like gardening, you can always be like Mendel and experiment with creating hybrid plants in your backyard. The possibilities are infinite and you might just end up creating something spectacular!

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