Isobel: I'm the only one in my family who can't roll my tongue. How come I didn't get that gene from my parents? 

Back in 1940, geneticist Alfred Sturtevant claimed that the ability to roll one's tongue was due to inheriting a dominant gene meant solely for that purpose. However, a few years later in 1952, another scientist named Philip Matlock led an experiment with identical twins. Since identical twins share the same DNA, if one can roll their tongue, the other should be able to as well. When Matlock ran his experiment he discovered that out of 33 pairs of twins, 7 did not share the tongue rolling ability with their sibling, disproving Sturtevant's earlier theory that tongue rolling was caused by a single dominant gene. Since this experiment, further research has been done that shows that tongue rolling is not a simple Mendelian trait. Tongue length and muscle tone may contribute to the ability, but it is primarily a learned trait. 

In Sturtevant's 1965 book, a History of Genetics, he said that tongue rolling had been disproven since his 1940 paper and was "practically useless as a genetic marker. But I am still embarrassed to see it listed in some current works as an established Mendelian case."

Further reading:

https://www.pbs.org/newshour/science/genetic-myth-textbooks-get-wrong

https://udel.edu/~mcdonald/mythtongueroll.html

Millie K-V: Is height genetic?

There are actually several factors that determine a person's height, one of which is genetics. Other factors, like nutrition, socioeconomic status, hormones, and disease all play a role as well. Eating healthy and having a properly balanced diet allows you to grow to the height your genes have coded. However, lack of nutrition can end up having the complete opposite effect, leaving you shorter than what may have been predicted. Studies have shown that lack of access to healthcare and nutritious foods contribute to individuals having a shorter height.

On average, males tend to be about 5.5 inches taller than females, due to the fact that during puberty, hormones cause a boy's body to grow at a faster rate for a longer time. 

Along with growth hormones, estrogen, testosterone, and thyroid hormones also contribute to height. Children who have hypothyroidism tend to be shorter than their parents, while children who have too many human growth hormones end up with a condition called gigantism, which causes them to be very tall.

If you are interested in trying to estimate what your adult height might be, one way is to add your mother's and father's heights. Add 5 inches (13 cm) for boys, or subtract 5 inches (13 cm) for girls, then divide that number by 2. The result is your predicted height. Another way is to dust off those old baby books! Looking back at what you were at age 2 for boys, and 18 months for girls, and then doubling that number will give you a good idea of how tall you will be. Are these estimations going to be 100% accurate? No, but they give an estimate and it can be fun to track and see if either proves to be correct!

C M​: How do twins happen?

Dizygotic (fraternal) twins start off as two individual eggs that are fertilized by two separate sperm. They do not share the same DNA. They are siblings who happen to share the same gestational space. Fraternal twins can be both boys, both girls, or one of each. Their genetics are no more similar than the genetics of any two siblings.

Monozygotic (identical) twins begin as a single egg that was fertilized by one sperm. This single egg then splits into two, basically cloning itself. Early in development, monozygotic twins will have nearly identical sets of genetic information. But, as mentioned in the Q&A from last week, as they grow each twin has more differences in how its DNA is expressed.

For more, see this article: https://learn.genetics.utah.edu/content/epigenetics/twins/

S C: With Mendel crossing different pea plants, why is it a times or multiplication symbol and not a plus or addition symbol? 

The multiplication symbol has been used in this way for a long time! The famous scientist Carl Linnaeus (inventor of scientific names) used x when talking about plant hybrids in the 1700s.

There are several reasons the multiplication symbol is a better fit than using the addition symbol. When drawing a cross between two pea plants, we're representing the fact that those two organisms are multiplying and creating offspring. Also, when we talk about probabilities of seeing different traits in the offspring, the way to calculate that is to multiply the probabilities of the alleles coming from each parent. 

G & T Videos​: Does the 3 to 1 ratio apply to humans too?

It all depends! Mendelian inheritance occurs when there is a single gene with 2 alleles, one dominant and one recessive. When two heterozygous individuals breed, the 3:1 phenotype ratio of dominant to recessive traits will show up in the offspring. Most human traits are controlled by more than 2 genes meaning they do not follow Mendelian inheritance.

Non-mendelian inheritance includes inheritance by multiple allele traits, codominance, incomplete dominance, and polygenic traits, which we will talk about more later on in this class.