5. Inheritance Explained

Amna Khalid: Didn't Mendel change some of his results but was correct in the end?

This is known as the Mendelian paradox! In 1936, a famous geneticist named Ronald Fisher analyzed Mendel's results with peas and said it wasn't statistically possible for Mendel's 3:1 ratio of traits to have been as clean and neat as it was. This gave rise to the idea that Mendel must have falsified his data -- yet everything we know about him as a scientist and person contradicts the idea that he would deliberately falsify his data. It's a paradox!

The two main resolutions for the Mendelian paradox are:

  1. Assume that Mendel or an unnamed assistant removed some of the pea plants that didn't make sense, allowing confirmation bias to impact the data.
  2. Fisher's analysis was flawed and, in reality, there is no paradox.

In 2008, several scientists wrote a 300-page book called Ending the Mendel-Fisher Controversy. The short version of their book is that Mendel's results were accurate. Fisher, although well-intentioned, misapplied statistics to Mendel's studies. So there isn't a paradox after all. 

(Despite the numerous articles and a book on the subject, you can still find lines in many genetics textbooks saying that Mendel's results were actually too good to be true.)

Shadowmation: Which animal has the most sets of chromosomes?

Humans have 2 copies of each chromosome (diploid). Bananas in the grocery store have 3 (triploid) and strawberries have 8 (octoploid). What's the organism with the highest number of chromosomes or highest degree of polyploidy? Somewhat surprisingly, it's a small butterfly:

(a picture of the Atlas blue butterfly)

The Atlas blue butterfly (Polyommatus atlantica) has over 400 different chromosomes. It's diploid (2 copies of each chromosome) but has an astonishingly high number of quite short chromosomes. A short open-access article in the journal Comparative Cytogenetics has more information if you're interested: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4698580/

The black mulberry tree has the highest degree of ploidy in plants, with 22 copies of each chromosome.

Rachel Wilkenfield: Are there any non-diploid animals?

Yes! Several species of insects have a system where males are haploid (one copy of each chromosome) and females are diploid (two copies). But being diploid is the most common genetic state for the animal kingdom.

Harper Huynh: Since there are 7,868,872,451 people on earth and some twins, triplets etc. can there be two people exactly the same?

This is a great question! The human genome is long enough and the variation is high enough, that it is statistically improbable. The odds of two siblings being genetically identical are less than 1 in 70 trillion. And identical twins, which start out with the same genetic information, soon have measurable differences in how their DNA is being expressed. Each individual truly is unique.

Val and Isa: How can a child get green eyes if the parents have brown and blue eyes?

Wiliam Wietman and Kenna Scott: How can a child have blond hair if both parents have brown hair?

From the comic on page 21 of the notes, you can see that eye color is a lot more complicated than just one gene and one trait! There are thought to be at least 16 different genes that play a role in eye color (not all of them are located on different chromosomes, I just drew them that way for the comic.) Two blue-eyed parents can have a brown-eyed child and two brown-eyed parents can have a blue-eyed child (or green).

This is also true for many other human traits such as hair color, hitchhiker's thumb, and rolling one's tongue. These traits don't follow simple Mendelian genetics!

John McDonald from the University of Deleware put together a great webpage that debunks some of the myths commonly associated with human genetics. I highly recommend it for further reading if you're interested. Here's a link to the section on hair color: http://udel.edu/~mcdonald/mythredhair.html

Harper Huynh: How do plants get two copies of genes?

In flowering plants, one copy of the genetic material comes from pollen, and the other comes from the ovary which is inside a flower. This is why it is so important that certain crops have pollinators. An apple orchard with no bees would produce no apples because their flowers require the help of a pollinator for fertilization. Corn, on the other hand, relies on wind to carry its pollen and it produces a LOT of pollen. As long as there is even a moderate breeze, a cornfield will have good pollination and produce plenty of corn.

Berrit Lebel: What is the answer to the matching game on page 21?

You can find it in the answer key at the top of the page here in teachable! But I'll also put it here for easy reference:

Kerry and Melody: Did you draw those flowers and cartoons yourself?

I drew most of them! The trumpet-shaped flower came from a website called free SVGs (https://freesvg.org/) where you can download editable graphics. I downloaded the flower on the left and then changed it to be black and white and have fewer lines for the notes. Then I colored it with purple and white versions for the slides we used in class.

Thanks for all the great questions! If yours wasn't answered here, it's likely because it will be answered in one of our upcoming classes in the next week or two.

-Science Mom

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