Question of the Week 06/14 – Egg Cells

I hope everyone is looking forward to summer vacation!  Here is our last question of the week for the summer- see you back here in September!

Dear Dr. Treatman-Clark,

A question occurred to me during the beginning of my class, but I forgot about it until recently. If an egg is a single cell until sperm fertilizes it, then would an unfertilized chicken egg, like the kind you get at the grocery store, be a single cell? If so, what would the different structures be? Would the yolk be the nucleus? And would there be other organelles in the egg as well?

I’m sorry if this is really a few questions. Thank you in advance


Hi Keylee,

This is a really interesting question!  There is actually some disagreement about this issue.  Let me start be saying that unfertilized bird eggs do contain a single cell.  The issue that causes confusion is whether the ENTIRE egg constitutes the cell, or whether the egg CONTAINS a single cell, as well as extracellular material.  For instance, eggs contain an air pocket so that the growing cell can respire- there is some question as to whether the air sac is inside the cell (in which case it would be like a vacuole) or outside the cell.  The nucleus of an egg is not the yolk  – the yolk is the food that would nourish a growing embryo if the egg were fertilized.  So in this sense, the yolk is most similar to a food vacuole.  The nucleus and most of the cytoplasm of the egg cell are located in a small disk (germinal disk) that lies on the side of the egg yolk.  If the egg gets fertilized, it is this part that begins to divide and forms the embryo.  So you can see why things are confusing- the yolk is part of the cell, but it doesn’t divide when the egg gets fertilized.  Most people simply call the egg a single cell and don’t worry about whether the air sac or amnion lie outside the cell membrane, but a few people insist that these are not actually inside the cell membrane so that they are just air or fluid spaces outside the cell. If you say that the unfertilized egg CONTAINS a single cell, rather than the egg IS a single cell, you are guaranteed to be right.  I hope that this makes sense to you- it is a confusing issue.

Dr. Treatman-Clark

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Question of the Week 6/7 – Fertilization

Hi, Dr. Treatman-Clark!

I have a question. In the lesson for reproduction, it says that for a normal individual to result, only 1 sperm can enter the egg. What happens if 2 sperm enter the egg?-


Hi Linnie,

There is a short answer to this question and a long answer.  The short answer is the egg will not develop properly and will not result in a living organism.

Here is the longer answer.  I will use a couple of fancy terms, but I will try to explain everything clearly.

The outer layer of the egg is called the “zona pellucida”, and it has a VERY important function.  It contains the receptors whose job is to recognize and react with sperm.  Getting through the zona pellucida is NOT an easy job for sperm.  Basically, the sperm have pointy heads, which they dig in to the zona pellucida and the sperm have a tail, which they move back and forth to try to force themselves deeper into the zone.  The head of the sperm is covered with something called the “acrosome”, which helps the sperm out by releasing enzymes that soften or dissolve the zona pellucida.

Now, as soon as 1 sperm makes it into the egg, it causes a chemical reaction inside the egg that changes the zona pellucida.  Several things happen:

1.  The zona pellucida hardens.  You can think of this like concrete setting.  The sperm are moving through thick wet concrete, but as soon as the first one gets all the way through, the concrete hardens and the rest are frozen in the hard cement.

2.  All those sperm receptors on the outside of the egg are destroyed, so no new sperm can bind to the egg.

3.  The head of the sperm inside the egg dissolves, and its DNA dumps out into the egg.

4.  The egg sort of “wakes up”.  Until this point, the egg has been in a kind of suspended animation, but once a sperm enters it begins to activate, and then it starts the process of division that will eventually lead to an organism.

So, for 2 sperm to get in, they would really have to break through the zona pellucida at exactly the same time, which is pretty rare.  If two sperm did manage it though, steps 1 and 2 would go as before, but when step 3 happened, there would be too many chromosomes in the egg.  It would not be able to pair up the chromosomes properly for cell division, so cell division would fail.

I hope this answers your question!

Dr. Treatman-Clark


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Question of the Week 5/28 – Animal Blood


I have a question.  What does the blood in animals do?


Hi Iniyan,

Well, blood has 2 main parts: cells and plasma.  The plasma is the liquid part of blood – it has lots of stuff dissolved in it, like nutrients, vitamins, and hormones, and the main function of the plasma is to transport these things around the body.  There are several cell types found in blood, but the two most important and well known are the red blood cells and the white blood cells.  Red blood cells carry oxygen throughout the body, and white blood cells are used to fight infection.  There is lots more to learn about blood, and here are some websites you might find useful:

Thanks for sending in the question!

Dr. Treatman-Clark

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Question of the Week 5/21 – Do Plants Have Blood?

I was just wondering if plants have blood. What’s the sticky stuff you touch when you break the stem in half?



Hi Linnie,

Great Question!  Plants do not have blood.  Blood is found only in animals, and mainly in the vertebrates.  Vertebrates all have a closed circulatory system in which blood carries oxygen (bound to hemoglobin in the red blood cells) through arteries and veins.  Some complex invertebrates, like squid and octopuses also have hearts and blood vessels.  Simple invertebrates, such as jellyfish, sponges, and roundworms don’t have a circulatory system – each cell is able to absorb nutrients and exchange gases with the cells directly touching it.

This system is similar to what happens in plants.  You learned in your lesson about xylem and phloem, the transport structures in plant stems.  Water, minerals, and glucose move up or down the plant stem through direct cell to cell contact.  So, the sticky sap that comes out when you break a stem is a mixture of these things.

Here’s a cool side note – plants have defenses!  Plant leaves or sap can contain chemical compounds that are poisonous to animals.  Plants with such defenses do better than their competitors because animals are less likely to eat a plant that makes them sick or causes a painful burn or rash.

Here’s a plant research question for you:  What is the relationship between mammals and flowering plants?

Dr. Treatman-Clark

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Solution to Inclusive Fitness Challenge and More Pedigree Fun

For ALL students:

Here is a fun online lab to help you practice working with pedigrees and inheritance;

For older students:

Last week’s challenge asked, “For how many second cousins would Haldane be willing to give up his own life?”

The answer is… 32!

Here’s a pedigree explaining how to calculate the coefficient of relatedness for first and second cousins:  first and second cousin pedigree

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Question of the Week 5/3 Inclusive Fitness

“I’d give my life for two brothers or eight first cousins!”– J.B.S. Haldane

Last week, we learned that in mammals and other diploid, non-inbred organisms, an individual typically shares 50% of its genes with its parents, offspring, and full siblings.  On the other hand, an individual is related to itself by 100%.  Haldane is doing some genetic arithmetic to determine how many siblings or cousins he would need to add up to 100%.  He would give up his life for two brothers, because each brother shares 50% of his genes.  So technically, the lives of two brothers are equal to his own life, as 50% + 50% = 100%.

Cousins are more distant relatives than siblings.  On average, an individual shares about 12.5% of his DNA with a first cousin.  So, in Haldane’s calculations, it would take 8 first cousins to equal the cost of his own life:  12.5% x 8 = 100%.

For Older Students:

Haldane’s quote is based on the idea of “inclusive fitness”.  Here is some important terminology and some links to help you better understand this concept:

Fitness:  The ability of an organism (or allele) to survive and reproduce in a given environment. 

Direct Fitness: The component of fitness gained through personal reproduction (i.e. production of offspring)

Indirect Fitness: The component of fitness gained from aiding the survival of non-descendent kin, such as siblings.

Inclusive fitness: The fitness gain through both routes – direct (personal production of offspring) and indirect (aiding the production and survival of siblings or other non-descendent kin).

Relatedness: The probability that two individuals share an allele due to recent common ancestry.  This probability is expressed as the coefficient of relatedness, denoted by the symbol r. It ranges from 0 (unrelated) to 1 (clones or identical twins).

Kin selection:  The evolutionary mechanism that favors the survival and reproductive success of an organism’s relatives, even at the cost of the organism’s own survival and reproduction.

To read more about all of these concepts, check out the following websites:

As a special challenge, can you tell me how many second cousins Haldane might be willing to give up his life for?

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Question of the Week 4/26 Eusocial Mammals

Lots of you did some research and discovered that naked mole rats are the only eusocial mammal.  Here is what some of your classmates had to report about naked mole rats:

Naked Mole Rats are arguably the only eusocial mamal.  Eusociality involves reproductive division of labor, overlap of generations, and cooperative care of young.


The language here is a bit complicated, but Max’s definition of eusociality is exactly right.  Reproductive division of labor means that one 1 or a few individuals reproduce, while the others have different jobs, such as gathering food, maintaining the tunnels, or caring for young.

The naked mole rat is the first eusocial mammal. One female in the entire community of 80 naked mole rats does all the breeding. It took ten years for scientists to truly consider the naked me rats as eusocial animals because they don’t have the permanent, physical trait that distinguish different levels of the colony. Then scientists found this physical difference. The queen naked mole rat’s spinal bone lengthens during it’s pregnancy so it can hold more babies and still fit into the tunnels of the home.

Really interesting,


Wow Rebecca, I thought I was an expert on naked more rats, but I did not know about the longer spine bone in the queens.  That is really neat!  I watch the naked more rats often at the zoo (you can see them at the Philadelphia zoo or the National Zoo in Washington D.C.) and it is very hard to see any difference – sometimes the queen looks a bit bigger than the others, but it is not a big difference.  Thanks for the information!

Naked mole Rats are a mammal with a eusocial system. they live in large colonies with only one female that breeds. The majority of the colony are workers. these can be both males and females and they spend their whole lives working for the colony. The normal colony size is about 80 but there have been colonies with a population up to 295 observed.

Inbreeding is very common in these animals which results in a lot of similar genetic make up.


Great job Annie!  You bring up a very important point.  In insects like ants and bees, eusociality works because of their very unusual haplo-diploid genetics.  It results in female workers being more closely related to their sisters than they would be to their own offspring.  As a result, they actually benefit more in terms of fitness (a fancy evolutionary term meaning how many of your own genes get passed down) from raising siblings than they would from raising their own children.  Naked mole rats are diploid animals like us, so for them it ought to be a real disadvantage to raise siblings instead of their own offspring.  BUT, because they are so inbred, siblings share a lot of the same DNA, so it isn’t a real disadvantage.

Normally, in diploid animals like humans, an individual shares exactly 50% of its genes with its offspring and an average of 50% of its genes with a full sibling.  Naked mole rats share more than 80% of their genes with siblings, and so can enjoy high reproductive success through kin selection- that is, contributing to the survival and reproduction of relatives rather than producing their own offspring.

I think that this also sort of answers Akki’s question below:

 ANTS have always made me curious since I was a baby. We had an ANT colony in our classroom when I was in 4th grade. We fed them and took notes and observed the colony the entire year. It was very interesting to see the ants organization.  Though it seems good that they work so hard and take care of their families, some ants sacrifice seems not good to me. It definitely sounds very unfair to intelligent creatures like us where everyone should be given equal rights. I always wondered why it is like that with ANTS? Probably because their species could survive under any circumstances.

Interesting Topic


Akki also sent some good links to information on mole rats, and I have included some of my own as well:

and this one is a zoo cam so you can watch the naked mole rats:

Thanks to everyone who participated in this fun research!

NEW Challenge

Here is a TOUGH question for you all to think about.  Normally, organisms act to maximize their reproductive success- that is, to ensure their own survival and reproduction and that of their descendants.  We’ve learned that eusocial insects (haplo-diploid) refrain from breeding in order to raise siblings because they actually share more genes with siblings that with their own offspring.  We’ve learned that naked mole rats are so inbred that once again, individuals can pass on a large proportion of their own genes by ensuring the survival of siblings.  But for normal, non-inbred diploid mammals, could it ever pay off to refrain from breeding in order to assist in the survival and reproduction of relatives?  Think about this question, and explain what this quote from the famous evolutionary biologist J.B.S. Haldane means:

“I’d give my life for two brothers or eight first cousins!”– J.B.S. Haldane

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