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During sexual reproduction, what happens if the egg lets in two sperm from 2 different men?
Question Date: 2020-07-02
Answer 1:

In humans, the entry of two sperm (whether from one or two males) would be a pretty rare event, but it could happen. Why rare? Usually, as soon as one sperm binds enters, there is a rapid "block to polyspermy" that prevents another sperm from entering.

Each sperm provides a haploid (1N = 23) complement of DNA which will pair with the 1N complement of egg DNA to create a diploid (2N = 46) zygote. A third set of chromosomes will result in a triploidy situation (69 chromosomes), which typically is lethal - the subsequent cell divisions just can't handle an entire extra set of chromosomes and it's thought that in most cases, the zygote will die after just a few cell divisions.

However, sometimes the embryo survives, but there are many problems. You can learn more about this rare Triploidy Syndrome here.

Answer 2:

That's a good question. The short answer is that the embryo would not be viable.

Here's the background. We each have 23 pairs of chromosomes. We get 23 from Mom and 23 from Dad. When a human makes a gamete (egg or sperm), a cell with 23 PAIRS of chromosomes divides TWICE instead of the usual once. So an egg or sperm cell has 23 chromosomes (not 23 pairs). This special division is called meiosis (MY o sis).

Sometimes a problem will happen in meiosis. That results in an egg or sperm with an extra chromosome, or missing a chromosome. If that gamete becomes part of a new embryo, the embryo will be missing a chromosome or have an extra one.

Usually, an embryo that is missing a chromosome will not develop long enough to be born. The exception is individuals who only have one X chromosome (X0 instead of being XX or XY).

An embryo that has an extra chromosome may or may not be viable. Having an extra chromosome #21 is what causes Down Syndrome (its technical name is trisomy 21). Some individuals have an extra X or Y chromosome. They may not even know it. Most other cases where a person has an extra chromosome may lead to a miscarriage or death soon after birth. So if you imagine an embryo getting an extra chromosome for all 23, I can't imagine that it would be viable.

Because of the need to limit fertilization to one sperm, the egg throws up a barrier as soon as one sperm cell contacts it. It is like a very thick coating of gelatin that forms immediately. This prevents a second sperm from entering the egg.

Which parent do you think provides more genetic information to the embryo? Is it the same? Do they provide different types of information?

You might be interested in a career in genetics or developmental biology.

Thanks for asking,

Answer 3:

An egg being fertilized by more than one sperm, or polyspermy, is a problem almost all sexually reproducing animals need to deal with. In many animals including human, polyspermic embryos are simply not viable.

The egg therefore has developed a defense mechanism to only allow one sperm to enter. In mammals, fewer number of sperms can reach the egg in the first place due to internal fertilization. After fertilization, the membrane of the egg is modified so that no other sperm can enter.

Genome duplication happens way more frequently in plants than in animals. It is not entirely surprising that polyspermy is much better tolerated in flowering plant.

Answer 4:

The resulting fetus would die before birth.

Adding an extra chromosome in an animal is generally fatal. Humans can have two Y chromosomes, since the Y chromosome has very few genes on it, but the second sperm would have all of the other chromosome, too. Three copies of chromosome 21 will cause Down Syndrome. Adding an extra copy of any other chromosome (apart from X, which is deactivated if there are more than one) will result in death for the fetus.

Answer 5:

There wouldn't be an embryo that divided and grew. The cell would die.

This is interesting, too.

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