Submitted by THRWLT t3_yvajq7 in askscience

So as I understand it, all sperms cells carry the same DNA. The fathers full set.

Likewise all egg cells carry the same DNA. The mothers full set.

When they meet some combination happens. I tried googling it, and its like half of each. How does this work?

With the same parents, how are children born not identical?



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junegoesaround5689 t1_iwdejpz wrote

Sperm and eggs carry half of each parent’s genome, not all of it. The process whereby sperm and eggs are formed is called meiosis.

Here are a few links to various levels of explanation:


eta: (thought I’d try to explain this a bit more to clarify how and why each child has a unique genome, except for identical twins.)

Each sperm and egg end up with half of a unique genome because of crossover during Prophase I of meiosis.

Your genome is made up of one DNA strand from your Mom and the other strand from your Dad. So exactly 50% from each parent.

During Interphase your cell duplicates your Mom‘s and Dad’s strands, still wound together but still separated as one strand from each parent in the DNA double helix. These are called sister chromatids.

In mitosis these sisters separate, travel to opposite sides of the cell, a new nucleus and cell wall are made and the cells separate. You end up with a clone of the original cell.

In meiosis this is where it starts being different. During Prophase I your Mom and Dad strands in each sister chromatid swap some of their homologous genes with each other (like the eye color gene from Mom swaps with the eye color gene from Dad) so that the DNA is now not a clone of either parents’ single strand, each strand is now unique. How many and which genes swap is mostly random, iirc, although it’s not a huge percentage.

The rest of the process you can get from the links.

One consequence of this is that, instead of each egg/sperm being either 100% from Dad or 100% from Mom, each of your gametes will be a random recombination of Mom and Dad’s contribution to your genome. Another consequence is that there won’t, generally, be exactly 25% of a child’s genome from each grandparent (remember your parents got exactly 50% from their parents, too). That can vary, roughly, from 20%-30% from each grandparent, depending on how much crossing over there was during Prophase I.

This is a the major contributor to why offspring have completely unique, though still related, genomes and why sexual reproduction usually produces more variety in a population.


cookerg t1_iwdeww2 wrote

Both the sperm and the egg carry a random half of the respective parent's DNA, so you get half from the mom and half from the dad. So you can get an approximately 50% different mix from your sibling who is not an identical twin.

(There are slightly different details for mitochondrial DNA and the X or Y chromosome from the father.)


WDYDwnMSinNeuro t1_iwek0ir wrote

To add onto this, if the only variation were which of each pair of chromosomes you got, that's 2^23 possible combinations per parent. But it's not limited to that, because crossover of genetic material happens at corresponding sites between chromosomes in a pair.


keerthanasl t1_iwdfj7m wrote

Consider the formation of the gametes (the sperm and the egg) in each of your parents. Each parent obviously has one set of chromsomes from the mother and another from their father. For each parent, only half of their overall genetic material is carried through to a particular gamete, and this could be a random combination of any of their parent's (your grandparent's) chromosomes (for example, chromosome 1 from the parent's mother, chromosome 2 from the father, etc). This occurs because the chromome pairs first replicate (doubling overall genetic material), line up randomly in their pairs, and are then split up into 2 daughter cells. The chromsomes in those cells can line up again and the sister chromatids of each chromsome can be split up into 4 gametes (each with half the genetic material of the original parent cell). This is an oversimplification of the process of meiosis So different chromsomes are inherited from each gamete, and different gametes combining in fertilisation leads to diversity. Also, fertilisation restores chromsome numbers (2 halves make a whole)

Also, when the homologous chromosomes (the chromosome pairs) line up in metaphase during gamete formation, they can undergo a process known as homologous recombination. In this entirely random process, sections of chromosomes are swapped between the homologous chromosomes. This means some of the paternal dna could be found on a maternal chromosome, and vice versa. Thus, different DNA itself is found on the same chromosome, which adds to diversity.

The randomisation of these events is further amplified when you consider that its happening in both parents, so when sperm and egg combine, there are many different genetic combinations possible from just two parents.

Thus, siblings look so different because they may inherit different chromosomes from each parent, with different combinations of DNA thanks to recombination. It's super super unlikely two siblings will inherit the exact same combination of DNA on their chromosomes and so genetic variety is allowed. This is also really important for species to survive in the long term as it gives us good diversity to handle different threats.


SweetBasil_ t1_iwdms6k wrote

  1. all sperm cells do not carry the same DNA, they each carry a different mixture of the two sets of genes the father got from his two parents (everyone has two copies of their genes, one from their mother, one from their father).
  2. all egg cells do not carry the same DNA. As above, they carry a random mix of the genes from the mother's two parents.
  3. the chance of any two sperm or eggs having the exact mix are so low that this never really happens. this is why children from the same parents are not identical.

carrottop317 t1_iwdtfhz wrote

Ooh I actually know this!

When cells split to make new cells, they normally are an exact copy of the old cell. This is mitosis. However, some cells near sex organs undergo meiosis.

The cell has 46 chromosomes. They double, and then some of the DNA gets swapped. The cell then splits into two slightly different cells of 46 chromosomes. This then splits again into two also slightly different cells of 23 chromosomes. Because of the DNA swapping, no two cells have the exact sequence of DNA and are genetically diverse.

When combined with the another sex cell (also genetically diverse from all others in that person’s body) you get a zygote of 46 chromosomes. No two zygote can be the same, even if they are from the same people.

Sometimes a zygote undergoes mitosis and that’s why identical twins are identical because mitosis produces a genetically similar copy. But meiosis doesn’t and that’s why non-identical siblings are genetically diverse.

EDIT: I kept writing gamete instead of zygote. Changed that.


GingerMau t1_iwfkbue wrote


All sperms and eggs do not carry the same DNA. They carry a random 50% of the parent's DNA.

Otherwise, all men would only have daughters (or sons).

Sometimes dad's sperm carries the Y; sometimes it carries the X.


Xanth592 t1_iwhlukj wrote

makes learning about historic persecution of women, especially royals who couldn't make boys tough to watch.


urzu_seven t1_iwe5etb wrote

Before we begin, an important correction: Each gamete (sperm and egg cell) contain HALF of the parents DNA.

Imagine you have two books, well call them Mom and Dad. Lets say for this example that each book has 23 pages.

You want to create a new book from those two books. To make the new book for each page you take half the text from the mom book for the same page, and half the text from the dad book for each page. Lets simplify it even more and say you take either the top half or the bottom half from each book.

So for any child page there are four possible combinations:

- Top Mom + Top Dad

- Top Mom + Bottom Dad

- Bottom Mom + Top Dad

- Bottom Mom + Bottom Dad

How many possible combinations of a 20 page book can you create from those four patterns for each page? A lot. 4^23 is over 70 TRILLION possible combinations. So you have a 1 in 70 TRILLION chance of making an exact duplicate using this method of making "new" books.

This is a VERY simplified version of how genetics works and already you can see how with even just a few options it becomes basically impossible to have identical copies from repeating the same process.


phryan t1_iwe60r4 wrote

Think of DNA as the blueprints for a house, down to flooring and paint color. Everyone is made from two sets of blueprints, for some features the blueprints compromise like blue paint and yellow paint becoming green paint for a room, and sometimes one set is dominate like tile floor instead of carpet. Children get a random set of blueprints from each parent.

So a mother could have green paint because of a copy of instructions for yellow and blue paint. But she passes along the instructions for yellow or blue paint, which then mixes with whatever the father contributes which might be white paint. So the children could have a dull yellow or light blue paint. With millions of pages of instructions it's unlikely two siblings would be identical but they will often share enough of the same traits to look similar.

And everyone once in a while something really weird happens and the instructions to build a window get spliced onto the instructions for a roof and then you get a skylight.


burke828 t1_iwffkjp wrote

Each parent has 2 sets of DNA, which aren't always the same. Lets say you have a father with one gene for red hair and one gene for blond hair, and a mother with one gene for red hair and one gene for brown hair. Brown hair has a dominant gene over blond or red hair, and blond hair is more dominant than red hair.

To determine the possibilities for hair color, you make a punnet square.


Red Hair Gene R Brown Hair Gene B
Red Hair Gene R RR (red hair) BR (brown hair)
Blond Hair Gene b bR (blond hair) Bb (brown hair)

See how you can have 3 different hair colors from 2 parents? Apply that to literally every gene and hopefully you can see how you end up with children with distinct genetics.


electric-angel t1_iwdsrag wrote

no Egg and sperm both have 50% of the dna. what goes into that 50% is randomised

so egg 1 and sperm 1 have the trait for brown hair and black hair. with black being dominant so black

and egg2 and sperm 2 have the traits brown and red hair. thus giving brown because brown is dominant over red


010011100000 t1_iwdvfw2 wrote

All sperms don't carry the same DNA. The father has two different versions of each chromosome, and each sperm cell randomly gets one of the two versions of each. Same with the egg. Each sperm+cell combination therefore gets a combination of chromosomes that is different from other sperm+cell combinations


Infamous_Weight_977 t1_iwf69s1 wrote

Egg and sperm has a combination of half the person dna, if it were full you would then theoretically not need sperm for pregnancy, but have dna wise a clone. With this would potentially cause abnormalities I would imagine.


Gavus_canarchiste t1_iwgn76o wrote

Other posts explained it well, so just adding a fun fact.
Basically, in theory for every gene, every parent can give one of their two versions, that differ in a few percent of cases.
If you take into account all possible combinations of alleles given by each parent, you get 10⁵⁰⁸ possibilities for the baby's genome.
That number makes no sense. For comparison, there's about 10⁸⁰ particles in the observable universe...
(Details: there's about 20,000 genes, 6,7% of which are heterozygous, so about 1,340. For every such gene, you have 2 choices: allel 1 or 2? So, total of 2^1340... squared, because there's two parents. This doesn't even take into account mutations that always occur.)


chesterbennediction t1_iwh1pfm wrote

Because of crossover. When you get half of each parents DNA it's not the same halves as your sibling but pretty random bits and pieces to make up each half. There is also DNA expression so even people with the same DNA will have different active genes over time which will alter there appearance.


MegavirusOfDoom t1_iwtbog9 wrote

Google.... Reproductive chromosome recombination methods..... You will find what you seek...

Basically It's like two pack of 24 cards where You mix 2 games together andYou have 10 billion possible mixes of just one card. On top of that, you can divide cards in the middle and gleam together with cello tape. The number of different possibilities then becomes a 100 billion.


[deleted] t1_iwdusey wrote



BatSensei t1_iwfwp3v wrote

Except that there are clonal species of everything from bacteria and fungi to plants, invertebrates, and even vertebrates (obligate and facultative parthenogenesis - Evolution favors what works, not just one "strategy". Radical recombination, like what happens during sexual reproduction, isn't required for survival, and is obviously not be the optimal way for all organisms to get alleles to the next generation...