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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.