How Are Genes Passed Down?
We know that genes are passed down from parents to offspring, but how exactly does that happen?
In some simple, single-celled organisms, like bacteria, an identical copy of the organism’s genes is passed down every time the organism replicates. Through a process known as binary fission, a single bacterium splits into two identical daughter cells, which each have a copy of the original bacterium’s genetic code.
When it comes to multicellular organisms that reproduce sexually the process becomes a bit more complicated. Most of the cells in the human body contain two copies of each chromosome and so two copies of each gene. However, sex cells (also known as germ cells) such as sperm or eggs only have one copy of each chromosome and one copy of each gene.
When a sperm cell fertilizes an egg cell, the resulting embryo will have one set of chromosomes and genes from the mother and one set of chromosomes and genes from the father. How these genes interact with each other determines what the ultimate phenotype, or outward physical traits, of the offspring will be.
It’s important to note that the X and Y chromosomes, also known as the sex chromosomes, are passed down a little differently. Female humans usually have two X chromosomes, while male humans usually have one X chromosome and one Y chromosome. Therefore every egg cell has an X chromosome, while a sperm may have either a Y chromosome or an X chromosome.
What this means for inheritance is that male humans usually only have one copy of any genes found on the X chromosome and only one copy of any genes found on the Y chromosome, rather than two copies as with all other genes.
Breher, Thomas (photographer). (2015). Sperm Fertilization Pregnancy [photography]. Retrieved from