Genetic Crossing Over: Definition & Concept

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  • 0:00 What is Crossing Over?
  • 1:07 Meiosis at Work
  • 1:55 Meiosis I
  • 2:42 Meiosis II
  • 3:13 The Gene Buddy System
  • 4:50 Lesson Summary
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Lesson Transcript
Instructor: Meredith Mikell
Crossing over of genes is a big factor in the genetic diversity of living things. Here we will define the process of crossing over, and further explore the mechanism behind it.

What is Crossing Over?

Ever know a large family with many children, all of whom are indistinguishable from each other? Unless they are all identical twins, you have not encountered such a family. Non-twin siblings typically have a range of physical differences, from subtle distinctions in features to looking unrelated. Even though they inherited equal chromosomes from the same two parents, the combination of genes is diversified due to crossing over.

Crossing over is the exchange of genes between two chromosomes, resulting in non-identical chromatids that comprise the genetic material of gametes (sperm and eggs). This process results in the millions of sperm or eggs that are produced by an organism, each being different from one another. In other words, every single sperm or egg cell in your body is completely unique!

Think of it like two traders meeting to exchange their goods, resulting in both leaving with a more diverse collection of wares than they had before. Thanks to this process, living things have high diversity within populations, allowing for better chances of adaptation to changing conditions and survival of the species.

The structure of a chromosome
chromosome gene

Meiosis at Work

Genetic information is stored in chromosomes, a coiled bundle of genes, which are segments of DNA that code for specific functions or features of an organism. Different species have different numbers of chromosomes; we humans have 46. When your body grows or replaces dead skin or hair, your cells duplicate during mitosis, and the 46 chromosomes are copied before the cell splits into two identical daughter cells, each with 46 chromosomes of their own. This works great for making clone cells. But when it comes to reproducing an entire organism, having a diversity of genes on those chromosomes is key. Meiosis is the process by which gamete cells are made. This occurs in two stages: meiosis I and meiosis II. Meiosis I is basically just like mitosis, except for one key step: crossing over.

Meiosis I

Crossing over occurs during prophase I of meiosis. The 46 chromosomes are duplicated (now 92 total chromosomes), just like during mitosis, but before separating to divide into two daughter cells, they commence in gene swapping. The chromosomes are essentially squashed together, during which time some genes from one are transferred to the other, and vice versa. When the cells divide after crossing over has occurred, the 92 chromosomes are distributed in half, with 46 going to each of the two daughter cells. Each daughter cell has a complete number of chromosomes, and now carry different combinations of genes on their chromosomes. Using our trader analogy, the traders have exchanged goods and now carry a diverse mix of each other's supplies.

Crossing over between chromosomes
meiosis crossing over

Meiosis II

During meiosis II, the 46 chromosomes are NOT duplicated. Instead, the cell divides into two daughter cells, each with 23 chromosomes. Each chromosome now has a different combination of specific genes due to crossing over in meiosis I, and each gamete represents a completely unique combination of genes. This halving occurs so that if an egg and sperm combine one day during fertilization, the offspring will have a complete set of 46 chromosomes.

meiosis chromosomes crossing over

The Gene Buddy System

Because of crossing over, genes can be inherited independently of each other in future generations. This is called independent assortment, and we see this every day in siblings with different combinations of their parents' genes. One might have mom's nose and dad's eyes, the other might have mom's eyes and dad's nose, another might have mom's eyes AND nose. This is because the traits for nose shape and eye color and eye shape are all different genes that aren't necessarily passed along together during meiosis.

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