Variation fROM SEXUAL REPRODUCTION

At the start of the history of life on earth, single celled organisms cloned themselves to make offspring. At that time, the only variation was through mutation. 

A major increase in variation occured when sexual reproduction evolved. The oldest fossil evidence of sexual reproduction comes from 1.2 billion years ago in a Canadian fossil of red algae or seaweed that had male and female spores. 

Explore the topics of mitosis, meiosis, and fertilization in the Learn Genetics introduction below. Then return to the lesson.

Sexual reproduction increases variation through 

1. Shuffling genes

2. random fertilization

Shuffling genes on chromosomes

Meiosis is the process of producing gametes which are eggs or sperm. Meiosis halves each paired chromosome into the gamete. In humans 23 pairs or 46 chromosomes is reduced to 23 chromosomes total. If this didn't happen, every time the egg and sperm fused, the number of chromosomes would double until the cell couldn't contain them all. We call the paired set 2N and the halved pairs N.

In organisms with many chromosomes, meiosis adds an additional level of variation. Genes are shuffled like a deck of cards. During the separation of chromosomes to reduce their number, whether the chromosome in the gamete comes from the mother or the father of the parent is a random process. This was named by Mendel as the Law of Independent Assortment.

You can see how this works in the paper sort activity for an organism with three chromosomes. 

Paper sort. Fold paper into half and then thirds, and cut at the folds. Mark with blue 1, 2, 3 (paternal) and red 1, 2, 3 (maternal). Sort the papers into each pair of sperm. Each sperm contains chromosomes 1, 2, and 3. How many ways are there to sort the colors of just three chromosomes? 

For humans, a sperm or egg with 23 chromosomes has 2 to the power of 23 possible variations of maternal or paternal chromosomes, which is 8,388,608. Just from this assortment. You can try this out with a two decks of cards, pairing the first 23 cards face up. Then shuffling the cards. Then dealing one of each card into a different bucket and seeing what color and value are in each bucket. 

Shuffling sections of chromosomes

During meiosis, sections of chromosomes containing many genes are exchanged with the matching chromosome pair in a process called crossing over. This will be studied during high school biology in more detail. The overall result is even more variaton.

Random fertilization of gametes

The chance of which exact sperm fertilizes each egg is random. This adds another level of variation.

The inheritance of one of two possible gene variations from each parent was named by Mendel as the Law of Segregation. When we start a Punnett Square, separating the two alleles from one parent at the side of the square represents this segregation. When we fill out a Punnett Square, the combinations shown within the square represent the probability of each random fertilization.

To summarize

Sexual reproduction provides an enormous increase in variation between each generation. This is due to three factors: shuffling of genes and chromosomes, and random fertilization.

Meiosis is the reduction and division of 2N cells to produce gametes with 1N chromosomes. This increases variation through gene shuffling. In an organism with three pairs of chromosomes, there are eight possible combinations for each gamete. Shuffling of of human cells with 23 chromosome pairs can result in over eight million possible combinations in one gamete. So gametes are non-identical. 

Fertilization is the combination of gametes to form the first cell of the offspring. Natural fertilization increases variation through random fertilization of gametes from non-identical parents.

The process of meiosis is studied in more detail later in high school biology. Until then, the names and diagrams of meiosis are not expected to be memorized. It is more important to understand the sources of variation rather than the names of each stage at this point.