Sexual Reproduction

Sexual reproduction ensures genetic variability by:

chromosomes swap segments (and genes) during synapsis
each reproductive cell receives only one of each kind of chromosome in any combination. Also known as INDEPENDENT ASSORTMENT.
sperm and egg that join are from two different individuals - RECOMBINATION

* Sexual reproduction contributes to evolution by creating diversity in populations.

Recall the structure of a chromosome.
Homologs match, but they are not exactly the same, both carry genes for the same trait, but individual genes may differ. One chromosome may carry the gene for blue eyes, the homologous chromosome may have the gene for brown eyes. Gametes (sperm or egg) will only receive one of those chromosomes.

How many different combinations are possible?

Each individual can create 2²³. That individual mates with another individual with 2²³ different gene combinations, the product of this match - the zygote - can then have (2²³)². That's a total of 70,368,740,000,000 different possible combinations in the zygote.

This is why siblings may look similar,but no siblings look exactly the same - except for identical twins, which is a whole different type of scenario.

Math problem: An organism has 4 chromosomes in total. If this organism sexually reproduces with another, what are the TOTAL number of different combinations that are possible in the zyote?

In this diagram, you can see how two genes located on separate chromosomes can combine in four different ways. Humans have 46 chromosomes and millions of genes, which create tons more different combinations.

Notice how all the puppies in this litter look different, despite having the same parents.

Why Did Sex Evolve?

There are other methods of reproduction, such as asexual reproduction and parthenogenesis. All in all it may be easier for an organism to divide and create offspring without sex.

Sex can be risky

Some offspring may not be as well adapted as the parents.
Sex can be dangerous - spreading disease, risk of predation
Sex requires energy to find a mate

With all that trouble, its a wonder why organisms started to sexually reproduce in the first place. There are several hypotheses to explain the origin of sex:

1. DNA Repair Hypothesis

diploid cells can repair damaged DNA, two chromosomes mean a spare set of genes
some single celled protists join together to form a diploid cell when they are stressed by radiation

2. Contagion Hypothesis

Infection of mobile genetic elements - transposons
transposons may have promoted chromosome pairing in order to copy themselves and "infect" other chromosomes

**The Mariner transposon of Drosophila (fruit fly) is responsible for a disorder in humans called Charcot-Marietooth Disease, which causes withering of the legs and feet. No one knows how a Drosophila transposon got into the human genome.

3. The Red Queen Hypothesis

Sex allows populations to "store" recessive alleles.
sexual species cannot get rid of these hidden alleles, even if they cause bad effects.
this may be nature's way of storing alleles that may have a future use if the environment changes.

Sickle Cell Anemia is caused by a recessive gene. In an individual gets two copies of the gene, they have the disease. Most people with Sickle Cell Anemia do not survive to adulthood.
However, if a person only receives one copy of the gene, the person has few ill effects AND they gain an immunity to malaria, as the protist does not thrive in their bloodstreams.
This may support the Red Queen Hypothesis, the sickle cell gene isn't good for most populations, but may be advantageous for people who live in tropical areas.

4. Miller's Ratchet

In asexual populations, when an individual gets a mutation, it will pass that mutation to all its offspring
Over generations, mutations can build up to a degree that the population is no longer viable
Recombination (and sex) may have evolved to reduce the mutation load on a population. Recombination can repair DNA.