Some organisms can choose to reproduce either by mating or by cloning themselves. However, many groups of organisms have only one option. Most animals only reproduce by mating. Bacteria and archaea can only reproduce by cloning.

Why this is important…The way an organism reproduces determines how fast they can produce offspring and how varied those offspring will be. Microorganisms usually reproduce wildly fast, but with little investment. Other organisms, like humans, invest a huge amount of time, effort and resources into children. Both have profound implications for human health.

Foundations

Cloning, or asexual reproduction, is a sure bet. It is simple and works almost every time. All offspring are genetically identical, so if the parents could survive then offspring probably can too.

Sexual reproduction is a bit of a genetic gamble. If you have played any card game, you know it starts by shuffling the deck to make sure everyone has an equal chance of winning…or losing. Everyone receives a random set of cards. Replace the cards in this analogy with genes and you have the essence of sexual reproduction. Two parents shuffle their genetic cards and give half to their offspring. This is a risky process that ends in failure more often than success. However, it does guarantee diversity or variation in the next generation. Each child will be different from all the others.

Separate genders are one way of assuring sexual reproduction. In mammals, gender is determined by the mix of sex chromosomes an individual receives – 2 X’s is female, while X and Y is male.

Key terms

Sexual reproduction – a way to produce offspring that combines the genetic material of 2 individuals
(verb: sexually reproduce)

Asexual reproduction – a way to produce offspring that are genetically identical to the parent; this only requires one parent (verb: asexually reproduce)

Autosome – chromosome 1 through 22; these play no role in determining gender

Sex chromosome – chromosomes that determine gender in some animals

Details

Most organisms that we are familiar with reproduce sexually. This is why birds choose mates and build nests, plants make flowers, and fungi make mushrooms. Sexual reproduction results in offspring that are genetically different from one another. Even if all the trees in the forest look the same, they each have a unique genome.

Most plants and fungi reproduce sexually, but still have the ability to reproduce without sex (asexually). This varies from species to species, but most plants or fungi can form new individuals from a fragment of the old one. If you have ever seen someone grow a new plant from a cutting, then you have seen an example of asexual reproduction. Plants that spread underground, like mint or turf grasses, are also examples of asexual reproduction.

Bacteria and archaea only reproduce asexually. When individual cells reproduce by binary fission, the resulting cells are the offspring. And offspring are clones of the parent – all the offspring from an individual bacterial cell are genetically identical. This is a very different mode of reproduction than humans, but it works for bacteria and archaea. They have been reproducing this way for a billion years longer than sexual reproduction has existed on Earth.

Stop & Think #1: What are advantages and disadvantages to sexual reproduction? To asexual reproduction? (Hint: part of the answer has to do with whether the environment is stable or changing.)

A few animals can reproduce without sex, but these are uncommon. For example, some worms can form two individuals if you split them in half. Most animal species are so dedicated to sexual reproduction that they make separate genders, assuring that one individual cannot reproduce alone. Animals need mates.

Gender is determined by sex chromosomes in mammals. A special pair of chromosomes – the sex chromosomes – determines gender in mammals. Humans have 22 pairs of chromosomes (autosomes) that are the same in every person. The 23^rd pair are the sex chromosomes. (see Figure 11-3)

Two X chromosomes (XX) results in female offspring.
A mix of X and Y chromosomes (XY) results in male offspring.

Diagram of 24 human chromosomes. Autosomes are numbered and shown as banded sticks. Sex chromosomes are also shown similarly, but X is much larger than the Y chromosome.

FIGURE 11-3. Human chromosomes. All unique chromosomes are shown here as diagrams that represent their size and staining patterns. Chromosomes 1 through 22 (autosomes) come in matched pairs. Sex chromosomes are not a matched pair in males, who inherit an X and Y chromosome. ("Sex chromosomes" by National Human Genome Research Institute is in the Public Domain)

How do X and Y combinations determine gender? The determining factor is the Y chromosome, specifically the SRY gene on the Y chromosome. During fetal development, this gene is expressed and starts making testosterone, resulting in development of male reproductive organs. If there is no Y chromosome, then the person will continue to develop female.

There are exceptions to the typical gender-determining formula when errors occur during reproduction. Sometimes individuals inherit 3 X chromosomes, in which case they are female. Some females have only one X chromosome. Some males have an extra X chromosome (XXY). This seems confusing, but underscores the role of the Y chromosomes in determining the male gender.

There are other exceptions as well. Sometimes individuals with a Y chromosome produce testosterone but cells don’t respond, resulting in an individual who appears female but does not have a functional reproductive system. In other cases, XX individuals can over-secrete masculinizing hormones from the adrenal glands. This may cause masculine-looking genitals to develop, giving the individual a male appearance even though they are chromosomally female. In addition to chromosomal and anatomical features of gender, there is also the psychological aspect of gender identity to consider.

Bottom line: Gender is more complex than the formula of XX is female and XY is male.

Learning objectives

Describe the difference between sexual and asexual reproduction
Give examples of sexual and asexual reproduction

Describe how gender is determined in mammals

Stop & Think Questions Explained

In a changing world, variation among offspring gives natural selection something to choose from. This improves the chance of survival for the species. In a stable environment, variation is less important and asexual reproduction is far more efficient. Sexual reproduction comes with many risks and hidden costs – finding mates, disease transmission, energy cost of nurturing offspring, and the chance that some variations in offspring may not thrive. Asexual reproduction avoids these risks and costs, but if the environment changes, the species risks extinction.