In chromosomes, nor is sex determination so cut

In humans, the Y chromosome denotes a male. In the model organism Drosophila, it is the number of X chromosomes and autosome sets that determine sex rather than the presence of a Y chromosome. In this lesson, we’ll learn about what makes normal male and female flies, as well as the super-normal!

Drosophila Chromosomes

In today’s lesson, we’ll start with doing a little comparison between humans and fruit flies. This isn’t the craziest idea, as Drosophila fruit flies are a common genetic model organism. We have a few things in common.

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We’re both animals, we both enjoy a ripe piece of fruit, and we both have chromosomes that determine if we’re male or female – just in different ways.There are 46 total chromosomes in a human diploid cell, with two of these 46 chromosomes, or one pair, being the sex chromosomes (either an X or a Y). The other 44 chromosomes, in 22 pairs, are all known as autosomes. It’s not so hard to determine sex in humans. In fact, you probably have this one down pat. Usually, it’s as simple as the fact that an XX genotype is female, while an XY genotype is genetically male.

The autosomes don’t influence human sex.In a Drosophila fly, there aren’t quite so many chromosomes, nor is sex determination so cut and dry. There are only three autosomes, called autosomes 2, 3 and 4. Autosome 4 is really itty-bitty in size. In a diploid cell, you would have three pairs of autosomes, or six non-sex chromosomes, and a pair of sex chromosomes. These two sex chromosomes could be either an X or a Y. However, you will soon see that it can be more complicated than this, leading to more sexes in flies than just your everyday female or male.

Sex Determination in Drosophila

In other lessons, we’ve discussed how Drosophila fruit flies make a superb developmental model organism. Although much of animal development is conserved, and there are identifiable patterns of gene expression that flies and humans share, sex determination is one area where the two have distinct differences. While the presence of a Y sex chromosome is what makes a human male a male, a Y chromosome does not determine sex in flies.

The Y chromosome does include some male sex genes, but it doesn’t exactly determine sex. In fact, sex is mostly determined by the ratio of X chromosomes to autosome haploid sets.This is the classic definition of sex determination in flies.

As with most things in science, this is actually more complicated than this simple definition. Sex determination also involves the expression of specific autosome and sex chromosome genes. We won’t get into the nitty-gritty here, but you should be aware that another layer of control exists.Generally speaking, a ratio of X chromosomes to autosome sets equal or greater to one produces a female, while a ratio less than one produces a male.We can summarize possible ratios and sex with the following table, where each A represents a haploid set of autosomes and each X represents one X chromosome.

Now remember that in a classic fly diploid, there would be two sets of autosomes, otherwise known here as AA. Because the Y chromosome does not determine sexuality, we are not including it on this table.

X Chromosomes Autosome Sets Ratio Sex
XX AA 1:1 Normal Female
XXX AAA 1:1 Normal Female
XXX AA 3:2 Metafemale
XXXX AAA 4:3 Metafemale
XX AAA 2:3 Intersex
X AA 1:2 Male
X AAA 1:3 Metamale

A one-to-one ratio of X chromosomes and autosome sets produces a normal female fly. Therefore, a fly with two X chromosomes but two sets of haploid autosomes would be a normal female. This is similar to humans, where an XX is a female. However, this is where the similarity ends.

The presence or absence of a Y chromosome doesn’t affect the sex. It also just so happens that some flies have a chromosome set number other than two. If the fly had two sets of sex chromosomes, three X chromosomes and a Y chromosome, and three haploid autosome sets, the ratio would remain one-to-one.

This is because the Y chromosome doesn’t matter. Three X chromosomes and three haploid autosome sets would still be a normal female.A higher than a one-to-one ratio of X chromosomes to autosomes sets, such as a three-to-two or four-to-three ratio, produces a metafemale. A metafemale sounds super cool, but it usually does not fully develop.A ratio less than one-to-one could produce an intersex fly that’s between a male and female with characteristics of both. An example of this is seen in flies with a two-to-three ratio of X chromosomes to autosomes.

A one-to-two ratio of X chromosomes to autosome sets produces a normal male. However, a one-to-three ratio of X chromosomes to autosome sets produces a metamale. Now, like a metafemale, this isn’t a good thing. A metamale is usually sterile.

Lesson Summary

In this lesson, we learned that sex determination in the model organism of Drosophila melanogaster is different than sex determination in humans. In humans, the presence of a Y chromosome determines a male. In Drosophila, the ratio of X chromosomes to autosomes plays a major role in sex determination rather than the presence of a Y chromosome.

Usually, a ratio of X chromosomes to autosome sets greater than one-to-one is a metafemale, a ratio of one-to-one is a female, and ratios of one-to-two of X chromosomes to autosomes is a normal male. In addition to an X and Y sex chromosome, usually Drosophila have three autosome pairs of chromosomes.

Learning Outcome

After watching this lesson, you should be able to discuss how sex determination in the model organism Drosophila is different from humans.


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