This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Teacher's Study Guide for Lesson Twelve
Sex Determination, Chromosomes and Mosaicism

by Dr Jamie Love Creative Commons Licence 2002 - 2010

Sex is determined in different ways depending upon the taxa.

Many plants and some animals (earthworms and hydra) have both male and female sex organs in the same individual and produce both male and female gametes (sperm and egg, respectively). These organisms are monoecious.
[In Greek monoecious means "one house".]
In animals we use the word hermaphrodite. There is no difference among the sexes because each individual is both sexes. To them, sex is just an extra organ system.

Dioecious organisms come in two sexes, male and female, and each individual will produce only one type of gamete.
[In Greek dioecious that means "two houses".]

Among some dioecious taxa (some species of fish, alligators and sea turtles) sex is determined by the environment, not genetics. Local concentrations of hormones or differences in temperature will cause the developing embryo to develop as either a male or a female.

The sex of most dioecious species is determined by genetics.

Insects have evolved some unusual sex determining systems.

In most dioecious taxa the sex is determined by specific sex chromosomes.
Most animals have a pair of sex chromosomes and, together or individually (depending on the taxa), these determine the sex of the developing embryo and thus the sex of the fully-grown individual.

The XY system of sex determination applies to all mammals and several other taxa, but not all taxa. Birds, butterflies, some reptiles and some fish use a sex determination system called the "ZW system" in which everything is "backwards".

We have 46 chromosomes or 23 pairs of chromosomes.
22 truly homologous pairs (chromosomes that share the same genes at the same loci). Those chromosomes are not involved in sex determination and are called autosomes. They form the familiar tetrads at meiosis I.
The 23rd pair of human chromosomes is NOT homologous, at least not in men.
All male mammals have one X chromosome and one Y chromosome. These are the sex chromosomes and they are NOT homologs. They do not share any loci or genes.
Female mammals do not have a Y chromosome. Women have two X chromosomes.
So, we have 22 pairs of autosomes (numbered from the largest #1 to the smallest #22) and one pair of sex chromosomes. Among the sex chromosomes, men have XY and women have XX.

At a particular position on the Y chromosome is a gene called the "sex determining region on the Y" or simply "SRY", responsible for turning a mammalian embryo into a male mammal.
We are genetically "pre-programmed" to become female.
SRY begins a developmental "cascade" in which other genes, many of them on autosomes, are also turned on.
SRY starts a development pathway producing male features in the embryo.
Embryos without a Y chromosome continue in their default pathway to produce female features in the embryo.

A Punnett square for sex determination using the sex chromosomes.

The female produces only one type of egg with respect to sex chromosomes.
She is XX so, with respect to sex chromosomes, she is "homozygous".
The male produces two types of sperm, one carrying the X and the other the Y. He is a "heterozyogte", with respect to his sex chromosomes.
Because men are "heterozygotes" it is their sperm that determines whether the woman gives birth to a son or daughter. Men determine the sex of their children.

Mating produces sons and daughters in a 1 : 1 ratio because the father produces equal numbers of sperm carrying an X or Y.

Aneuploidy among the sex chromosomes is more common than autosomal aneuploidy. Aneuploidies come about due to errors in meiosis during the gamete formation in one of the parents so the zygote starts out with the wrong chromosomal count.

Klinefelter syndrome a person has two Xs and one Y.
XXY and has a total of 47 chromosomes
What would be this person's sex?
There are rarer variations of Klinefelter syndrome in which the individual has even more extra Xs (Like XXXY and XXXXY.)

The "opposite" sex chromosome aneuploidy would be to have more than one Y chromosome.
These XYY people are male, and this condition is XYY syndrome. These men do not have an abnormal phenotype.

YY and YO are fatal.
We all must have at least one X chromosome to survive.

Trisomy X is a condition where the woman has three Xs, so she has a total of 47 chromosomes. There are rarer variations on this aneuploidy in which a woman has four or five extra X chromosomes! With each extra X the mental retardation becomes more severe.

Women with Turner syndrome have only one X chromosome. This is the only (viable) human aneuploidy involving less than 46 chromosomes.

The Y chromosome is the smallest chromosome and it is made mostly of useless "junk DNA". The only gene it carries (of any importance) is SRY.
The X chromosome is one of the largest chromosomes and it is packed with very important genes.
Genes on the X chromosome are said to be X-linked and they present us with a very special puzzle.

There are (roughly) equal amounts of gene products (proteins) from X-linked genes in men and women.

Women avoid being "over dosed" by turning off one of their X chromosomes in a process called X inactivation. Which X chromosome they turn off is determined randomly during embryo development.

  1. Female mice heterozygote for two coat-colors do not show a simple pattern of dominance. Instead, their fur is made up of patches of the two colors arranged randomly.
    Male mice never have this arrangement. They have coats of uniform color.
  2. A dark pack of sex chromatin, called a Barr body, is seen in the interphase nucleus of all female mammals.
    Normally - males never have a Barr body and women always have one.
    Women with Turner syndrome (only one X) do not show a Barr body! All the other conditions involving extra X chromosomes always showed one Barr body for each extra X chromosome.
Mary Lyon proposed an explanation that we now called the Lyon hypothesis. In the cells of female mammals only one X chromosome is active. The other X becomes (and remains) inactivated. This inactivated X chromosome is the Barr body.
So men have no Barr body but women have one.
Women with Turner syndrome (2n = 45, with only a single X) don't show a Barr body. Klinefelter syndrome (2n = 47, with the sex chromosomes of XXY) show a Barr body because they inactivate the extra.

Early in development the X chromosomes are randomly inactivated in each cell. In any particular cell it could be the paternal (Xp) or the maternal (Xm) that remains active.
The inactivated X chromosome becomes a tiny blob of dense, useless chromatin called the Barr body (shown here as a dark oval).
These cells pass their inactivation patterns to all their descendants.

The embryo, and the organism it becomes, is a mixture of two types of cells. We call such an organism, a mosaic (because it is like each cell contributes a "tile" to make the final "image").

X inactivation allows for a dosage compensation and explains the mosaicism seen in coats of heterozygous female mice and cats.

Females that are heterozygous for X-linked genes are mosaic for those genes and, therefore, are not "truly" heterozygotes on a cell-to-cell basis.
Males, on the other hand cannot be heterozygote at all for any X-linked genes because they have only one location (one locus) to place only one allele.