Sex Chromosomes
Humans have 23 pairs of chromosomes (46 total). Two of these are called sex chromosomes, the other 44 are called autosomes.
There are two kinds of sex chromosomes, called the X chromosome and the Y chromosome. The X chromosome is larger and contains many genes. The Y chromosome is much smaller and contains very few genes.
Normally, human females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).
Occasionally, an accident happens in which a person is born with too many or too few sex chromosomes. In these cases, the person will be male if they inherit a Y chromosome and female if they do not.
Examples of four different possibilities that produce males are shown below. The last three are abnormal.
XY
XXY
XXXY
XYY
Examples of four different possibilities that produce females are shown below. Normal females are XX.
X
XX
XXX
XXXX
The cross below shows that normal females produce eggs that have one X chromosome. Half of the sperm produced by normal males have an X chromosome and the other half have a Y chromosome.
XX x XY
Sex Chromosomes
Humans have 23 pairs of chromosomes (46 total). Two of these are called sex chromosomes, the other 44 are called autosomes.
There are two kinds of sex chromosomes, called the X chromosome and the Y chromosome. The X chromosome is larger and contains many genes. The Y chromosome is much smaller and contains very few genes.
Normally, human females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).
Occasionally, an accident happens in which a person is born with too many or too few sex chromosomes. In these cases, the person will be male if they inherit a Y chromosome and female if they do not.
Examples of four different possibilities that produce males are shown below. The last three are abnormal.
XY
XXY
XXXY
XYY
Examples of four different possibilities that produce females are shown below. Normal females are XX.
X
XX
XXX
XXXX
The cross below shows that normal females produce eggs that have one X chromosome. Half of the sperm produced by normal males have an X chromosome and the other half have a Y chromosome.
XX x XY
¯
This analysis shows that half of the offspring are expected to be male, half are expected to be female.
X-Linkage
Morgan (Columbia U):
P1 red-eyed X white-eyed
¯
F1 all red-eyed
F2 3:1 (red:white) but all white were male
explanation:
These genes are found on the X chromosome but not on the Y chromosome. An XrY male will therefore have red eyes. Details of this cross are below.
P1 XRXR X XrY
female male
gametes: XR (female) and Xr, Y (male)
The offspring produced from the above cross are crossed with each other (below):
F1 XRXr X XRY
¯
gametes: XR and Xr (from female); XR and Y (from male)
F2:
Notice that there are three possible genotypes for females and two possible genotypes for males.
Females
Males
Genotypes
Phenotypes
Genotypes
Phenotypes
XRXR
red
XRY
red
XRXr
red
XrY
white
XrXr
white
X-Linked Inheritance
Males inherit their X chromosome from their mother. Their Y chromosome comes from their father. A male, therefore, cannot pass an X-linked trait to his sons. Males inherit all of their X-linked traits from their mother.
If a male inherits an X-linked recessive trait, it will be expressed because males do not have a homologous X chromosome.
Females can be carriers of X-linked traits without expressing them because they might carry the dominant allele on the other X chromosome. For example, the following genotype will have a dominant phenotype: XAXa.
Dosage Compensation
Although females have twice as many X-linked genes, the amount of protein produced by these genes is the same in females as it is in males.
Reduced protein production (called dosage compensation) occurs as a result of inactivating one X chromosome by coiling and condensing it. When condensed, it cannot be transcribed, that is, it cannot be used to produce mRNA…
Condensed X chromosomes, called Barr bodies, are visible using ordinary light microscope techniques.
The table below shows the number of Barr bodies in normal cells and in the cells of people with an abnormal number of X chromosomes.
Genetic Condition # Barr Bodies per Cell
normal male 0
normal female 1
XXX female 2
XXXX female 3
XXY (Klinefelter male) 1
In summary, one X chromosome remains active, the others are inactivated by forming Barr bodies.
Inactivation
Inactivation occurs early in embryonic development (12-16 days).
In females, each cell normally contains two X chromosomes. The X chromosome that is inactivated is determined randomly.
Once inactivation occurs, all daughter cells of a particular cell have the same X chromosome inactivated.
All of the “pink” chromosomes in the drawing below (left side of diagram) have been inactivated. All future cells produced by this cell will have the pink chromosome inactivated. In the diagram on the right, all of the blue chromosomes have been inactivated. All future generations of this cell will have the blue chromosome inactivated.
Females are therefore mosaics with respect to the X chromosome. Patches of body cells will have the maternally inherited X chromosome inactivated and other patches will have the paternally inherited one inactivated.