Another way to determine the 4 different combinations
of the genes for a trait (two from each parent) is to use a Punnett
diagram. In this method one makes a square that is divided into
four quadrants. Write the genes of one parent across the top with
one gene over each quadrant as in the figure below. Write the genes
of the second parent along the left side. In the figure below, one
parent has genes X and Y, while the other parent has genes A and
B. This is a slightly different notation than I used in the previous
drawing. Now, in each quadrant write a gene from each parent. Write
the gene from each parent that is physically nearest the quadrant
you are presently working in. Another way to say this is to put
the 'A' gene in the top row of quadrants, the 'B' gene in the bottom
row of quadrants, the 'X' gene in the first column of quadrants
and the 'Y' gene in the second column of quadrants. Hopefully the
figure below makes this process clear
Sex-linked traits:
Genes responsible for sex-linked traits are on the
long sex chromosome, Z, but not the short sex chromosome, W. For
that reason, I prefer to use a blank or underscore to represent
the missing gene when representing the genes (genotype) of the female.
Some people write a 'W' to indicate that the presence of the W sex
chromosome which lacks the locus for the gene in question. Rather
than writing (B, -) to represent a barred female, some authors write
(B, W) or B/W with the slash separating the gene symbols for the
different chromosomes.
A Punnett diagram (or Punnett square) for determining
the mating outcomes for sex-linked traits is below:
In this example the fact that
the female can have only one gene is represented with a dash
or underscore in one column of quadrants. The dash or underscore
actually represents the W chromosome which is what makes her
female. The other gene symbols represent genes that are all
allelic to each other (that means that they all belong to
the same location on the chromosome). The chicks that 'inherit
the dash' really inherit the W sex chromosome and are therefore
female. In the example in the Punnett square above, the (A,_)
and (B,_) chicks are the females.
As an example of sex-linked barring (I use it a different
way below), the A and B gene could both represent the wild-type
gene, b+, which is lack of barring. The X gene could represent the
barring gene, B. So this is a mating between a barred female and
a nonbarred male. The Punnett square predicts that all the male
chicks will be barred [(B, b+) genotype] and the females will all
be nonbarred [(b+, _) genotype]. Punnett square below is this mating
example
Saying the same thing a little bit differently, Cuckoo
barring or sex-linked barring, B, is one of these genes that is
located on the Z chromosome (recalling that the male has two Z sex
chromosomes and the female has one Z and one W sex chromosome and
there is no barring gene on the W chromosome so females can have
only one). A male with two copies of the barring gene might be represented
as (B, B). This is his ‘genotype’ with respect to barring.
A male with one copy is represented as (B, b+) where lower-case
b+ indicates a lack of the barring gene and the b+gene is the gene
that the wild red jungle fowl has instead of the barring gene, B.
A male with no barring is represented as (b+, b+). Since the female
has one long chromosome and one short chromosome and the barring
gene is on the long chromosome and not the short one, a female can
have only one copy of the barring gene. A female with barring is
represented as (B, _). A female without barring is represented as
(b+, _). The underscore indicates her short chromosome lacks the
locus (location) of that gene.
I distinguish between the terms ‘sex-linked’ and ‘sex-indicating’.
A gene is sex-linked when that gene is on the long sex chromosome
and not the short sex chromosome. A sex-indicating trait is one
that arises from a sex-linked gene if the cross is carried out properly.
The following example indicates this.
When a barred female, (B, _) is crossed with a non-barred male,
(b+, b+), the four possible outcomes (see the figure above) are:
(B, b+), (b+, _), (B, b+) and (b+, _). Here I have written all four
combinations even though some are the same. The order of writing
the genes of the pair is usually to write the dominant gene first
and the blank last. Of the four possible outcomes, the males are
barred and the females are non-barred. So, when the cross is carried
out this way, barred female x non-barred male, the barring is a
sex-indicating trait, and indicates male offspring.
When a barred male, (B, B) is crossed with a non-barred female,
(b+, _), the four possible combinations of the genes are: (B, b+),
(B, _), (B, b+) and (B, _). Therefore all the chicks will be barred.
The barring is still a sex-linked gene, but the cross was carried
out in a way that leads to both males and females being barred.
In this situation the barring is not indicative of the sex of the
offspring.
The Sil-Go-Links (for silver-gold-sex-link) are similar except
that the dominant sex-linked gene is the silver gene, S, which has
the function of inhibiting the red pigment, pheomelanin. The lack
of the silver gene is represented with lower-case s+. Here again,
s+ is the gene that the wild-type fowl or red jungle fowl has instead
of the silver gene. Crossing a red male lacking silver, such as
a Rhode Island Red (s+, s+), with a silver female, such as a Delaware
(S, _), gives (S, s+), (S, s+), (s+, _) and (s+, _). So, the males
are silver (which means mostly white) and the females are red and
can be sexed after hatching. Carrying out the cross the other way,
a silver male (S, S) and a red female (s+, _) gives as possible
combinations: (S, s+), (S, s+), (S, _) and (S, _) so that the red
pigment in both males and females is inhibited and they will be
mostly white.
Black sex-links can be made by crossing Barred Rock females with
a red male, such as Rhode Island Red or New Hampshire Red. The Barred
Rock females have one barring gene, (B, _) and should have two nigrum
genes (E, E). The nigrum gene extends black by changing red to black.
The red male will be (b+, b+) for barring and lacking in nigrum,
(e+, e+). Here the lack of nigrum is represented with the symbol,
e+, which indicates that, instead of the nigrum gene, E, the bird
has the wild-type gene that the red jungle fowl has.
With respect to the barring, the four combinations of the genes
are: (B, b+), (B, b+), (b+, _) and (b+, _). So the males are barred
and the females are not. With respect to the nigrum gene, all four
combinations are the same, (E, e+). So, for this set of genes, all
the chicks will be black and the males will be barred.
/
Set As Homepage 
/ Copyright
/ Disclaimer / Top 
Glossary: 
