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Essential
Genetics for the Terrified
by Chris Rutt
Part 9: Sex
& Linkage (1)
Last time we concluded with
the idea that some genes for colour mutations are carried on the sex
chromosomes, which we called "X" & "Y". One of these is known as
the American Cinnamon, which is referred to as a SEX LINKED RECESSIVE.
In order to explain its inheritance I need to approach the question
from the results we obtain when breeding.
We mate two birds of normal, Light Green phenotype, and find a ratio of
1:3 cinnamon birds to Light Greens in the F1 generation. We have seen
this ratio before. 3:1 is the proportion of colours we get when we mate
two genetically identical birds which are HETEROZYGOTES carrying a
dominant:recessive pair of genes. But the HEN of this pair comes from a
line of pure breeding Light Greens. The cock therefore must be carrying
some factor as a recessive. If you have remembered earlier sections you
will now be feeling confused.
If only one parent is passing a recessive gene to its offspring, it
cannot show in the phenotype in the examples we have examined so far. I
now want you to look at the formation of the actual letters "X" &
"Y", and get you to think of the "Y" as if it were an "X" with one of
its "legs" missing. "Whatever is the idiot on about?", I hear you cry.
"What on earth have letter shapes got to do with bird genetics?". Don't
give up yet! All will be explained.
Next I want to consider what happens to a gene on that leg of the "X"
which is missing in the "Y". American Cinnamon is such a gene. No
problem in a cock bird with its two "X" chromosomes. A simple recessive
pattern - if only one of the "X" chromosomes has the cinnamon gene the
cock will appear normal, but be a "split" and if both have it the bird
will show the cinnamon colouring. In the HEN however, you might be
tempted to think that since she can never have two "X"s, she can never
show the characteristic. NOT SO! In fact the recessive gene DOES show
in a single "dose" in a hen BECAUSE ITS DOMINANT PARTNER GENE IS NOT
THERE TO SUPPRESS IT. You can think of this as if the gene is carried
on that part of the "X" chromosome which is missing in the "Y". How can
we deal with this complication when drawing up our grids? We need to
incorporate "X"s & "Y"s AND show which "X"s are carrying these "sex
linked" genes. So, we will return to our pairing of a Cinnamon carrier
cock (= split for Cinnamon) to a pure normal Light Green Hen.
| Parents
Contribution |
(Xc) |
X |
| X |
(Xc)X |
XX |
| Y |
(Xc)Y |
XY |
In this case I have shown
the fact that the "c" for American Cinnamon is "tied" to the "X" by
putting them together inside brackets. What phenotypes have we
produced? How many birds can pass cinnamon to their offspring? Look
carefully at the grid, and remember to work out the sex of the bird.
(XX = Cock & XY = Hen).
Phenotypes first. Only one "dose" of cinnamon is possible to the
offspring, and the only birds that can possibly show the cinnamon in a
single dose are the hen chicks as we saw above. Hens have to have a "Y"
so are all in the bottom row, where we see that in half of these hens
the "X" has a "c" attached, so will be "visual cinnamons". Looking at
it another way, we know that any Hen which is not visually a cinnamon
does not have the gene for it. That may seem to be stating the obvious,
but it is a useful concept when trying to exclude cinnamon from a line
of birds whose other characteristics you wish to keep. Half the cocks
also have an (Xc) but these will NOT be visual, since the "NOT
cinnamon" gene IS present in the other X chromosome. These cocks are
therefore "splits" or "carriers" but indistinguishable from their
brothers who have no cinnamon gene. So we have the useful situation
that we know that any visual cinnamon chicks from this pairing MUST BE
HENS.
Before you read the next section of this genetics primer, work out the
result of pairing a Cinnamon Hen to a Homozygous Light Green Cock, and
to a "Cinnamon Carrier".
Part
10: Sex & Linkage (2)
All rights reserved
© 1997-2000 Chris Rutt
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