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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