Genes on the same chromosome are ‘linked’ and usually
inherited together. Two genes that are always inherited together
would be linked 100% of the time. However, linkage is never 100%
because crossover events occur when the body manufactures sperm
and egg cells. The figure below illustrates a crossover event.
The two bars on the left side represent two chromosomes having
three genes each. The genes in the middle ‘cross over’
during the process of sperm or egg cell formation. The end result
is that new ‘linkage’ relationships exist for the genes
on the chromosomes on the right.
Crossover events are actually very common. The rate of crossover
events occuring between a gene at locus A and a gene at locus B
is proportional to the distance between the two genes on the chromosome
(or equivalently, the crossover rate is proportional to the distance
between the two loci). A rule-of-thumb for the rate of crossover
events in poultry is 1% for every 10 map units in separation between
the genes. A map unit is a distance along a chromosome. The actual
distance in length units is not really relevant since all chromosome
maps are written with distance expressed in map units rather than
more familiar units of length.
In the Punnett diagram above describing the gene combinations for
two traits, silver and barring, the loci of the genes are linked
because they are both on the same chromosome, the Z sex chromosome.
This means that the wild-type genes, s+ and b+ genes of the red
and non-barred female will be inherited together most of the time.
Occasionally a crossover event will occur and the genes will be
inherited separately. So, while the Punnett square above gives all
the possible combinations of the four genes, it can not be used
to determine the percentages that the gene combinations will appear
in the progeny that arise from a cross between a male that is a
heterozygote for silver and barring and a red, non-barred female.
Since linked genes are inherited together, they can be treated
as single entities in the Punnett square. The Punnett diagram below
shows the inheritance of silver and barring in the mating of a red,
non-barred female and a male that is heterozygous for both barring
and silver. This corresponds to the larger Punnett diagram above,
but here I consider that fact that the genes are linked and I treat
them as a single object:
The results of this Punnett square (the gene combinations inside
the square) are what one would get from the mating if no crossover
events occurred. The number of times a given gene combination appears
inside the Punnett diagram divided by the number of squares in the
Punnett diagram is the probability or percent frequency of occurrance
of that combination in the progeny. For example, the males are 50%
barred and silver and 50% red and non-barred. The females are also
50% barred and silver and 50% red and non-barred. The red, barred
male progeny (B b+ s+ s+) that is indicated in the large Punnett
diagram above only occurs when a crossover event has taken place,
if the parents have the genotype we assumed.
This is an important point, namely that we assumed that the barred,
silver male had the B and S genes on the same chromosome. If his
genotype had been: Z1 = B s+ and Z2 = b+ S for his two Z chromosomes,
the red barred male would have been present in the progeny without
requiring a crossover event. The Punnett square for this mating
is:
In this mating, half the males are red and barred.
Inbreeding
There are varied opinions regarding the issue of inbreeding. One
school of thought contends that inbreeding is a negative thing and
brings about depression in traits such as fertility, hatchability,
rate of lay and others. Another school of thought maintains that
the negative aspects of inbreeding can be controlled and even eliminated
to a large extent through intelligent selection.
Several studies were conducted in the early part of the twentieth
century (for a brief synopsis, see Crawford, Elsevier, 1990, Chapter
39) that showed essentially disasterous results when full sibling
fowl were mated for several generations. However, even in the first
generation of progeny from full sibling matings in these early studies,
traits such as hatchability and rate of lay were seriously depressed.
These early studies are largely responsible for many people believing
that inbreeding in poultry is universally negative.
Other poultry enthusiasts are aware that inbreeding in plants is
a very successful strategy in developing hardy strains with desirable
traits. They also recognize that most lines of show-quality poultry
are inbred. Research performed in the 1970s and later (see Crawford)
on inbreeding in chickens (Leghorns), turkeys, quail, pheasants
and partridge fowl showed that desirable traits such as rate of
lay, hatchability and fertility can be selected for in inbred lines.
These traits can recover from the initial depression due to inbreeding,
sometimes even to the same level as the non-inbred lines. A 1988
study by Ameli and co-workers showed that long-term selection against
the negative effects of inbreeding can be successful in recovering
traits such as high rate of lay and fertility in Leghorn populations.
The depression in traits seen in (random, nonselective) inbreeding,
such as fertility, hatchability and rate of lay, is often due to
recessive genes. If the depression of these traits were due to dominant
genes, the depression would be expressed and observed in non-inbred
lines and would not be a phenomenon associated with inbreeding.
Epistasis or Epistacy (the interaction of genes at different locations
on chromosomes) is sometimes invoked to explain aspects of inbreeding
depression.
As of this writing, inbreeding experiments ongoing at the University
of Arkansas have associated the greater part of inbreeding depression
on hatchability to the male. The evidence for this is the following.
Inbred females were mated to a range of different males and the
hatchability of their eggs was observed. Inbred males were bred
to a range of different females and the hatchability of their eggs
were observed. The hatchability of eggs from inbred males was substantially
lower than the hatchability of eggs from inbred females, regardless
of the cross. So, for example, the hatchability of eggs from a father-daughter
cross in which the father is an inbred individual was about the
same as the hatchability of eggs from a mating of the same male
with non-inbred females. This is strong evidence that the inbreeding
depression of hatchability is largely a property of the male birds.
The fact remains that, if the backyard fancier allows inbreeding
to take place and does not actively select against the negative
effects of inbreeding, the entire population will perform at a lower
level with respect to fertility, hatchability, rate of lay and and
so on. On the other hand, the objective evidence is convincing that
it is possible to develop successful inbred lines of poultry through
active selection for desireable traits.
Gametes, meiosis, mitosis
This presentation of genetics tries to limit the use of jargon.
However, the interested reader may well want to participate in discussions
on the Poultry Genetics discussion board, for example, and will
need to know the meanings of some basic terms. Some have already
been defined, but others have not.
Cell Differentiation and Reproduction
In the early stages of the development of the embryo, the cells
proliferate as they must to grow the early embryo, but they remain
essentially identical in that there is no difference among the cells.
At a certain point, cells begin to differentiate into specific tissues.
Some make heart and circulatory cells, some make kidneys, liver,
intestines and so on. What controls cell differentiation is not
well understood.
A gamete is the ‘sex cell’. In other words it is the
sperm of the male or the unfertilized egg (ovum) of the female.
In general, the gamete has only half the chromosomes of a mature
individual.
Mitosis: There are two types of cell division
processes. One process, mitosis, is the division of mature cells
in the body…cells that have the full compliment of chromosomes
(two pairs of chromosomes).
The prophase is an initial organization phase in which the ‘centrioles’
(small centers from which fibers originate…small yellow squares
in the figure above) form and become organized. In the metaphase
spindle fibers emminate from the centrioles and attach to the chromosomes.
The anaphase is characterized by the separation of the chromosomes
by the spindle fibres and the centrioles…they essentially
pull the chromosomes apart. In the telophase the cell wall closes
and new cells are evident.
Meiosis: The process of cell division that produces
gametes or ‘sex cells’ (sperm and ovum) . The cells
that initiate meiosis contain the full set of chromosomes. However,
the process of meiosis yields gamete (sperm and ovum) cells that
have half that number of chromosomes. Which chromosomes of the original
ones find their way to the gamete cells is essentially a random
process. In this process, the chromosomes (of the chromosome pairs
of the parents) get mixed or ‘scrambled’ in a random
fashion. This is also the point at which crossing over of genes
from one chromosome of a chromosome pair to the other chromosome
can occur.
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