Written
by the Sellers family of Brookings, South Dakota
The purpose of this document is to provide
a poultry genetics reference to interested poultry enthusiasts who
may not have any formal training in genetics.
DNA, genes and chromosomes:
A gene
is a piece of DNA that carries information about a specific trait.
A chromosome
is a string of genes connected together (although most
of the chromosome is DNA that has no known function or no genetic
activity).
An allele
is a gene that is a member of a set of genes that all belong to
the same locus, or location, on a chromosome. These genes are often
thought of as being related to each other through mutations (one
allele could be a mutation of another allele) or they could be mutations
of an ancestor gene.
Chickens, like people, usually have two of every chromosome. The
chromosomes in a chromosome pair are not identical, since one comes
from each parent. A gene is said to be dominant when only one gene
(rather than two) is sufficient for the expression of that trait
to which the gene corresponds. Some genes are referred to as incompletely
dominant. The expression of these genes is inhibited by (usually
unknown) modifying genes. When the inhibiting, modifying genes are
not present, the incompletely dominant gene expresses. This interaction
with modifying genes is responsible for the seemingly random nature
of the expression of incompletely dominant genes.
The sex chromosomes are unique in that there are two types, a
long sex chromosome, the Z chromosome, and a short sex chromosome,
the W chromosome. The female has one long and one short sex chromosome,
she has ZW sex chromosomes. The male has two long sex chromosomes,
he has ZZ sex chromosomes. For this reason, the female has only
one copy of some genes that are on the long, Z, sex chromosome.
The genes that are not on the sex chromosomes are called ‘autosomal’
or autosomes. Both male and female chickens have two of these genes.
Chickens have 39 pairs of chromosomes (78 individual chromosomes).
Most of them are tiny and referred to as ‘dot’ or micro
chromosomes.
An important point is that, when we talk about adding or removing
a gene, say frizzle, F, we don’t intend that the chromosome
is lengthened or shortened by the addition or deletion of that gene.
Rather the frizzle gene, F, replaces the gene of the wild-type jungle
fowl, f+, when it is added, or, it is itself replaced by the wild-type
jungle fowl gene, f+, when frizzle is removed. I used the frizzle
gene as an example here, but the statement applies to all genes.
Generation notation:
The original members of a mating are referred to as the parental
(P) generation. The first generation of progeny from the parental
cross is referred to as the first filial generation, F1. The progeny
of a cross in which one or both of the parents are from the F1 generation
is an F2 generation (F1 x F1 = F2) and so on.
Homo / hetero / hemi – zygous…genotype and
phenotype
For the interested reader who might like to know the meaning of
these terms, I have included this brief description. A bird that
has one gene, rather than two, for a specific trait is said to be
heterozygous for that trait. A bird that has two genes for a given
trait is homozygous for that trait. The genotype is the actual set
of genes. The phenotype is the appearance or visual characteristics…what
you can see. For example, a bird that is heterozygous (has one gene
instead of two) for a given dominant trait may look the same as,
or similar to, one that is homozygous (has two genes) for that trait.
They both have the same appearance or phenotype. Because the female
fowl have differing sex chromosomes, the long one, Z, and the short
one, W, the Z chromosome has gene locations that the W chromosome
does not (see above). Sometimes when referring to these genes that
have no counterpart on the W chromosome, the female is said to be
hemizygous. Since the female can have only one copy of these genes,
there is an apparent overlap in the meanings of 'heterozygous' and
'hemizygous'.
How to predict the outcomes of breeding events for non-sex-linked
and sex-linked traits
Non sex-linked traits:
Both parents have two genes for a given trait. Let’s consider
the gene for frizzle plumage, F, and agree that we will represent
the lack of the frizzle gene with f+. The superscript ‘+’
indicates that the gene is present in the wild-type fowl which,
with respect to chickens, is the red jungle fowl. Here, I apply
the jargon immediately above, but will minimize the use of it from
now on. A bird is said to be heterozygous for frizzle if her genotype
is (F, f+) and homozygous if her genotype is (F, F). Since frizzle
is dominant, both genotypes will have the same (or similar) appearance
or phenotypes. (In this particular case, frizzle shows a 'dose effect'
and the frizzle homozygote has brittle feathers that usually break
off so the homozygotes can be almost bare. There is a common recessive
modifying factor, mf, that reduces the influence of the frizzle
gene.)
To determine the genetics of the offspring, one takes the four
possible combinations of the genes of one parent with the genes
of the other parent. For example, let’s consider a cross between
a bird that has two frizzle genes, homozygous for frizzle, (F, F)
and one that is without frizzle, (f+, f+). It helps with the bookkeeping
for our purposes here if we (artificially) number the genes: (F1,
F2) and (f+1, f+2) so that F1 is the first frizzle gene of the first
parent, F2 is the second frizzle gene of the first parent and so
on. The four possible pairs that can be made by combining these
genes are: (F1, f+1), (F1, f+2), (F2, f+1) and (F2, f+2). Since
frizzle is a dominant trait, these four gene combinations will result
in chickens with frizzle plumage (they will all have the same or
similar phenotypes). In practice one would not number the genes
as I have done in this paragraph. I numbered them to distinguish
the four combinations, since they are all genetically the same.
One would normally write: (F, F) crossed with (f+, f+) gives (F,
f+) times 4.
So, in order to get the four combinations
of the genes of the two parents, just take the first gene
of the first pair with each gene of the second pair, then
do the same thing with the second gene of the first pair.
The figure below illustrates how to get the combinations of
genes of one parent, (A, B), and the genes of another parent,
(C, D). The four possible combinations are (A, C), (A, D),
(B, C) and (B, D).
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Glossary: 
A gene
is a piece of DNA that carries information about a specific trait.
