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Types of Rumpless Chickens

By Alan Stanford, Ph.D.
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Based Upon "The Genetics of the Fowl" by Frederick B. Hutt
Norton Creek Press ISBN 0972177035

Please refer to the definitions at the end of this article.
Anatomy and genetics are not difficult just hidden behind unusual words.

Rumpless birds appeal to many people for lots of reasons. Some people like the rumpless look, the Araucana people think these birds better escape predators, and others believe rumpless birds do well in fights. If you have a rumpless bird and want to raise more, it is important to know which rumpless type you have.

The simplest definition of a rumpless bird is a bird without a tailbone. Rumpless birds can have tail feathers but the standards of the American Poultry Association and the American Bantam Association do not allow tail feathers. There are many types of rumplessness in addition to the kind in these standards.

Rumplessness is difficult to understand because many different genes interact to determine whether a bird is rumpless. In addition, there are at least three distinct types of rumpless chickens: dominant, recessive, and accidental, and the dominant type can be divided into two types: dominant complete and dominant incomplete. If you know which of these types is a particular bird, you can make good decisions on how to raise more.

Unfortunately, you can't look at a living bird to learn its type of rumplessness; it is also very difficult to figure out the type of rumplessness when you dissect a dead bird. You can only disguish live accidental and genetic rumpless birds by breeding tests. However, rumpless birds from normal parents most likely are not genetic nor from a mutation.

Test matings have shown there must be a considerable number of genes preventing the dominant rumpless gene from producing rumpless birds. There are two interpretations of this effect.

1. Fischer suggested that whenever an undesirable dominant mutation occurs, individuals with genes that suppress the effects survive best. If a dominant mutation happens a lot, these modifying genes will accumulate and completely prevent the effects.

2. There must be a lot of genes forcing normal tail vertebrae . Gradually these genes accumulate and suffice to inhibit the dominant rumpless gene.

We have a rumpless Silkie from tailed parents; this might mean our rumpless Silkie's is the accidental type. However, the same parents produced three rumpless offspring in 2 years and no other parents produced any. This might mean our rumpless Silkie is (improbably) a mutation, is recessive rumpless, or that modifiers hid a parent's but not the offspring's rumplessness. Only test crosses will sort this out.

Rumpless Type Dominant Complete Dominant Incomplete Recessive Accidental
Anatomical Differences from Normal Tail
Outward Appearance No tail or tail feathers.
No outward visible difference between complete genetic and accidental rumpless.
Difficult to distinguish types by dissection.
More variable than other types. Known as "roachback".
Fused caudal vertabrae often bend downward and appear laterally compressed.
Some birds show curvature of the spine and humped backs.
No tail or tail feathers.
No outward visible difference between complete genetic and accidental rumpless.
Difficult to distinguish types by dissection.
Synsacro-caudal Vertabrae One or two vertebae missing from center of the 5 synsacro-caudal vertebrae. One or two vertebrae missing from center of the 5 synsacro-caudal vertebrae. No missing synsacro-caudal vertebrae. Two (rarely one) vertebrae missing from end of the 5 synsacro-caudal vertebrae
Bony Knob Small bony knob on last vertebra (caudal end of vertebral column ). Small bony knob on last vertebra (caudal end of vertebral column). Pygostyle and free caudal vertebrae commonly fused into a bony knot. No small bony knob on last vertebra (caudal end of vertebral column)
Pygostyle Some consider the small bony knob on last vertebra homologous with the last two pygostyle vertebrae. Some consider the small bony knob on last vertebra homologous with the last two pygostyle vertebrae. Sometimes absent, more commonly fused with free caudal vertebrae into a bony knot. No pygostyle
Uropial Gland No uropial gland 75% no uropial gland, less than 2% functional uropial gland Absent or rudimentary No uropial gland
Free Caudal Vertebrae No free caudal vertebrae Distinguishing feature: all five caudal vertebrae usually present but irregularly fused together. Free caudal vertebrae commonly fused with pygostyle into a bony knot No free caudal vertebrae
Retrices No retrices 2-17 retrices 0-17 retrices No retrices
Supernumerary Ribs None None Up to 17% of males and 8% of females have supernumerary ribs, mostly rudimentary, on one or both sides None
Genetic Basis Genetic Genetic Genetic Environmental, not transmitted to progeny.
Caused by accidents when embryo developed
Dominant or Recessive Dominant Dominant Recessive Not genetic
Genetic Symbol Rp Rp rp 2 None
Modifying Genes Sometimes unaffected by modifying genes Modifying genes can cause incomplete rumplessness even in birds homozygous for Rp Strongly influenced by modifying genes.
Low penetrance , only 33 to 38% of the offspring from two recessive rumpless birds are rumpless
Not genetic

allele - Alternate form of a gene. One of the different forms of a gene that can exist at a single locus.

caudal - 1) Of, relating to, or being a tail, 2) In or directed toward the hind part of the body.

chromosome - Linear or sometimes circular part of a cell that controls how a cell is built, functions, reproduces, interacts with its environment, and dies. Chromosomes contain most or all of an organism's genes.

dominant - An allele that determines phenotype even when heterozyrous. Also the phenotype controlled by a dominant allele.

gene - Unit of inheritance; usually part of a chromosome. Genes control how traits are expressed and transmitted to offspring.

genotype - Genetic make up of an organism, and is different from the organism's phenotype (physical appearance).

heterozygote - An organism with a heterozygous pair of genes.

heterozygous - Having different genes at corresponding loci (position) on the two homologous chromosomes; having different genes at the same spot on paired chromosomes.

homologous - Having the same relative position, value, or structure.

homozygote - An organism with a homozygous pair of genes.

homozygous - Having identical genes at corresponding loci (position) on the two homologous chromosomes; having the same gene at the same spot on paired chromosomes.

ilium - (plural ilia) a bone that is part of the hip, or pelvic girdle.

locus - (plural loci) Position of a gene on a chromosome.

lumbar - Of, near, or situated in the part of the back and sides between the lowest ribs and the pelvis. Of or relating to the vertebrae between the thoracic vertebrae and sacrum.

penetrance - The proportion of individuals of a particular genotype that express its phenotypic effect in a given environment.

phenotype - Physical appearance of an organism; different from the organism's genotype (genetic make up).

pygostyle or tailbone - A plate of bone at the posterior end of the vertebral column in most birds. Several of the last caudal vertebrae form the pygostyle. It supports the uropygium.

recessive - An allele that is not expressed when heterozygous but is only expressed when homozygous. Also the phenotype of the homozygote of a recessive allele.

retrix - (plural retrices) Tail feather, any of the stiff quill feathers in a bird's tail used to control the direction of flight.

sacral - Of or relating to the sacrum.

sacrum - The part of the vertebral column directly connected with or forming a part of the pelvis.

supernumerary - Exceeding the usual number.

synsacro-caudal - Of or relating to the synsacrum and caudal vertebrae.

synsacrum - The fused part of a bird's vertebral column (backbone) between the thoracic vertebrae and the caudal vertebrae (the pelvic region near the base of the tail). The synsacrum is fused vertebrae and includes some thoracic, lumbar, sacral, and caudal vertebrae. The number of vertebrae in the synsacrum varies from species to species. The ilium attaches to the synsacrum. The synsacrum probably developed for rigidity and so muscles can attach to it, possibly helps fine control of the tail and feet, and might have unknown functions for the feet.

thoracic - Of, relating to, located within, or involving the thorax.

thorax - the part of the body between the neck and the abdomen.

uropial gland or oil gland - A large gland in most birds. It opens toward the rear, is at the base of the tail feathers, and usually secretes an oily fluid, which the bird uses in preening its feathers.

uropygium - The fleshy posterior end supporting the tail.

vertebra - (plural vertebrae) Any of the bones or cartilaginous segments that form the vertebral column (or backbone or spinal column).

vertebral column - Backbone, spinal column.

Fischer, R. A. 1930 . "The Genetical Theory of Natural Selection." Oxford: Claredon Press.

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