Jennifer

I wrote this article for Jennifer, my white crested black Polish hen. Before I talk about crests and vaulted skulls, it’s important that I share Jennifer’s story.

Jennifer was among the 20 chicks I brought home in 2013—my original flock. Until recently, she’s been near the top of the pecking order. That’s an unusual status for a Polish hen, but not for Jennifer. This old girl is in her tenth year. She has outlived most of her peers. She has seen many new chickens join the flock. And one-by-one, Jennifer has put them in their place.

But one day last August I noticed that Jennifer wasn’t acting like her usual self. She seemed confused and spent the day staring into space. Her head would slowly droop until it was almost on the floor, then she would snap to attention. The she would repeat the same head-drooping action. Over and over. When I noticed her strange behavior, I knew that I would have to keep a close eye.

The very next day I found her pressed into a corner trying to make herself very small. She’d attracted the attention and the pecks of the flock bullies and she’d forgotten how to defend herself. When I reviewed coop cam footage, I discovered that she’d spent the night on the coop floor. She’d also forgotten how to get onto the roost. Jennifer needed her own space. I separated her from the flock.

At first, I couldn’t put my finger on Jennifer’s specific problem. I did wonder if time and age had finally caught up with this old bird. Maybe her time had come. But after watching her on her own for a week, I formed a new idea. Jennifer was eating, drinking, and appeared healthy. But she also spent all her time in one spot, staring at things that only she could see. She seemed mentally confused. I came to realize that Jennifer had sustained a brain injury.

When people think of Polish chickens, they think of those elaborate, beautiful crests. Most people don't realize that Polish chickens also have “vaulted skulls.” Vaulted skulls are misshapen, with a large knob or vault sticking up from the top. A misshapen skull results in a misshapen, unusual, “hourglass” shaped brain. Vaulted skulls are often filled with holes. The only thing protecting parts of the chicken’s brain is a bit of skin and some feathers. Vaulted skulls and large crests are probably related traits—two sides of the same coin.

Because of their fragile, misshapen skulls, Polish chickens are vulnerable to brain injury. And it can be fatal. Jennifer, fortunately, is not dead. But now, in February, months after her symptoms first appeared, Jennifer still has some major issues. She still eats, drinks, and appears healthy. She’s relearned how to get herself onto the roost every night. But she can’t figure out how to get down in the morning. So, I gently lift her down every day. She continues to live alone, and spends most of her day staring at nothing. She may be like this for the rest of her life.

I’ve been vaguely aware, for a while, of vaulted skulls in certain chicken breeds . But Jennifer’s condition started me on a quest for more information. If you find any of the information that I’m sharing here useful, you can thank Jennifer.

Polish Chickens and Their Crested Cousins

The American Poultry Association first recognized Polish chickens in 1874. But there are references to crested chickens in Europe, described as “polish” or “pollish,” at least as far back as the 16th century. The breed most likely had its beginnings in the Netherlands. There are many examples of crested chickens in 16th century Dutch paintings. There is some disagreement about the origin of the Polish chicken’s name. Most scholars agree that it has nothing to do with the country of Poland, but instead refers to the crest.

The word pol is a Middle Dutch word that means head or top. Pol found its way into English as “poll,” the process of “counting heads.” It’s not surprising that the Dutch would name these chickens for their heads. Their heads are the launching point of their crests; their most striking attribute.

There are, of course, other crested chickens. Crèvecœurs, Houdans, Appenzeller Spitzhaubens and Sultans. are a few of the better-known breeds. All Polish chickens and all other crested breeds that have been tested share the same genetic mutation. The mutation alters the expression pattern of the cluster of HOXC genes on chicken chromosome 33. I’ll say more about that later and it will make perfect sense. Don’t be afraid!

Since crests are an inherited trait, all crested chickens share a common ancestor - unless this mutation has occurred more than once. The crest mutation occurred a long, long time ago. The Roman author Claudius Aelianus included a description of crested chickens in De Natura Animalium, his 3rd century treatise on natural history. British archeologist Don Brothwell discovered physical evidence. In 1979 he presented a specimen found in in a rubbish pile in the buried ruins of a 4th-century Roman temple complex in Somerset, Great Britain. Feathers usually don’t survive for fifteen centuries. They did not in this instance. What Professor Brothwell found was a skull. And that skull had a vault; proof the skull belonged to a crested chicken. Vaulted skulls and crests do have a connection. That’s the point of this article. But I’m getting a bit ahead of myself.

The Genetics of Crests

It’s impossible to talk about the genetics of crests, or the genetics of anything, without using some of the terminology that geneticists use. As I explain the genetics of crests, I’ll also define some important genetics terminology. If you know this stuff, feel free to slide right past it. Or read it anyway. Because it’s fun!

Genes

are the basic unit of heredity. They’re built out of DNA and they contain all the information that make your chicken (and you, and everybody!) who they are. They come in pairs. Your chicken (and you, and everybody!) gets one gene from each parent for every trait. For example, think feather color. There’s a gene that determines whether a chicken is red, black, or some other color.

Mutation

is a word that everybody probably knows. Because something is always mutating in all those sci-fi movies. DNA can get scrambled during cell division, resulting in a mutation. Or sometimes DNA-damaging chemicals or radiation cause the mutation. Vaults are the result of a mutation. Sometime in the past, DNA moved around, and a baby chick hatched with a big bump on his head. Crests? Same story. Crested chickens have crests because their mutated Crest gene causes body feathers to replace the much smaller feathers that normally grow on their heads. Crested chickens show the same sex-related feather differentiation on their crests that most chickens show on their body feathers. Hens have large, neat, feathery bouffant hair-dos. Roosters sprout wild “troll king” crests with pointed feathers going every which way.

Alleles

are one of two or more alternative forms of a gene. Think about feather colors again. There’s an allele for red feathers. There’s another one for black feathers.

Homozygous

means that a chicken has inherited the same allele of a gene from both parents. Thus, a chicken getting two black feather alleles, one from each parent, will have black feathers. A chicken getting two red feather alleles will have red feathers. Makes sense, right?

Heterozygous

means the inheritance of different alleles of a gene. Thus, if a black rooster and a red hen love each other very, very much, get married and have chicks, their baby chicks will be heterozygous for feather color. Because each chick will get a black feather gene from their dad and a red feather gene from their mom.

Dominant

is about relationship between alleles. In heterozygous chickens, the allele that produces a physical trait is called the dominant gene. The black feather allele is dominant. So, all the babies of daddy black roo and mama red hen will be black. Even though they all carry copies of their dad’s back gene and their mom’s red gene.

Recessive

is the opposite of dominant. Recessive genes are masked in the presence of a dominant gene. Since the red feather gene is recessive, Red Hen’s and Black Roo’s babies are all black. Bear in mind though, that all those babies are heterozygous. What happens if one of those heterozygous chicks grows up and has babies with another heterozygous chicken? Some babies will get black feather genes from both parents and will be black. Some will get black feather genes from one parent and red feather genes from another parent. They will be heterozygous, like their parents. These chicks will also be black – black is dominant. But some chicks will get red feather genes from both parents and will be red, even though both parents are black. Exactly one fourth will be red. Why one fourth? Read on.

Incomplete Dominance

is a situation where both alleles of a gene are partially expressed. This results in intermediate physical characteristics. The Crest gene in chickens is incompletely dominant. Homozygous crested chickens have those beautiful full crests. But heterozygous chickens, with one homozygous crested parent and one noncrested parent, will have crests as well – just not as full or developed.

Doctor Punnett’s Square

Back in the olden days when I was in college, I took a Genetics 101 course. Picture me sitting on my three-legged stool in the drafty, candle-lit lecture hall rapidly writing notes on parchment with my quill. Those were the good old days!

One useful tool I learned about in that class of yore was devised by British geneticist, Dr. Reginald C. Punnett in 1905. The Punnett Square is a square diagram that geneticists and breeders use to predict the genotypes of a particular cross.

Here’s an example using Crest genes. In the diagram Cr represents the incomplete dominant Crest gene. And cr represents the recessive Noncrested gene. The Cr-Cr across the top of the box represents the alleles of one parent who is homozygous Crested. The cr-cr down the left side of the box represents the alleles of the other parent who is homozygous noncrested. Each square within the box gets assigned the allele for its row and its column. Thus, it gets one from the top and one from the side. In this case, each box gets a Cr from the top and a cr from the left side. So all four boxes end up with a Crcr gene set. This Punnett Square shows that the offspring of a homozygous noncrested chicken and a homozygous crested chicken will all be heterozygous. Since crests are an incompletely dominant trait, all offspring are crested. But those crests will not be full.

Here’s the Punnett Square for the offspring of two heterozygous chickens. In this case, two squares are Crcr – heterozygous. One square is CrCr homozygous. And square is crcr homozygous. Thus, we can predict that half the offspring of heterozygous chickens will have partial crests. A quarter will have full crests. And a quarter will have no crests.

Does the Punnett Square hold up in real life? Gregor Mendel, a Moravian monk, did genetics experiments with pea plants in 1866. This was long before Dr. Punnett developed his square and before anybody knew there were things called genes. And Mendel proved the same statistical outcome of traits that the Punnett Square generates.

In a 2012 study with chickens, heterozygous crosses of Silkies (crested) and White Plymouth Rock (noncrested) were mated. The 3000 offspring included 789 noncrested birds - not significantly different from the expected 750 (a quarter of 3000). Let’s have a round of applause for science and the Punnett Square!

Before we leave Dr. Punnett and move on to other things, it is worth noting that besides his square, Punnett also developed the Cambar – the first autosexing chicken. He also developed all the Legbar chickens, including the recently-extremely-popular Crested Cream Legbar.