This Australian Shepherd puppy's father is a black-tri; mother is a blue merle. What would cause this dog to have so much white color?
Are there any general explanations as to why this might happen in nature? Other examples of this kind would also be greatly appreciated.
Cat coat genetics
Cat coat genetics determine the coloration, pattern, length, and texture of feline fur. Understanding how is challenging because many genes are involved. The variations among cat coats are physical properties and should not be confused with cat breeds. A cat may display the coat of a certain breed without actually being that breed. For example, a Siberian could wear point coloration, the stereotypical coat of a Siamese.
Why Do Some People Have Curly Hair and Others Straight?
Either environmental or sexual selective pressure began acting on hair after humans began dispersing out of Africa.
A Curious Reader asks: Why do some people have curly hair and others straight?
The short answer is: selective pressure acting on the genes responsible for hair type. Humans evolved on the African continent, and from there dispersed to the rest of the world. Muthukrishnan Eaaswarkhanth and colleagues, writing in Human Biology, noticed a trend in human hair genetics while testing out a new analytical technique.
Eaaswarkhanth et al. noticed significant changes in certain genes of the KAP cluster. These genes do not directly cause hair curl, but are responsible for keratin structure. Thus, they play a significant role in the final shape of an individual’s hair. The group’s analysis revealed that certain variations in the KAP cluster genes are present in African populations but virtually absent elsewhere. They suggest that either environmental or sexual selective pressure began acting on hair after humans began dispersing out of Africa, helping shape the wide range of human hair types present today. The limited evidence points to temperature as a factor. Curly hair may help keep the head cool in warm climates. These days, environmental pressure is less of an issue, but cultural factors may impact the prevalence of curly hair in a population over time. Ethnicity is not connected.
Selection explains why different hair types exist at all, but for any given person, inheritance studies suggest that curly hair mostly follows the rules of Mendelian genetics. Curly hair is dominant, so someone is more likely to have curly or wavy hair if at least one of their parents does. Recent research points to trichohyalin, a protein in hair follicles, as having primary influence over hair curl. However, there are many genes contributing to hair curliness, most of them unknown. Ultimately, hair texture remains a phenomenon that, like so many in genetics, is not yet fully understood.
Fur genetics - what are the causes of fur colour decision? - Biology
R ecently I have had a number of requests for information on rat genetics. While there is quite a bit of very good information out there, none of it covers the more recent colors which are so popular with AFRMA members. Because of the interest in breeding these colors I have repeatedly been asked for my opinion on their genetics, therefore I will attempt to cover these to the best of my abilities.
I am not a geneticist, just a hobbyist with a particular interest in this aspect of the rat fancy. I have based this article on the writings of other authors (in particular those of Ann Storey and Nick Mays) and I highly recommend you reading their original works. Unfortunately, most of the more recent colors which have appeared in AFRMA breeders&rsquo stock have not yet made their way to the U.K., and those authors who know the most about this topic have not had the opportunity to work with them. These new colors have not appeared in any of the scientific literature that I have been able to find (please let me know if they have, my genetics resources are very limited.) The information I am presenting on the newer colors is based on my own theories, as well as those of other breeders, and is at best an educated guess which may well change as we get more information. I will denote these with a * whenever I talk about them.
Genetics is not some mystical system for predicting the color of animals, nor is it an elaborate mathematical equation which can only be solved by a brilliant scientist. Instead it is a straightforward way of finding out the likely results of breeding two animals of known or suspected background.
In this article I am not going to go into the mechanics of how genetics work, because it has already been done quite a few times by people who are much more knowledgeable (and better writers) than I will ever be. Instead I will be discussing the specific loci involved in determining the colors, markings, coat types, and other features we recognize in fancy rats. I recommend that anyone who does not already understand how genetics work, or anyone who needs a brush up on the topic, read Nick Mays&rsquo book The Proper Care of Fancy Rats, pp. 226&ndash236 before going any further.
A rat&rsquos color is caused by the pigment in its hair. There are two basic pigments, eumelanin which causes black/brown color, and phaeomelanin which causes yellow/red color. The distribution and density of these two pigments throughout a rat&rsquos coat is what causes it to be a particular color.
The distribution and density of pigment in a rat&rsquos coat is dictated by various genes. The position which these genes occupy on the chromosome is called its locus. Each locus is responsible for a distinctive effect on the pigment in the coat, and all the loci taken together &ldquospell out&rdquo what the rat will look like.
Polygenes are also important to be aware of. These &ldquomodifying genes&rdquo cause all the variations of each color. They make the difference between a good black and a bad black, determine the amount of white on our marked animals, and control the amount of silvering. Once you breed a color you then select for the correct shade of that color, and in doing so you are selecting for the combination of polygenes you want.
LOCI Which Regulate
The Agouti locus controls the distribution of yellow pigment throughout the coat.
A&ndashBanded hairs (Agoutis)
a&ndashSolid hairs (Selfs)
AA and Aa animals have a band of yellow near the top of each hair, thus giving the hair a striped appearance. This causes the agouti effect and can be seen on Agoutis, Cinnamons, and Blue Agoutis (and others). aa animals are non Agouti (self) and have hairs which are solid colored (no band). These animals include Black, Chocolate, Blue, Champagne, and Lilac.
This locus simply determines whether eumelanin is black or brown.
B causes eumelanin pigment in an animal&rsquos coat to be black. We see this in Agouti and Black animals.
bb animals are brown, a color we call Chocolate. Chocolate is recessive to Black, and as babies they are very distinct when compared with their Black littermates. Unfortunately, Chocolate varies immensely, and dark ones often look like poor Blacks as adults. Chocolate Agoutis are dark Cinnamons who have chocolate for a base color, and are too dark to show. Chocolate Blues (bbdd and bbgg) are a very pretty silver color with no blue in it, a color AFRMA calls Platinum.
C causes the dilution of both yellow and black pigment but each is affected differently. There are three known and a forth suspected allele at this locus. Yellow pigment is completely eliminated by the third one, but black pigment shows a more gradual reduction, not being completely eliminated until the fourth allele.
C&ndashFull color (no dilution)
C ch &ndashChinchilla
c h &ndashAcromelanism (Siamese)
c&ndashAlbino (Pink-Eyed White)
In other animals c ch c ch is chinchilla, basically an Agouti animal with the brown in its coat diluted to cream or white. AFRMA currently has an animal in its Standards which fits this description (Chinchilla*) however, in recent years few have been bred. I am placing them here based on their appearance rather than any genetic evidence, and I am very interested in hearing from anyone who breeds this color. If this truly is Chinchilla, it opens up a whole realm of possibilities, like Blue Chinchilla and Sable Siamese. This gene appears to be linked with the Berkshire gene, and Chinchillas are often very good Blaze Berkshires. Those who were around when this color first appeared say that it brought with it Lynx, so the two may be related. Some of these animals have been exported to England, and in the November/December &rsquo95 Number 90 issue of Pro-Rat-A, Ann Storey reports that &ldquoIt (c ch ) has a slight bleaching effect on black. This gene appears incompletely dominant to C. AACc ch rats resemble bleached and color-paled Agoutis and aaCc ch are poor blacks.&rdquo
c h c h causes yellow pigment to disappear, and dilutes black to a warm brown. It also causes acromelanism (fancy name for the Siamese pattern). This pattern is created because the color of this animal&rsquos fur is determined by the temperature. The cooler it is, the darker the fur grows therefore, the extremities (ears, nose, feet, and tail) are dark, and the rest of the body is light. Currently we have Seal Point Siamese (aaBBc h c h ) a beige rat with sepia points Blue Point Siamese* (aaBBc h c h gg), ivory with light blue points and Russian Blue Point Siamese* (aaBBc h c h dd), gray with dark blue points. Potentially almost any non-yellow color could be combined with Siamese. Because yellow completely disappears, we can never create Fawn Point Siamese (in cats this would be Flame Point).
c h c further dilutes the rat&rsquos color causing a white body with lighter points, what we call Himalayan.
In cc animals, both yellow and black pigment is completely diluted. Since both pigments are eliminated, this animal appears white with pink eyes, an albino. Albino animals do carry other colors, though we don&rsquot see them because all the pigment is gone, and depending on what alleles they have at the other loci, they can produce any other color.
This locus causes pigment to be globbed together in big clumps instead of being distributed in the normal, even manner. This lightens the appearance of the color despite the fact that there is actually more pigment in each hair. The pigment in these animals usually gathers at the base of the hair, and the tips tend to be light, which gives a ticked/silvered effect.
D&ndashNormal color (normal distribution of pigment)
d&ndashDiluted color (clumped pigment)
dd animals, on a black background, are what AFRMA calls Russian Blue*. These animals are very close in color to Blue mice or Russian Blue cats. In many other animals this is the only locus that causes a blue color, but in rats we appear to have two separate blues&mdashRussian Blue and Blue* (I will deal with Blue next). This is a new color so there hasn&rsquot been much experimenting but Russian Blue Point Siamese* has been created (very pretty) and it is possible that Silver Blue* may be created by combining Russian Blue and Blue. Russian Blue originally appeared out of breedings for Velvet-furred rats.
Don&rsquot go looking for this one in the genetics books, I made it up. To all appearances we have two loci that produce a blue color in rats. Multiple blue genes have been reported in hamsters and mice, but not in rats to my knowledge. (If anyone knows of further information on multiple blue genes in single species of animals, please let me know.) Our Russian Blue corresponds best to the &ldquoMaltese&rdquo blue in other animals, which leaves the genetics of our Blues in question. I do not know what exactly this locus does to pigment, other than diluting black to a blue-gray.
G&ndashFull color (no dilution)
g&ndashGray (black pigment diluted to blue-gray)
aaBBgg animals are a very pretty, even, gray color which can range from very light to very dark, a trait which it shares with Mink/Lilac. AFRMA standardized the darkest shade and calls it Blue while the lightest shade is called Powder Blue (unstandardized) and the medium shade is referred to as Sky Blue (unstandardized). gg has also been combined with Agouti creating Blue Agouti. These animals have the black band in their fur diluted to blue, but there is little or no effect on the yellow band, creating a very pretty combination. Blue has also been crossed with Siamese and the resulting Blue Point Siamese are a very light ivory with pale blue points. It is interesting to note that in the U.K. there have been many major health and temperament problems reported in their Blues. In the U.S. this does not appear to be the case, though many of our Blues do tend to be overly sensitive to fat/protein in their diet and there have been some problems with scabs. It is proving fairly easy to breed this problem out, and it responds well to simple selection.
(Because &ldquoRussian Blue&rdquo corresponds more closely to the blues in other animals which are labeled D I put it there and created a new place for &ldquoBlue.&rdquo In Ann Storey&rsquos article, &ldquoBlue&rdquo is represented as D.)
- &ldquoGenetics of Fancy Rats&rdquo by Ann Storey. Pro-Rat-A, The National Fancy Rat Society Journal, November/December 1995, Number 90, pp.7&ndash11 (This is the most current and up to date information available on the genetics of fancy rats. She includes much more information than I have been able to present and I highly recommend anyone interested in this subject get a copy. There will be a more detailed version of this article appearing in the next NFRS Handbook which will be out in 1997. I am anxiously awaiting its publication.)
- &ldquoGenetics&rdquo by Ann Storey, National Fancy Rat Society Handbook, a Pro-Rat-A publication 1991, pp 30&ndash35. (Also published in various issues of Pro-Rat-A and AFRMA Rat and Mouse Tales, this article was the old standby for genetics information. It contains most of the basics and is well worth reading.)
- The Proper Care of Fancy Rats by Nick Mays, T.F.H. publications, Inc., 1993 pp. 226-53. (Contains an excellent section on how genetics work and is a must read for anyone just being introduced to the world of genetics.)
- &ldquoGenetics for Mouse Breeders&rdquo series by Bonnie Walters, published in various issues of AFRMA Rat and Mouse Tales. (If you ignore the fact that it is talking about mice, this is the best information on how genetics work that is available. This series of articles contain the most complete and detailed explanations of the workings of genetics, how and why breedings produce what they do, and what to expect when crossing one thing to another. If you ignore the information on color genetics, which are different for mice than they are for rats, it&rsquos a very valuable resource.)
- The Inheritance of Coat Color in Dogs by Clarence C. Little, Sc. D., Howell Book House 1971.
Copyright © 1995&ndash2021 American Fancy Rat and Mouse Association
All text, artwork, and photos are copyright to AFRMA, and/or the author, artist, or photographer.
Unauthorized copying of any part constitutes a breach of copyright law.
Most aspects of the fur phenotypes of common cats can be explained by the action of just a few genes (Table 6-2). Other genes, not described here, may further modify these traits and account for the phenotypes seen in tabby cats and in more exotic breeds, such as Siamese.
For example, the X-linked Orange gene has two allelic forms. The O O allele produces orange fur, while the O B alleles produce non-orange (often black) fur. Note however, that because of X-chromosome inactivation the result is mosaicism in expression. In O O / O B female heterozygotes patches of black and orange are seen, which produces the tortoise shell pattern (Figure 6-13 A,B). This is a rare example of co-dominance since the phenotype of both alleles can be seen. Note that the cat in part A has short fur compared to the cat in part B recessive alleles at an independent locus (L/l) produce long (ll) rather than short (L_) fur.
Figure (PageIndex<13>): Representatives of various fur phenotypes in cats: Tortoise shell (A,B) pigmentation in cats with short (A) and long (B) fur black (C) and grey (D) cats that differ in genotype at the dilute locus. The pure white pattern (E) is distinct from piebald spotting (F). (flickr-cwbuecheler&ndashCC:AN, flickr-=-.0=-CC:AND, flickr-Leonardo Zanchi CC:AS, flickr-malfet_ CC:AN, flickr- valentinastorti-CC:AN, flickr- Denni Schnapp-CC:AS)
Alleles of the dilute gene affect the intensity of pigmentation, regardless of whether that pigmentation is due to black or orange pigment. Part C shows a black cat with at least one dominant allele of dilute (D_), in contrast to the cat in D, which is grey rather than black, because it has the dd genotype.
Epistasis is demonstrated by an allele of only one of the genes in Table (PageIndex<2>). One dominant allele of white masking (W) prevents normal development of melanocytes (pigment producing cells). Therefore, cats with genotype (W_) will have entirely white fur regardless of the genotype at the Orange or dilute loci (part E). Although this locus produces a white colour, W_ is not the same as albinism, which is a much rarer phenotype caused by mutations in other genes. Albino cats can be distinguished by having red eyes, while W_ cats have eyes that are not red.
Piebald spotting is the occurrence of patches of white fur. These patches vary in size due to many reasons, including genotype. Homozygous cats with genotype ss do not have any patches of white, while cats of genotype Ss and SS do have patches of white, and the homozygotes tend to have a larger proportion of white fur than heterozygotes (part F). The combination of piebald spotting and tortoise shell patterning produce a calico cat, which has separate patches of orange, black, and white fur.
|fur length||short||LL or Ll||L is completely dominant|
|all white fur (non-albino)||100% white fur||WW or Ww||If the cat has red eyes it is albino, not W_. W is epistatic to all other fur color genes if cat is W_, can&rsquot infer genotypes for any other fur color genes.|
|piebald spotting||> 50% white patches (but not 100%)||SS||S is incompletely dominant and shows variable expressivity|
|< 50% white patches||Ss|
|no white patches||ss|
|orange fur||all orange fur||X O X O or X O Y||O is X-linked|
|tortoise shell variegation||X O X B|
|no orange fur (often black)||X B X B or X B Y|
|dilute pigmentation||pigmentation is intense||Dd or dd||D is completely dominant|
|pigmentation is dilute (e.g. gray rather than black cream rather than orange light brown rather than brown)||dd|
|tabby||tabby pattern||AA or Aa||This is a simplification of the tabby phenotype, which involves multiple genes|
Health status by gender, hair color, and eye color: Red-haired women are the most divergent
Red hair is associated in women with pain sensitivity. This medical condition, and perhaps others, seems facilitated by the combination of being red-haired and female. We tested this hypothesis by questioning a large sample of Czech and Slovak respondents about the natural redness and darkness of their hair, their natural eye color, their physical and mental health (24 categories), and other personal attributes (height, weight, number of children, lifelong number of sexual partners, frequency of smoking). Red-haired women did worse than other women in ten health categories and better in only three, being particularly prone to colorectal, cervical, uterine, and ovarian cancer. Red-haired men showed a balanced pattern, doing better than other men in three health categories and worse in three. Number of children was the only category where both male and female redheads did better than other respondents. We also confirmed earlier findings that red hair is naturally more frequent in women than in men. Of the 'new' hair and eye colors, red hair diverges the most from the ancestral state of black hair and brown eyes, being the most sexually dimorphic variant not only in population frequency but also in health status. This divergent health status may have one or more causes: direct effects of red hair pigments (pheomelanins) or their by-products effects of other genes that show linkage with genes involved in pheomelanin production excessive prenatal exposure to estrogen (which facilitates expression of red hair during fetal development and which, at high levels, may cause health problems later in life) evolutionary recentness of red hair and corresponding lack of time to correct negative side effects or genetic incompatibilities associated with the allele Val92Met, which seems to be of Neanderthal origin and is one of the alleles that can cause red hair.
Conflict of interest statement
Competing Interests: The authors have declared that no competing interests exist.
Fig 1. Gradations of hair redness: Population…
Fig 1. Gradations of hair redness: Population frequencies for men and women.
Fig 2. Eye colors: Population frequencies for…
Fig 2. Eye colors: Population frequencies for men and women.
Fig 3. Incidence of any cancer by…
Fig 3. Incidence of any cancer by gradation of hair redness, for men and women.
Fig 4. Mean seriousness of cancer by…
Fig 4. Mean seriousness of cancer by gradation of hair redness, for men and women.
The gene for curly hair in Caucasians
It has been long established that curly hair is a dominant trait in Caucasians and straight hair is recessive. This means two things: 1) if a person carries one allele for curly hair and another for straight hair, this person will have curly hair 2) curly hair is a simple trait and is most likely determined by one single gene. However, a single gene has yet to be found to be solely responsible for the curly (or straight) hair trait in Caucasians.
A recent genome wide association scan has found a SNP (single nucleotide polymorphism) called rs11803731 in the TCHH gene which accounts for about 6% of hair curliness. The TCHH gene encodes a protein called trichohyalin, which is known to be expressed at high levels in hair follicles and has been shown to be involved in the cross-linking of the keratin filaments found in hair. The ancestral allele of this SNP (the A-allele) is present in the worldwide population. Sometime during human history, a mutation lead to the emergence of the T-allele (called the derived allele in Fig. 1A). The T-allele causes an amino acid to change from leucine to methionine at position 790 of the TCHH gene. The function of this change is not clear. Nevertheless, Caucasians carrying the T-allele are more likely to have straight hair (about 70%) than those without the T-allele (about 50%) . From this we can infer that curly hair is the ancestral trait while straight hair evolved much later.
What to know about Genetic Health Risk reports
Possible test results
Variant(s) not detected
You do not have the variant(s) we tested. Since these tests do not include all variants that may impact your risk of developing a condition, you may still have another variant that could affect your risk. Non-genetic factors may also affect your risk.
You have one or more of the variants we tested. You may be at increased risk for the condition based on this result. This does not mean you will definitely develop the condition. Other factors may also affect your risk.
Result not determined
Your test result could not be determined. This can be caused by random test error or other factors that interfere with the test.
In some cases, the laboratory may not be able to process your sample. If this happens, we will notify you by email and you may request one free replacement kit.
Other companies offering genetic risk tests may include different variants for the same health condition. This means that it's possible to get different results using a test from a different company.
What to do with the results
If your report says you have variants associated with increased risk
- Consider sharing the result with a healthcare professional.
- Certain results, such as having a variant detected for the BRCA1/ BRCA2 (Selected Variants) report, may warrant prompt follow-up with a healthcare professional, since effective options may exist to prevent or reduce risk for disease. Each report will provide more specific guidance.
- Consider sharing your results with relatives. They may also have these variants. Keep in mind that some people may not want to know information about genetic health risks.
If your report says you do not have any risk variants detected
- Continue to follow screening and other healthy behaviors recommended by your healthcare provider. This is because our reports do not cover all factors that might influence risk.
Concerned about your risk?
- If you have other risk factors for the condition, you should discuss the condition with a doctor.
- You can also discuss your results with a genetic counselor (this link takes you to a page managed by the National Society of Genetic Counselors to find a genetic counselor near you: http://www.aboutgeneticcounselors.com/).
Genetic Health Risk reports are intended to provide you with genetic information to inform conversations with a healthcare professional. These reports should not be used to make medical decisions. Always consult with a healthcare professional before taking any medical action.
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I'm 13 and have brown, straight, slightly textured hair. My mum has black straight hair while my dad had black very curly hair. Could my hair become curlier as I grow up or will it remain straight?
- A middle school student from New Zealand
It’s possible that your hair could change as you get older. But it also might not! Let’s look first at why you have wavy hair, and what causes different hair textures.
The texture of our hair is mainly determined by our DNA. And a child can have a different hair texture than their parents! Unlike with some traits, you can get a hair texture that is a little bit of both parents. This means that if someone with curly hair and someone with straight hair have a child, this child would have wavy hair.
But what causes curly or wavy hair in the first place? When hair grows, it comes out of tiny holes (or “pores”) in our skin. These pores are called hair follicles. The texture of a person’s hair is determined by the shape of their hair follicles.
Your follicle shape is partly determined by what genes you have. Your DNA might cause you to have perfectly round follicles. In this case you would have straight hair. Or your DNA might cause you to have oval-shaped follicles, leading to curly hair. Or you might have a little bit of round follicle DNA and a little bit of oval follicle DNA. In this case, your follicles would be a little bit in the middle! You would have wavy hair.
The shape of your hair follicle determines if you have straight, wavy, or curly hair
Genes can turn on and off
While we know genes influence hair texture, scientists still don’t know all of the genes that contribute to hair texture or what exactly those genes do.
But we do know a little bit about how these genes can get turned on or off. While a person might have cells with the instruction manuals or “genes” for curly hair, those instruction manuals might not always be in use. If a cell is not using an instruction manual, we call the gene “turned off.” The genes for making hair curly might turn on or off over the course a person’s lifetime.
Genes can turn on and off for lots of different reasons. These factors are not even completely understood by scientists! But we do know that hormones can turn genes on and off.
Several hormones have been implicated causing changes in hair texture. Hormones are chemical signals that the body uses to send messages between body parts. Your hormone levels can change a lot during your life.
Your hormones change a lot when you’re still growing, especially once you hit puberty. But it can also change when you’re an adult! Changes in age, nutrition, temperature, sun exposure and various other factors can cause our bodies to change the amounts or types of hormones we make.
Some women report that their hair became thicker and glossier during pregnancy. This might be a result of increased levels of estrogen and progesterone in the body.
Why does hair become curlier in humid climates?
You may have noticed that hair tends to curl or wave when it’s humid outside. This has less to do with genetics and more to do with the physical properties of hair.
Humid air has a lot of water molecules floating around in it. Each water molecule has a slightly positively charged side and a slightly negatively charged side. Water molecules are just like tiny magnets! The proteins in your hair are the same way. So when you walk into a humid room, it’s like you’re surrounding your hair with tiny magnets pulling your hair in different directions. This can make your hair curly or wavy that day.
In humid climates, your hair can become wavy or frizzy
Image from pxhere
The genetics of merle coat patterns in dogs
Merle coat patterns, prevalent in breeds like Dachshunds, Great Danes, and Collies, can vary greatly. In this blog, researchers from Clemson University examined how these coat pattern varieties differ on the genetic level.
Like other domesticated species, dogs show great diversity in the coloring and patterning of their coats. During domestication and breed formation, genes responsible for these phenotypes underwent strong selective pressure, including the pigmentation gene PMEL (aka SILV). PMEL is a protein found in specialized organelles responsible for producing the black and brown pigments that give hair its color.
The genetics of the merle coat
Heterozygosity for a Short INterspersed Element (SINE) insertion in canine PMEL causes a striking pigmentation pattern, known as merle, that is unique to domesticated dogs. A SINE is a type of retrotransposon, a “mobile” DNA element that can be copied and pasted into a new location in the genome. The most common canine SINE is about 200 base pairs (bp) long and has a stretch of adenine bases at its tail end. The insertion of the PMEL SINE is an event that occurred once in a common ancestor of the diverse breeds that have the phenotype, which include several herding breeds, Great Danes, and Dachshunds.
A standard merle coat has two characteristics: a diluted base color and random patches of full pigmentation. In recent years, two spontaneous variations of merle have been recognized: dilute and harlequin. Dilute merles have a milder coat dilution with no patches. Harlequin merles have a white background with large patches of full pigmentation.
In this study published today in Mobile DNA, we sequenced PMEL in dilute and harlequin merle dogs and found that variation exists only in the number of adenine bases in the tail of the SINE. We refer to this variable region as “oligo(dT)” because the SINE was inserted in reverse orientation relative to PMEL, resulting in a run of thymine bases at the beginning of the insertion. The Merle (M) SINE has an unusually long and pure oligo(dT).
Understanding variations in coat dilution and patches
To understand the relationship between length of the variable region of the SINE (termed oligo(dT)) and the physical characteristics of merle coats, we studied 259 merle dogs (with the heterozygous genotype Mm) in which we observed 36 different oligo(dT) lengths. Dogs having the shortest lengths did not display the merle phenotype these dogs are termed cryptic merles (since they are technically genetically merle, but don’t appear to be). Standard merle dogs possessed lengths from 78 to 86 bp. Dilute merles had lengths intermediate to cryptic and standard merles, while harlequin merles had the longest lengths (See Figure above. Note: We report an individual dog’s oligo(dT) length as a superscript number in the genotype (e.g., an oligo(dT) length of 79 bp is reported as M 79 m.))
When we considered the two characteristics of merle patterning separately, we found that dilution or lightening of the base coat and patches of full color occur at unique oligo(dT) length thresholds. Coat dilution starts to occur somewhere between 56 and 66 bp and increases as the oligo(dT) length increases, such that dogs having lengths of 87 bp or longer have some or complete hypopigmentation (i.e., loss of color) of their base coat.
Fully-pigmented patches begin to appear with oligo(dT) lengths between 75 and 78 bp. We hypothesize that during development, replication slippage on these long tracts causes frequent, random truncations to stable cryptic merle oligo(dT) lengths. Somatic cell populations in which the oligo(dT) length has reverted to cryptic merle lengths produce normal PMEL protein, resulting in patches of full pigmentation.
Longer oligo(dT) lengths are less stable and experience more frequent, earlier reversions during development. Some dogs have patches of full pigmentation that are so large they appear at first glance to be non-merle dogs. Others have multiple somatic reversions to various oligo(dT) lengths, yielding large patches with an array of dilution intensities, a pattern sometimes referred to as tweed. Oligo(dT) lengths representing somatic reversions are denoted in parentheses (e.g., a dog that inherited an oligo(dT) of 86 bp but has a detectable somatic reversion to 45 bp is reported as M 86 m (M 45 )).
Implications of merle phenotypes for dog owners and breeders
Spontaneous changes in oligo(dT) length can also occur in the germline, such that a standard merle dog can produce a “surprise” dilute or harlequin merle puppy. The dilute merle phenotype resembles the characteristic grey coat of a recessive disease of collies, cyclic neutropenia, that requires humane euthanasia of puppies. Harlequin merle can resemble another common coat pattern, piebald, which can be troublesome for breeders in countries or breed clubs where this phenotype is prohibited.
Although merle is one of the more popular coat patterns of dogs, it is not without controversy. Merle-to-merle matings can result in double merle (MM) offspring that have ocular (including blindness and microphthalmia) and auditory defects this is possible even for merle-to-cryptic merle matings. We hope this work will raise awareness among breeders that there are merle varieties that do not fit the standard phenotypic description and that this knowledge will prevent undesirable matings and mischaracterizations.