Barbara,
There are genes that can cause black to extend into areas on a bird that are normally red or gold. An example is a birchen bird that is solid black. Normally male birchen birds have gold or silver in thier hackles, back and saddle feathers. The solid black birds have black intensifiers or extenders that cause black to replace the gold or silver.
There is also a gene called the patteren gene (Pg) that works with other genes to cause lacing, spangling or autosomal barring to form on a feather. But the coloration on your birds is different than that found in birds that have Pg.
Most lacing is a black(blue) lace on the end of the feather not a red lace on the end of a black(blue) feather.
The area of color that is effected on your birds is normally determined by the E locus gene and in this case the dominate wheaten or recessive wheaten gene. The feather under color of your birds looks to be gray so I would say your birds are recessive wheaten.
If the uncharacteristic coloration is found on adult birds it has a genetic component but nothing I have read about. Most likely it is the expression of a gene or gene interaction and it is caused by a black intensifier. If the coloration was only found in one bird, then it could be environmental.
There are many genes that are not identified in chickens. You may have one of those unidentified genes in your birds.
Tim
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