Could someone explain a chromosomal crossing over?

[tadkerson]

Svarthöna :

Thank you so much everybody for your explanations! It was a very interesting read and I think it's starting to make sense for me... hopefully lol.

I read that a single combed blue egg layer has a chance of about 3%, does anybody know how that was calculated? I have no idea how to link the math part of such a calculation with the biology part...
And did anybody try to calculate how unlikely a crossing over between barring / mulberry skin gene is? Unlikely in the range of 3% or 0,3% does make a difference after all. I like crazy projects, but there's a difference between crazy and next to impossible, so it'd be nice to find out how unlikely this really is.

The chance is determined by the number of cross over units or centiMorgans between two genes. The gene for blue egg shell and pea comb are about 3 centiMorgans apart from each other ( located on the same chromosome). If you crossed a bird that was heterozygous ( hybrid or split) for pea comb and blue egg shell ( P/p and O/o) with a homozygous single combed white egg layer (p/p an o/o), three percent of the time the offspring will have a new combination of genes.

In the hybrid bird (during prophase l of meiosis) , the pea comb gene on one chromosome and the single comb gene on another chromosome will exchange places to produce an egg or sperm that contains a single comb gene linked to a blue egg shell gene. The egg or sperm will contain a chromosome that has a single comb gene linked to a blue egg shell gene ; this is the new combination. The new combination will only occcur 3% of the time because the two genes are 3 centiMorgans apart.

For simplicity, lets say a male bird produces 100 sperm ( they actually produce millions) , three out of the 100 sperm will contain a chromosome that has a single comb gene linked to a blue egg shell gene. So, three chances out of 100 that the sperm will unit with an egg and produce an offspring that carries the new combination.



The barring gene is a powerful inhibitor of the production of eumelanin ( black pigment) in the dermis and epidermis (skin). The barring gene prevents the addition of black pigment to the skin so you do not get the mulberry face. No one has worked out the genetics of mulberry skin- some believe it has a relationship with the birchen gene. What that genetic relationship is I do not know.

It is just like trying to get black, blue or green legs on a chicken that is barred. You can not do it. Two barring genes in a male and one barring gene in a female will prevent the addition of black pigment to the legs. In the case of females because they only have one barring gene they will many times have some black pigment in the legs. How much black is dependent upon the E locus allele the bird carries.

Tim​

 

[Sonoran Silkies]

I wonder if Svarthona meant black skin (rather than mulberry)?

 

[Arielle]

wow, great read! tried reading this thread the other night. At midnight I definitely didn't understand a thing. But now, I understood some of it . . . .

How is the egg color combe type used in breeding. For example:

What is the result of crossing my silver spangled hamburg with EE hens?

What is the result of using the same rooster over a brown egg hen like a SLW?

 

[Svarthona]

Thank you so much tadkerson! Your explanation totally made sense for me

so that's how it's calculated.
Is there any literature I could read online about the location of genes, to figure out how many centiMorgans certain genes are apart? English genetic terms are kind of tricky for me, but I'd like to try reading more about gene charts for chickens... but in lack of correct search terms google isn't my friend in finding much.

Sonoran Silkies, I'm actually not sure if I meant black skin or mulberry. Those terms seemed to describe the same gene to me

we don't even have mulberries here and those nuances are lost in translation

I have no idea if my chickens would be considered black skinned or mulberry in English.

When I got chickens some years ago I got the hens from a landrace/barnyard flock where many birds resembled Swedish black hens. My avatar is showing one of the look alike hens I have. My first rooster was from another landrace/barnyard flock and dark barred B/b. So now I'm having a flock of barred birds or non barred ones with very dark skin and every once in a while I get barred birds with varying amounts of dark blueish/black spots (think Holstein cows

) on their skin.
I'd really like to figure out what's going on there and how likely/unlikely it is to ever get a fully dark skinned bird with barring. The beaks are usually black already and the legs show increasing amounts of dark spots from generation to generation, so it seemed not hopeless at least

and I thought I'd ask about the crossing over after I read about it in a Cuckoo Silky project.

 

[tadkerson]

You would have to get the information from research papers. I am not working on the subject at this time or I would be able to tell you exactly what papers to find. I get most of my literature from a university library. I do not know when I will start working on the subject concerning linkage.


Black skin is due to more than one gene. There are three genes that contribute to the dark skin. A bird must have a recessive sex linked gene called dermal melanin( females have one and males have two) , the dominant gene for white skin and a dominant gene called fibromelanosis.

I do not believe any of the three genes are linked.

There are some breeds that express a mulberry face. Sumatra and birchen game are a couple. I imagine under the correct genotype any black skinned bird will express mulberry face.



Tim

 

[Svarthona]

Don't worry about the link

this thread provided me with so many new search terms that I just plan to have some entertaining days googling. Good that it's a rainy Sunday here in Sweden, because I plan to start right away

Thank you for clearing up the different pigmentation terms, I didn't realize that mulberry just referred to facial skin, oops. And to know that it's different non linked genes causing the pigmentation in different areas makes things easier since I can work on the legs separately from the other traits so to say. It will be interesting to see how long apart barring is from those three genes... theoretically the "easier" to achieve traits like leg coloring should be further away than the others. Hmm. Lots of food for thought! It's great to watch nature work its magic

 

[Henk69]

Start: http://www.edelras.nl/chickengenetics/linkages.html

 

[Mahonri]

This thread has provided me with a couple of months worth of reading. LOVE IT.