Crossover???

[Tjerk]

Hey,

afgelopen weekend ben ik naar Yvonne geweest en heb daar een erg mooie Witmasker Bonte man weggehaalt. Ook heeft zij een valkparkiet zitten wat een crossover gen bevat. Maar hoe gaat dat nou precies in z´n werk.

Groeten Tjerk

[Lin]

Precies kan ik het ook niet uiteggen maar het houdt in dat de 1x en de 2x wisselen van pop naar man.

Groeten, Lin

[Ludo]

Dat is nu eens iets wat ik niet snap zie ;-)
Hoe kan je nu weten dat een valkje een crossover gen heeft ??
Kan je mij daar wat meer uitleg over geven ?

Groetjes

Ludo

[Lin]

tja misschien kan je het merken als je je ook een crossover krijgt als je een andere partner er bij zet en je hebt het weer.

[Tjerk]

Hey,

dit kun je zien als je elke keer weer iets eruit krijgt wat volgens de mutatiecalculator niet kan. De splits zouden dus wisselen. Meer weet ik er dus ook niet van, daarom hoopte ik dat iemand hier iets over zou kunnen vertellen.

Groeten Tjerk

Misschien dat Yvonne hier ook nog is iets neer kan zetten daarover.

[Karin]

Voor de liefhebbers  ;D :

Genetic Crossover
 
Imagine you have two dogs, standing side by side. One dog has two fleas, the other has none. One of the fleas jumped from the first dog to the second dog, and now each dog has a flea on his back!
Crossover is the "jumping" of one sex-linked gene (the flea) from one X chromosome (dog) to another! Since you need two X chromosomes for crossover to occur, it is obvious that it only happens with males. There are only a few rules regarding crossover:

 

1. The gene cannot occur on BOTH X chromosomes. For example, this means that a male with a genetic description XC XC cannot experience a crossover.
2. It is not necessary for the entire clutch to be affected by crossover. Only one bird out of a clutch, for example, could have sex-linked genes that result from a crossover.
3. Crossover occurs, on the average with Cinnamon, Pearl and Lutino, only 25-30% of the time. (Yellowcheek has a much lower percentage of crossover.)  Let's look at crossover a little closer, using a normal male split to cinnamon and pearl, mated to a normal female.
 
MALE: XP XC FEMALE: X Y
XP X XP Y
XC X XC Y

 
This would be the expected outcome of the above mating...50% of the males will be split to Pearl, 50% of the males will be split to Cinnamon; 50% of the females will be pearl and 50% will be cinnamon. But how do you think we ever got the beautiful double mutation Cinnamon-Pearl? It was the result of crossover.

X P   X C = X   XCP
This makes the male's genetic coding X XCP, and the mating chart like this:
 
MALE: X  XCP FEMALE: X  Y
X X X Y
XCP X XCP Y

 
Suddenly, he has produced Normal and Cinnamon-Pearl female offspring. Without crossover, he could only produce Pearl and Cinnamon hens, with all Normal males.
There is also such a thing as REVERSE crossover, where a male that contains a double mutation on one X chromosome passes on only a single sex-linked mutation!

X C P   X  =  XC 

Groetjes, karin.

[Gerda]

Nog meer info (van cockatiels4U en door Peter op een rijtje gezet):

LINKAGE AND CROSSOVER

When two or more genes reside on the same chromosome, whether sex chromosome or autosome, they are said to be linked. Linked genes do not independently assort during gamete formation. However, linked genes do not always stay together because homologous non-sister chromatids may exchange segments of varying length with each other. This chromatid exchange, referred to as crossover, occurs during prophase I of meiosis when homologous chromosomes synapse. In the Figure below, the sex-linked genes for Pearl (pl) and Cinnamon (c) are used to demonstrate crossover. In this case, the male is heterozygous for both traits. You will also notice that the mutations are arranged such that they do not both occur on the same chromosome. This arrangement ([+,pl][c,+]) is referred to as repulsion. If both dominant (or wild-type) alleles are on one chromosome and both recessive (or mutants) are on the other the linkage relationship ([+,+][pl,c]) is referred to as coupling.


Op ware grote: http://www.valkparkiet.org/forumupload/Figurea.gif

From the above example, the male can provide four different types of gametes to the fertilization process ([pl,+] [pl,c] [+,+] and [+,c]). If coupling was applied to the above example the type of gametes produced would be the same. The only difference between the two situations is the statistical outcome.

Crossover rate can be effected by sex, age, temperature, proximity of allele to the centromere, inversion and many other factors. The highest probability of crossover that could be seen between any two genes for any species is no more than 50%. If crossover was to occur at its maximum frequency (~50%) then the alleles would appear to be independently assorting.

With sex-linked traits, crossover only occurs in males because only males have a homologous set of sex chromosomes. Currently, I am unaware of any linked mutations on the autosomes. If such a condition was to arise, then crossover could be observed and it would be at the same frequency for both sexes.

The physical point at which chromosomes exchange during crossover is referred to as a chiasma (plural chiasmata). Each type of chromosome within a species has a characteristic (or average) number of chiasmata. The frequency with which a chiasma occurs between any two genes also has a characteristic or average probability. The further apart two genes are located on the chromosome, the greater the opportunity for a chiasma to occur between them. The probability of chiasma occurring between two linked genes is calculated as follows:

Chiasma % = 2(crossover %) or  Crossover % = 1/2(chiasma %)

Therefore, if you observe 30% of the crossover genetic linkage in your offspring, such as [pl, c] and [+, +] in the above example, then the Chiasma frequency is 60%. Chiasma frequency is twice the crossover frequency because when a chiasma forms between two genes, only half of the meiotic products will be of the crossover type. In other words, chiasma is the probability that chromosomes (i.e. two chromatids) will exchange between a synapsed pair of chromosomes (bivalent) containing four chromatids - also referred to as a tetrad.

The chromosomes that are not involved in the crossover are referred to as non-crossover or parental types. Those involved with the crossover are referred to as recombinant or crossover types.

In the above example they are as follows:
[pl , +]
[+ , c]  Parental 
   
[pl , c]
[+ , +]  Recombinant

Next, I will discuss application of crossover to the Punnett square. When applying sex-linked genes to the Punnett square, the crossover gametes produced by the male should be included. Again, the female cannot produce any crossover gametes for sex-linked genes. I will use the example above with a Gray Male heterozygous for Pearl and Cinnamon in repulsion phase (X+,plXc,+) crossed to a Gray Female (X+,+Y). See Figure 18.


Op ware grote:
http://www.valkparkiet.org/forumupload/Figure18.gif

In the above case, I arbitrarily assumed a 30% crossover rate for the two loci. I have been unable to locate genetics information on cockatiels that provides crossover frequencies for the various linked mutations. I suspect we are still in the data collecting phase.

Let us assume we have a male heterozygous for four sex-linked traits. This would then present three various regions for crossover to be observed. See Figure 19.


Op ware grote:
http://www.valkparkiet.org/forumupload/Figure19.gif

The possible Parental and Single Crossover gametes produced by the example in Figure 19 would be:
Xa,b,c,d
X+,+,+,+  Parental 
 
Xa,+,+,+
X+,b,c,d
Xa,b,+,+
Xa,b,c,+
X+,+,+,d  Single
Crossover
Recombinants

The maximum frequency of recombinant gametes for any two loci is 50%, because this would represent 100% single chiasma frequency. Gene distances are expressed as map units. One map unit is equivalent to 1% recombinant progeny. If the a-b distance is 50 map units, b-c is 30 map units, and c-d is 40 map units, the distance a-d would be 120 map units. But in a cross involving segregration of only the two most distant markers (a-d), the number of recombinant progeny would not be expected to exceed 50% because of mulitple crossover, some of which produce the equivalent of noncrossover.

When discussing more than two genes on a chromosome that experience crossover then you must also factor in two strand double crossovers and multiple even-numbered crossovers. The figure below shows the sequence of events involved in double crossover using a male heterozygous for three sex-linked traits.


Op ware grote:
http://www.valkparkiet.org/forumupload/Dcrossfi.gif

The double crossover frequency is the product of each single crossover. Thus, if the crossover frequency between genes a and b is 10% and between b and c is 20%, then the double crossover rate is expected to be 2% (i.e., 10% x 20% = 2%) if there is no interference. Interference is a phenomenon in which a given crossover tends to inhibit the formation of a nearby crossover. The gametes produced by this male will be as follows:

Parentals Xa,b,c
X+,+,+ 72%

Single
Crossover (b-c) Xa,b,+
X+,+,c 18%
Single
Crossover (a-b) X+,b,c
Xa,+,+ 8%

Double
Crossover Xa,+,c
X+,b,+ 2%

Note that in the above example, the rate of double crossover (2%) has been subtracted from the total crossover frequency for a-b (10%-2%=8%) and b-c (20%-2%=18%) to derive the single crossover rates. Additionally, the following can be stated:

The a-b distance on the chromosome = 18%+2%=20%=20 map units.
The b-c distance on the chromosome = 8%+2%=10%=10 map units.
The a-c distance on the chromosome = 20%+10%=30%=30 map units.

Double crossover has been studied in fruit flies and from these studies, it has been determined that double crossover usually does not occur between fruit fly genes less than 5 map units apart (1 map unit=1% crossover rate). In the above example, the distance between genes a and c is 30 map units (i.e., 30% total crossover rate including single and double crossovers = 30 map units).).

[Marije]

Ik heb 't er nu net met Bio over gehad! :D
Het is idd dat als twee chromosomen dicht bij elkaar liggen, een chromatide van beide chromsomen over elkaar heen kunnen komen te liggen. Zo kan een stukje van de chromatide van het ene chromosoom overgenomen door het andere chromosoom, en kan je dus verassende nakomelingen krijgen...
Groetjes,
Marije

[Ludo]

wat zit de natuur toch mooi in elkaar ;-)

[Tjerk]

Hey,

het is idd heel raar, maar toch heel mooi.

Groeten Tjerk

[Yvonne_R]

Hoi Tjerk,

Wellicht ten overvloede, maar ik heb nog even een stukje tekst opgedoken over crossover bij 1 van mijn koppels (waarvan de man dus degene is die je zaterdag gezien hebt)
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Ik heb een heel bijzonder koppel valkparkietjes die mij elke keer weer verrassen met wat voor jongen ze krijgen.
De man was mij verkocht als een cinnamon witmasker en de pop als een bonte pop. Nou kregen ze vorig jaar 2 nestjes waarin 2 lutino jongen zaten (1 die cinnamon opscheen) en een cinnamon jong. Navraag bij Kirsten Munson leerde mij dat de man dan in ieder geval split moest zijn voor lutino en bont. Nou hebben ze momenteel weer jongen waarvan 2 geparelde! Ik heb dit weer nagevraagd bij Kirsten Munson en blijkt dus het volgende:

Mother: Pied Pearl
Father: Pied Whiteface Cinnamon Split to (X1: Pearl) (X2: Lutino)

Male offspring:
50 % Pied Pearl Split to Whiteface (X1: Cinnamon)
50 % Pied Split to Whiteface (X1: Cinnamon Lutino) (X2: Pearl)

female offspring:
50 % Pied Cinnamon Pearl Split to Whiteface
50 % Pied Cinnamon Lutino Split to Whiteface

Dit verklaarde echter nog niet dat ik vorig jaar een Cinnamon bonte pop heb verkregen uit dit koppel. Nu blijkt dus dat er sprake moet zijn geweest van crossover. Dat houdt in dat de geslachtsgebonden mutaties van de man verplaatst zijn ten opzichte van hun normale positie op de X chromosomen. In dit geval moet het Lutino gen verplaatst zijn van X2 naar X1, zodat X1 Lutino Pearl was en X2 geen mutatie had. Dan krijg je dus het volgende:

Mother : Pied Pearl
Father: Pied Whiteface Cinnamon Split to (X1: Lutino Pearl)

male offspring:
50 % Pied Pearl Split to whiteface (X1: Cinnamon Lutino)
50 % Pied Split to Whiteface (X1: Cinnamon) (X2: Pearl)

female offspring:
50 % Pied Cinnamon Lutino PEarl Split to Whiteface
50 % Pied Cinnamon Split to whiteface
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Misschien dat je hier nog wat aan hebt. Veel succes met Diesel en White trouwens. O, ja ik hoop ook voor jullie dat Diesel split is voor lutino!


Groetjes Yvonne