In the first article of the Genetics Mini-Series, I discussed the genes responsible for egg shell color, the compounds which are involved in producing those colors, and how those colors behave during the egg laying cycle. In the second article on egg shell color, I will discuss breeding for blue egg color as a means to explain simple autosomal dominance.
Genetics of Egg Shell Color Part 2: Breeding for Blue Eggs
Genetics Mini-Series Article #2
Let us first discuss the simple dominance of blue egg shells. Take a look at the table produced by Punnett in the 1930s. (You can read the original article by Punnett on my Google Docs.) When he crossed a blue egg layer and a white egg layer, why did half of the resulting offspring lay blue and half white? This is because each chicken (both roosters and hens) have a pair of genes that determine the base shell color. They inherit one gene from their sire (father) and one gene from their dam (mother).
Remember from GMS1 that blue egg shells contain oocyanin; thus, the gene symbol for blue egg shells is O and for white egg shells is o. If they inherit two genes for blue egg shells, they are OO. If they inherit two genes for white egg shells, they are oo. When a chicken (or any animal, for that matter) carries two of the same gene for a trait, they are homozygous for that trait. (Homo means “same” and zygote refers to the combination of the two parents’ genetic material.) If they have a dominant gene from each parent, they are homozygous dominant for that trait. If they receive two recessive genes, they are homozygous recessive.
If they inherit one gene for blue egg shells and one gene for white, they are Oo; heterozygous. (Hetero means “other” or “different.”) Now, we see autosomal dominance take place – autosomal because these genes are not on the sex chromosomes. The blue-egg-shell gene is stronger and is dominant over the white-egg-shell gene. The heterozygous hen will lay blue eggs, but the white-egg-shell gene will sit quietly in the background and is just as likely to be passed along to offspring as the blue-egg-shell gene.
Take another look at the first cross in Punnett’s table. He indicates that the cross was between a white egg layer (oo) and a heterozygous blue egg layer (Oo). We can use a common tool, named, in fact, for this same man – the Punnett Square – to help us determine how this cross resulted in 50% white egg layers and 50% blue egg layers.
In the Punnett square above, we see what happens when each parent gives one half of its genetic material to its offspring. In Punnett’s cross, the Golden Hamburg rooster is homozygous recessive and carries two copies of the same gene for white egg shells. The “Chilean” hen is heterozygous and can contribute either a dominant blue-egg-shell gene or a recessive white-egg-shell gene. She gives the dominant gene to half of her offspring and the recessive gene to the other half. Thus, half will lay blue eggs and half white. Half will be Oo and half oo. Because the offspring are the first generation resulting from this cross, they are called F1, the “F” from the Latin filial meaning “generation,” so literally ‘first generation.’
But suppose we decided to cross these F1 chicks to each other with the goal of producing a true-breeding or homozygous blue egg layer (OO). First, we need to figure out which of the cockerels is heterozygous. He won’t lay an egg, so we need another way to find out. Because the pea comb is closely related to the blue egg shell gene, we might simply gamble and choose an F1 rooster with the tightest pea comb.
A safer, albeit more time-consuming and work-intensive method would be to test cross the males to determine whether they carry an Oo or oo genotype or genetic code. By crossing our ?o male to a white-egg-laying oo F1 hen, we can determine whether or not he is heterozygous. If he is heterozygous, their offspring will be approximately 50% blue egg layers and 50% white egg layers, just like the cross above but with the male and female reversed. If he is homozygous recessive oo, all of their offspring will be oo white egg layers.
To create our true-breeding homozygous dominant OO chickens, we can begin by crossing two heterozygous F1 offspring. (Breeding full brothers and sisters can quickly lead to inbreeding depression because they share approximately 50% of their genetic material. If the initial cross can be made with more than one hen or even more than one rooster, then the F1 generation will not be so closely related. It would be best to track the matings by marking the birds to make sure full siblings are not mated to create the F2 generation.) We will take an F1 blue-egg-laying hen who must be Oo and an F1 tightly-pea-combed or test-crossed heterozygous Oo rooster and put them together.
Twenty-five percent of the F2 offspring will be homozygous recessive, oo. Fifty percent will be heterozygous Oo. Finally, 25% will be homozygous dominant, OO. They will be unable to give anything but a blue-egg-shell gene to their offspring. These are the keepers for the breeding program – but it will be a headache figuring out which these are.
White-egg-laying pullets can be culled, meaning sorted into a group and removed from the breeding gene pool but not necessarily killed; many people cull by simply selling unwanted stock. Then, we will need to sort the blue-laying heterozygotes from the blue-laying homozygotes. It appears that two copies of the dominant blue egg shell gene sometimes results in a deeper blue egg, so you might try to identify these early in the laying cycle and select these hens for test crosses. The best gamble with the cockerels would be to eliminate non-pea combs.
Test cross F2 pullets and cockerels by putting them with known F1 heterozygotes, carrying Oo. If the resulting offspring are approximately 25% white egg layers, then you know your F2 bird was also a heterozygote; your results match the Oo x Oo chart above. If 50% are white egg layers, your F2 was homozygous recessive, oo. If, however, all offspring lay blue eggs, then your F2 bird is homozygous as demonstrated in the chart below.
Once you have identified your homozygous dominant males and females, you can cross these F2 birds to create your F3 generation which will be entirely OO.
Thus, your F3 generation is all homozygous dominant for blue egg shells. They will all only be able to hand down O to their offspring, and any crosses between members of this generation will continue to yield homozygous dominant offspring, repeating the cross in the chart above. Finally – we have true breeding blue egg layers. At this point, we could continue to select for the best egg color, mating only hens with the bluest eggs and roosters who themselves hatched from hens capable of laying the bluest eggs while in their prime. Alternately, we could breed for other traits such as feather, leg color, comb shape,… No wonder breeding can be the job of a lifetime. And we haven’t even mentioned type, which is the most basic and important of breeding jobs – and should be bred for before egg color. Egg color is only a small detail in the bigger picture!
This was just an exercise to explore the basics of autosomal dominance using the first cross in Punnett’s experiment. If your only goal was blue egg shells, your best bet would be to buy stock as close to your goal as possible and begin from there. But, if one wanted to use a white-egg-laying breed as part of a blue-egg-laying project, this is a strategy one could follow.
Attaining the above percentages
Each chick hatched has its own roll of a theoretical four-sided dice. If you only hatch four chicks, by no means are you assured one representing each quartile. In fact, you could end up with four chicks with the same genetic code or genotype! If you want to assure yourself a reasonably-sized group representing each type, you need to hatch as many chicks as you possibly can. In fact, this is the best way to create or improve your breed; the larger the number of chicks hatched, the more “choosey” you can be when selecting your breeders. If you hatch out 20 chicks and half are female, the top four females will represent the top 40%. If you hatch out 100 chicks and half are female, the top four are the best 8% of females. If you hatch out 200 chicks and half are female, the top four are the best 4% of females. You can see how this would lead to much quicker progress in all aspects of your breeding program – if you have the space and resources to raise so many birds and the parents to make them with!