bee biology

Diploid drones are a result of inbreeding

In bee school you learned that the sex of a bee is determined by the fertilization status of the egg. A fertilized egg becomes a female; an unfertilized one becomes a male (or drone). You may have also learned that this phenomenon is called haplodiploidy, and that all the hymenoptera (bees, wasps, and ants) are haplodiploid organisms.

What they didn’t tell you was that some of the fertilized eggs become diploid drones. This happens because the thing that actually determines sex is not the presence or absence of fertilization but the presence or absence of heterozygous alleles at the sex locus. Don’t go away yet; this isn’t difficult.

You see, instead of having an entire chromosome that determines sex (like the X and Y chromosomes in humans) bees have one gene on one chromosome that determines sex. Specific places on chromosomes are called loci (the singular is locus), so the “sex locus” is just the place (think location) on the chromosome where the sex gene is found.

The European honey bee species has about 18-20 different alleles of the sex gene. An allele is just a variation of a gene. All the sex alleles do basically the same thing, but the genetic coding is a little different in each one. Compare this to having 18 different recipes for chocolate cake—the end products are similar but the instructions for getting there vary.

  • Haplodiploidy: Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid.
  • Allele: One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
  • Locus: The specific location of a gene or DNA sequence on a chromosome.

So different bees are running around with different alleles (or instructions) for the sex gene. If an egg is not fertilized, there is only one set of instructions and the bee becomes a drone. If an egg is fertilized and has two different sets of instructions, the bee becomes a female. But—and here’s the rub—if the egg is fertilized but receives two identical sets of instructions (two identical sex alleles) the bee becomes not a female but a diploid drone. Think of it like this: one set of instructions twice is not the same as two different sets of instructions.

These diploid drones do not survive. In colonies of social insects such as honey bees, the worker bees eat the diploid drones soon after the eggs hatch. Many diploid drones in a colony result in “shot brood” or “scattered brood”—brood combs that have lots of empties or brood of many different ages. In solitary bees, the diploid male may die in the cell, or may emerge and mature but be sterile.

The chart below shows what would happen when a honey bee queen (with two alleles) mates with five different drones, each with one allele. In this case, two of the drones have the B allele and the rest have different alleles.

DronesQueen Allele AQueen Allele B
Drone Allele AAABA
Drone Allele BABBB
Drone Allele CACBC
Drone Allele BABBB
Drone Allele DADBD

Wherever you have homozygous alleles for the sex gene (two of the same alleles), you get a diploid drone. This chart shows an extreme example because it has a small number of alleles and a small number of matings, but it illustrates how homozygous alleles happen.

In real life, honey bees have about 18 alleles for the sex gene and a queen may mate twelve or more times, both of which lessen the likelihood of diploid males. But inbreeding decreases the number of alleles in a population and thereby increases the occurrence of diploid drones. Large numbers of diploid drones weaken a colony because the nurse bees waste resources raising these bees only to kill them later, and because the presence of so many drones reduces the number of worker bees that the colony can raise.

Large numbers of diploid males hasten a species toward extinction. Studies have shown that when population sizes of haplodiploid organisms become small, they go extinct more quickly than other species (Zayed and Packer 2004, 2005). With feral honey bee populations declining and inbreeding becoming even more common, the presence of diploid drones is a major concern to honey bee breeders.


Shot brood often signals high numbers of diploid drones.


  • Wonderful explanation, Rusty! I have tried to explain this concept in a few presentations but never did it so well as you’ve done here. Hope you don’t mind that I’m going to try to commit this to memory for future presentations.


    • Since the queen that produced the shot brood won’t be mating again, I would replace her ASAP. She has genetics similar to the drones she mated with, so the situation won’t get any better on its own.

  • Great genetics lesson. Weird to think in what might sound to be a simpler system (haploidy), that sexual determination is more complex and occasionally inefficient.

    I also wanted to thank you for this site. I check in daily. It looks like a labor of love. Your work is appreciated.

  • And when you say, “shot” brood, are you referring to the scattered nature of the cells containing brood?

  • Hi Rusty,

    I believe I’m experiencing this drone problem. It’s mid-winter and on warm sunny days there are lots & lots of drones coming out of my hive entrance. Is this a good time to shake out, forget, and start over come spring? Maybe you could give me some idea on how to remedy this problem sinse I shouldn’t open the hive to just inspect.

    I’m also wondering about extensive spotty drone cells throughout 2 brood boxes. Understand on requeening, but I think it’s the wrong time.

    Thank you for the knowledge, another disadvantage of failing queens.

    • Dave,

      I think the drones you are seeing are the result of laying workers. You probably went queenless some time ago and the workers, without queen or open-brood pheromone to suppress their ovaries, began to lay. Laying workers can lay only drones. This is difficult situation to deal with and you are probably best off to just start over in the spring.

  • If you’d like to promote the genetics of a particular queen, is a good approach to use foundationless frames and let them make as many drones as possible to attempt to flood the area with her drones? Or what would you suggest? (I feel so smart after reading your articles! 🙂 )

    • Kim,

      Right. Queen breeders are just as concerned with the drones as the queens. It makes perfect sense. And yes, foundationless frames will give you a much higher (and much more natural) proportion of drones.

  • Hi Rusty,

    It’s the captain from down under here. Thank you for your more in depth description of diploid drones. As a small hobbyist with 20 colonies, I have great interest in learning more about my colony management. Next year I might come over and visit some bee clubs and sit in to some club night talks.

    We do not have varroa yet that we know of but we are already in full study and by degrees preparing for its ultimate arrival. The medicinal values are being more closely studied as our native flora and fauna are amongst the oldest species on the planet and my theory about medicinal values falls along the lines of adaption to the environment for such long time frames, that millions of generations of trees have simply adapted for longer than any other plants to have produced unique antibacterial, antiseptic properties and concentrations. Honeys unique delivery system is another key to its effectiveness in healing.

    Thank you for your insights I hope to meet you one day,

    David Smith . The Captain . .

  • Hi Rusty

    “an unfertilized one becomes a male (or drone)” quoted from the first paragraph.

    May I ask if the drones are genetically identical or dissimilar?

    Mr Choo

    • Choo HG,

      The drones of a queen are related but not identical. The genes from the queen’s mother and those she mated with are assorted in different ways in each egg, just like brothers and sister except there is only one set of chromosomes.

  • Hello Rusty,

    I wonder if the irregular sprinkling of few a eggs is laying worker. I know that I need to wait a few days to find out how they are capped but question is how do the workers know the eggs are unfertilised if produced by a laying worker? I am keen to queenright my hive and fear that the hatched supersedure cells that I noted a week ago did not produce a mated queen. I think maybe not so I would like to prepare to either merge another hive; add more eggs from other hive for the new supersedure cell being built, or order a queen. I really don’t want the bees to be any longer with a queen.

    Kindest regards, Lyn

  • Hi Rusty! Appreciate all the great information produced here. I did find your response to Choo HG a little confusing-

    “The drones of a queen are related but not identical. The genes from the queen’s mother and those she mated with are assorted in different ways in each egg, just like brothers and sister except there is only one set of chromosomes.”

    My understanding is the genes of the unfertilized eggs are the queen’s alone albeit spliced and diced differently in each egg. Any drone characteristics would have to come from the queen’s dad not her mates because by definition a drone egg is unfertilized by a mating drone sperm so no genes are passed on from drones that have mated with the mother queen.

    Here’s my cited source to check my math!

    “You’ll notice that the egg can only carry half of the queens 32 chromosomes so she can only pass on half of her genes to her offspring. Each egg contains a unique collection of her genes, so each egg is different. Drones on the other hand only have 16 chromosomes to begin with, so each sperm must contain all the genes of the drone. This means that each sperm from a drone is exactly identical, they are clones. This is different from most other animals, where each sperm is unique, just like each egg is.”

  • A question was presented by one of our local club members. Would two sibling drones be clones any more that two sibling humans?

    My thinking is yes if they are haploid. Is this correct?

  • Oops, I didn’t read through all of the comments/responses. I see the answer that even haploid drones from the same queen are not identical or clones. Kind of like “identical” twins. They is always individuality.

  • Rusty,

    I am seeing a lot of drone comb in a 3-frame observation hive with an active queen.

    I also notice what “appears” to be a couple of possible queen cells being capped.

    In doing the math, it appears that both the drones and these queen cells will be erupted and fertile on about the same day.

    Is this normal, due to weather and nectar flow? Can I assume all the other hives are on the same timeline, so that the observation hive new queen will be mated by other hive’s drones?

    Or do I need to worry that the new queen will be fertilized by her brothers from the same hive?

    Thank for the help!

    • Kathy,

      Honey bees have several mechanisms to limit inbreeding. Drones are produced throughout the reproductive season, so there will be some emerging almost every day. Some of those will coincide with a queen emergence, but it is normal and of no consequence. Other colonies are the same: drones emerge every day, queens whenever it’s necessary.

      Queens fly to drone congregation areas to mate. Drones from many colonies from many miles away mill around in the air and wait for virgin queens to appear, so the likelihood of mating with a sibling is small. Furthermore, they mate 12 to 16 times, so even if one mating is with a close relative, it is unlikely they all will be.

      This is something you really don’t need to worry about.

    • Addis,

      Diploid drones don’t survive very long, so I imagine the workers can sense they are not right. They eat them in order to dispose of them yet conserve the protein that is in them.

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