Science project

Observing Dominant and Recessive Traits in Angelfish

The purpose of this experiment is show experimentally that two fish with heterozygous genes for color will produce offspring that of which 50% show the heterozygous coloration, 25% the dominant coloration and 25% recessive coloration.

Research Questions:

  • How are genes passed down from parents to their offspring?
  • How many color alleles does each angelfish have?
  • How many color alleles can one angelfish pass on to their offspring?
  • Are dominant or recessive genes passed down more frequently?
  • What is the difference between genotype and phenotype?
  • How can you determine possible genotypes?
  • What is co-dominance?

Selective breeding has produced the variety of colorations we observe in tropical fish purchased from a pet store. In the wild, many of these species come in one or two varieties, but by selecting for certain traits; humans have expanded the range of color options available to collectors. Fish breeders keep track of fish genetics by carefully recording which fish they breed and keeping notes on the resultant offspring. By continuing to select for rare or unusual traits, breeders continue to expand the number of colorations available to collectors. 

Angelfish come in a number of different colorations, one of the most common being the Black Hybrid. In this variation, the fish has the genotype Dg. Following a simple gene chart, we can predict the outcome of breeding two of these heterozygous fish.

Father à
gold (g)
Mother ↓

DD Black Lace

Dg Black Hybrid

gold (g)

gD Black Hybrid

gg Gold


  • An aquarium with a heater and a filter
  • 3-5 Black Hybrid angelfish over 9 months old
  • A broad-leafed aquarium plant (for the fish to lay their eggs on)
  • Fish food

You will need to talk to your local aquarium store or a fish breeder (many are listed on the internet) for information about where to acquire Black Hybrid angelfish. Both of these resources will also help you set up a tank if this is your first aquarium.

Experimental Procedure:

  1. Set up the aquarium.
  2. Add the fish to the aquarium.
  3. Feed the fish daily and monitor their health. If you have a new aquarium, some of the fish may die as you cycle the water. This is normal, but you will want to replace the fish with healthy ones in order to have a good sized breeding population. It is possible to start the tank using 1/3 used aquarium water from a friend. Make sure that the fish in the tank are healthy so you do not spread disease to your new fish.
  4. Observe the fish for mating behaviors and check around the tank daily for eggs. Most angelfish make good parents but some will eat the eggs or young fish. If your fish are eating their eggs, you will want to separate the parents from their offspring.
  5. The eggs will hatch within 2 weeks. Once they have hatched, the young fish will display their adult coloration. It may not be possible to tell the Black Heterozygotes from the Black Lace, however, until the fish are older.
  6. Record the genotypes of the offspring you observe and compare them to the predicted results.
  7. It is possible to take this experiment to the next level by breeding fish of different phenotypes. In this case, use your knowledge of angelfish genetics to predict the possible phenotypes and then compare the observed results to your predictions.

Terms/Concepts: Mendel; Dominant gene; Recessive gene; Genetics; Inherited traits; Eye colors; Genotype; Phenotype; Allele


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