Chapter 7: Genetics
Genes- Genes are the basic units of heredity and make up the characteristics which are inhereted and passed on to offspring. Genes are found at specific points (called loci) on chromosomes. Locus refers to the pair of loci on each chromosome.
Chromosomes- Found in pairs, chromosomes are composed of thousands of genes which control characteristics.
Allele- A genes that occupies a loci is referred to as an allele. Alleles from each parent can be found on the chromosomes inhereted by the offspring.
Dose- Each parent contributes an allele to their offspring and each contribution of a particular allele is considered a dose. For example, the characteristics for pearlscale will only occur if both parents contribute a pearlscale allele.
Hybrid- When two different alleles for a particular characteristic is contributed, this is referred to as a hybrid. Hybrids can occur in fish color, scale, or fin varieties. When crossed with a similar hybrid, they will produce a percentage of offspring that are double doses (truebreeding) in a characteristic and some that are hybrid.
Mutation- A permanent modification of a gene. Usually stable over generations, genes may be altered which are transferred to future generations. This is a rare occurance. An example of a mutant is the albino angelfish.
Homozygous- Two alleles for a particular characteristic that appear at the same locus results in a true breeding for that particular trait. For example, a standard fin (as opposed to veil) angelfish is homozygous for that fin trait.
Heterozygous- When one or more genes of a pair are not alike at a locus, the hybrid does not breed true and will be a "carrier" of that trait to its offspring.
Recessive- Characteristic when a gene is carried by a heterozygous individual but not expressed. This is a heterozygous individual. For example, if a pearlscale angelfish breeds with an angelfish without that characteristic, they will produce offspring that will not appear pearlscale. This fish, however, will carry that trait.
Dominant- When a gene is carried by a heterozygous individual and the characteristic is expressed by that individual. For example, when a pearlscale angelfish breeds with an angelfish without those characteristics, the normal scale pattern is dominant.
Intermediate Inheritance- This occurs when neither gene dominates the same locus and there is a combination of traits contributed by each allele in lesser degree. An example is Black Lace angelfish that have a blendingof the two characteristics, however, each characteristic is not fully expressed.
Inbreeding- This includes the combination of individuals (brother to sister, mother to son) that are closely related. This method can be used to magnify traits known to a certain line of fish (German Blues, Half Blacks), however, a strain can deteriorate rapidly due to the lack of genetic diversity.
Linebreeding- Similar to inbreeding, this technique is also used to develop the attractive qualities of a strain, however, it involves mating fish that are more distantly related (brother to grandmother, mother to cousin, etc...). Negative characteristics may also be magnified due to a small gene pool, however, this method does not produce as severe problems as inbreeding.
Outcrossing- This technique strengthens the general vigor of a strain of angelfish by bringing in unrelated fish every few generations. Outcrossing can curtail the advances of a particular trait being worked on through linebreeding. A good idea is to keep two separate, but related, lines of the same strain and cross these lines each fourth or fifth generation. This postpones the urgency to bring in unrelated lineage for more generations.
CROSSING
Angelfish genetics follow the same genetic principles discovered by Mendel and his beans. Characteristics of an angelfish are determined by its array of genes. Dominant genes express themselves when crossed with other genes it can predominate or act in codominance with. For example, the genotypes for the true breeding black angelfish are considered a dominant gene when crossed with a gold angelfish. Recessive genes express themselves only when crossed with another related recessive gene. The genotypes for the pearlscale angelfish is considered a recessive gene just as the gold angelfish genotype whereas the characteristic desired will only express itself if both parents carry the allele.
A simple way to determine phenotypes is to designates single letters for individual characteristics. A common silver angelfish could be designated as "S" while the marble angelfish may be designated "M". Both of the marble characteristics from a mother and father fish would give the progeny "MM" genotypes (true breeding). Both the female and male genotypes are combined to form the offspring's characteristics. Simple multiplication can be used to give theoretical ratios. A diagram can be used to visualize what characteristics a breeding pair would produce in what is called the Punnit Square. This is useful to help identify the outcome of a spawning. Place the alleles of one parent across the top and the alleles of the other parent on the side vertically. Now copy the respective allele down from the top row and position it in each box directly beneath it. From the side rows, bring each allele across into each box to the right of the allele. Each box represents 25% of the possible outcomes of each cross. This represents the genotype of each fish. The phenotype (how it appears) is decided by the dominant and recessive characteristics of each allele in its respective box.
If a marble/silver hybrid were to be bred to a true breeding marble, the resulting progeny would theoretically include 50% true breeding marble angels and 50% marble/silver hybrids (See Figure 1). Most of the marble/silver hybrids would be expressed as silver angels with light marbilization however most of them would have a dominantly silver body. The marble/silver hybrid is not a very attractive characteristic to maintain in a breeding program unless you are crossbreeding your fish to strengthen your strain.
| Sperm | |||
|---|---|---|---|
| M (marble) | S (silver) | ||
| Egg | M (marble) | MM | MS |
| M (marble) | MM | MS | |
The same principles can be used in reference to finnage. The veiltail finnage is dominant over the standard finnage. If one were to cross a true breeding veiltail to a true breeding standard-tail (See Figure 2), the resulting offspring would be 100% hybrid veiltails. The offspring would have both a veil allele and a standard allele yet the phenotype (characteristic expressed) would be veiltail. This hybrid appears as a veiltail, however, the tail is not as long as a true breeding veiltail. It is good to strengthen a true breeding veiltail strain by crossing it with the stronger standard-tail angel strains since the resulting cross produces relatively robust veiltails. A cross between two true breeding veiltails results in a weaker fish with very long finnage susceptible to fungal diseases.
| Sperm | |||
|---|---|---|---|
| S (standard) | S (standard) | ||
| Egg | V (veil) | VS | VS |
| V (veil) | VS | SS | |
If one were to cross two hybrid veiltails previously discussed, the spawn would theoretically produce 25% true breeding veiltails (observed as longer fins), 50% hybrid veiltails (medium length veiltails not as long as true breeders), and 25% standard tails (See Figure 3). This cross produces a variety of finnage types.
| Sperm | |||
|---|---|---|---|
| V (veil) | S (standard) | ||
| Egg | V (veil) | VV | VS |
| S (standard) | VS | SS | |
Certain characteristics always remain dominant over others. Gold marbles and silver angels both have a codominance. If you were to breed these, the would look like silver angels with marbalizations on them. For finnage, veiltails are dominant over standard tails. The pearlscale gene is a recessive gene both parents must carry in order for it to be expressed in the offspring.
DETERMINING ALLELES Often when receiving fish from a petshop, it is not possible to determine the alleles of certain fish just by appearance. Sometimes you have to breed the fish before ascertaining what alleles are present in the fish. For example, if you have a marble angelfish (Fish X) that pairs with a gold marble (Fish Y) angelfish and produces 50% marble and 50% gold marble, then Fish X is a marble/gold marble hybrid. The Punnit Square can be used in a reverse fashion to determine alleles. Hybrid angelfish that give off such variations provide good variety in a breeding program with limited space.
For fish with recessive genes (gold, albino, blushing, etc...), it is obvious that both parents had the gene for a recessive trait therefore a fish that displays such characteristics can be determined to have both alleles and is true breeding if paired with a similarly colored fish.
Visit Chapter 8: Varieties of Angelfish