Simple Genetics for Us Simple Folks
By Tom Rood, Penn Yan, NY
Originally published in
the 1998 Spring/Summer AHS Region 4 Newsletter
Winner 1999 "Best Scientific Article"
(All daylilies in these articles are Hemerocallis )
A few folks wrote to me privately to say that a lot of this gene, chromosome, and allele stuff was pretty deep. So what follows is for the folks who would like to understand but are not willing to take Genetics 101. Genes are made of DNA if that helps any. Chromosomes carry DNA, which is why the excitement kicks in for those messing around with tetraploids and their 44 chromosomes, twice the number of diploid chromosomes.
You should first understand that a lot of things make life complicated and we are going to assume for simplicity's sake that everything is simple, or can be with just a little effort.
Gregor Johann Mendel (1822-1884) was an Austrian Monk who published a paper in 1866 entitled "Experiments in Plant Hybridization." Nobody gave a hoot in a hat about it and it was promptly lost for over 30 years. Reportedly, Charles Darwin had a copy buried in his personal book shelf, which he may never have seen and certainly did not understand. If he had, his Origin of Species work might have had some corroboration; but that is another story.
When Mendel died, his personal papers were destroyed. So we do not know very much about him or his work. Nevertheless, he is considered the father of the study of genetics. Mendal, a mathematics teacher, worked with garden pea plants, carefully hybridizing under controlled conditions and recording the results.1
Probably the first daylily hybridizer of note who had some familiarity with Mendel's principles was Arlow B. Stout. He worked actively on the hybridization of Hemerocallis at the New York Botanical Garden, where he was director of laboratories from 1911 until his retirement in 1948. There were very few hybrid daylilies created prior to the 1930's. The explosion in daylily hybridization didn't begin until the 1950's, so we are working in a relatively new area from a historical point of view.
At the heart of genetics is sexual reproduction, which is a form of genetic information resuffling. Take two playing card decks of 52 cards each, for example, and shuffle them together. Lay the results out on a large table, one card at a time. The result is just one possible combination. The maximum number of different possible combinations from reshuffling the 104 cards is staggering: 250,000,000,000,000,000,000,000,000,000.2
Fortunately, the numbers in daylilies are smaller and simpler than those for two decks of cards. Genes are the determinants for the characteristics within a plant. When a plant reproduces, each parent provides one half of its alleles to the genes in the offspring; the other half of the alleles in the offspring come from the other parent. Alleles are different forms of a gene. Take the determinant gene for controlling plant height for example. Let's say that the controlling gene has two contrasting forms of alleles. One is the dominant allele for tallness, and the other is a recessive allele for dwarfness.
The dominant form is the overriding allele. It is strong enough to "cover up" or hide the effect of the recessive allele. Although unseen, the recessive allele is still there. We express dominant alleles with a capital letter (A) and recessive alleles with a lower-case letter (a). So in the above case, the genes for controlling plant height in a diploid hybrid would be written Aa (a letter for each allele). If the dominant allele (A) is for tall, than the plant would be tall, with the recessive allele (a) masked.
Another plant might have both dominant alleles. We would then express the situation as AA. Since both alleles are the same dominant alleles for tall, the plant would again be tall. Conversely, if a gene's alleles were both recessive we would express the combination as aa, In this case, there is no dominant allele to override the recessive allele and the plant would be dwarf. The double recessive allele combination for a gene is the most important and most overlooked discovery in daylily genetics.
Mendel said that if we were able to take a large enough representative sample of seeds from a given cross, we would expect to see a predetermined distribution of alleles in our gene pool. Let's follow the height gene to keep things simple, again assuming that the allele for tall is dominant. Let's also agree to determine that we wish to take a tall plant (AA) and reduce its size. We cross blossoms on the tall cultivar with pollen from a single bloom on a dwarf cultivar. Assuming the wind or bugs haven't messed up our experiment, we happily collect 100 seeds from the cross.
When we bloom these seedlings, Mendel said we would expect to see them all tall, since all received the dominant allele from the tall (AA) parent. The offspring would all have the Aa combination of alleles. One allele for dominant tall (A) and one recessive allele for dwarf (a) would be present in each plant. All 100 seedlings' genes for height would look the same. We call this the "F1 generation." Note that the recessive allele is still there but not in evidence in any of our F1 seedlings. Many folks compost F1's when they fail to show the desired recessive gene. What a waste! Mendel tells us it will not generally be possible to see recessive characteristics in F1 generations.
So how do we get those elusive recessive genes to surface? The answer is that we have to be sure that we hybridize blooms carrying the recessive allele. That's no problem in our hypothetical example, since we have 100 seedlings, all carrying the Aa combination for height control. We can also be sure that our dwarf cultivar parent is also carrying the recessive aa combination. To keep things simple, we can cross any two of the siblings. This is acceptable, since they all have the Aa combination. Again assume that we collect and bloom 100 seedlings from our second cross. These seedlings are known as the "F2 generation."
According to Mendel's experiments, he found that when two plants carrying the Aa alleles were crossed, the results were in a predetermined numerical distribution. The distribution was 25% of the plants had two dominant alleles (AA), 50% of the plants carried one dominant and one recessive allele (Aa), and 25% of the plants had two recessive alleles (aa). He expressed this as a ratio of 1:2:1 or AA-2Aa-aa. What it means to us is that 75% of the plants would be tall, since they carry the dominant A allele for tall. However, 25% of the plants (aa) should be dwarf, since no dominant allele for tall is present to override the recessive allele. In our 100-seed sample, we should wind up with 25 plants showing dwarfness.
This is very basic. In the real world, there are many things that influence genetics. It isn't all that simple. Nevertheless, it still gives us some insight into things we must think about when deciding what pollen to put where. We must keep records if we want to see any kind of success with our F2 generations. A list of plant characteristics (genes?) before and after will help us identify dominant and recessive traits.
When we make a cross, we should write down what it is we trying to do and how we are trying to do it. If you are trying to get a red edge on a cream flower with a red eye for example, you might use a flower that has a reddish picotee for a pollen parent or vice versa. The red picotee edge might be recessive and this means going to the F2 generation to recover the trait and find out if it is. If we are going to evaluate only the F1 for a red edge from this cross and trash our failures, then why bother to make the cross in the beginning?
Tetraploids carry 44 chromosomes or twice that of diploids. Since we know the chromosomes are the gene carriers, we can use our imagination to see tremendous possibilities that tetraploids have opened up to us. Someone took me to task for mentioning that 100 seedlings is a good representative sample of a given diploid cross to assess its gene pool characteristics. I can only quote what I have read from others who have been working with diploids for 30 or more years. It is not necessary to work a cross to death to find out if it is in fact a failure. With 40,0003 registered plants to use, when a cross does not show promise, one should move on.
1Principles of Genetics, Tamarin & Leavitt, 3rd Edition, Wm. C. Brown, 1991
2The Way Life Works, Hoagland & Dodson, Random House, 1995
The AHS Publishes three booklets that go into more detail on Hemerocallis Genetics (See any AHS The Daylily Journal):
The Art of Hybridizing by Oscie Whatley
The Elusive Blue Daylily by several authors
Some Basic Hemerocallis Genetics by Joanne Norton