The Fundamentals of Genetics

Basic principles of genetics are identical for plants and animals.

Man has been improving his crops and domesticated animals for 10,000 years or longer. In some instances he has modified the crop so much that we are not sure what wild organism was the original parent. That is true for Atlantic Giant pumpkins.

Even before man, the plants and animals themselves and selection by nature had been practicing genetic improvement. When any animal selects a mate, the future for the species may be slightly different than if a different mate had been selected. It is common to suppose that animal matings in nature occur randomly, but nature photographers have film which show animals work very hard to attract mates. Many plants have self-sterility factors which force cross pollination.

The cole crops present an interesting demonstration of the power of selective breeding. It is said that most cole crops came from a common origin and that you can begin with any one of these crops and derive any other. The cole crops are cabbage, cauliflower, rutabaga, etc.

In the old days, breeders thought the traits of an offspring were due to a blending of the traits of the parents. Some folks still speak of a cow being 1/4 blood Holstein. We now know that blood is not the basis of heredity and Mendel proved inheritance occurs by lumps of genetic matter which we now call genes. I still hear gardeners using the blending theory; they are wrong! Due to the unit nature of genes, it is possible, but unlikely, that you do not have a single gene from your grandfather. The same is true for your grandmother. However, you do have cytoplasm from your materal grandmother! Same is true in plants because no sperm carries much cytoplasm.

I shall never forget a lecture I heard during the summer of 1960 at Murray State University. An anatomy professor from the University of Lousiville itemized 10 evidences that DNA carries the genetic information from generation to generation. I was there as one of the teachers, but that lecture played a part in my going back to Purdue to earn my Ph.D. in molecular genetics.

Inheritance of a Single Locus Trait

There is a lethal yellow seedling trait in Cucurbit maxima. Let's use it to begin our study of genetics. There is a location (locus) on some chromosome of Atlantic Giant where this gene resides. If there is a good allele (gene) in that location then green chlorophyll gets made, else there is no good chlorophyll and the plant is yellow and dies when all the food stored in the seed is exhausted.

A homozygous (pure) normal plant has this genotype +/+ at that locus. I use + to indicate the normal effective gene. A homozygous yellow seedling plant has this genotype ys/ys. A heterozygous (hybrid) plant is +/ys. Notice that the heterozygous plant has one good allele and one defective allele. We often use + to denote a good allele. Some people use YS to denote the good allele that has some unknown role in making good chlorophyll. We say + is the dominant allele because it only takes one + allele to make a full amount of chlorophyll. On a molecular basis + is dominant because it makes a good enzyme (or something) and one dose of + is often just as good as two doses of +.

We must now consider the genetic content of eggs and sperm. The +/+ plant makes only + eggs and + sperm. The +/ys can make + eggs and ys eggs. It also makes + pollen and ys pollen grains.

What phenotypes do we expect when we self a +/ys plant? We have + and ys eggs and we have + and ys sperm. They will join randomly to produce +/+, +/ys, and ys/ys zygotes. How many of each do we expect?

Below is a Punnett Square which shows what is expected when you self a heterozygous plant ys/+ where + denotes the allele for normal green plant. Since we are selfing, the male and females gamets will be the same. There will be + and ys eggs and + and ys sperm. To determine the genotype type of the invidvidual represented by any square, look at the two headers for that square. I am using the @ sign in each square to donote the expected phenotype (appearance) of the plant grown from the seed represented by the square.

Female Gametes
+ ys
Male

Gametes

+ +
+  @
+
ys  @
ys +
ys  @
ys
ys  @

Notice that the Punnet Square above predicts 3 green plants and one yellow seedling which will die. The phenotypic ratio is 3 green to 1 yellow. Also written as 3 green : 1 yellow. The expected genotypes are 1 ++ : 2 +ys : 1 ysys. Which is often written 1 +/+ : 2 +/ys : 1 ys/ys. You should practice drawing some Punnett Squares so that you will recall how to use them when you are trying to predict the outcome of a monohybrid cross.

Punnett Square for a Test Cross

Suppose you want to know whether a plant is free of a recessive allele. Just mate it to a homoygous recessive individual. If all the offspring are normal, then the plant was free of the recessive trait. If the plant was heterozygous, half the offspring will be normal and half will show the recessive trait. As an exercise draw modify the above Punnett Square to demonstrate a test cross. You can't yellow seedling because ys/ys plants die and no pure ys pollen can ever be produced. Use a different trait such as B/b x b/b where B is brown eyes and b is blue eyes in man.

Punnett Square for a Two Loci Cross

Most of the time two or more gene loci are required to make a normal trait. I do not know what loci are needed to make Orange pumpkins. I suspect several are needed. Let's suppose a yellow and a red genes are neeed (this is not true, it is for colorful illustration only). The Punnett square below is drawn to illustrate selfing a plant heterozygous at the red and yellow loci when the two loci are on different chromosomes. Let's pretend the recessive genes make no color and rr yy plants produce white fruit. We often call such a plant a double reccessive because it has only recessive genes at the two loci. Remember this is all for illustration and fun. There is no evidence that Orange pumpkins get their color in his manner. However, the facts are very similar to this hypothesis, except that their are probably several genes required not two as illustrated here.

Let us begin by supposing we cross a homozygous red pumpkin plant with a homozygous yellow plant.

The red plant has this genotype R/R ; y/y genes. We are assuming the Red and Yellow are on different chromosomes. The red pumpking makes these gamete R y only. The yellow plant makes only r Y gametes.

All the childern (called F1) will be orange and have this genotype: Ry/rY . Notice I have switched to the other system where I call the good alleles R and Y and the defective alleles r and y. I use which ever system of symbols is easiest for me to understand in a given problem.

The gametes produced by the F1 will be Ry, rY, RY, and ry. You can now enter those in the Punnett square. The red and orange are too close I am using these colors:
Red = @   Yellow = @   Orange = @

           Female gametes
Ry rY RY ry
Male

gametes

Ry Ry/Ry
  @
RY/ry
  @
Ry/RY
  @
Ry/ry
  @
rY rY/Ry
  @
rY/rY
  @
rY/RY
  @
rY/ry
  @
RY RY/Ry
  @
RY/rY
  @
RY/RY
  @
RY/ry
  @
ry ry/Ry
  @
ry/rY
  @
ry/RY
  @
ry/ry
  @

Notice that we predict 9 orange : 3 Red : 3 yel : 1 white, but remember the genetics of pumpkin color are unknown. This is for illustration only. However, the 9:3:3:1 ratio is the common phenotypic ratio seen in a dihybrid cross. Often there is a modidfication of the ratio. Suppose the ry/ry genotype is lethal then there would be no white pumpkins. Suppose the red and orange were indistinguishable then the ratio would be 12 orange : 3 yellow : 1 white. Suppose a double dose of R gives a redder color then the RYRy would be a redder orange. In practice, colors of fruit can be difficult to classify. Often there are modifying genes which cause some or all colors to paler, more intense, or absent.


I welcome comments and any kind of information for this site please send your comments today while this site is being designed. You may have observations which along with others can help us identify more Pumpkin Genes.

You may send private e-messages to Dr. Eddleman and he will reply, usually within 24 hours.


First installed 1999 April 1      Revision #0 1999 April 1       indbio@disknet.com
Written by Harold Eddleman, Ph. D., President, Indiana Biolab, 14045 Huff St., Palmyra IN 47164
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