5. A little bit of genetics - segregation of traits and hybrid vigor
Let’s explore the laws of genetic inheritance initially proposed by J. G. Mendel. What is meiosis? And how is it responsible for the segregation of traits?
Life cycle in plants
During their life, plants, as sexual reproductive organisms, alternate vegetative and reproductive growth phases. The vegetative organs (roots, leaves, flowers, seeds, fruit) are the sporophyte, and every cell of these organs has two quantities of DNA (2n DNA), one from the pollen and one from the ovule. Ovule and pollen are called gametophytes because they contain the gametes: the sperm cells and the egg cells. The gametes are formed by meiosis and contain one quantity of DNA each (1n DNA). After fertilization, because of the fusion of the sperm nucleus and the egg cell nucleus, the embryo (in the seed) will have equal parts DNA from the male and the female parents, 2n DNA again. So, the life cycle of these plants is an alternate of sporophyte and gametophyte, an alternate of 2n and 1n DNA.
All the (animal and plant) cells divide to propagate. They do so by mitosis. But to make the sperm cells in the pollen grains and the egg cell in the ovules where the quantity of DNA needs to be reduced, the cellular division is a bit different. It is meiosis. And meiosis is a reducing division. This works in animals like in plants!
DNA is compacted into chromosomes. Each species may have a different number of chromosomes. However, they come by pair, one from the mother and one from the father. The compacted DNA in the chromosome carries genes. Each gene, when it is expressed into protein, will have a particular function. Some proteins may be responsible for visible traits, like the color of the seed, the color of the flower, the size of the plant, etc. We call these visible traits phenotype. The genes have two copies, one of each chromosome, from the father or the mother. Each copy is called an allele. Because each allele originates from each of the parents, they may produce similar proteins or proteins that may have slight differences. We speak of genotype. Check the part about the inheritance of traits to know more.
During meiosis, the number of chromosomes is reduced by two, from 2n to 1n DNA. Meiosis consists of two nuclear divisions with only one DNA replication, Meiosis I and Meiosis II. During DNA replication, each chromosome pair duplicates to have two arms (chromatids). During mitosis, each chromatid of each chromosome pair is distributed to the two daughter cells. So each daughter cell will have one chromatid per chromosome, like the parental cell. During meiosis I, the chromosome pairs (with two chromatids each) are separated in each daughter cell. So, each daughter cell has only one chromosome from each pair. During meiosis II, the chromatids are separated in each daughter cell. It results in four cells with 1n DNA, one of the chromosome pairs with one chromatid. Therefore, each of the four gametes will have a different composition, with the same amount of DNA. This will generate diversity during fertilization.
Find out more: Meiosis in more details
Suppose you wish to understand more about meiosis. In that case, some fascinating videos explain why meiosis is different from mitosis and why meiosis is essential for reproduction and, thus, for seed and fruit formation. The videos give examples with the number of chromosomes in human cells. The same happens in plants with a different number of chromosomes. Remember that sexual reproduction is essential to bring diversity and segregation of characters!
Inheritance of traits and segregation
It is said that children look like their parents. And traits are passed from generation to generation. It is the heredity and inheritance of traits from parents to offspring. This is not only for humans. It works for any organisms which reproduce sexually, including plants. Gregor Johannes Mendel performed the first experiments about the inheritance of traits in the greenhouse of the Augustinian monastery in Brno around 1860.
J.G. Mendel postulated that the traits are carried by “unit of inheritance” that we now know to be the genes. Each gene has two alleles. One allele is inherited by the mother and one by the father. And those alleles may be different. If like Mendel, you would follow, for example, the color (green or yellow) or the shape (smooth or wrinkled) of peas, you would cross a plant producing yellow peas with a plant producing green peas. The color of the peas is carried by one gene with two alleles, one for yellow (let’s name it Y) and one for green (let’s name it y). The offspring of this cross will be yellow peas only. This is their phenotype. We say that the yellow allele is dominant over the green allele, which is recessive. Let’s write this with the allele code:
Parent 1 YY (yellow peas) is crossed with Parent 2 yy (green peas). So the gametes of each parent will have the Y allele for parent 1 and the y allele for parent 2. The Offspring 1 is Yy, the mix of both alleles. This is the genotype. Because Y is dominant, the peas will be yellow.
Then the offspring 1 (Yy) plant is selfed. The gametes may carry either the Y allele or the y allele at a 50% chance. Therefore, the peas of offspring 2 will be either yellow or green. Look at the 2x2 square in the picture. The mother can give the Y or the y. The father also does the same. The offspring will be YY (1 chance out of 4), Yy or yY (2 chances out of 4), or yy (1 chance out of 4). The proportion for the phenotype is 3 yellow peas for 1 green pea (3:1), while the proportion for the genotype is 1:2:1.
How Mendel's pea plants helped us understand genetics
Take coins or stones of two different colors or a set of lego pieces of two different colors or shapes. Choose which character(s) will be dominant and which will be recessive. If you use pieces from draughts, black may be dominant over white; if you use lego, blue over red, or size 4 over size 2.
And reconstruct the allele distribution for one character (the shape or the color).
If you feel like it, you may try with two characters!
To find out if the peas are Yy or YY, as the peas are yellow in both cases, you can do a test-cross. It is a cross done with the parent carrying the recessive alleles yy. So crossing Yy and yy will give peas Yy (yellow) and yy (green) in the same proportion, 1 to 1. And crossing YY and yy will only give yellow peas Yy.
Look at the picture below where hairless and hairy guinea pigs have replaced the yellow and green peas traits.
„If the two alleles are identical (YY or yy), we say the plant is homozygous for the Y or y allele. If the two alleles are different, we say the plant is heterozygous.“
Crop breeding is performed by crossing varieties of the same species to remove unwanted traits and bring together the traits of interest: growth rate, yield, stress resistance, etc. Some traits result from a combination of multiples alleles of various genes, which makes understanding the genetics beyond it quite challenging!
Some other traits are enhanced in the hybrid offspring because of mixing the genetic information of both parents. This is the hybrid vigor. These hybrids tend to be bigger, grow faster and be more fertile than either parent.
Name the offspring and find the name of the Father of Genetics
Find out more: Not all traits follow the Mendelian segregation rules
Some traits are codominant or the results of the presence of multiple genes. To find out more about this, look at the video of the Amoeba Sisters on this topic.
Test your vocabulary in Genetics