In the last essay (Part LI), we found that Mendel’s brilliant work was lost for some thirty two years. The April 1900 issue of Berichte der Deutschen Botanischen Gesellschaft (Reports of the German Botanical Society), University of Amsterdam’s botany professor Hugo de Vries (1848-1935) reported that he had previously discovered the same principles of heredity that Mendel had discovered; de Vries gave Mendel credit but had become aware of that fact only after his own discovery. His paper was titled Spaltungsgesetz der Bastardo (the law of segregation in hybrids). Not to be left out, Carl Correns (1864-1933) of the University of Tubingen claimed that he too had recently found the same laws but also had no knowledge of Mendel’s discovery beforehand. Finally, in the June issue, Erich Tschermak von Seysenegg, (1871-1962) made the same claim.
All three apparently came to the same conclusion independently of each other. Over the years many researchers have investigated each of the claims and have come to different conclusions. Since then, many papers and books have been written on the subject. (Zevenhuizen, Erik)
Moving on, within two years Archibald Garrod confirmed that transmission of alkaptonuria followed Mendelian principles. At this point, a clarification of Mendelian genetics needs to be made. Mendelian genetics refers to single gene traits. Over 7,000 human diseases are known to be caused by a single gene.
Before proceeding further, we need to understand two very important processes that take place in all sexually reproductive organisms—meiosis and fertilization. We have two kinds cells, somatic and sex. Somatic cells have a full set of chromosomes (diploid, 46 in humans), the normal number whereas the sex cells (egg and sperm), have the haploid number, 23 in humans. Meiosis is the process of reducing the number of chromosomes from the diploid to the haploid number. Also called reduction division, the process occurs in the sex cells (gametes) and involves two stages meiosis I and meiosis II. The real reduction in chromosome number occurs in meiosis II. This must happen so that later when fertilization occurs, the offspring will have the proper (diploid) number of chromosomes in the body for that species. The details of meiosis can be somewhat complicated; the main thing to remember is that the number of chromosomes has to be halved to maintain the correct diploid number of chromosomes from generation to generation which is very important in genetics. Most students found meiosis to be a difficult concept to understand and without some visual aids, they found genetics to be very difficult. We biology teachers often became frustrated and one year I attempted to just skip that chapter (meiosis) but eventually had to go back and cover it before continuing on with genetics.
For an overview of the principal events of meiosis including differences between spermatogenesis and oogenesis, see “For further understanding” at the end of this essay. Of extremes importance is Mendel’s Principle of Segregation which states that the alleles of a given locus segregate into separate gametes during meiosis.
At this time, allow me to discuss the typical and historical way to represent a genetic cross using a Punnett square. In another essay I will present an alternative method of doing this but for now we will use Punnett squares. Let’s do a cross between a homozygous (purebred) tall pea plant with a short plant.
Homozygous tall = TT X short (homozygous) = tt
The result of meiosis is shown by the separation of alleles in the two TT’s representing ,say, the pollen (male sperm , ♂) and tt in, say, the egg (in the ovary of the pistil, female ♀) The offspring are represented in the interior squares. As a result of this mating, you would expect 100% of the offspring to be hybrid tall (dominate)*. (National Human Genome Research Institute) This was Mendel’s original mating and represents Mendsl’s Principle of Dominance.
The next cross typically produces 25% homozygous tall (TT purebred), 50% hybrid tall (Tt), carrying one recessive allele, form of a gene and represented by a letter (i.e. T or t), and 25% short (tt) and represents Mendel’s second generation and principle which says that recessive traits are always dominated or masked by dominant trait. :For example, when pea plants with round seeds (RR) are crossed with plants with wrinkled seeds (rr), all seeds in F1 generation were found to be round (Rr).
Will you always get a 3: l ratio of tall to shout plants? No; I always told my students there is a 75% chance that any seed produced will grow up to be tall and a 25% chance of a short plant but if you have a bag of 100 seeds you may have a different ratio. (You may pull out, say, four short seeds in a row). It’s all a matter of chance. Punnett squares will work for two or three trait crosses but become very cumbersome. A monohybrid (one trait) cross such as those above require 4 interior squares(41 = 4). The superscript represents the number of traits being considered, in this case, height of plant. If we jump to a dihybrid cross (i.e. height and say, seed color) we need 42 =16 total and for a trihybrid let’s say, round vs. wrinkled seeds added, we need 43 = 64 squares.
Principal of independent assortment
A good example of independent assortment is Mendelian dihybrid cross. The presence of new combinations – round green and wrinkled yellow, suggests that the genes for the shape of the seed and color of the seed are assorted independently
Glossary
Allele An allele is one of two or more versions of DNA sequence (a single base or a segment of bases) at a given genomic location. An individual inherits two alleles, one from each parent, for any given genomic location where such variation exists. If the two alleles are the same, the individual is homozygous for that allele. If the alleles are different, the individual is heterozygous
Gene The gene is considered the basic unit of inheritance. Genes are passed from parents to offspring and contain the information needed to specify physical and biological traits. Most genes code for specific proteins, or segments of proteins, which have differing functions within the body. Humans have approximately 20,000 protein-coding genes.
Genotype A genotype is a scoring of the type of variant present at a given location (i.e., a locus) in the genome. It can be represented by symbols. For example, BB, Bb, bb could be used to represent a given variant in a gene. Genotypes can also be represented by the actual DNA sequence at a specific location, such as CC, CT, TT. DNA sequencing and other methods can be used to determine the genotypes at millions of locations in a genome in a single experiment. Some genotypes contribute to an individual’s observable traits, called the phenotype.
*Dominant, as related to genetics, refers to the relationship between an observed trait and the two inherited versions of a gene related to that trait. Individuals inherit two versions of each gene, known as alleles, from each parent. In the case of a dominant trait, only one copy of the dominant allele is required to express the trait. The effect of the other allele (the recessive allele) is masked by the dominant allele. Typically, an individual who carries two copies of a dominant allele exhibits the same trait as those who carry only one copy. This contrasts to a recessive trait, which requires that both alleles be present to express the trait. Dominant, as related to genetics, refers to the relationship between an observed trait and the two inherited versions of a gene related to that trait. Individuals inherit two versions of each gene, known as alleles, from each parent. In the case of a dominant trait, only one copy of the dominant allele is required to express the trait. The effect of the other allele (the recessive allele) is masked by the dominant allele. Typically, an individual who carries two copies of a dominant allele exhibits the same trait as those who carry only one copy. This contrasts to a recessive trait, which requires that both alleles be present to express the trait. National Human Genome Research Institute.
For further understanding:
Meiosis I
Prophase I
Metaphase I
The homologous pairs of chromosomes are aligned on the equatorial plate.
Anaphase I
The homologous chromosomes are pulled on the opposite poles. The sister chromatids remain attached to each other.
Telophase I
The nuclear membrane reforms and chromosomes decondense.
Cytokinesis I
The cell divides into two haploid daughter cells.
Meiosis II
Prophase II
Chromosomes condense and nuclear envelope breaks.
Metaphase II
The non-homologous chromosomes aligned on the equatorial plate.
Anaphase II
Sister chromatids move to opposite poles.
Telophase II
Chromosomes decondense and nuclear membrane reforms.
Cytokinesis II
The cell divides into four haploid daughter cell.
Credit to: BYJU’S
Note that in spermatogenesis four viable sperm cells are normally produced but in oogenesis only one viable egg cell is normally produced. Why? See answers below references.
References
- Zevenhuizen, E. Recognizing Mendel’s Rediscovery, Discovery, or Neither? American Biology Teacher, Vol 84 No 1 January 2022
- Talking Glossary of Genetic Terms – NHGRI – National Human …https—www.genome.gov › genetics-glossary National Human Genome Research Institute.
Why Does Spermatogenesis Result in Four Spermatids but Oogenesis Only in One Ovum?
Spermatogenesis leads to the formation of four spermatids from each primary spermatocyte, whereas each primary oocyte produces only a single ovum during oogenesis. This is due to the fact that during oogenesis meiotic division is unequal. The primary oocyte produces one larger secondary oocyte and a tiny polar body. Similarly, the secondary oocyte produces one large ovum and a tiny second polar body. The ovum retains the bulk of the cytoplasm and polar bodies degenerate.
Credit to: BYJU’S
And to that I would add that it is nature’s method of birth control.