Meiosis

Meiosis is a process of cell division used to produce gametes (sperm and egg cells). Meiosis differs from mitosis in the fact that gametes have half the number of chromosomes as the parent cell (if you are not familiar with mitosis, visit the cell signaling and cycle page). Meiosis is divided into two portions: meiosis 1 and meiosis 2. In meiosis 1, the cell progresses through prophase 1, metaphase 1, anaphase 1, and telophase 1. These processes are similar to mitosis. Differences begin to appear during prophase 1 when chromosomes condense but also align with their homologous pair (that is, the chromosome that contains the same DNA). During metaphase 1 of meiosis, these homologous pairs line up next to each other in the center of the cell. Then in anaphase 1, the homologous chromosomes separate to each side of the cell, rather than sister chromatids separating as they would in mitosis. Next, meiosis 2 occurs. This process is identical to mitosis except for the fact that the cells are haploid (have half the number of chromosomes) when they begin prophase 2. Again in metaphase 2 the chromosomes line up in the center of the cell, and again in anaphase 2 the sister chromatids are split. After telophase 2, there are 4 new gamete cells, each with genetic variation and a haploid number of single chromosomes.

Genetic diversity within a species is the foundation of natural selection and adaptability to the environment, so a species that can maximize genetic diversity has the best chance of survival. Meiosis increases genetic diversity in three ways: crossing over, independent assortment, and random fertilization.

Crossing over occurs during prophase 1. When the homologous chromosomes pair up, they also exchange bits of genetic information. The two chromosomes form a tetrad and a chunk of DNA is swapped between the two chromosomes. This occurs randomly and ensures that the DNA in the daughter cells is not identical to the DNA in the parent cell.

Independent assortment occurs in metaphase 1. When the chromosome pairs line up in the center of the cell, their orientation determines which daughter cell they will be a part of. Each individual chromosome pair can line up in one of two possibilities, with each chromosome being on either side of the dividing center line. This means that there are 2^n possibilities for orientations for the chromosomes in this step where n is the haploid number of chromosomes. In humans for example, we have 46 chromosomes which form 23 pairs, so our haploid number is 23. This means that we have 2^23 or 8388608 genetic possibilities from independent assortment in one parent alone.

Random fertilization occurs after the meiosis process but is made possible by meiosis. When fertilization occurs, the haploid genetic material from each parent’s gametes combine into a genetically distinct diploid child. Since any sperm cell can fertilize the egg, this process is random. Accounting only for independent assortment in each parent (and neglecting crossing over which is more difficult to accurately calculate), there are 2^2n possibilities from random fertilization given the independent assortment that occurs within each parent. This means that in humans there are 2^46 (a really big number) of genetic possibilities from two parents without even counting for crossing over.

Mendelian Genetics

Gregor Mendel and his experiments on pea plants laid the foundation for our modern understanding of genetics. Mendel’s research was based on discrete characteristics, that is characteristics that had only two options such as tall/short or green/yellow.

Law of Segregation

The Law of Segregation states that each gamete cell contains only one copy of each gene. A normal cell in G1 has 2 copies of each gene, while a normal cell in G2 has 4 copies (for more information of G1 and G2 visit the cell cycle page). These single copies of genes are separated randomly into each gamete, which is the Law of Segregation.

Law of Independent Assortment

The Law of Independent Assortment states that the gene for one trait (known as an allele for that trait) is sorted into a gamete in a way that does not depend on the sorting of any other alleles.

This theory allows us to use Punnett Squares to predict genetic models. In a Punnett Square, the letters represent certain alleles. The capital letters represent dominant traits while the lower case letters represent recessive alleles. Punnett Squares show both the genotype and the phenotype of an organism. The genotype is the genetic code of a trait, and the phenotype is the outward expression of the trait. In the image below, we look at Mendel’s original model as an exploration into the use of Punnett Squares.

Non-Mendelian Genetics

The idea of dominant always “covering up” recessive traits is not always an accurate model. Such is the case in codominance and incomplete dominance. Codominance occurs when both alleles are expressed in the phenotype. One common example is blood type. The alleles for blood are A, B, and O. None of the alleles are dominant; rather, in AB blood, for example, both the A and B factors are present together. Similarly, some flowers express both of the alleles in their genotype by having flowers with two colors.

Incomplete dominance occurs when traits are “mixed” in the phenotype. A common example is a red and white flower being crossed to make a pink flower. In incomplete dominance, the heterozygous genotype results in a different genotype than the homozygous dominant genotype as the dominant and recessive traits are mixed.

Sex linked traits are characteristics with genes on the X and Y or sex chromosomes. Sex linked recessive traits are more common in men because men only have one X chromosome. Therefore, if a man has a recessive trait on his X chromosome, it will always be expressed since there is no other X chromosome to carry the dominant gene. This is true in colorblindness and hemophilia. Men are more commonly afflicted with these conditions, but women who are homozygous recessive on their X chromosomes can also have these conditions.

Mitochondrial/chloroplast DNA

Mitochondria and chloroplasts contain small amounts of DNA. Though the use of this DNA is still relatively unknown, it is not passed on through Mendelian means. Mitochondria and chloroplasts are only passed through the mother/female gametes as the father/male gametes (sperm or pollen) only carry genetic material. Therefore, the random distribution of mitochondria or chloroplasts in the egg or ovule determines how these genes are passed on, but they are always passed through the mother.

Physical characteristics

When genes are close together on the same chromosome, they can appear as if they are genetically linked. When genes on the same chromosome are farther apart, they can more easily cross over and be separated onto different chromosomes. However, when genes are physically close, they will more often remain linked together. This information can be used to create a gene linkage map which shows the order of certain genes on the chromosome as seen in the example below.

Environmental Impact on Genes

Epistasis is when a gene at one location on a chromosome impacts the expression of other genes. One example is if a gene on one chromosome codes for black or brown fur, but a gene on another chromosome codes for the creation of fur pigment or not. In order to have black or brown fur, the organism must also code for creation of pigment; otherwise it will have white fur with no pigment no matter what the other chromosome codes for.

Epigenetics is the material that we pass on with our DNA that is not part of our DNA sequence which is determined by our environment. The most common forms of epigenetics is DNA imprinting: methylation and acetylation. Methylation occurs when methyl groups are added to the DNA strand which tightens the coil and represses gene transcription. Conversely, acetylation occurs when acetyl groups are added to the DNA which widens the coil and increases gene transcription