Early in Prophase I, as the nuclear envelope begins to break down, homologous chromosomes pair up with one another so genes are aligned precisely with each other. As in mitosis, spindle fibers from centrosomes will attach to the centromeres, but in this case they attach to a set of homologous chromosomes. While the chromosomes are lined up in this way, a process called crossing over can take place, in which chromosomal segments from maternal DNA can be exchanged with paternal DNA. The crossover events are the first source of genetic variation in the nuclei produced by meiosis. When sister chromatids are separated, each will carry some DNA from one parent of the individual and some DNA from the other parent. Each recombinant chromatid has a combination of maternal and paternal genes that did not exist before the crossover.

During Metaphase I, homologous chromosomes are arranged randomly along the equator of the cell. This is important in determining the genes that will be passed along to the final daughter cell, as each will only receive one of the two homologous chromosomes. This randomness is the physical basis for the creation of the second form of genetic variation in offspring. Because there is an equal chance that a spindle fiber will encounter a maternally or paternally inherited chromosome, this can produce a unique combination of maternal and paternal chromosomes that will make their way into the gametes.

In Anaphase I, the spindle fibers pull the linked chromosomes apart, while the sister chromatids remain tightly bound together at the centromere.

In Telophase I, the separated chromosomes arrive at opposite poles, after which the cells may separate or not, depending on the species.

In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II, but no S phase, nor chromosomal duplication, will take place.

In Prophase II, if the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed. Each sister chromatid attaches to microtubules from opposite poles.

In Metaphase II, the sister chromatids are aligned at the equator of the cell.

In Anaphase II, sister chromatids are pulled apart by the spindle fibers, move toward opposite poles.

Finally, in telophase II and cytokinesis, the chromosomes arrive at opposite poles and, and nuclear envelopes form. Cytokinesis separates the two cells into four unique haploid cells. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombining of maternal and paternal segments of chromosomes (with their sets of genes) that occurs during crossover.

Genetics is the study of heredity. Genes, carried on chromosomes, are the basic functional units of heredity with the capability to be replicated, expressed, or mutated.

Johann Gregor Mendel (1822–1884) was an Augustinian monk who researched patterns of inheritance in pea plants. He demonstrated that some traits are transmitted faithfully from parents to offspring in predictable patterns. Mendel performed hybridizations, which involve mating two true-breeding individuals (those that always produce offspring that look like the parent), and tracking the phenotypes (appearance) of subsequent generations. His research demonstrated that there are different versions of genes – what we now call alleles – that determine the traits offspring will have.

Some terms to understand:

• Character – an observable feature (for example, hair color)
• Trait – a variation of a character (for example, blonde, brunette, red)
• Allele – a version of a gene
• Genotype – the combination of alleles that an organism inherited (one from each parent)
• Phenotype – the way the genotype is expressed, usually thought of as the physical appearance for the character in question
• Homozygote – the condition of having inherited two copies of the same allele at a given gene or locus
• Heterozygote – the condition of having inherited two different alleles at a given gene or locus

Not all genes are transmitted from parents to offspring according to Mendelian genetics, but Mendel’s experiments serve as an excellent starting point for thinking about inheritance.

Phenylthiocarbamide

In 1931, a chemist named Arthur Fox was pouring some powdered phenylthiocarbamide (PTC) into a bottle. When some of the powder accidentally blew into the air, a colleague standing nearby complained that the dust tasted bitter. Fox tasted nothing at all. Curious how they could be tasting the chemical differently, they tasted it again. The results were the same. Fox had his friends and family try the chemical then describe how it tasted. Some people tasted nothing. Some found it intensely bitter, and still others thought it tasted only slightly bitter.

Soon after its discovery, geneticists determined that there is an inherited component that influences how we taste PTC. Today we know that the ability to taste PTC (or not) is conveyed by a single gene that codes for a taste receptor on the tongue. The PTC gene, TAS2R38, was discovered in 2003. There are two common alleles (versions of a gene) of the PTC gene, and at least five rare forms. One of the common forms is a tasting allele, and the other is a non-tasting allele. Each allele codes for a bitter taste receptor protein with a slightly different shape. The shape of the receptor protein determines how strongly it can bind to PTC. Since all people have two copies of every gene, combinations of the bitter taste gene variants determine whether someone finds PTC intensely bitter, somewhat bitter, or without taste at all. The ratio of tasters to non-tasters varies between populations, but every group has some tasters and some non-tasters. On average, 75% of people can taste PTC, while 25% cannot.

Although PTC is not found in nature, the ability to taste it correlates strongly with the ability to taste other bitter substances that do occur naturally, many of which are toxins. Plants produce a variety of toxic compounds in order to protect themselves from being eaten. The ability to discern bitter tastes evolved as a mechanism to prevent early humans from eating poisonous plants – those humans who had the ability to avoid toxic plants were better able to survive and pass on their alleles to offspring. Humans have about 30 genes that code for bitter taste receptors. Each receptor can interact with several compounds, allowing people to taste a wide variety of bitter substances. Plants are much more likely than animals to contain toxins. Because avoiding bitter plants would severely limit their food sources, strict herbivores have fewer bitter taste genes than omnivores or carnivores. Instead, animals that graze on plants have a high tolerance to toxins. Grazers have large livers that can break down toxic compounds.

If the ability to taste bitter compounds conveys a selective advantage, then shouldn’t non-tasters have died off long ago? Why do so many people still carry the non-tasting PTC variant? Some scientists believe that non-tasters of PTC can taste another bitter compound. This scenario would give the greatest selective advantage to heterozygotes, or people who carry one tasting allele and one non-tasting allele. The ability to taste PTC shows a dominant pattern of inheritance. A single copy of a tasting allele (T) conveys the ability to taste PTC. Non-tasters have two copies of a non-tasting allele (t).

The chemical structure of PTC resembles toxic alkaloids found in some poisonous plants, like ricin, which can be extracted from castor beans. It may also be the reason that some people find broccoli and cabbage bitter to the taste.

The PTC gene is one example of an inherited trait in humans that is determined by a single gene. Today, we’ll see which allele of this gene you possess.

For centuries humans have selected plant varieties and animals for specific traits.

• Plant breeders select plant varieties which produce more seed or fruit
• Livestock producers select animals with specific traits such as increased milk production, ample muscle mass or structural correctness
• Dogs have been bred for physical traits like eyesight and speed, as well as personality traits such as aggression or friendliness

Heredity is the passing on of traits from parents to offspring. Most plants and animals have two of every kind of gene, one from their mother and one from their father.

• Only one allele from each parent is passed to each offspring for a particular trait.
• Alleles are forms of the same gene with small differences in their DNA sequence. These small differences contribute to each organism’s unique physical features.
• These physical features are called phenotypes.
• The genetic makeup of an organism is its genotype.

Some alleles are dominant while others are recessive.

• Dominant alleles are always expressed in offspring
• Recessive alleles are only expressed in offspring if both parents contribute a recessive allele

For example, in humans, the allele for freckles is dominant.

• Children who receive a “freckle” allele from either parent will exhibit the trait
• Children who receive two recessive alleles will be freckle-less.