In animals, the gametesproduced in females are calledeggs and those produced in males are calledsperms. The definition of the word "sex" is based on the existence of males and females.Varioustypes of marriage have become accepted in humanculture, but from a biologicalperspective,everyone must have one father and one mother.Humansconsist of two sexes—male and female—and canproduceprogenythroughreproduction of gametes from both sexes.
This factseemsobvious, but it allowsonly one possiblearrangement of sexes when viewed in context of nature as a whole.Unicellular organismsusuallyreproduceonly by simplecell division.Malehoneybeesdevelop from unfertilized eggs, and therefore, have onlyhalf the amount of DNA of females.Water fleas(genusDaphnia)go throughparthenogenesis*9, which producesonlyfemales. In nematodes, sperm-producing and egg-producing organsbothexist in the sameindividualorganism, and fertilizationoccurswithin the sameorganism*10. In water fleas and nematodes,males first appear when the environmentdeteriorates.Somefishes are known to exhibit dichogamy or sequential hermaphrodism, i.e., sometimes they change from male to female (protandry) and sometimes from female to male (protogyny).
The human genomeincludes the sex-determining X and Y chromosomes. If the organismreceivesX chromosomes from bothparents and contains the combination XX will be female, and if it contains the combination XY, it will be male.In other words, the biologicalsex of humans is determined by the combination of chromosomespassed down from the parents. In the above examples of honeybees and fish,sex is determined by the amount of DNA and the environment,respectively.

*9 Reproduction without fertilization.
*10 Calledhermaphroditism

3.3.2

Origin of Sexes

Even in the unicellular organismsmentioned above,cellssometimesfuse,shuffle their chromosomes, and divideback into individual cells. The combinations of the cells that fuse with one another are prescribed by the proteins they express or the kinds of chemical compounds they synthesize;incompatiblecells do not fuse. The origin of primitivesexescan be supposed from such phenomena.Furthermore,after the chromosomes of these organisms are shuffled, they exchange their DNA sequences by genetic recombination. This genetic recombination is thought to have an importantsignificance for the existence of sexes.

3.3.3

ReproductiveCells and Meiosis

Let us takehumans as an example:sperm is provided by the father and egg is provided by the mother; when the spermfertilizes the egg, the development of an individualhumanbegins(seeChapter 5, Fig. 5-1). DNA sequences from both the father and mother are present in the cells of the human body.Therefore, the sperm and egg must eachcontainhalf of the parentalDNAbeforefertilization.In fact,germ cells(gametes) with half of the parentalDNA are produced by a processcalled "meiosis" from a primordial germ cell11 that has differentiatedspecifically for this purpose.DNA from the father and mother is reshuffled during meiosis. One chromosome comes from the father,another from the mother, and these two chromosomes are randomlyredistributed in their offspring. More importantly,even when the two chromosomes are identical,DNA sequences from the father and mother are intermixed by genetic recombination (Fig. 3-8). In other words, the production of gametesinvolvesgenetic recombination and reshuffling of the chromosomes. The sexes of bothhumans and unicellular organisms are the same in the sense that they bothperform the importantrole of genetic recombination.Furthermore,sincegenetic recombination disarranges the precisereplication of DNA, the existence of sexes and the accompanyinggenetic recombinationcreateflexibilitywith respect tochanges in genetic information.

Fig. 3-8. Genetic Recombination Occurring during Meiosis

3.3.4

ArtificialGenetic Recombination and Gene Therapy

With the progress in science and technology, it is nowpossible to manipulateartificialgenetic recombination (Fig. 3-9). Furthermore, by effectivelyutilizingviruses that infectcells,gene therapy is nowperformed by expressing a normallyfunctioninggene in the body of a patient(seeSection 1 of Chapter 11 [11.1.7]).
Today, gene therapy for humans is applied to somatic cells(cellsother than germ cells).In other words,gene therapy is performed on the somatic cells of patients to treatdiseases.
There is a big differencebetweengene manipulation in somatic cells and in germ cells.Gene manipulation in somatic cells does not transmitartificialrecombinantgenes to subsequentgenerations.In contrast,genetic engineering in germ cellseasily reminds us of a superman born with incredibleintelligence and legs as fast as bullets(seeColumn in Section 1 of Chapter 11) *12.
In a scientificsense,genetic recombination that has been established in experimental animals is probably also possible in humans.However,gene manipulation, which introducesrecombinantgenes into germ cells, has not yet been performed, and the introduction of foreigngenes into germ cells is nowstrictlyprohibited*13.

Fig. 3-9. Schematic Diagram of Gene Therapy

VirusescontainingartificiallyconstructedDNA are infected into human cells. The infected cellexpressesrecombinantgenes

*11 Cellbefore it becomes a germ cell
*12 In prenatalgenetic testing,genes are taken from a fetusbefore it is born.Although this process is not gene manipulation, it has already been accomplished(seeSection 1 of Chapter 11) .
*13 Even with moderngene therapy, the possibility that an insertedforeigngeneentersgerm cellscannot be completelydisregarded.

Sex Chromosomes and Genetic Diseases

Muscular dystrophy is a disease in which musclesshowprogressiveatrophy and become unable to function in patients.In particular,Duchenne muscular dystrophy,namedafter its discoverer, is caused by mutations in a gene for a proteincalleddystrophin.Duchenne muscular dystrophy is a recessive disorder.In general, if there is even one dominant gene in either of the pair of chromosomes from the father and mother, the phenotypewill also be dominant. If bothalleles of the gene are recessive, the phenotypewill not be expressed.Therefore, one wouldexpectrecessive diseases to have lowerphenotypeexpression frequencies than dominant ones. However,Duchenne muscular dystrophy is known to occur at an especiallyhigh frequency in males.
This is because the dystrophingene is on the X chromosome.Females have the XX combination of sex chromosomes;thus,as long asmutations do not occur in bothsex chromosomes,muscular dystrophywill not develop. In males,however, the sex chromosomecombination is XY. Eventhough the disease is recessive, its phenotypewill be expressed if there is a mutation on just one X chromosome.As such, the manner in which the phenotypes are transmitted by genes on the sex chromosomesdiffersbetweenmales and females. This mode of inheritance is referred to as "sex-linked inheritance." The Duchennetype of muscular dystrophymentioned here is an example of a genetic disease that is caused by a sex-linkedrecessive gene.In contrast,Huntington's disease(seeSection 1 of Chapter 11 [11.1.6]) is caused by a mutation of a gene on chromosome 4, and is known to be autosomal dominant.