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It has been postulated that the critical events leading to major differences between humans and the great apes are associated with major changes on sex chromosomes. Regions of homology between the human sex chromosomes have arisen at different points during mammalian evolution. The two largest blocks are specific to hominids, having appeared at some time after the divergence of humans and chimpanzees. These are the second pairing region found at the telomeres of the sex chromosome long arms and a region of homology between Xq21.3 (X chromosome long arm) and Yp11 (Y chromosome short arm). Questions arise as to whether these regions of the sex chromosomes contain functional genes and these genes might be candidates for the differences in cognitive function that distinguish modern humans from their ancestors. Furthermore, divergence between functional sequences on the X and the Y may lead to a more subtle sexually dimorphic variation.

This chapter provides a conceptual and methodological overview of relevant issues in sex differences/sexual differentiation research, and provides guidance to investigators studying the role of sex and/or gonadal steroids in a variety of physiological or behavioral functions in experimental animals and man. Topics discussed include the difference between sex and gender, testing organizational effects of gonadal steroids, testing activational effects of gonadal hormones, conditions that can affect the endocrinology of the estrous cycle, and testing sex-specific effects of sex chromosome genes.

The chromosomes carrying the genes that determine sexes or mating types often show strong heteromorphy. This chapter discusses the evolutionary trajectories of sex chromosomes, from the initial acquisition of a sex-determining gene, to the linkage of sexually antagonistic genes, and the suppression of recombination. The ensuing degeneration of non-recombining regions may occur in several steps, as testified by evolutionary strata. This process may end in the loss of the degenerated chromosome and its replacement by a proto sex chromosome evolving from an autosomal pair. The several genomic processes stemming from sex linkage and recombination arrest differ between the haploid (U or V), homogametic (X or Z) and heterogametic (Y or W) chromosomes, resulting in specific and divergent evolutionary fates. Several mechanisms (such as gene conversion and retention of X-Y recombination) may oppose the degeneration, whereas various forms of dosage compensation may accommodate it. Although strongly heteromorphic in some lineages, sex chromosomes have remained homomorphic in others, which requires further study.

The study of sex differences in the brain has a long, rich history and remains a vibrant and controversial topic that is central to the field of neuroscience both for its obvious relevance and its heuristic value. This chapter provides a brief historical perspective, largely by directing the reader to the many excellent reviews already available, while emphasizing emerging paradigm shifts in our view of the origin and functional significance of brain sex differences. It highlights two major new initiatives: the direct role of sex chromosome genes in determining brain sex differences, and, the novel theoretical view indicating that sometimes the sexes are striving to be the same. The chapter reviews 10 recent discoveries that have changed our thinking about sex differences in the brain.

Many of the early observations on unusual chromosome pictures concerned the number of sex chromosomes. Chapter 3 argues that a combination of technical, bio-medical, and cultural reasons were responsible for the first string of observations of sex chromosome anomalies and for the continuing research on these cases. It then focuses on two contested areas: firstly, the presumed connection between the XYY karyotype and aggression and violence and its potential impact on criminal justice cases and, secondly, the use of karyotyping for gender verification practices in the competitive sports context. Especially the first case on the “criminal chromosomes” has become notorious in the history of human genetics and is often cited as an example of biased science. The chapter aims to explain the length and vehemence of the debate as well as its role in an emerging ethical discussion on clinical research and prenatal and newborn genetic screening and prospective studies in particular. Chromosome testing for female athletes was first introduced at the Olympic Games in Mexico City in 1968 and became immediately controversial. Both cases serve to gauge the weight laid on biology and human heredity to explain social behavior and inform social policies in the long 1960s.

Although clonality is often discussed in reference to whole organisms, the phenomenon also applies to (and is underlain by) genetic processes operating within each individual. All forms of clonal reproduction begin with the faithful replication of genetic material. This chapter discusses the clonal propagation of nucleic acids (via DNA replication) and of entire nuclear genomes and chromosome sets (via mitosis) in populations of somatic cells. It also describes how mitochondrial genomes, as well as particular kinds of sex chromosomes, provide special examples of genetic systems that abstain from recombination. The net result of such micro-asexual processes is a multicellular individual, which can thus be viewed as a tightly knit colony of clonemate cells.

Transitions among sex-determination systems and mechanisms are manifold and surprisingly frequent. The ultimate causes for such transitions are classified in three main categories: i) neutral processes, ii) fitness differences between sex phenotypes (stemming e.g. from sexually antagonistic mutations or accumulating mutational load), and iii) sex-ratio selection, arguably the most important evolutionary force triggering transitions. Sex-ratio selection may result from changes in population structure or environmental conditions, or from conflicts between genetic elements. Intergenomic conflicts may arise from differences between parents and offspring over optimal sex allocation, and intragenomic conflicts from differences in inheritance modes. The latter may have led to the control of sex determination by endosymbiotic microbes, and ensuing evolution towards haplodiploidy. Open questions in this area include why turnover rates differ between lineages, whether particular sex-determination systems are more labile and some transitions more likely to occur, and whether particular chromosome pairs are more likely to evolve into sex chromosomes. It is argued that experimental evolution approaches offer a promising way forward.

Investigations into the evolution of sex chromosomes and sex-determination have attracted wide attention, with voluminous literature reporting interesting findings in the evolution of individual X and Y (Z and W) chromosomes empirically and theoretically. Further examinations into the duplications involved in both sex chromosomes and autosomes have revealed a class of previously unknown processes and related mechanisms of genomic evolution: evolutionary interactions between sex chromosomes and autosomes. A plethora of data and analyses have shown that the gene traffics between the two types of chromosomes are the consequence of natural selection on the sex-related genes and may contribute to the peculiar chromosomal distribution of sex-biased genes, e.g., the dominance on the autosomes, rather than the previously predicted X chromosome, of male-biased genes. This chapter reviews the evidence for the generality of the gene traffic in different sex-determination systems and copying mechanisms of gene duplication, and discusses the underlying evolutionary mechanisms leading to the gene traffics.

Genetic variation among individuals within populations and among populations can be assessed at the chromosomal, protein, or DNA sequence level. The best tool or approach depends on the question being asked. Variation in the number or structure of chromosomes can result in reproductive incompatibilities and reduced fitness that influences the success of conservation efforts. Differences in amino acid sequence that alter the electrophoretic mobility of proteins, termed allozymes, were widely used to measure genetic variation and population differentiation on a gene-by-gene basis prior to advances in DNA sequencing. Mitochondria and chloroplasts contain circular DNA molecules that are usually inherited from one parent and are useful for assessing population history and structure. Most studies of genetic variation now rely on the analysis of single nucleotide polymorphisms (SNPs)—variations in nucleotides at a single location within the genome—to understand both selectively neutral and adaptive processes.

This chapter suggests that the most important factors that diminished Esther Lederberg’s scientific career and legacy were her gender and marriage. The fact that her famous collaborator was also her husband doubled the chances that her own scientific achievements were overshadowed. The chapter goes on to explain how the so-called Matthew and Matilda Effects altered the history of science right at birth of genetics as a distinct branch of biology. As an example of the Matilda Effect, the chapter presents Nettie Stevens whose discovery of the XY sex-determining chromosomes in 1905 and establishment of the two patterns of sex chromosomes in various beetles, flies, and bugs was credited to Edmund Wilson, a better-known scientist. In an example of the Matthew Effect, Thomas Hunt Morgan, the most famous geneticist of the early twentieth century, eventually received most of the credit for discovering sex chromosomes. Finally, the careers and legacies of three other Matildas who worked in the early days of microbial genetics—Martha Chase, Laura Garnjobst, and Daisy Dussoix—are presented.

This chapter on gender-related aspects of consumer neuroscience provides an overview of the differences in brain function between men and women. Consideration is given to the biological basis of gender differences that are manifest at the genetic, neurotransmitter, hormonal, neurophysiological, and neuroanatomical levels. Specifically, differences are discussed that can be detected before birth and the early stages of life. This is followed by a presentation of gender differences observed in studies of neuroeconomics, advertising, decision-making, and purchasing choices. A recurring theme throughout the chapter is the issue of whether the differences in neuronal and cognitive function between men and women are categorical or a matter of degree. Acknowledging the complexity of topic, the chapter presents a biopsychosocial perspective of gender differences in conjunction with a discussion of contemporary critiques of gender neuroimaging research.