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Chapter 7: Sex Determination and Sex Chromosomes A wide variety of reproductive methods exist. Some organisms never reproduce sexually, some use both asexual and sexual methods, while some rely strictly on sexual reproduction. Organized transmission of genes relies on the processes of ____________________________________________________________________. Meiosis thus produces a variety of gametes with ____________ the number of chromosomes of the parents, where after fertilization; the resulting offspring regains the total chromosome number. In higher life forms, the differences between the sexes are more pronounced as ___________________________. _________________________________chromosomes, those that are dissimilar such as the X and Y, represent the different sexes and are labeled sex chromosomes. Primary sexual differentiation – Secondary sexual differentiation – Organisms who are only one sex can be referred to as Organism who are both male and female are considered ________________________ refers to those of intermediate sexual differentiation, most of which are sterile. Edward Wilson was experimenting with a grasshopper (Protenor) and found that the female while the male only had___________________. He then was experimenting with a milkweed bug (Lygaeus) and found that the females have ______ chromosomes, while males have only ______ and a smaller heterochromosome called the ____. In the first model, called the Protenor model, the genders are written _________________________________. In the second model, Lygaeus, the genders are written ________________________. In some cases, the male is not the heterogametic sex. In the cases where the female is heterogametic, the notation _____________________________ is used to denote the female as the sex determining gender. H. von Winiwarter Theophilus Painter In 1956, Hin Tjio and Albert Levan CE Ford and John Hamerton Within the 23 pairs, it was found that one pair had a different configuration in males and females. The human female has XX and the human male has XY. Klinefelter Syndrome – Turner Syndrome – 47, XXX Syndrome 47, XYY Condition Sexual differentiation in humans During early development, the human embryo is essentially hermaphroditic. By the end of the fifth week, the _______________________________________ (what will become the gonads) arise and primordial germ cells migrate to the area. An outer ______________________ and inner ____________________ form. The cortex can develop into an ________________________, while the medulla can become the ____________________. If the cells of the genital ridge have an XY setup, then male development is initiated by the seventh week. If no Y is present, then no male development occurs and the ridge forms ovarian tissue. The Y chromosome and male development The human Y chromosome was originally believed to be mostly _____________________ genetically. It is now known that it is not blank, but it does contain fewer genes than the X chromosome. In the Y chromosome is a sexdetermining region Y (____________) gene. The absence of this gene leads to the development of a female. SRY encodes a gene product that triggers the development of testes. 7.4 The ratio of males to female in human is not 1.0 Provided that the X and Y chromosomes are equally viable during development, a ______________________ of male to female births would be expected. Yet, the human sex ratio has been investigated. It is assessed in two ways: _______________________ Sex Ratio – the proportion of males to females conceived in a population _______________________ Sex Ratio – the proportion of each sex that is born. It is unclear why the secondary ratio is so different, and many hypotheses have been suggested. One hypothesis suggested that Y bearing sperm may be more motile than X bearing.