Download Chapter 7: Sex Determination and Sex Chromosomes

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.