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 segregation and independent assortment. Meiosis thus produces a variety of gametes
with half 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 phenotypic dimorphism. Males
are often represented with the shield and spear of Mars (♂) and the female is represented by the mirror of Venus
(♀).
 Heteromorphic chromosomes, those that are dissimilar such as the X and Y, represent the different sexes and
are labeled sex chromosomes. However, it is the genes in these chromosomes that ultimately serve for sex
determination.
7.1 Sexual differentiation and life cycles
 Primary sexual differentiation – involves only the gonads where the gametes are produced.
 Secondary sexual differentiation – overall appearance of the organism and clear differences in the male and
female organs.
 Organisms who are only one sex can be referred to as unisexual, dioecious, and gonochoric
 Organisms that are both male and female are considered bisexual, monoecious, or hermaphroditic.
 Intersex refers to those of intermediate sexual differentiation, most of which are sterile.
7.2 X and Y chromosomes were first linked to sex determination early in the 20th century
 The presence or absence of the X chromosome in male gametes provides a mechanism for sex determination. In
1906, Edward Wilson was experimenting with a grasshopper and found that the female had 14 chromosomes,
the last of which was an X, while the male only had 13 chromosomes. He then was experimenting with a
milkweed bug and found that the females have 2 X chromosomes, while males have only one X and a smaller
heterochromosome called the Y. In the first model, called the Protenor model, the genders are written XX or
XO. In the second model, Lygaeus, the genders are written XX or XY.
 In some cases, the male is not the heterogametic sex. In the cases where the female is heterogametic, the
notation ZZ and ZW is used to denote the female as the sex determining gender.
7.3 The Y chromosome determines maleness in humans
 The first attempts to determine sex determination in humans involved the observation of chromosomes in
dividing cells. In 1912, H. von Winiwarter counted 47 chromosomes in a dividing spermatogonial metaphase.
He believed that sex determination must be dependent on an extra chromosome in the female, giving her 48. In
the 1920s, Theophilus Painter observed between 45 and 48 chromosomes in cells from the male, and found a
small Y chromosome. Painter originally believed 46 to be the diploid number in humans, but changed his ideas
to 48.
 In 1956, Hin Tjio and Albert Levan found a better way to prepare chromosomes and thus determined 46 was the
human diploid number. CE Ford and John Hamerton confirmed this finding later in the year.
 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 and Turner Syndrome
Around 1940, two abnormalities were found to cause abnormal sexual development.
 Klinefelter Syndrome – have the physical structure of a male, but the testes are rudimentary and fail to produce
sperm. They are generally tall and have large hands and feet. Some masculine development occurs, but the
female traits are not fully suppressed, resulting in slight breast enlargement and rounded hips. Intelligence may
fall below the normal range. Chromosome change is usually XXY, but it can occur with XXXY, XXXXY, or
XXXYY. The manifestations can be more severe with the more X chromosomes that are present. It can present
in 2 out of 1000 male births.
 Turner Syndrome – the outer appearance is female, but the ovaries are rudimentary. These females are generally
short in stature (under 5 feet); they have skin flaps on the back of the neck and underdeveloped breasts.
Intelligence is usually normal. The chromosome change is normally X. Turner occurs in 1 of 2000 female
births. It occurs less than Klinefelter due to spontaneous miscarriages in the offspring.
47, XXX Syndrome
The presence of an extra X results in a female. Occurs about 1 in 1200 female births. This syndrome has variable
expression. In many cases, the female is normal. In other cases, underdeveloped secondary sex characteristics,
sterility, and mental retardation may occur.
47, XXY Condition
Only deviation is the presence of the additionally Y chromosome. It was initially believed that males with this
genotype were more likely to end up in prison, but it was later determined that these males were no more likely to
end up in prison than any other male. Conditions include above average height and subnormal intelligence.
Sexual differentiation in humans
During early development, the human embryo is essentially hermaphroditic. By the end of the fifth wee, the gonadal
primordial (what will become the gonads) arise and primordial germ cells migrate to the area. An outer cortex and
inner medulla form. The cortex can develop into an ovary, while the medulla can become the testes. Two
undifferentiated sets of ducts are also present. 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 blank 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 sex-determining region Y
(SRY) 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 one to one ratio of male to
female births would be expected. Yet, the human sex ratio has been investigated. It is assessed in two ways:
 Primary Sex Ratio – the proportion of males to females conceived in a population
 Secondary Sex Ratio – the proportion of each sex that is born.
 In 1969, the secondary sex ratio of humans was calculated. It was found to be 1.06 in the Caucasian population,
or 106 males were born for every 100 females. In the African American population it was found to be 1.025,
and in Korea, it was found to be 1.15. 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.