HGEN: Chapter 5 Sex Determination and Chromosomes
Quest on Thursday
Vocabulary to Know:
heteromorphic chromosomes: sex chromosomes that are different; XY
homomorphic chromosomes: sex chromosomes that are the same; XX
Primary sex differentiation: gonad development for production of gametes
Secondary sex differentiation: other characteristics of overall appearance; mammary glands, facial hair, etc.
Unisexual: “one sex”; organisms are either male or female
bisexual/hermaphroditic: “two sexes”; containing both male and female sex characteristics
intersex: between two sexes; neither fully male or female
heterogametic sex: gametes look different from one another; males produce an X and Y chromsome
homogametic sex: gametes look the same; females produce 2 X chromosomes
isogamete: gametes produced by male and female look the same.
Heterogamete: gametes produced by male and female look different; ex: sperm and egg
Bipotential gonads: gonadal ridges can become ovaries or testes
Mullerian ducts: become fallopian tubes and uterus of the female reproductive system
Wolffian ducts: become vas deferens, glands of male reproductive system
Testis-determining factor: gene found on Y chromosome in SRY that leads to development of testes.
Absence of this gene (or misreading) will permit development of female reproductive gonad even though Y
chromosome is present.
What is gender? Is it different than sex? Describe.
Gender is the culture role applied by society. Sex is biological; determined by presence of XX or XY in genetic
code.
Understand the process of sex differentiation in humans (p. 99)
o
Gonadal ridges are produced during the 5th week of gestation. The embryo is potentially hermaphroditic; at this stage the gonads can become
ovary or testis.
o
Cortex portion can develop into ovarian tissue. Mullerian ducts will become fallopian tubes and uterus
o
Medulla portion can develop into testis. Wolffian ducts will become vas deferens, ejaculatory duct, and male glands.
o
Humans have bipotential gonads which can form either ovary or testis.
o
Presence or absence of the Y chromosome determines the transition of gonadal ridges into ovaries or testes.
o
In the absence of Y chromosome, no male GONADAL development occurs.
o
During the 12th week of fetal development, ovaries begin meiosis for the production of eggs.
How should gametes be produced in normal situations? Describe MEIOSIS (Chapter 2)
Spermatogenesis produces 4 mature spermatozoa that are capable of fertilizing an egg. In humans, the sperm
SHOULD contain 23 single chromosomes = the haploid number.
Oogenesis produces 1 mature ova and 4 polar bodies. Only the ova is capable of sustaining fertilization due to
the presence of organelles and cytoplasm that will permit embryonic development until the placenta and
amniotic sac is formed later in gestation. The ova SHOULD also contain the human haploid number of 23 = the
haploid number.
Human DIPLOID number is 46 ; HAPLOID number is 23.
Remember, chromosomes are only visible during MITOSIS and MEIOSIS. Visualization of chromosomes during
cell division will allow for the construction of a karyotype, seen below.
Describe nondisjunction and the impact it CAN have on gamete chromosome numbers.
Nondisjunction is a mistake that can happen during meiotic division in the production of gametes.
If nondisjunction occurs during Meiosis 1 – Metaphase, the tetrad does not separate into individual cells. As the
cells continue into Meiosis 2, the resulting gamete will have a dyad in one of the gametes formed.
If nondisjunction occurs during Meiosis 2 – Metaphase, the centromere fails to separate allowing a dyad to
remain joined in the resulting gametes. This also results in too much genetic information.
Nondisjunction does NOT typically occur in every chromosome pair; it usually occurs in one set of chromosomes.
The focus of THIS chapter was nondisjunction of SEX CHROMOSOMES.
Describe the clinical differences between Turner and Klinefelter syndromes. Classify each as a monosomy or a
polysomy according to # of sex chromosomes.
(It is KEY to remember that both of the above syndromes have a NORMAL number of autosomes, chromosomes #1-22.)
Turner: 45,X; a monosomy condition of the 23 pair having received only 1 X chromosome during conception.
Normal external female genitals but undeveloped ovaries; short stature; normal intelligence.
Klinefelter: 47, XXY; a trisomy condition of the 23 pair having received 2 copies of the X chromosome and 1 copy
of the Y during conception. Normal external male genitals but undeveloped testes; tall with large hands and
feet; rounded hips and breast enlargement is common; below normal intelligence.
Be able to identify the critical areas of the Y chromosome and its influence in sex differentiation
PAR: Psuedoautosomal region allows for synapsis with X chromosome during conception. Only part of X and Y
that are the same.
SRY: Sex-determining Region of Y chromosome; carries TDF gene for testes development.
MSY: Male specific region of Y chromosome; does NOT match with X chromosome; carries both functional
chromatin (euchromatin) for male development and nonfunctional chromatin (heterochromatin).
What allows the X and Y chromosomes to synapse during conception? What term is used to describe the
relationship between the X and Y chromosomes?
The PAR region is the similarity between the X and Y chromosomes.
The X and Y chromosomes are non-homologous or heteromorphic.
What area some of the reasons the sex ratio at birth slightly favors males over females?
Y sperm have less mass and are lighter and MAY travel more quickly to reach the egg for fertilization first.
Eggs may be more receptive to penetrance by Y sperm.
Female embryos may not have a high success rate due to increased estrogen elvels.
Discuss the issues arising from sex selection and gender preference in countries like China and India.
Gender preference allowing female babies to be either killed, left to die, adopted to other countries, or willfully
terminated during pregnancy has brought a significant gender imbalance to their culture. Now not enough
women of child-bearing age to have children will cause decrease in population numbers and productivity.