Development and Sex Determination

1
Development and Sex Determination

• Chapter 7

2
The human life cycle
3
7.1 The Human Reproductive System

• The human reproductive system
• Gonads produce gametes (testes in males, ovaries
  in females)
• Ducts to transport gametes
• Genital structures for intercourse and
  fertilization

4
The male reproductive system
5
Spermatogenesis
6
The female reproductive system
7
Oogenesis and ovulation
8
Timing of Meiosis and Gamete Formation in Males
and Females

• Males
• Spermatogenesis begins during puberty
• Millions of sperm are always in production
• Spermatogenesis takes about 48 days
• Meiotic division produces 4 sperm
• Females
• Primary oocytes produced during embryonic
  development remain in meiosis I until ovulation
• Ovulation begins during puberty
• Meiotic division produces 1 oocyte and 2-3 polar
  bodies

9
Single-celled Zygote

• We all begin as a single cell, the zygote,
  produced by the fusion of a sperm and oocyte
• Zygote
• Fertilized egg that develops into a new individual

10
Stages of development
11
Embryonic Membranes and Placenta

• Chorion
• Two-layered structure formed from trophoblast
• Releases human chorionic gonadotropin (hCG)
  hormone which maintains uterine lining and
  stimulates endometrial cells to produce hormones
• Grows and forms villi that exchange nutrients and
  wastes with maternal blood circulation,
  eventually forming the placenta

12
Human development
www.healthof children.com
13
Development is Divided into Three Trimesters

• First trimester
• First month basic tissue layers form most of
  the body is divided into paired segments
• Second month most major organ systems are formed
• Third month embryo becomes a fetus sexual
  development is initiated

14
Development is Divided into Three Trimesters

• Second trimester
• Increase in size and organ-system development
• Bony parts of skeleton form
• Heartbeat is heard with a stethoscope
• Fetal movements begin
• Third trimester
• Rapid growth
• Circulatory and respiratory systems mature
• Birth is a hormonally induced process at the end
  of the 3rd trimester

15
7.3 Teratogens Are a Risk to the Developing Fetus

• The embryo and fetus are sensitive to chemical
  and physical agents that can cause birth defects
• Teratogen
• Any physical or chemical agent that brings about
  an increase in congenital malformations
• Radiation, viruses, medications, alcohol

16
Sensitivity to teratogens
Rohan.sdsu.edu
17
Alcohol is a Teratogen

• Fetal alcohol syndrome (FAS)
• Alcohol is the most common teratogenic problem
  and leading cause of preventable birth defects
• There is no safe amount of alcohol consumption
  during pregnancy

nsnet.org
18
Other teratogens

• Important not all have severe effects
  consultation with health care provider critical!
• Anticonvulsants
• Antipsychotics
• Antimicrobials
• Antihypertensives
• Anticoagulants
• Factors maternal health maternal infections
  (esp. viruses), radiation exposure

19
Human Sex Ratios

• Sex ratio
• The proportion of males to females, which changes
  throughout the life cycle
• The ratio is close to 11 at fertilization
• The ratio of females to males increases as a
  population ages

20
Factors in Sexual Differentiation

• The formation of male and female reproductive
  structures depends on
• Gene action
• Interactions within the embryo
• Interactions with other embryos in the uterus
• Interactions with the maternal environment

21
Three Levels of Sexual Determination

• The chromosomal sex of an individual (XX or XY)
  can differ from the phenotypic sex
• Sex of an individual is defined at three levels
• Chromosomal sex
• Gonadal sex
• Phenotypic sex

22
Chromosomal basis of sex determination
Male or female?
23
Y Chromosome and Testis Development

• SRY gene
• Sex-determining region of the Y chromosome
• Located near the end of the short arm of the Y
  chromosome
• Plays a major role in causing the
  undifferentiated gonad to develop into a testis

24
Embryonic development of genitalia
No SRY Gonads develop into ovaries
Under influence of SRY gene product Gonads
develop into testes
8 weeks
12 weeks
16 weeks
25

• Testosterone
• A steroid hormone produced by the testis
• Male sex hormone
• Müllerian inhibiting hormone (MIH)
• Hormone produced by developing testis that causes
  breakdown of Müllerian (female) ducts in the
  embryo

26
Androgen Insensitivity

• A mutation in the X-linked androgen receptor gene
  (AR) causes XY males to become phenotypic females
• Testosterone is produced, but not testosterone
  receptors cells develop as females
• Androgen insensitivity (CAIS)
• An X-linked genetic trait that causes XY
  individuals to develop into phenotypic females

27
Mutations can cause Sex Phenotypes to Change at
Puberty

• Pseudohermaphroditism
• An autosomal genetic condition that causes XY
  individuals to develop the phenotype of females
• Caused by mutations in several different genes
• Affected individuals have both male and female
  structures, but at different times of life
• At puberty, females change into males

28
7.7 Equalizing the Expression of X Chromosomes in
Males and Females

• Human females have one X chromosome inactivated
  in all somatic cells to balance the expression of
  X-linked genes in males and females

29
Dosage Compensation

• Females have two X chromosomes, males have one
  yet the amount of gene product is the same
• Dosage compensation
• A mechanism that regulates the expression of
  sex-linked gene products

30
Barr Bodies and X Inactivation

• Lyon hypothesis (proposed by Mary Lyon)
• Dosage compensation in mammalian females
• Random inactivation of one X chromosome in
  females equalizes the activity of X-linked genes
  in males and females
• Barr body
• A densely staining mass in the somatic nuclei of
  mammalian females
• An inactivated X chromosome, tightly coiled

31
Female Mammals are Actually Mosaics for X
Chromosome Expression

• In females, some cells express the mothers X
  chromosome and some cells express the fathers X
  chromosome
• Inactivated chromosome can come from either
  mother or father
• Inactivation occurs early in development
• Inactivation is permanent all descendants of a
  particular cell have the same X inactivated
• Genetic regulation (Xic) of which chromosome is
  inactivated

32
Effects of Random X-Chromosome Inactivation

• Random X inactivation can cause twins with
  identical genotypes to have different phenotypes
  (also calico cats!)

nitro.biosci.arizona.edu
Fig. 7-20, p. 171
33
7.8 Sex-Related Phenotypic Effects

• In sex-influenced and sex-limited inheritance,
  the sex of the individual affects
• whether the trait is expressed
• the degree to which the trait is expressed
• This is true for autosomal and sex-linked genes
• Sex hormone levels modify expression of these
  genes, giving rise to altered phenotypic ratios

34
Sex-Influence Traits

• Sex-influenced traits
• Traits controlled by autosomal genes that are
  usually dominant in one sex but recessive in the
  other sex

35
A Sex-Influenced Trait

• Pattern baldness
• Acts like an autosomal dominant trait in males
  and an autosomal recessive trait in females

The difference is testosterone
Fig. 7-21, p. 172
36
Sex-Limited Traits

• Genes that produce a phenotype in only one sex
• Example Precocious puberty in heterozygous males
  but not in heterozygous females
• Traits expressed only in females because males
  die before birth
• Example Male-lethal X-linked dominant traits
• Traits expressed only in males
• Example Duchenne muscular dystrophy (X-linked
  recessive) males do not have offspring and dont
  pass their X onto daughters
• Result very rare in females

37
Imprinting

• One copy of a gene is inactivated, depending on
  whether it comes from the father or the mother
• Example NOEY2 gene paternal copy is expressed
  in normal breast and ovarian cells
• Imprinting
• A phenomenon in which expression of a gene
  depends on whether it is inherited from the
  mother or the father
• Chemical modification of DNA (more common)
• Uniparental disomy (rare)

38
Summary

• Male and female reproductive systems look
  different, but both include gonads for gamete
  formation, systems to transport gametes, and
  structures for intercourse and fertilization
• Human development starts with a single cell
• Certain physical and chemical agents can cause
  birth defects
• Human sex determination depends on the presence
  or absence of the Y chromosome (hormones)
• Some phenotypes are influenced by gender
  (hormones)