• Chromosomal sex is determined at fertilization
• Sexual differences begin in the 7 th week
• Sex is influenced by genetic and environmental factors
• Females (generally XX) do not have a Y chromosome
• Males (generally XY) have a Y chromosome
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Fig. 7.10

• Chromosomal sex
• Gonadal sex
• Phenotypic sex
• Formation of male or female reproductive structures depends on
– Gene action
– Interactions within the embryo
– Interactions with other embryos in the uterus
– Interactions with the maternal environment
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

• In early embryo there are two internal duct systems
– Wolffian (male)
– Müllerian (female)
• At 7 weeks, developmental pathways activate different sets of genes
• Cause undifferentiated gonads to develop as testes or ovaries
• Determine the gonadal sex of embryo
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

Appearance of “uncommitted” duct system of embryo at 7 weeks
Y chromosome present Y chromosome absent
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Stepped Art
Fig. 7-12b, p.167

Appearance of “uncommitted” duct system of embryo at 7 weeks
Y chromosome present
Testes
Y chromosome absent
Ovaries
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Stepped Art
Fig. 7-12b, p.167

Appearance of “uncommitted” duct system of embryo at 7 weeks
Y chromosome present
Testes
Y chromosome absent
Ovaries
Testis
Ovary
Uterus
Stepped Art
Fig. 7-12b, p.167

Appearance of structures that will give rise to external genitalia
7 weeks
Y chromosome present Y chromosome absent
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Stepped Art
Fig. 7-12c, p.167

Appearance of structures that will give rise to external genitalia
7 weeks
Y chromosome present Y chromosome absent
10 weeks 10 weeks
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Stepped Art
Fig. 7-12c, p.167

Appearance of structures that will give rise to external genitalia
7 weeks
Y chromosome present Y chromosome absent
10 weeks 10 weeks
Penis
Vaginal opening
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Birth approaching Birth approaching
Stepped Art
Fig. 7-12c, p.167

• Cause the indifferent gonad to develop as a testis
• Sex determining region is the SRY gene
• Other genes on the autosomes play an important role
• Once testes develop they secrete two hormones
– Testosterone
– Müllerian Inhibiting Hormone (MIH)
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

• Embryonic gonads develop into an ovaries
• Testosterone not produced
– Wolffian system degenerates
• MIH is not produced
– Müllerian duct system develops to form oviduct, uterus and parts of the vagina
• Sexual phenotype develops
– Hormones are important
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

Male Egg with X sex chromosome Female
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Stepped Art
Fig. 7-13, p.168

Male
Sperm with Y chromosome
Egg with X sex chromosome
Fertilized by Fertilized by
Female
Sperm with X chromosome
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Stepped Art
Fig. 7-13, p.168

Male
Sperm with Y chromosome
Egg with X sex chromosome
Fertilized by Fertilized by
Female
Sperm with X chromosome
Embryo with XY sex chromosomes
Genetic sex
Embryo with XX sex chromosomes
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Stepped Art
Fig. 7-13, p.168

Male
Sperm with Y chromosome
Egg with X sex chromosome
Fertilized by Fertilized by
Female
Sperm with X chromosome
Genetic sex
Embryo with XY sex chromosomes
Sex-determining region of the Y chromosome (SRY) brings about development of undifferentiated gonads and testes
Gonadal sex
Embryo with XX sex chromosomes
No Y chromosome, so no
SRY. With no masculinizing influence, undifferentiated gonads develop into ovaries
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Stepped Art
Fig. 7-13, p.168

Male
Sperm with Y chromosome
Egg with X sex chromosome
Fertilized by Fertilized by
Female
Sperm with X chromosome
Genetic sex
Embryo with XY sex chromosomes
Sex-determining region of the Y chromosome (SRY) brings about development of undifferentiated gonads and testes
Testes secrete masculinizing hormones, including testosterone, a potent androgen
Gonadal sex
Embryo with XX sex chromosomes
No Y chromosome, so no
SRY. With no masculinizing influence, undifferentiated gonads develop into ovaries
No androgens secreted
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Stepped Art
Fig. 7-13, p.168

Male
Sperm with Y chromosome
Egg with X sex chromosome
Fertilized by Fertilized by
Female
Sperm with X chromosome
Genetic sex
Embryo with XY sex chromosomes
Sex-determining region of the Y chromosome (SRY) brings about development of undifferentiated gonads and testes
Testes secrete masculinizing hormones, including testosterone, a potent androgen
Gonadal sex
Embryo with XX sex chromosomes
No Y chromosome, so no
SRY. With no masculinizing influence, undifferentiated gonads develop into ovaries
No androgens secreted
In presence of testicular hormones, undifferentiated reproductive tract and external genitalia develop along male lines
Phenotypic sex
With no masculinizing hormones, undifferentiated reproductive tract and external genitalia develop along female lines
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Stepped Art
Fig. 7-13, p.168

• Hemaphrodites
– Have both male and female gonads
• Androgen insensitivity
– XY males become phenotypic females
• Pseudohermaphroditism
– XY males at birth are phenotypically female; at puberty develop a male phenotype
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

• Equalizes the amount of
X chromosome products in both sexes
• In XX females an inactivated X chromosome forms a
Barr body in each cell
• XY males do not contain
Barr bodies
Fig. 7.15
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

• One X chromosome is genetically active in the body cells; the second is inactive and tightly coiled
• Either the maternal or paternal chromosome can be inactivated
• Inactivation is permanent (reset in germ cells)
• Inactivation of second X equalizes the activity of X linked genes in males and females
• ROSENSTIEL AWARD - Mary Lyon (+ others) 2007
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

Cytological correlates of X-inactivation in mammals
Barr body:
•Present in somatic XX nuclei
•Not present in XY nuclei
•In X-chromosome aneuploids, all but one X become Barr bodies
Females Barr Bodies Active X
XX 1 1
XO
XXX
XXXX
0
2
3
1
1
1
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

Cytological correlates of X-inactivation in mammals
Barr body:
•Present in somatic XX nuclei
•Not present in XY nuclei
•In X-chromosome aneuploids, all but one X become Barr bodies
Females Barr Bodies Active X
XX 1 1
XO
XXX
XXXX
0
2
3
1
1
1
Males Barr Bodies Active X
XY 0 1
XXY
XXXY
1
2
1
1
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

• Some cells express the maternal X and others express the paternal X
• Cats heterozygous for orange and black gene must carry two X chromosomes
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

TEM of Barr Body
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Fig. 7.17

• Contains several genes
• The XIST gene causes the chromosome to become coated with
XIST RNA and inactivated.
• Occurs at approximately 32-cellembryo stage
Fig. 7.18
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

I
II
1 2
1 2 3 4
III 1 2 3 4
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
Fig. 7-19, p.174

The cloned calico cat or why your clone may look different from you cc or “Carbon Copy”
Born Dec 22, 2001
Rainbow
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

Dosage Compensation
Mechanisms that generate the same amount of X-linked gene product regardless of chromosome dosage
Mammals : One of two X chromosomes in the female cell is inactivated
Drosophila : X chromosome in males generates twice the amount of gene product when compared to females
C. elegans : Activity of genes on BOTH X chromosomes is halved to equal activity of genes on singleX chromosome in males.
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

• Expressed in males and females
• Usually controlled by autosomal genes
• Generally phenotypic variations are due to hormonal differences between the sexes
• An example is male pattern baldness
Fig. 7.20
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

Sex-influenced traits
Some autosomal genes govern traits that show up in both sexes but their expression differs because of hormonal differences example : pattern baldness in males. b allele is recessive in one sex and dominant in the other b
+ b
+ b/b
/b
+
/b
Male Female non-bald non-bald bald non-bald bald bald
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning

• Genes that produce a phenotype in only one sex
• Examples
– Precocious puberty
– Secondary sex characteristics
Chapter 7 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning