Ch 6 Matter Of Sex Flashcards
Sex Chromosomes
Human males are the heterogametic sex with different de chromosomes ( XY)
Human females are the homogametic sex (XX)
In other species sex can be determined in many ways ex. In birds snakes males are (ZZ) females are (ZW)
Sexual development
During the fifth week of prenatal development all embryos develop
Gonads
Reproductive duct mullerian female specific
And wolffian male specific
An embryo develops as a male or female based on the absence or presence of the Y chromosomes
Specifically the SRY gene (sex determining region of the Y chromosomes) male determine the SRY gene
X and Y chromosome
X chromosome
Contain >1500 genes
Larger than the Y chromosome
Y chromosomes
Contain 231 genes
2/3 is heterochromatin (very condensed type of DNA packed to tight that transcription can not occur
Euchromatin a little more spread out
SRY Gene
Activated dozens of genes in the Genome create males
Encodes a transcription factor protein
Controls the expression of other genes
Stimulates male development
Developing testes secrete anti mullerian hormone and destroy female structures
Testosterone and dihydrotesterone (DHT) are secreted and stimulate males structures
Abnormalities in sexual development
Pseudohermaphroditism have some structures that associate with male and females
Androgen insensitivity syndrome = lack of androgen receptor - chromosomally XY
Congenital adrenal hyperplasia = high levels of androgens
Doesn’t develop in female has 46 chromosomes produces testosterone but has no receptor to bind to the gonads develop in the abdomen
Y-linked Traits
Genes on the Y chromosomes are said to be Y-linked
Traits are very rare
Transmitted from male to male
No affected females
Currently identified y-linked traits involve infertility and are not transmitted
X-linked traits
Possible genotypes
X+X+ homozygous wild-type female
X+Xm heterozygous female carrier
XmXm homozygous mutant female
X+Y Hemizygous wild-type male
XmY Hemizygous mutant male
X linked Recessive Inheritance
Always expressed in the male
Expressed in a female homozygote but very rarely in a heterozygote
Passed from heterozygote or homozygote mother to affected son
Affected female has an affected father and a mother who is affected or a heterozygote
Ex. Ichthyosis= deficiency of an enzyme that removes cholesterol from skin
Color blindness= inability to see red and green colors
Hemophilia A= disorder of blood clotting
X-Dominant Inheritance
Expressed in female in one copy
Much more severe effects in males
High rates of miscarriage due to early lethality in males
Passed from males to all daughters but to no sons
Ex incontinentia pigmenti
Congenital generalized hypertrichosis - werewolf syndrome
Sex- limited Traits
Traits that affect a structure or function occurring only in one sex
Sex-limited traits are autosomal 0% penetrance in one sex usually
Ex hypospadias (congenital defect of the male urethra opening on the undersurface of the penis)
Sex-Influence Traits
Autosomal trait in which the phenotype expressed by heterozygote is influenced by sex
Allele is dominant in one sex but recessive in the other sex
Ex. Pattern baldness in humans
A heterozygous male is bald but heterozygous female is not
X- inactivation
Females have two alleles for X chromosome genes but males have only one
In mammals X inactivation balances this inequality and one X chromosome is randomly inactivated in each cell
It becomes inactivated by densely condensing so that enzymes that make RNA from DNA cannot bind in that X chromosome
The inactivated X chromosome is called Barr body
X inactivation continued
Occurs early in prenatal development
An example of an epigenetic change
One that is passed from one cell generation to the next but does not alter the DNA base sequence
The XIST gene encodes an RNA that binds to and inactivates the X chromosomes making it a Barr body
Manifesting Heterozygote
A female that expresses the phenotype corresponding to an x-linked gene
Ex. X-activation is obvious in calico cats
A males cannot express this because it means he has an extra X = XXY
Genomic imprinting
The phenotype of an individual differs depending on the genes parental origin
