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sex determination

involves genes, rather than chromosomes; heterogametic: male
homogametic: female

two modes of sex determination

only 1/2 of the gametes have x chromosome; xx-female x0- male
all females gametes have an X chromosome; male gametes have either an X or Y chromosome
-XX female; SY male

heteromorphic chromosomes

dissimilar, XY pair in mammals, characterize one sex or the other in a wide range of species


individuals containing both male and female reproductive organs, a commor occurence in both plant and animal kingdoms


individuals of an intermediate sexual condition; sterile

Klinefelter syndrome

47, XXY, 47, XXX (highly variable in expression) 48 XXXY, 48XXXX (rare), 49 XXXXY, 49 XXXYY, 49 XXXXX (rare) tall male, testes underdeveloped and fail to produce sperms; feminine sexual development: slightly enlarged breasts and rounded hips; intersexuality often leads to abnormal social development; intelligence below normal

Turner syndrome

45, X; affected individual has female external genitalia nd internal ducts, but ovaries are rudimentary; short stature; skin flaps on back of neck; underdevelped breasts and widely spread nipples; broad shieldlike chest; normal intelligence; many die in utero

47, XYY

1965, Patricia Jacobs, 9 out of 315 males in Scottish maximum security prison have this karyotype; above average height and subnormal intelligence; Stanley Walzer and park Gerald, 1974 screen babies after they are born and do follow-up studies; abandoned in 1975 because there was no significant correlation and a self-fulfilling prophecy


the failure of the X chromosomes to segregate properly during meiosis

pseudoautosomal regions (PARs)

share homology with regions on the X chromosome and synapse and recombine with it during meiosis;

male specific region of the Y (MSY)

the remainder of the chromosome, about 95% does not synapse or recombine with the X chromosome

euchromatic regions

contain functional genes

heterochromatic regions

lack genes

sex- determinig region Y (SRY)

critical gene that controls male sexual development; absent from X chromosome; encodes testis-determining factor (TDF) which encodes a protein that causes the undifferentiated gonadal tissue of the embryo to form testes

sexual detetermination in humans

the Y chromosoem contains far fewer genes than the X chromosome; (PARs) on two ends share homology with regions on the X chromosome; the X and Y chromosomes synapse and can recombine during meiosis; this synapse is critical to the segregation of the X and Y chromosomes during male gametogenesis
Y chromosome contains the (MSY) and (SRY)- determines sexual differentiation in humans

sexual determination in humans 2

early human embryos are sexually indifferent; in the presence of SRy, bi potential gonads form testes and male reproductive organs and ducts; in the absence of SRY, the female reproductive organs and ducts are formed; once teste differentiation is initiated, the embryonic testicular tissue secrets two hormones that are essential for continued male sexual differentiation

testicular feminization

androgen receptor mutation; will be female but have XY chromosomes

Ratios in offspring

sex ratio: the actual proportion of male to female offspring
primary sex ratio: reflects the proportion of males to females conceived in a population
secondary sex ratio: reflects the proportion of each sex that is born
(more males than females are born)


repeat unit in DNA sequence; contained within seven segments scattered across the euchromatic regions of both the short and long arms of the Y chromosome


sequences of base pairs that read the same but in the opposite direction on complementary strands- present throughout the MSY

Why sex ratio of 1.0 does not occur?

1) segregation, males produce equal numbers of X and Y bearing sperm
2) each type of sperm has equivalent viablility and motility in the female reproductive tract
3) the egg surface is equally receptive to both X and Y bearing sperm

dosage compensation

balances the dose of X chromosome gene expression in females and males; one of the X chromosomes will be inactivated; barr bodies are physical evidence

Barr body

inactivated X chromosome; chromosome structure or sex chromatin body; number of Barr bodies follows N-1 rule, when N is the total number of X chromosomes present
(as many as 15% of the human X chromosomal genes actually escape inactivation)

Lyon hypothesis

early in development, inactivation is random; all subsequent progeny cells inactivate the same X as their progenitor cell; large patches expressing color allele on one X and other patfches expressing the allele on the other X, different X chromosomes were inactive in adjacent patches of cells (coat color genes in female mice and does not occur in males); (absence of sweat glands in a female heterozygous for the X linked anhidrotic ectodermal dysplasia; locations randomly based on random pattern of X chromosome inactivation during early development)

generation anticipation

disorder will get worse from generation to generation


culture of cells derived from a single cell

mechanism of inactivation

Xic (X-inactivation center) major control area on the X chromosome; located on the proximal end of the p arem in humans (the end toward the centromere)

Sex determination in flies

the ratio of X chromosomes to sets of autosomes determines its sex; female = 1; male= 0.5; normal: 2X:2A; different dosage compensation mechanism

genic balance theory

male gametes containing one of each autosome plus a Y chromosome result in male offspring not because of the presence of the Y but because they fail to contribute an X chromosome; threshold for maleness is reached when X:A ratio is 1:2 (X:2A)

RNA splicing

portions of the RNA are removed and the remaining fragments are spliced bac together prior to translation into a protein

alternative splicing

RNA transscript may be spliced in different ways; different RNA transcript is produced in females than in males

temperature dependent sex determination (TSD)

in many reptiles, the incubation temperature at a critical time during embryonic development is responsible for sex determination; can't distinguish male and female sex chromosomes in some reptiles


converts androgens (male hormones such as testosterone) to estrogens (female hormones such as estradiol)

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