33 terms

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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
hermaphroditic
individuals containing both male and female reproductive organs, a commor occurence in both plant and animal kingdoms
intersex
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
nondisjunction
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)
amplicon
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
palindromes
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
clone
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
aromatase
converts androgens (male hormones such as testosterone) to estrogens (female hormones such as estradiol)