sex determination and chromosomal changes

sex determination

process by which as organism differentiates into one of two distunguishable sexes

autosomes

chromosomes not directly involved in the determination of sex

sex chromosomes

chromosomes that directly help determine sex

primary sexual differentiation

formation of the gonadss

secondary sexual differentiation

formation of all other visible traits that are indicative of a given sex (facial hair, genitalia, etc.)

unisexual

individuals who have only male OR female reproductive organs

bisexual/hermaphroditic

individuals who contain both male AND female sex organs

haplodiploidy

• have no sex chromosomes

• sex is detrmined by the # of chromosome sets found in the nuclei

  • males develop from unfertilized eggs (haploid)

  • females develop from fertilized eggs (diploid)

• (bees, ants, wasps)

genic sex determination

• no obvious differences between chromosomes of males and females

• sex is determined by several different genes found on the autosomes

• (some plants and protozoans)

environment controlled sex determination

• environment partly or fully controls sex determination

• (some reptiles)

pseudoautosomal region of the Y (PAR)

• homologus region exists on the X chromosome

• PARs on X and Y contain homologus genes

• allows the Y chromosomes to pair up with the X chr. during prophase I (allows crossing over)

• critical for proper segregation of X/Y during meiosis

• genes found in the region exhibit similar patterns of inheritance as genes located on the autosomes

nonrecombining region of the Y (NRY)

• rest of the Y

• can be divided into several sub- regions

  • heterochromatin

  • euchromatin

heterochromatin

• lacks “functional” genes

• stays hypercondensed in interphase (when transcription should be occuring)

• genes, if present, will never be transcribed

euchromatin

• active areas that contain genes that are constantly being transcribed

• a section of this near the PAR on the p arm contains a critical gene that controls male development called the sex determining region of Y (SRY)

sex determining region of Y (SRY)

• encodes a protein called testis-etermining factor (TDF)

testis-etermining factor (TDF)

• binds to DNA and causes it to bend (only known function)

• present in all male mammals

how do individuals obtain an abnormal of sex chromosomes?

nondisjunction

klinefelter syndrome (XXY)

• individuals have at least 2 X chromosomes to go along with at least 1 Y chr.

• have male genitalia and internal ducts, but their testes are underdeveloped and fail to produce sperm

  • they are sterile

• are usually very tall, with long arms and legs

• have several feminine characteristics including enlarged breats, rounded hips, and sparse hair

• can give regular testosterone shots to reduce feminine characteristics

turner syndrome (XO)

• individuals have only 1 X chromosome (and no Y)

• most die during embryonic development

• female genitalia and ducts, but infertile

• have short stature, underdeveloped feminine traits, and webbed neck

Triplo X syndrome (XXX)

• females that have at least 1 extra X chromosomes

• highly variable phenotype

• some women are normal, some have several mental impairment, some are infertile

• the more X’s, the worse the symptoms

XYY (superman or supervillian)

• very controversial condition

• scientists hypothesized that such individuals may be overly aggressive and have behavioral problems (not true)

• men with this are taller than average (usually over 6’)