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Autosomes versus Sex Chromosomes. In some organisms , there are 1 or 2 specialized chromosomes sex chromosomes involved in sex determination X and Y or Z and W Autosomes - all the others!. Sex Linkage.

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autosomes versus sex chromosomes
Autosomes versus Sex Chromosomes
  • In some organisms, there are 1 or 2 specialized chromosomes
    • sex chromosomes
      • involved in sex determination
        • X and Y or Z and W
  • Autosomes - all the others!
sex linkage
Sex Linkage
  • In organisms with X and Y chromosomes, the Y carries no genes absolutely necessary for survival
  • The X chromosome is necessary for survival
    • Analogous for ZZ, ZW
sex linkage3
Sex Linkage
  • Females and males differ in possible genotypes for X - linked loci
    • Females may be heterozygous or homozygous
      • AA, Aa, aa
    • Males are hemizygous
      • A , a
sex linkage4
Sex Linkage
  • For sex-linked (or X-linked) traits, a male’s genotype is known by his phenotype!
    • Males have the trait or don’t
    • No chance for heterozygous males
sex linkage and reciprocal crosses
Sex Linkage and Reciprocal Crosses
  • Sex linkage is often identified by the results of reciprocal crosses
    • Reciprocal crosses yield identical results with autosomal loci
      • AA x aa --> Aa
    • Reciprocal crosses give very different results w/ sex-linkage!
      • AA x a versus aa x A
sex linkage and reciprocal crosses7

Aa x A

Aa x a

Sex Linkage and Reciprocal Crosses

F2 results:

A

A

a

a

A

a

white eyes in drosophila

red female x white male

RR r

White eyes in Drosophila

white female x red male

rr R

r

R

R

r

white eyes in drosophila9

Rr x R

Rr x r

White eyes in Drosophila

F2 results:

R

R

r

r

R

r

sex linkage in humans
Sex Linkage in Humans
  • Hemophilia
    • lack of blood clotting factor
      • blood does not clot properly
      • simple cut or bruise can lead to uncontrollable bleeding
      • fatal until recently
hemophilia
Hemophilia
  • Females
    • HH normal, non-carrier
    • Hh normal, carrier
    • hh hemophiliac
  • Males
    • H normal
    • h hemophiliac
hemophilia12

Hh X H

H

H

h

HH

H

Hh

h

Hemophilia

The disease is usually passed from a carrier

mother to a son

hemophilia13

Hh x h

Hemophilia

For a female to have hemophilia, she must

have a carrier mother and a hemophiliac

father

h

H

h

patterns of sex linked inheritance
Patterns of sex linked inheritance
  • traits appear to “skip” generations
    • passed from carrier mother to 1/2 sons
    • 1/2 daughters expected to be carriers also
  • trait cannot be passed from father to son
    • all daughters of a father with the trait will be carriers
patterns of sex linked inheritance16
Patterns of sex linked inheritance
  • Crosses producing males of one phenotype and females of another are a strong indication of sex linkage
  • The NUMBERS of each sex are not affected! The sex ratio remains the same.
heterogamy
Humans, mammals, many others:

XX = female

XY = male

Lepidopterans, most birds, some others:

ZZ = male

ZW = female

Heterogamy
  • Homogametic sex: 2 like chromosomes Heterogametic sex: 2 unlike chromosomes
sex determination in drosophila
Sex Determination in Drosophila

Gender is determined by the ratio of X’s to the

sets of autosomes - “Genic Balance”

Normal female: 2 X’s, 2 sets of autosomes; X : A = 1.0

Normal male: 1 X, 2 sets of autosomes: X : A = 0.5

X : A > 1.00 ----> “metafemale,” sterile, reduced viability

X : A ~ 0.67 ----> intersex

X : A < 0.50 ----> “metamale,” sterile, reduced viability

sex determination in drosophila19
Sex Determination in Drosophila
  • Sex determination also affected by heat and cold shock
  • Several genes impact on sex determination
    • Doublesex; produces intersexes
    • Transformer; females converted to sterile males
    • Sex ratio genes; alter 1:1 sex ratio
sex determination in humans
Sex Determination in Humans
  • Simplest terms: presence of Y = male absence of Y = female
  • ONE GENE on the Y needed for development of testes from rudimentary embryonic gonadal tissue
    • testosterone from testes causes “cascade” in development of genitalia, etc.
  • many minor genes involved
slide21

Just what is the “male” gene?

SRY = sex-determining

region of the Y (Tdf)

Master regulatory gene for

sex determination

SRY

ZFY

ZFY needed for sperm

production

dosage compensation and x inactivation
Dosage Compensation and X Inactivation
  • Dosage Compensation
    • Females do not have twice the number of active X linked genes as males
    • Mechanism exists so that only one X functions in females
      • Barr bodies
dosage compensation and x inactivation23
Dosage Compensation and X Inactivation
  • In any individual w/ more than 1 X, the extra(s) condense into a Barr body
    • dark spot on nuclear envelope
    • inactivated X chromosome
  • When does inactivation occur? Is the Barr body completely inactive?
    • active at some time, at least partially
dosage compensation and x inactivation24
Dosage Compensation and X Inactivation
  • XO - Turner Syndrome
    • female
    • no Barr body
  • XXX - female
    • two Barr bodies
  • XXY - Klinefelter Syndrome
    • male
    • one Barr body
dosage compensation and x inactivation25
Dosage Compensation and X Inactivation
  • Lyon Hypothesis
    • Which X inactivates is initially a random event
      • Early in embryonic development, one X shuts down; cannot reactivate
      • All descendents of that cell have same X inactivated
dosage compensation and x inactivation26
Dosage Compensation and X Inactivation
  • Considering one locus; homozygotes have identical cells:

AA

AA

AA

AA

AA

AA

dosage compensation and x inactivation27
Dosage Compensation and X Inactivation
  • Heterozygotes are mosaics of phenotypes

Aa

Aa

Aa

Aa

Aa

Aa

Aa

Aa

Aa

Aa

Aa

Aa

Aa

dosage compensation and x inactivation28
Dosage Compensation and X Inactivation
  • Some mosaics are visible
    • Ectodermal dysplasia
      • Absence of sweat glands (absence of some teeth, early baldness)
      • Heterozygotes show patches of skin w/ sweat glands, patches w/o sweat glands
mosaicism
Mosaicism
  • Hemophilia
    • Heterozygous females have half the amount of clotting factor as homozygous dominants
  • Lesch-Nyhan Syndrome
    • Carrier females show some symptoms from not having a full complement of gene product
mosaicism30
Mosaicism
  • Tortoiseshell/Calico cats
    • sex linked gene
    • B = black; b = yellow
    • Possible genotypes:males: B7b7females: BB Bbbb
slide31

Heterozygous females:

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

slide32

Heterozygous females:

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb