Download Sex Linkage Reciprocal Cross Exceptions

8/26/10
Sex Linkage
Reciprocal Cross
•  Organisms that have different sexes can be
crossed in two different ways Phenotype A male X Phenotype B female Phenotype B male X Phenotype A female •  Mendel concluded that reciprocal crosses
are equal Exceptions
•  1906 - Doncaster and G.H. Raynor –  Working in magpie moths
–  Light winged females X dark winged males =
all dark wings
–  Light winged males X dark winged females =
dark winged males and light winged females
•  1908 - W.M. Bateson
–  Working with chicken feather patterns
–  Barred male X nonbarred female = all barred –  Nonbarred male X barred female = barred
males and nonbarred females
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Thomas Hunt Morgan - 1909
•  Working with the fruit
fly - Drosophila
melanogaster •  White-eyed male X
red-eyed female –  F1 all red-eyed
offspring
–  F2 all red-eyed
females and males are
1/2 white-eyed and 1/2
red eyed
Heteromorphic Chromosomes
•  A chromosome pair in which the two
members of the pair have some homology
but differ in shape, size or staining
properties 2
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Terms
•  Homogametic - the sex that produces a
single type of gamete
•  Heterogametic - the sex that produces two
types of gametes
Now remember:
•  1906 - Doncaster and G.H. Raynor –  Working in magpie moths
–  Light winged females X dark winged males =
all dark wings
–  Light winged males X dark winged females =
dark winged males and light winged females
•  1908 - W.M. Bateson
–  Working with chicken feather patterns
–  Barred male X nonbarred female = all barred –  Nonbarred male X barred female = barred
males and nonbarred females
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More Terms
•  Hemizygous - only one copy in a diploid
organism
•  X linkage -genes located on X chromosome
•  Y linkage - genes located on Y chromosome
•  XY linkage - genes located in the common
XY region
Sex determination
•  Y chromosome mechanism - the Y
chromosome actively determines sex
•  X chromosome - autosome balance
mechanism -ratio between the number of X
chromosomes and the number of sets of
autosomes
Turner Syndrome
•  Monosomic - Sex
chromosome
complement of 44
autosomes + 1 X
chromosome
•  1 in 5,000 female
births
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Turner Syndrome
Kleinfelter Syndrome
•  Trisomic - 44
autosomes
+XXY sex
chromosomes
Klinefelter Syndrome
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X Chromosome Inactivation
•  One X chromosome
condenses and becomes
“inactive” in female cells
early in development Barr body
•  Dosage Compensation Adjusts the dosage of Xlinked genes Lyon Hypothesis
•  Proposed by Mary Lyon and Lillian Russell
-1961
•  Barr body is highly condensed and mostly
genetically inactive X chromosome is
“lyonized” •  Inactivation occurs on Day 16 of development
•  X chromosome to be inactivated is randomly
chosen from the maternal and paternal X
chromosome in an event that is independent
from cell to cell. Mosaic
•  Females are a mixture
of cells-some with one
X chromosome
inactivated and some
with the other X
chromosome
inactivated
•  Forms sectors of
phenotypically
different cells. 6
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Anhidrotic Ectodermal Dysplasia
X-linked Inheritance
•  Dominant
–  Affected males pass the trait to all daughters
and no sons
–  Affected heterozygous females pass the trait to
1/2 both sons and daughters
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X-linked recessive
•  Males display trait more often than females
•  Offspring of affected males are not affected
but his daughters are carriers
•  None of the sons of affected males display
the trait nor do they pass it to their offspring
Testicular feminization syndrome
•  X-linked recessive disorder –  1 in 65,000 male births
•  Affected males are phenotypically female
with a chromosome complement of a male
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Y-linked Inheritance
•  Traits transmitted from father to son
•  TDF - name given to hypothetical gene that
encoded a hypothetical product called testisdetermining factor
•  Causes the gonadal primordia to
differentiate into testes instead of ovaries
•  SRY - gene that has been isolated in
humans that can induce testis determination
Holandric traits
•  Traits associated with Y-chromosome “wholly male” traits
Environmental Determination of
Sex
•  A mechanism of sex determination that is
not genetically determined at fertilization
•  Directed by environmental factors –  related to association or lack of association with
other members of species
–  temperature determination
•  turtle eggs incubated above 32 oC produce females,
below 28 oC produce males
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Sex-limited traits
•  Autosomal genes that determine a trait that
appears in one sex but not the other
–  Hen feathering/cock feathering in birds
–  Milk yield in dairy cattle
–  Formation of breasts and ovaries in human
females
–  Facial hair distribution
–  Ability to produce sperm and oocytes
Sex-Influenced Traits
•  Controlled by autosomal genes
•  Traits appear in both sexes but the frequency
of occurrence in the two sexes is different or
relationship between genotype and phenotype
is different
–  Pattern baldness - determined by alleles b and b+
•  b/b is bald in both sexes
•  B/b in males = bald, females =nonbald
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