The Chromosomal Basis The Chromosomal Basis of Inheritanceof InheritanceChapter 15
The Chromosomal Theory of The Chromosomal Theory of InheritanceInheritancechromosomes and genes are
present as pairs in diploid cellshomologous chromosomes
separate during meiosisfertilization restores chromosome
number to diploid conditionThe chromosomal theory of
inheritance claims it is the chromosome that segregates and assorts independently.
MorganMorganChose fruit fliesWhy are fruit flies good
organisms to do genetic studies on?
Prolific breeders◦New generation every 2 weeks◦Hundreds of offspring
Only 4 chromosomesTraits easy to see
Morgan Morgan and and Linked Linked TraitsTraits
Morgan and sex linkageMorgan and sex linkageCrossed red eye female with white
maleF1 showed all red eye offspringCrossed red eye offspring from F1Noticed that all males in F2 were
white eyed, females were red eyedDetermined that eye color is sex
linked
Show a Punnett square of a cross between a white eyed male with a carrier female fly
Sex Linked TraitsSex Linked TraitsEye color is linked on the X
chromosomeFemales carry 2 copies of the gene
while males have only one copyIf mutant allele is recessive, white
eyed female has the trait on both X’s
White eyed male cannot hide the trait since he has only one X
Who is responsible when a boy inherits a sex linked condition?
the motherA carrier of a condition is
heterozygous for the trait, so only females can carry sex linked traits
In flies gray body(b+) is dominant to black body (b), and long wings (vg+) are dominant to vestigial wings (vg).
What is the expected phenotypic ratio from a cross between a heterozygous gray, heterozygous normal winged fly with a black vestigial fly?
1:1:1:1 ratioBut what he got was…..
What explains the non-Mendelian results?There was no independent assortment!The traits are linked on the same
chromosome. If the traits are linked, then all of the offspring should resemble the parental types, black vestigial or gray normal.
How did we get the recombinant traits of black, normal and gray vestigial?
Linkage mapsLinkage mapsSturtevantMap shows sequence of genes on
a chromosome, not precise location
Genes farther apart have a greater recombination frequency
Using Recombination Using Recombination Frequency to Map TraitsFrequency to Map TraitsRecombination frequency
between black and vestigial traits is 17%
Recombination frequency between cn and black is 9%
Recombination between cn and vg is 9.5%
Map these traits on the chromosome
Determine the order of genes along a chromosome based on the following recombination frequencies:
A - B 8%A - C 28%A - D 25%B - C 20%B - D 33%DABC
X Inactivation in FemalesX Inactivation in FemalesHow does an organism compensate
for the fact that some individuals have a double dosage of sex-linked genes, while others have only one?
X inactivation , the inactive X condenses into a Barr body
The inactivation is random and approximately half the X’s from Mom and half the X’s from Dad will inactivate
The calico cat is the result of multiple alleles on the X chromosome combines with X inactivation.
One allele causes orange fur and the other causes black fur.
The male cat will be either orange or black.
Only a heterozygous female will be calico with regions that are orange and regions that are black based on the X inactivation.
Nondisjunction of sex Nondisjunction of sex chromosomeschromosomesKlinefelter Syndrome XXY
◦Male, sterile, some female secondary sex characteristics
Turner Syndrome XO◦Female, short stature