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