The Chromosomal Basis of Inheritance

Chromosomal Theory of Inheritance

Genes are physically located at loci on chromosomes

Additionally, it is chromosomes that undergo segregation and independent assortment

Thomas Hunt Morgan

Very important geneticist, studied Drosophila melanogaster (fruit flies)

8 chromosomes New generation every

2 wks XY sex determination

Wild type

He focused on fruit fly eye colorWild type phenotype: the phenotype that

occurs most commonly in a natural population

In fruit flies, red eyes were the wild type (notation for red eyes is w+)

White eyes are considered the mutant phenotype (notation for white eyes is w)

Sex-linked Genes

Genes located on the X chromosome are sex-linked (XY system)

Therefore, males only had 1 allele for eye color

Inheritance of Sex-linked Traits

In humans, males are the heterogametic sex (XY).

Males inherit sex-linked traits from their mother

Males are hemizygous (only 1 allele)

Sex-linked Diseases

Examples of sex-linked genesColor-

blindnessHemophilia

LINKED GENES

There are many genes located on chromosomes

Genes that are located on the same chromosome tend to be inherited together

These are called LINKED GENESNOTE: Linked genes do not follow

Mendel’s Law of independent assortment

LINKED GENES EXCEPTIONS

Even though genes on the same chromosome are USUALLY inherited together, they are not always.

Think about meiosis . . . Why do you think that genes on the

same chromosome are not always inherited together.

GENETIC MAPPING

ANSWER: Crossing overBecause of the discovery of

crossing over, scientists developed a system to map the order of genetic loci, genetic mapping

GENETIC MAPPING

Assumptions:Crossing over is a

random eventFarther apart

genes are on the chromosome, the higher the probability that they will be recombined

Therefore:With enough

recombination data, you can map the genes on the chromosome in relation to each other

TYPES OF GENETIC MAPS

Linkage map: a genetic map based on recombination frequencies

Cytogenic map: a genetic map based on staining genes and observing them under the microscope

X-Inactivation

In females, 1 X chromosome in each cell is randomly inactivated and becomes a Barr body

Genomic Imprinting

Most times the expression of the allele is not dependent on which parent donates the traitPTC taster is dominant (it doesn’t matter if

you received the trait from mom or dad)Sometimes the expression of a trait

depends on which parent you receive the trait from . . . Genomic imprinting

Genomic Imprinting

In gametogenesis, some alleles are “silenced” in one parent

Therefore, depending whether the allele is silenced in the sperm or egg, the trait is only active in one gamete (mom’s or dad’s)

Mostly occurs in developmental genes Individuals heterozygous for a disease on an

imprinted allele show different phenotypes depending on which parent it was inherited from

Angelman Syndrome/Prader-Willi

Caused by mutation on chromosome 15Angelman:

inherited from mother

Jerky movements, Happy disposition, seizures

Prader-Willi – inherited from father

Obesity, learning disabilities, delayed motor development