Upload
monica-york
View
229
Download
2
Tags:
Embed Size (px)
Citation preview
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