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 color Wild 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 genes Color- blindness Hemophilia

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 GENES NOTE: 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 over Because 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 event Farther 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 trait PTC 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 15 Angelman: inherited from mother Jerky movements, Happy disposition, seizures Prader-Willi – inherited from father Obesity, learning disabilities, delayed motor development