The Chromosomal Basis of Inheritance

1. The Chromosomal Basis of Inheritance

2. Chromosomal Theory of Inheritance • Genes are physically located at loci on chromosomes • Additionally, it is chromosomes that undergo segregation and independent assortment

3. Thomas Hunt Morgan • Very important geneticist, studied Drosophila melanogaster (fruit flies) • 8 chromosomes • New generation every 2 wks • XY sex determination

4. 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)

5. Sex-linked Genes • Genes located on the X chromosome are sex-linked (XY system) • Therefore, males only had 1 allele for eye color

6. 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)

7. Sex-linked Diseases • Examples of sex-linked genes • Color-blindness • Hemophilia

8. 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

9. 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.

10. 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

11. 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

12. 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

13. X-Inactivation • In females, 1 X chromosome in each cell is randomly inactivated and becomes a Barr body

14. 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

15. 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

17. 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