1. Single Gene Inheritance
Chapter 2

2. Gene are located on chromosomes
- the two members of an homologous pair carry alleles for the same genes and, therefore, affect the same traits.

4. The stuff that makes up chromosomes
Chromatin condenses into chromosomes at cell division
Chromatin – 25% protein + 75% DNA

5. Packaging of DNA

6. Helix Nucleosome Fibers Loops

7. Loops Coils Chromosome

8. A chromosome,
A closer view

9. Definitions
Phenotype - the visible expression of information contained in the genetic make-up of an individual
Genotype - the genetic make-up, latent or expressed. The sum of all genes present in an individual.
P1 - the parental lines
F1 - the first filial generation, the offspring resulting from a cross. F2/ F3 the second and third generations.

10. Genetic polymorphisms The stuff of genetic variation
1 locus (monogenic),
2 different alleles
3 combos possible

11. Mendel’s Principles of Inheritance
1) Genetic characteristics are controlled by unit factors (elemente) that exist in pairs in individual organisms.
These unit factors represent units of inheritance today called genes.
Alternative forms of a single gene are called alleles.
Individuals can have identical alleles (homozygous) or different alleles (heterozygous) for a single gene.

12. An example: seed color
P1 (parental) X + o o F1

13. Self-pollination of the F1X + o o 3 1

14. Dominance/Recessiveness
When two unlike alleles responsible for a single character are present in a single individual, one is dominant (expressed) to the other which is said to be recessive (silent). X P1 + o o GG gg F1 Gg

15. Segregation
During formation of the gametes, the paired alleles separate or segregate randomly so that each gamete receives either with equal likelihood.
Possible sperm alleles G g GG Gg G
Possible
egg alleles Gg gg g
Punnett square

16. Sample Problem
Flower position in pea plants is another single gene trait. Axial flower position is dominant to terminal. If we cross a true breeding plant with axial flowers to one with terminal flowers, what phenotypes can we expect in the F1 progeny? in the F2 of a selfing of the F1?

17. Sample Problem
Having dimples is a dominant trait in humans. I have no dimples but my father and mother do. All my siblings have dimples as well. Am I adopted??

18. More on Single Gene Inheritance

19. Query
P1 red X white
F pink

20. Self F1 pink X pink
red pink white F2

21. Incomplete (partial) dominance
Self F1 pink X pink
(R1R2) (R1R2)
R R2
Possible sperm cells
Possible
egg cells R1 R2
R1R2
pink
R1R1
R2R2
Intermediate expression of the phenotype.

22. But what about the yellow ?
This could be the effect of multiple alleles involved in color production.

23. Multiple alleles An example: the ABO blood types in humans
Genotype Phenotype
IAIA
IAIO
IBIB
IBIO
IAIB
IOIO A B AB O

24. Sample problem
Red-green color blindness is a sex linked, recessive allele of a single gene trait. My brother-in-law is red-green color blind. My sister is not, but my father might have been. My sister and her husband have 4 sons. My mother (their grandmother) was worried that they would be color blind. Should she have been concerned?

25. Genes can be Pleiotropic
single gene, multiple effects
ex. dwarf

26. Chromosomal Basis of Inheritance

27. Meiosis (the prelude to sexual reproduction)
For sexual reproduction to occur, chromosomes must be duplicated and divided between the gametes.

28. Meiosis I

29. Meiosis II

30. Each gamete contains only one member of each homologous pair.
Meiosis
There are 2 steps of meiosis - 2 cell divisions,
but only
1 replication of chromosomes.
Each gamete contains only one member of each homologous pair.

31. Summary Mendel’s “rules of the game”
1) Genes occur in pairs - Genetic characteristics are controlled by genes that exist in pairs.
2) Dominance/Recessiveness- When two unlike alleles responsible for a single character are present in a single individual, one is dominant (expressed) to the other which is said to be recessive (silent).
3) Segregation- during formation of the gametes, the paired unit factors separate or segregate randomly.

32. Discovering genes via Mutant analysis
Generating mutants
Chemical mutagenesis (EMS)
base transition, point mutation
Radiation
deletions
Transposons/ T-DNA tags
insertion/deletions (indels)

33. And observing segregation ratios
Planned crosses and Punnett squares, Pedigree analysis
Mutant or polymorphism analysis
Autosomal dominant/recessive
Sex-linked genes