Presentation on theme: "Single Gene Inheritance Chapter 2. - the two members of an homologous pair carry alleles for the same genes and, therefore, affect the same traits. Gene."— Presentation transcript:
Single Gene Inheritance Chapter 2
- the two members of an homologous pair carry alleles for the same genes and, therefore, affect the same traits. Gene are located on chromosomes
Chromatin – 25% protein + 75% DNA Chromatin condenses into chromosomes at cell division The stuff that makes up chromosomes
Packaging of DNA
A chromosome, A closer view
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. P 1 - the parental lines F 1 - the first filial generation, the offspring resulting from a cross. F 2 / F 3 the second and third generations.
Genetic polymorphisms The stuff of genetic variation 1 locus (monogenic), 2 different alleles 3 combos possible
Mendels 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.
An example: seed color + o o X P 1 (parental) F1F1
Self-pollination of the F 1 + o o X 3 1
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). + o o X GG gg P1P1 Gg F1F1
Segregation During formation of the gametes, the paired alleles separate or segregate randomly so that each gamete receives either with equal likelihood. G G g g Possible egg alleles Gg GGgg Punnett square Possible sperm alleles
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?
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??
More on Single Gene Inheritance
P 1 red X white F 1 pink Query
Self F 1 pink X pink red pink white F2F2
Self F 1 pink X pink R 2 ) R 2 ) (R 1 R 2 ) (R 1 R 2 ) Incomplete (partial) dominance R 2 R 1 R 2 Possible sperm cells Possible egg cells R1R2R2R1R2R2 R 2 R 1 R 2pink R1R1R1R1R1R1 pink R2R2R2R2R2R2 Intermediate expression of the phenotype.
But what about the yellow ? This could be the effect of multiple alleles involved in color production.
Multiple alleles An example: the ABO blood types in humans Genotype Phenotype IAIAIAIOIBIBIBIOIAIBIOIOIAIAIAIOIBIBIBIOIAIBIOIO A B AB O
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?
Genes can be Pleiotropic -single gene, multiple effects ex. dwarf
Chromosomal Basis of Inheritance
Meiosis (the prelude to sexual reproduction) For sexual reproduction to occur, chromosomes must be duplicated and divided between the gametes.
Meiosis Each gamete contains only one member of each homologous pair. There are 2 steps of meiosis - 2 cell divisions, but only 1 replication of chromosomes.
Summary Mendels rules of the game 3) Segregation- during formation of the gametes, the paired unit factors separate or segregate randomly. 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).
Discovering genes via Mutant analysis Generating mutants – Chemical mutagenesis (EMS) – base transition, point mutation – Radiation – deletions – Transposons/ T-DNA tags – insertion/deletions (indels)
And observing segregation ratios Planned crosses and Punnett squares, Pedigree analysis –Mutant or polymorphism analysis Autosomal dominant/recessive Sex-linked genes