Single Gene Inheritance Chapter 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.
The stuff that makes up chromosomes Chromatin condenses into chromosomes at cell division Chromatin – 25% protein + 75% 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. P1 - the parental lines F1 - the first filial generation, the offspring resulting from a cross. F2/ F3 the second and third generations.
Genetic polymorphismsThe stuff of genetic variation 1 locus (monogenic), 2 different alleles 3 combos possible
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.
o o + An example: seed color P1(parental) X F1
o o + Self-pollination of the F1 X 3 1
o o + 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). P1 X gg GG F1 Gg
Gg GG gg Gg 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 G Possible egg alleles g Punnett square
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??
P1 red X white F1 pink Query
Self F1 pink X pink red pink white 1 2 1 F2
Possible sperm cells R1 R2 R1R2 pink R1R1 R1 R2 Possible egg cells R2R2 R1R2 pink Incomplete (partial) dominance Self F1pink X pink (R1R2) (R1R2) 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 IAIA IAIO IBIB IBIO IAIB IOIO 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
Meiosis(the prelude to sexual reproduction) For sexual reproduction to occur, chromosomes must be duplicated and divided between the gametes.
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.
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.
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