Presentation on theme: "Classical Genetics Gregor Mendel. Gene versus Allele Gene - a sequence of DNA in a specific location on a chromosome Determines traits in an organism."— Presentation transcript:
Gene versus Allele Gene - a sequence of DNA in a specific location on a chromosome Determines traits in an organism Ex. – eye color Allele – a single gene that is specific for a trait (usually represented by a letter) –“Gene” determines eye color exists –“Allele” determines the color: blue eyes
Blending Theory of Inheritance Offspring were the average of their parents –A tall parent and a short parent have a medium child Parental genes were lost in favor of offspring’s genes –The tall gene and small gene from the parents are lost. The offspring carries a gene for medium height. X Incorrect!!
Gregor Mendel Wanted to breed flowers for selective traits Chose one trait at a time Applied mathematics to his results PEAS!!
Why use pea plants? Peas had been shown to be pure breeding –all offspring will have the same characteristics for every generation variety of contrasting traits –purple vs. white flowers –round vs. wrinkled seeds reproduce by self-pollination –pollen produced by a flower fertilizes eggs in the same flower –can be prevented and hand pollinate. Pea plants grow quickly and do not require much space
1 st Basic Experiment Parental (P) Generation Pure purple and Pure white are crossed (mated) P Generation produces the (first filial) “F1” Generation – all purple
Next Experiment Cross 2 individuals from F1 Generation X Produces 2nd Filial (F2) Generation
Why the difference in color? Dominant trait – a trait that needs only 1 gene to be expressed (shown) in the organism –Capital letter Recessive trait – a trait that must have 2 of the same genes to be expressed –Lower case letter Example: Flower color Purple vs. White Using the letter “F” to represent allele Purple flower – Dominant trait “F” White flower – Recessive trait “f”
Using the allele F… Purple is the dominant trait –Possible genotypes: FF, Ff –Phenotype: purple flower White is the recessive trait –ONLY possible gentoype: ff –Phenotype: white flower
Dominant Genotypes Both produce the same Phenotype –Homozygous Dominant – FF ; purple –Heterozygous – Ff ; purple Recessive Genotypes Only one genotype will produce phenotype –Homozygous Recessive – ff ; white
Punnett Squares and Peas! Recall our P Generation plants (Parents are Pure) crossed to produce our F1 generation which are all purple How can this be predicted? PUNNETT SQUARES!!! Can be used to predict offsprings’ genotypes and phenotypes
Let’s complete a punnett square for our purple and white flowers… Recall: 1) Dominant trait is purple: F 2) Recessive trait is white: f 3) in the P Generation all flowers are “Pure”
Dihybrid Cross Two traits are examined in a 4 x 4 Punnett square Each conceivable combination of gametes is used to determine possible offspring
Mendel’s Law of Segregation Genes are normally paired Separated or Segregated in the formation of gametes (eggs and sperm).
Mendel’s Law of Independent Assortment Each factor's distribution in the gametes is not related to the distribution of any other factor. This principle is not strictly true due to the organization of genes on chromosomes.