Mendelian Genetics

Heredity – the passing of traits from parents to offspring Genetics - The study of heredity

Gregor Mendel “Father of Genetics” Austrian monk who studied patterns of inheritance in Pisum sativum, the garden pea. Used math to discover patterns of inheritance and developed genetic rules to explain them.

Only 2 traits for each characteristic (Table 1) Why Study Peas? Only 2 traits for each characteristic (Table 1) Characteristic Contrasting Traits Alleles Dom vs. Rec Dom vs. Rec P p A a Y y R r I i G g T t

Genotype PP (homozygous dominant) Pp (heterozygous) The genetic make-up of an organism Examples: PP (homozygous dominant) Pp (heterozygous) pp (homozygous recessive) We’re the SAME We’re the SAME Homo = “Same” Hetero = “Different”

Phenotype The external appearance of an organism Genotype Phenotype PP (homozygous dominant) = Purple Pp (heterozygous) = Purple pp (homozygous recessive) = White Curly Straight

A cross involving 1 characteristic or gene. Mendel’s Experiments Monohybrid cross A cross involving 1 characteristic or gene.

Predicting the Outcomes of Monohybrid Crosses: Punnett Square A chart devised by Reginald Punnett in 1905 Used to simulate a cross & predict the likelihood of all possible outcomes. Sperm Eggs

Steps For Using a Punnett Square: Write the genotypes of the individuals to be crossed. Ex #1: Perform a P1 cross for plant height: TT X tt

Steps For Using a Punnett Square: Draw the Punnett square below them. TT X tt

Steps For Using a Punnett Square: Simulate meiosis by writing the types of sperm the male can produce above the square and the types of eggs the female can produce to the left. T Eggs t Sperm Law of Segregation X T T t t

Steps For Using a Punnett Square: Simulate fertilization by filling in the boxes of the square TT X tt T Eggs t Sperm Genotypic ratio of F1: TT : Tt : tt 0 : 4 : 0 100% Tt Phenotypic ratio of F1: Tall : short 4 : 0 100% Dominant Tt Tt F1 Tt Tt

Mendel’s Conclusions: Organisms inherit 2 copies of a gene (one from each parent) 2. Genes exist in alternative forms (P vs. p). * Today the different versions of a gene are called alleles. When an organism inherits alleles that are different hybrid, one may totally mask or hide the other. * Mendel described the trait that was expressed in the F1 the dominant trait - capable of masking its contrasting trait. * He called the trait that was lost in the F1 only to reappear in the F2 the recessive trait - the trait that is masked. 4. When gametes are formed, the alleles for each gene separate independently (Law of Segregation)

Mendel’s findings were published before chromosomes were discovered, so scientists couldn’t explain them. Walter Sutton (1900) Proposed the Chromosome theory States that genes are located on chromosomes. p P Chromosomes occur in pairs, therefore, genes also occur in pairs – one allele per homologue

Simulate an F1 cross: Ex #2: Given: B = Black fur b = brown fur (heterozygous X heterozygous) Bb X Bb Genotypic ratio of F2: BB : Bb : bb : 2 : 1 Phenotypic ratio of F2: Black : Brown 3 : 1 B b BB Bb B Bb bb b

How can we find out which one? Dwarfism in dogs is caused by an undesirable recessive allele: D = Normal height d= dwarfism Normal and dwarf German Shepherd litter mates. What is the genotype of a dwarf dog? dd What is the genotype of a normal dog? DD or Dd How can we find out which one?

Perform a test cross Procedure used to determine the genotype of an individual with a dominant phenotype. Mate the unknown with a homozygous recessive individual. Enables us to determine the genotype of unknown individual.

Test Cross If we have a normal dog, we can breed it with a dwarf to determine if it carries a hidden dwarf allele or DD X dd Dd X dd d d d d D D Dd Dd Dd Dd D Dd Dd d dd dd If unknown is heterozygous, offspring will be 50% normal, 50% dwarfs. If unknown is homozygous dominant, offspring will be 100% normal.