- Slides: 20
Mendelian Genetics §The term ‘Mendelian genetics’ typically relates to the outcomes of simple dominant and recessive gene pairings §Shows specific ratios or patterns of inheritance within a lineage of offspring generations (e. g. , F 1 and F 2 generations)
Early ideas of heredity 1) Constancy of species – heredity occurs within the boundary of the species; not so prior to the Middle ages (Ex: giraffe and minotaur)
View held thru time of Darwin • Direct transmission of traits – child is formed after hereditary material from all parts of parent’s body come together – blending occurs
Gregor Mendel (1822 – 1884) • Studied garden peas • 1 st to use mathematics to examine outcomes of crosses • Large # of pea varieties with at least 7 easily distinguished traits • Peas are small, easy to grow, short generation time • Peas can self-fertilize; bisexual
Some definitions for tracking traits via Mendelian inheritance • • Genotype/Phenotype Gene/allele Dominant/Recessive alleles Homozygous/Heterozygous P/F 1/F 2 generations Genotypic ratio/Phenotypic ratio Monohybrid cross/Dihybrid cross
Mendel conducted studies in 3 stages 1. Self-crossed flowers to make sure white/purple flowered plants were true-breeding 2. Crossed true-breeding plants (white X purple) (X means “crossed with”) 3. Crossed F 1 plants to see traits in future generation (F 2 generation)
Mendel came to understand…. • Plant progeny (offspring) did not show blending of traits • For each pair of alternative traits, 1 was not expressed in F 1 generation, but reappeared in F 2 generation • Traits segregate among the progeny • Alt, traits are expressed in 3: 1 ratio in F 2
Punnett squares allow analysis using symbols for gametes and genotypes
Outcome of crossing true breeding purple-flowered and white-flowered pea plants F 1 progeny: All purple flowered F 2 progeny: 3 purple to 1 white Self cross each of the F 2’s
The Mendelian ratio • Phenotypic ratio of 3: 1 yet, • Genotypic ratio of 1: 2: 1 When crossing heterozygous individuals of trait controlled by simple dominant/recessive alleles
Mendel proposed a simple model of heredity – 5 parts: 1. Parents transmit “factors’ to offspring 2. Each individual receives 2 factors which code for the same trait 3. Not all factors are identical – alternative gene forms are called alleles 4. Alleles do not influence each other as alleles separate independently into gametes 5. The presence of an allele does not insure that its trait will be expressed
Monohybrid Crosses genotype: total set of alleles of an individual PP = homozygous dominant Pp = heterozygous pp = homozygous recessive phenotype: outward appearance of an individual
Monohybrid Crosses Principle of Segregation – Mendel’s first Law of Heredity Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization.
Dihybrid Crosses Dihybrid cross: examination of 2 separate traits in a single cross -for example: RR YY x rryy The F 1 generation of a dihybrid cross (Rr. Yy) shows only the dominant phenotypes for each trait.
Dihybrid cross between two heterozygous parents Instead of 4 possible outcomes, there are now 16!!
Dihybrid Crosses Principle of Independent Assortment: Mendel’s 2 nd Law. In a dihybrid cross, the alleles of each gene assort independently.
Probability – Predicting Results Rule of addition: the probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities. When crossing Pp x Pp, the probability of producing Pp offspring is probability of obtaining Pp (1/4), PLUS probability of obtaining p. P (1/4) ¼ + ¼ = ½
Probability – Predicting Results Rule of multiplication: the probability of 2 independent events occurring simultaneously is the PRODUCT of their individual probabilities. When crossing Rr Yy x Rr. Yy, the probability of obtaining rr yy offspring is: probability of obtaiing rr = ¼ probability of obtaining yy = ¼ probability of rr yy = ¼ x ¼ = 1/16
Testcross: a cross used to determine the genotype of an individual with dominant phenotype -cross the individual with unknown genotype (e. g. P_) with a homozygous recessive (pp) -the phenotypic ratios among offspring are different, depending on the genotype of the unknown parent