Presentation on theme: "Mendelian Genetics The term ‘Mendelian genetics’ typically relates to the outcomes of simple dominant and recessive gene pairings Shows specific ratios."— Presentation transcript:
1 Mendelian GeneticsThe term ‘Mendelian genetics’ typically relates to the outcomes of simple dominant and recessive gene pairingsShows specific ratios or patterns of inheritance within a lineage of offspring generations (e.g., F1 and F2 generations)
2 Early ideas of heredity Constancy of species – heredity occurs within the boundary of the species; not so prior to the Middle ages(Ex: giraffe and minotaur)
3 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
4 Gregor Mendel (1822 – 1884) Studied garden peas 1st to use mathematics to examine outcomes of crossesLarge # of pea varieties with at least 7 easily distinguished traitsPeas are small, easy to grow, short generation timePeas can self-fertilize; bisexual
5 Some definitions for tracking traits via Mendelian inheritance Genotype/PhenotypeGene/alleleDominant/Recessive allelesHomozygous/HeterozygousP/F1/F2 generationsGenotypic ratio/Phenotypic ratioMonohybrid cross/Dihybrid cross
6 Mendel conducted studies in 3 stages Self-crossed flowers to make sure white/purple flowered plants were true-breedingCrossed true-breeding plants (white X purple)(X means “crossed with”)3. Crossed F1 plants to see traits in future generation (F2 generation)
7 Mendel came to understand…. Plant progeny (offspring) did not show blending of traitsFor each pair of alternative traits, 1 was not expressed in F1 generation, but re-appeared in F2 generationTraits segregate among the progenyAlt, traits are expressed in 3:1 ratio in F2
8 Punnett squares allow analysis using symbols for gametes and genotypes
9 Outcome of crossing true breeding purple-flowered and white-flowered pea plants F1 progeny: All purple floweredF2 progeny: 3 purple to 1 whiteSelf cross each of the F2’s
10 The Mendelian ratio Phenotypic ratio of 3:1 yet, Genotypic ratio of 1:2:1When crossing heterozygous individuals of trait controlled by simple dominant/recessive alleles
11 Mendel proposed a simple model of heredity – 5 parts: Parents transmit “factors’ to offspringEach individual receives 2 factors which code for the same traitNot all factors are identical – alternative gene forms are called allelesAlleles do not influence each other as alleles separate independently into gametesThe presence of an allele does not insure that its trait will be expressed
12 Monohybrid Crosses genotype: total set of alleles of an individual PP = homozygous dominantPp = heterozygouspp = homozygous recessivephenotype: outward appearance of an individual
13 Monohybrid CrossesPrinciple of Segregation – Mendel’s first Law of HeredityTwo alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization.
14 Dihybrid CrossesDihybrid cross: examination of 2 separate traits in a single cross-for example: RR YY x rryyThe F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait.
15 Dihybrid cross between two heterozygous parents Instead of 4 possible outcomes, there are now 16!!
16 Dihybrid CrossesPrinciple of Independent Assortment: Mendel’s 2nd Law.In a dihybrid cross, the alleles of each gene assort independently.
17 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 isprobability of obtaining Pp (1/4), PLUSprobability of obtaining pP (1/4)¼ + ¼ = ½
18 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 RrYy, the probability of obtaining rr yy offspring is:probability of obtaiing rr = ¼probability of obtaining yy = ¼probability of rr yy = ¼ x ¼ = 1/16
19 TestcrossTestcross: 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
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