INTRO TO GENETICS, MENDELIAN ANALYSIS

1/5/94, rvsd 1/26/94, 1/4/95, 1/3/96, 1/9/96, 1/6/97, 1/5/98, 1/5/00, 3 Jan 01, 7 Jan 02, 7 Jan 04, 9 Jan 08

GMSLG, P 22-32, 7^th: 28-37, Griffiths, Wessler, Lewontin and Carroll, 9^th Ed: 31-42

Gregor Mendel (1822-1884)          Czech from Brno, trained to be teacher, failed teacher's test 2x, retreated to monastery.

Took science and math courses there.

http://biology.clc.uc.edu/Fankhauser/Travel/Berlin/for_web/Mendel_in_Brno.html

Convinced abbot to place him in charge of the garden. Took to breeding Pisum sativum.

Peas were an excellent choice for several reasons:

              1) They were naturally self pollinating, and therefore tended to breed true

              2) Many varieties were readily available commercially (p 38)

              3) Several crops could be grown in a single season (esp in green house), also, got many progeny (seeds)/cross

How to perform cross (see p. 38):    1) emasculate immature flower

2) pollinate by brushing stigma of emasculated plant with pollen from male parent

3) save seeds, plant, record phenotype of progeny

MONOHYBRID CROSS

                               parental generation             = P                 (from pure lines) crossed to produce F₁

P phenotypes:        white times purple flowered plants (also did reciprocal cross, purple x white)

                              (alternate forms of same trait = allele)

                              first filial generation            = F₁                self pollinates (selfed) to produce F₂

F₁ phenotype:        all purple F₁ (purple is therefore the dominant allele, white the recessive)

                              second filial generation:      = F₂                progeny of F₁ x F₁

F₂ phenotypes 929 seeds, planted:                 705 purple 

                                                                        224 white             (close to 3:1 ratio)

repeated for other six pairs, same 3:1 ratio (table on p. 40):

He selected seven pure-lined pairs of traits to cross:                  F₂ numbers of phenotypes:

PHENOTYPE                           Dominant         Recessive                       Dom                  Recessive

              seed shape                  round                 wrinkled                          5474                        1850

              seed color                  yellow                green                               6022                        2001 

              flower color               purple                white                                 651                          207

              pod shape                  inflated              pinched                             882                          299

              pod color                     green                  yellow                               428                          152 

              flower position             axial                   terminal                             705                          224

              stem length                long                   dwarf                                 787                          277

Blending is disproved:          1)         F₁ showing dominant phenotype

                                              2)         reappearance of recessive phenotype

He proposed that the dominant phenotype in F₂ is composed of two distinct genotypes:

              519 F₂ selfed, progeny scored: 166 only yellow, 353 yellow and green in 3:1 ratio

Mendel did TEST CROSS: cross dominant phenotype with unknown genotype times recessive phenotype: (p 41 far right)

              predicted:                1:1 yellow to green when F₁ is back crossed to green

              result:                      58 yel and 52 grn peas

These were monohybrid crosses, for single set of alleles.

MENDEL'S FIRST LAW OF EQUAL SEGREGATION: Each parent carries two copies of each hereditary determinant (gene) for each trait (diploid). When gametes are formed, these pairs of alleles segregate in a 50:50 ratio.

              If two copies are identical, homozygous

              If two copies are different, heterozygous

DIHYBRID CROSS: Round green X wrinkled yellow (RRyy X rrYY)

              Got 100% RY F₁, but 9:3:3:1 RY, Ry, rY, ry pattern in F₂ (p 93)

MENDEL'S SECOND LAW OF INDEPENDENT ASSORTMENT

During gamete formation, alleles of one gene assort independently of those of another gene.

Mendel's laws, proposed in 1866

            MENDEL’S FIRST LAW EQUAL SEGREGATION):                    1. hereditary factors which

2. segregated and

            MENDEL’S SECOND LAW INDEPENDENT ASSORTMENT:   3. independently assorted

PROBABILITY laws to predict progeny

Published in 1866, Ignored for 34 years, [Rediscovered in 1900, 34 yr later)