A. Experimental genetics began in an abbey garden

1. in the 1860’s, an Austrian monk named Gregor Mendel discovered the fundamental principles of genetics by breeding garden peas.

2. Mendel argued in a paper published in 1866 that parents pass on to their offspring discrete heritable factors (today called genes)

3. he stressed that these factors retained their individuality generation after generation

4. Mendel studied garden peas- easy to grow, lots of different traits to distinguish between them

5. he could also have strict control over mating

6. in nature, pea plants often self-fertilize; pollen from stamens (male part) falls into carpel (female part)

7. (Fig. 9.2c) to cross fertilize, he would cut immature stamens, brush on pollen from male parent of choice

8. after pollination, carpel grows into pod with seeds (peas) of next generation, which he could grow

9. so Mendel could always be sure of parentage and offspring they produced

10. Mendel worked with plants until he had true breeding varieties in 7 characteristics

11. (Fig. 9.2D) each of the seven characteristics had 2 distinct forms

12. true breeding, again, means varieties that when self fertilized, produced offspring all identical to the parent - eg- parent w. purple flowers produces all offspring with purple flowers

13. now Mendel could see results of breeding experiments

14. hybrids - offspring that result from breeding 2 different varieties; cross fertilization of 2 different varieties is called hybridization, or a cross

15. Parents are called P generation

16. hybrid offspring are called F1 generation

17. their offspring are called F2 generation

B. Mendel’s principle of segregation describes the inheritance of a single characteristic

1. Mendel performed many experiments where he tracked the inheritance of a single characteristic such as flower color

2. Fig. 9.3A & B - a cross between a plant with purple flowers and one w/ white flowers - only differing characteristic of truebreeders

3. F1 plants were all purple, not light purple so blending was not going on

4. Then he mated the F1 plants - of 929 plants, Mendel found 705 (~.75) to be purple, 225 were white (~.25)

5. So Mendel decided that the F1 plants mush have had the factor to make both purple and white flowers even though only purple was expressed in F1

6. From these results, and many others, Mendel developed 4 hypotheses

1. there are alternative forms of genes, the units that determine heritable traits; e.g. the gene for flower color comes in purple, white and other colors. We call alternative forms alleles.

2. for each inherited characteristic, an organism has two genes, one from each parent. Alleles may be the same or different.

3. a sperm or egg carries only one allele for each inherited trait, because allele pairs separate (segregate) from each other during the production of gametes. Moreover, when sperm and egg unite at fertilization, each contributes its allele, restoring the paired condition in offspring 

4. When 2 genes of a pair are different alleles and one is fully expressed while the other has no noticeable effect on the organism’s appearance, the alleles are called the dominant allele and the recessive allele

7. Fig. 9.3b shows how Mendel explained the results

a. P represents the dominant allele, for purple flowers

b. p represents the recessive allele, white flowers.

c. true breeders have 2 of the same alleles = homozygous

d. F1 plants have inherited one of each of the alleles - 2 different alleles = heterozygous

e. Punnett square shows 4 possible combinations of gametes, and the proportions of genes in offspring

8. Organisms appearance doesn’t always reveal genetic composition

a. genotype is genetic makeup - PP, pp or Pp, in this case

b. phenotype is expressed or physical traits or appearance, purple or white in this case

c. in example 9.3b, the phenotypic ratio is 3:1 purple to white,

d. genotypic ratio is 1 PP : 2 Pp : 1 pp

9. Mendel found that each of the traits he examined exhibited the same inheritance patterns

10. in fig. 9.2d, all the left hand traits are dominant traits, right hand are recessive

11. The principle underlying the inheritance patterns he noted is called Principle of Segregation

12. Principle of segregation = pairs of genes segregate during gamete formation; the fusion of gametes at fertilization pairs genes once again

C. Homologous chromosomes bear the two alleles for each characteristic

1. Fig. 9.4 - Every diploid individual, peas and humans, has 2 sets of chromosomes - 1 moms, 1 dads

2. labeled bands of chromosomes are gene loci, locations of genes along the chromosomes

3. matching colors of bands represent loci for same gene, different uppercase or lowercase represent different alleles

4. Alleles (different forms of the same gene) can reside at the same locus on homologous chromosomes

D. the Principle of Independent Assortment is revealed by tracking two characteristics at the same time

1. 2 of Mendel’s 7 characteristics were seed shape and seed color

2. shape could be either round or wrinkled

3. color of peas could be either green or yellow

4. Mendel new that allele for round was dominant to allele for wrinkled

5. allele for yellow was dominant to allele for green

6. Mendel did dihybrid crosses - plants differing in 2 traits

7. Mendel crossed homozygous plants having round yellow seeds (RRYY) with plants homozygous for wrinkled green (rryy)

8. (Fig. 9.5A) crossing a parent homozygous dominant for both traits with a parent homozygous recessive for both traits gave all heterozygotes in the F1

a. this shows that traits are inherited separately or you’d have both traits always be together in the offspring

b. the 4 gamete possibilities are RY, rY, Ry, ry from F1

c. 9 different genotypes in F2; only 4 different phenotypes, 9:3:3:1 ratio

d. 12 plants with round seeds, 4 with wrinkled = 3:1

e. 12 yellow seeds, 4 green = 3:1

f. Mendel always observed a 9:3:31 ratio of phenotypes in dihybrid crosses in the F2

g. These results support hypothesis that each pair of alleles segregates independently during gamete formation = Principle of Independent Assortment

9. Labrador retrievers show same principle - B = densely packed pigment = black; b = brown

a. Progressive Retinal Atrophy = PRA = n is recessive to allele N, which is normal vision

b. only nn dogs are blind; If we mate 2 heterozygous labs, both of genotype BbNn, the F2 phenotypic ratio = 9 black normal vision with 3 black, blind; 3 brown normal, 1 brown blind

 

E. Geneticists use the testcross to determine unknown genotypes

1. lets say you have a new black lab and you want to determine its genotype - BB, or Bb

2. mate it with a brown dog (bb for sure)

3. if puppies are all black, then its probably BB; if some are brown, then its probably Bb

4. a Bb x bb cross = exhibit a 1:1 phenotypic ratio

5. Mendel used testcrosses to determine whether he had truebreeding varieties of plants.

6. testcrosses are still important tools in genetics

F. Connections sections - 9.8 - 9.10

1. genetic traits in humans can be tracked through family pedigree (fig. 9.8b)

2. many human inherited disorders are controlled by a single gene (fig 9.9)

3. fetal testing can spot many inherited disorders early in pregnancy

a. amniocentesis

b. ultrasound

c. chorionic villus sampling (fig. 9.10