Gregor Johann Mendel 1822 1884 Austrian monk Experimented

Gregor Johann Mendel • 1822 - 1884 • Austrian monk • Experimented with pea plants • He thought that ‘heritable factors’ (genes) retained their individuality generation after generation

Terms to Know and Use • Gene – A DNA blueprint controlling synthesis of a protein • Trait - variant for a gene: i. e. a purple flower, determined by alleles • Dominant trait - expressed over recessive trait when both are present • Recessive trait - not expressed when the dominant trait is present • Co-Dominant – expressed as blended traits

• Allele - a variation of a gene responsible for different traits, often represented as A or a • Locus - location of a gene, or allele, on a chromosome • Chromosome - strand of DNA containing the genes • Haploid - one copy of a chromosome • Diploid - two copies of a chromosome • Gamete - a spermatozoa or oocyte (egg) cell, they are haploid

• Zygote - cell resulting from the fusion of two gametes, they are diploid • Genotype - the type of alleles on a chromosome: genetic makeup • Phenotype - The way a genotype is expressed: i. e. the color of a flower

dihybrid crosses, mating of parent plants that differ in two traits in plants that were true breeding for two traits. A test cross is a breeding or a mating between an individual of dominant phenotype, who could be either homozygous dominant (SS) or heterozygous (Ss), with an individual that MUST be homozygous recessive(ss)

Garden Pea Experiments 1856 -64 • Mendel disagreed with the “Blending Theory” of inheritance. • Worked on peas Pisium sativum

Mendel chose garden peas as his experimental organism because: ØStrict control over mating was possible to ensure the percentage of new seeds. Pea flowers enclose the carpel and stamens, which prevents cross-pollination. Immature stamens can be removed to prevent self-pollination. Mendel hybridized pea plants by transferring pollen from white flower to a purple flower. ØMendel chose to track only characters in pea plants that varied in an “either – or” (or clear cut) manner. For example, his plants had either purple or white flowers; there was nothing in between these two varieties.

Flower Parts

Pea Characteristics Trait on the left is dominant. Trait on the right is recessive.

Mendel’s Hypotheses • There alternate forms of ‘genes’=alleles • For each trait, organisms have 2 genes, one from mom & one from dad • Pollen and egg each carry 1 allele/trait because alleles segregate • When only one allele is expressed & other has no noticeable effect, it is dominant

Mendel's Laws of Inheritance • Law of Segregation during gamete formation allele pairs separate or segregate, into different gametes (Demonstrated with a “test cross”). • Law of Independent Assortment suggested that each allele pair segregates independently of other gene pairs during gamete formation (Demonstrated with a dihybrid cross).

Mother contributes: A A or A AA AA or Father contributes: True Breeding A

Mother contributes: A a or a Aa Aa or Father contributes: Cross Breeding A

Monohybrid Cross

Test Cross

Dihybrid Cross

By the law of segregation, the two alleles for a characters are packaged into separate gametes When Mendel crossed true-breeding plants with different character traits, he found that the traits did not blend. Ø Using the scientific process, Mendel designed experiments in which he used large sample sizes and kept accurate quantitative records of the results. Ø For example, a cross between true-breeding varieties, one with purple flowers and one with white flowers produced F 1 progeny (offspring)with only purple flowers. Hybridization: Mating or crossing of two varieties. Monohybrid cross: A cross that tracks the inheritance of a single character. Fig. 14. 2

HYPOTHESIS: Mendel hypothesized that if the inhertitable factor for white flowers had been lost, then a cross between F 1 plants should produce only purple flowered plants. EXPERIMENT: When Mendel allowed the F 1 plants to self-fertilize (self-pollinate). RESULTS: Based on a large sample size, Mendel recorded 705 purple-flowered and 224 white-flowered in the F 2 generation a ratio of 3: 1 The inheritable factor for white flowers was not lost (reappeared in the F 2). , so the hypothesis was rejected. CONCLUSION: From these types of experiments and observations, Mendel concluded that since the inheritable factor for white flowers was not lost in the F 1 generation, it must have been masked by the presence of the purple flowers factor. Fig. 14. 2

Mendles factor are now called genes and in Mendel’s terms Purple flower is the dominant trait White flower is the recessive trait. Mendel repeated these experiments with six other characters and found similar 3 to 1 ratios in the F 2 generations.

v Mendel developed a hypothesis to explain these results that consisted of four related ideas. 1. Alternative version of genes (different alleles) account for variations in inherited characters. Ø For example the gene for flower color in pea plants exists in two alternative (forms); one for purple color and one for white color. Ø These alternative versions for a gene are now called alleles. Ø Today we know that each gene resides at a specific locus on a specific chromosome. The DNA at that locus can vary somewhat in its sequence of nucleotides, and hence in its information content. The purple-flower allele and the white flower allele are two DNA variations possible at the flowercolor locus on one chromosome of Fig. 14. 3 one pea plant.

2. For each character, an organism inherits two alleles, one from each parent. § Mendel deduced that each parent contributes one “factor” even though he did not know about chromosomes or meiosis. § We now know that Mendel’s factors are genes. Each genetic locus is represented twice in diploid organisms, which have homologous pairs of chromosomes, one set from each parent § These homologous loci may be identical, (have the same allele, AA or aa) as in the true-breeding plants of the P generation. § Alternatively, the two alleles may differ ( Aa ) § In the flower-color example, the F 1 plants inherited a purple-flower allele from one parent and a whiteflower allele from the other.

3. If two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance, the other is completely masked (recessive allele) and has no noticeable effect on the organism appearance. ØMendel’s F 1 plants had purple flowers because the purple-flower allele is dominant and the whiteflower allele is recessive. Ø Dominant alleles are designated by a capital letter. A purple flower color ØRecessive alleles are designated by a lowercase letter. a white flower color

4. The two alleles for each character segregate (separate) during gamete production. ØWithout any knowledge of meiosis, Mendel deduced that a sperm cell or ovum carries only one allele for each inherited characteristic, because allele pairs separate (segregate) from each other during gamete production (meiosis) ØGametes of true breeding plants will all carry the same allele. If different alleles are present in the parent, there is a 50% chance that the gametes will receive the dominant allele and a 50% chance that it will receive the recessive allele. § The separation of alleles into separate gametes is summarized as Mendel’s law of segregation.

§ Mendel’s law of segregation accounts for the 3: 1 ratio that he observed in the F 2 generation. § The F 1 hybrids (Aa) will produce two classes of gametes, half with the purple-flower allele (A) and half with the white-flower allele (a). § During self-pollination, the gametes of these two classes unite randomly. Ova containing purple-flower alleles have equal chances of being fertilized by sperm carrying purple-flower alleles or sperm carrying white-flower alleles. § Since the same is true for ova containing white flower alleles, there are four equally likely combinations of sperm and ovum.

v The combinations resulting from a genetic cross may be predicted by using a Punnett square ¼ of the plants with two alleles for purple flowers (AA) ½ of the plants with one allele for purple flowers and one allele for white flowers (Aa). Since the purple-flower allele is dominant, these plants have purple flowers. ¼ of the plants with two alleles for white flower color (aa), which will have white flowers since no dominant allele is present. Fig. 14. 4

The pattern of inheritance for all seven of the characteristics studied by Mendel was the same: one parental trait disappeared in the F 1 generation and reappeared in ¼ of the F 2 generation Genetics has some unique, useful vocabulary. Homozygous: having two identical alleles for a given character or gene (e. g. AA or aa). § All gametes carry that allele § Homozygotes are true breeding Heterozygous: having two different alleles for a character or gene (e. g. Aa). § Half the gametes carries one allele (A) and the remaining half carries the other (a) § Heterozygotes are not true- breeding.

Dihybrid Cross Mother contributes: Father contributes: SB Sb SB SSBB s. B sb SSBb Ss. BB Ss. Bb Sb SSb. B SSbb Ssb. B Ssbb s. B s. SBb ss. BB ss. Bb sb s. Sb. B s. Sbb ssb. B ssbb

Because of dominance and recessiveness, an organism’s appearance does not always reveal its genetic composition, therefore we should distinguish between: Phenotype: an organism’s expressed trait or organism’s appearance (e. g. purple or white flower) Genotype: An organism’s genetic makeup (AA, Aa or aa) Fig. 14. 5 §For flower color in peas, both PP and Pp plants have the same phenotype (purple) but different genotypes (homozygous and heterozygous). §AA and Aa plants have the same phenotype (purple) but different

§ The Test cross: because some alleles are dominant over others, the genotype of an organism may not be apparent, eg: a pea plant with purple flowers may be either homozygous dominant, AA or heterozygous, Aa. So we use § A test cross, breeding a homozygous recessive (aa) with dominant phenotype, but unknown genotype, AA § or Aa, so we can determine the identity of the unknown allele. Fig. 14. 6

By the law of independent assortment, each pair of alleles segregates into gametes independently § Mendel’s experiments that followed the inheritance of flower color or other characters focused on only a single character via monohybrid crosses. § He conduced other experiments in which he followed the inheritance of two different characters, a dihybrid cross.

§ In one dihybrid cross experiment, Mendel studied the inheritance of seed color and seed shape. Ø The allele for yellow seeds (Y) is dominant to the allele for green seeds (y). Ø The allele for round seeds (R) is dominant to the allele for wrinkled seeds (r). § Mendel crossed true-breeding plants that had yellow, round seeds (YYRR) with true-breeding plants that has green, wrinkled seeds (yyrr). § The resulting F 1 dihybrid progeny were heterozygous for both traits (Rr. Yy) and had round yellow seeds, the dominant phenotypes. § From the F 1 generation, Mendel could not tell if the two characters were inherited independently or not, so he allowed the F 1 progeny to self-pollinate.

Mendel considered two alternate hypothesis: Hypothesis 1: If the two characters segregate together, the F 1 hybrids can only produce the same two classes of gametes (RY and ry) that they received from the parents, and the F 2 progeny will show a 3: 1 phenotypic ratio. Hypothesis 2: If the two characters segregate independently, the F 1 hybrids will produce four classes of gametes (RY, Ry, r. Y, ry) And the F 2 progeny will show a 9: 3: 3: 1 ratio Experiment: Mendel performed a dihybrid cross by allowing selfpollination of the F 1 plants (Rr. Yy X Rr. Yy) Results: Mendel categorized the F 2 progeny and determined a ratio of 315: 108: 101: 32 which approximates 9: 3: 3: 1. Conclusion: The experimental results supported the hypothesis that each allele pair segregates independently during gamete formation

Fig. 14. 7 a Fig. 14. 7 b

§ Mendel repeated the dihybrid cross experiment for other pairs of characters and always observed a 9: 3: 3: 1 phenotypic ration in the F 2 generation. § Each character appeared to be inherited independently. § The independent assortment of each pair of alleles during gamete formation is now called Mendel’s law of independent assortment. § One other aspect that you can notice in the dihybrid cross experiment is that if you follow just one character, you will observe a 3: 1 F 2 ratio for each, just as if this were a monohybrid cross.