Introduction to Genetics GENETICS – branch of biology that deals with heredity and variation of organisms. Chromosomes carry the hereditary information (genes) A gene is a coding region of DNA DNA  RNA  Proteins

Allele for purple flowers Figure 14.4 Allele for purple flowers Pair of homologous chromosomes Locus for flower-color gene Figure 14.4 Alleles, alternative versions of a gene. Allele for white flowers

Gregor Johann Mendel Austrian Monk, born in what is now Czech Republic in 1822 Son of farmer, studied theology and joined the St. Augustine Order. Taught science classes and experimented with honeybees and plants. Settled on peas as a suitable model for several reasons Starting in 1856, spent a decade studying genetics using pea plants as the model organism Prior to Mendel, heredity was regarded as a blending process and the offspring were essentially a dilution of the different parental characteristics (as it frequently appears to be)

Figure 14.1 Figure 14.1 What principles of inheritance did Gregor Mendel discover by breeding garden pea plants?

In 1866 he published Experiments in Plant Hybridization, (Versuche über Pflanzen-Hybriden) in which he established his three Principles of Inheritance He tried to repeat his work in another plant, but didn’t work because the plant reproduced asexually! Work was largely ignored for 34 years, until 1900, when 3 independent botanists rediscovered Mendel’s work.

Mendel was the first biologist to use mathematics (simple statistics) to explain his results quantitatively. Mendel predicted the concept of genes, which he called “unit factors”. Each unit factor occurs in pairs (tall-short, purple-white, etc), and one unit factor is present in each gamete. This is remarkable considering we didn’t know what DNA even did, not to mention what a gene was, for many many years after Mendel

Genetics terms you need to know: Gene – a unit of heredity; a section of DNA encoding a protein Genome – the entire set of genes in an organism Alleles – two genes that occupy the same position on homologous chromosomes and that cover the same trait (purple-white, smooth-wrinkled). Locus – a fixed location on a strand of DNA where a gene or one of its alleles is located.

Homozygous – having identical genes (one from each parent) for a particular characteristic. Heterozygous – having two different genes for a particular characteristic. Dominant – the allele of a gene that masks or suppresses the expression of an alternate allele; the trait appears in the heterozygous condition. Recessive – an allele that is masked by a dominant allele; does not appear in the heterozygous condition, only in homozygous. Mendel used “latent” and “expressed”

Genotype – the genetic makeup of an organisms Phenotype – the physical appearance of an organism (Genotype + environment) Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single trait. P = Parental generation F1 = First filial (daughter) generation. F2 = Second filial generation of a genetic cross

Mendel looked at seven traits or characteristics (phenotypes) of pea plants:

Cross-fertilization pollen-dusting technique Stamen = sperm producing, Carpel = egg producing 1 2 Parental generation (P) Stamens Figure 14.2 Research Method: Crossing Pea Plants 3 Carpel 4

First filial generation offspring (F1) Figure 14.2b RESULTS 5 First filial generation offspring (F1) Figure 14.2 Research Method: Crossing Pea Plants

Mendel chose to track only those characters that occurred in two distinct alternative forms He also made sure they were true-breeding (plants that produce offspring of the same variety when they self-pollinate) © 2011 Pearson Education, Inc.

The true-breeding parents are the P generation In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are the P generation The hybrid offspring of the P generation are called the F1 generation When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the F2 generation is produced © 2011 Pearson Education, Inc.

(true-breeding parents) Figure 14.3-1 EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers Figure 14.3 Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation?

White flowers F1 Generation (hybrids) Figure 14.3-2 EXPERIMENT P Generation (Mendel made sure these were true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers Self- or cross-pollination Figure 14.3 Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation?

(true-breeding parents) Purple flowers White flowers Figure 14.3-3 EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers Self- or cross-pollination Figure 14.3 Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation? F2 Generation 705 purple- flowered plants 224 white flowered plants

PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Figure 14.6 Phenotype Genotype Purple PP (homozygous) 1 3 Pp (heterozygous) Purple 2 Pp (heterozygous) Purple Figure 14.6 Phenotype versus genotype. pp (homozygous) 1 White 1 Ratio 3:1 Ratio 1:2:1

Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants

Monohybrid cross Parents differ by a single trait. For example, crossing two pea plants that differ in stem size T = allele for Tall t = allele for dwarf TT = homozygous tall plant t t = homozygous dwarf plant T T  t t

Monohybrid cross for stem length: P = parentals true breeding, homozygous plants: T T  t t (tall) (dwarf) T t (all tall plants) F1 generation is heterozygous:

Using a Punnett Square for a monohybrid cross STEPS: 1. determine the genotypes of the parent organisms 2. write down your "cross" (mating) 3. draw a p-square Parent genotypes: TT and t t Cross T T  t t

Punnett square: F1 generation 4. "split" the letters of the genotype for each parent & put them "outside" the p-square 5. determine the possible genotypes of the ‘F1’ offspring by filling in the p-square 6. summarize results (genotypes & phenotypes of offspring) T T T T  t t T t t Genotypes: 100% T t Phenotypes: 100% Tall plants

Monohybrid cross: F2 generation If you let the F1 generation self-fertilize, the next monohybrid cross would be: T t  T t (tall) (tall) Genotypes: 1 TT= Tall 2 Tt = Tall 1 tt = dwarf Genotypic ratio= 1:2:1 T t T T T t t t T t Phenotype: 3 Tall 1 dwarf Phenotypic ratio= 3:1

Secret of the Punnett Square Key to the Punnett Square: Determine the gametes of each parent… How? By “splitting” the genotypes of each parent: If this is your cross T T  t t The gametes are: T T t t

Once you have the gametes…  t t t t T t T T

Another example: Flower color For example, flower color: P = purple (dominant) p = white (recessive) If you cross a homozygous Purple (PP) with a homozygous white (pp):  P P p p ALL PURPLE (Pp) P p

Cross the F1 generation: P p P p  F2 Genotypes: PP Pp 1 pp P p P P P p p p P F2 Phenotypes: 3 Purple 1 White p

Mendel’s Principles 0. Mendel showed that hereditary factors are inherited as discreet units, contradicting the belief that they were blended. 1. Principle of Dominance: One allele will mask or conceal another allele (and therefore trait) in a heterozygote 2. Principle of Segregation: When gametes (egg/sperm) are formed, the pairs of hereditary factors (genes) become separated, so that each sex cell (egg/sperm) receives only one kind of gene. That’s the reason we can use a Punnett square to predict the offspring.

3. Principle of Independent Assortment: Members of one gene pair segregate independently from other gene pairs during gamete formation. Gene’s do not influence each other.

Violations of Mendel’s Laws Linked Genes: many genes are located closely to neighboring genes on the same chromosome, and are inherited as a pair. This violates ‘independent assortment’. Epistasis: most observable characteristics are controlled by multiple genes. Eye color is controlled by eight separate genes. This violates “segregation”, and it negates “dominance”. Different genes (and alleles of the same gene) help each other, oppose each other, operate in complex sequences, etc. Cells, and the body plan, are complex.

Dihybrid crosses Matings that involve parents that differ in two genes (two independent traits) For example, flower color: P = purple (dominant) p = white (recessive) and stem length: T = tall t = short

Dihybrid cross: flower color and stem length TT PP  tt pp (tall, purple) (short, white) Possible Gametes for parents T P and t p F1 Generation: All tall, purple flowers (Tt Pp) tp tp tp tp TP TtPp

Dihybrid cross: flower color and stem length (shortcut) TT PP  tt pp (tall, purple) (short, white) Possible Gametes for parents F1 Generation: All tall, purple flowers (Tt Pp) T P T P t p t p T t P p

(tall, purple) (tall, purple) Dihybrid cross F2 If F1 generation is allowed to self pollinate, Mendel observed 4 phenotypes: Tt Pp  Tt Pp (tall, purple) (tall, purple) TP Tp tP tp Possible gametes: TP Tp tP tp Four phenotypes observed Tall, purple (9); Tall, white (3); Short, purple (3); Short white (1) TP Tp tP tp TTPP TTPp TtPP TtPp TTpp Ttpp ttPP ttPp ttpp

Dihybrid cross 9 Tall purple 3 Tall white 3 Short purple 1 Short white TP Tp tP tp TP Tp tP tp TTPP TTPp TtPP TtPp TTpp Ttpp ttPP ttPp ttpp Phenotype Ratio = 9:3:3:1

Dihybrid cross: 9 genotypes Genotype ratios (9): Four Phenotypes: 1 TTPP 2 TTPp 2 TtPP 4 TtPp 1 TTpp 2 Ttpp 1 ttPP 2 ttPp 1 ttpp Tall, purple (9) Tall, white (3) Short, purple (3) Short, white (1)

Test cross When you have an individual with an unknown genotype, you do a test cross. Test cross: Cross with a homozygous recessive individual. For example, a plant with purple flowers can either be PP or Pp… therefore, you cross the plant with a pp (white flowers, homozygous recessive) P ?  pp

Test cross If you get all 100% purple flowers, then the unknown parent was PP… P P P p p If you get 50% white, 50% purple flowers, then the unknown parent was Pp… P p P p p p p

Dihybrid test cross?? If you had a tall, purple plant, how would you know what genotype it is? tt pp ?? ??  1. TTPP 2. TTPp 3. TtPP 4. TtPp