Mendelian Genetics

What is Heredity? The passing on of traits from one generation to the next X Female normal wing Male double wing P- generation F 1 – normal wing

Early Ideas about Heredity Blending Theory (19 th century): –Each parent contributed “factors” that were blended in the offspring What was the problem with this theory? –All individuals of a population would eventually look the same. –Once blended, traits would never separate and show up in later generations. Chapter 9, Section 1: Pre-Mendel

Gregor Mendel (1822 – 1884) Augustinian monk who studied pea plants Established the particulate theory of heredity Significance: –Developed pure lines –Counted results and kept statistical notes (data) His work remained undiscovered until Chapter 9, Section 1: Mendel

CharacterDominant TraitRecessive Trait Flower Color Flower Position Seed Color Seed Shape Pod Shape Pod Color Stem Length PurpleWhite SideTop YellowGreen RoundWrinkled InflatedConstricted GreenYellow TallDwarf Mendel’s Research Why did he research pea plants? 1.They are normally self-pollinated, but can be cross-pollinated. 2.They have several qualitative traits that are easy to distinguish i.e.,Tall vs. Short Self Pollination Cross Pollination

Mendel’s Research 1.Removed stamens from purple flower. 2.Transferred pollen from stamens of white flower to pistil of purple flower. 3.Pollinated flower matured into a pod. 4.Planted seeds from pod. All purple flowers 5.Examined offspring: All purple flowers… Watch this video clip, and see if you can explain why…video clip

Mendel’s Conclusions 1.Law of Segregation a.Factors (genes) for a particular trait occur in pairs pairs b.For each trait, an organism inherits two genes, one from each parent.each parent c.Dominant alleles mask recessive onesmask i.Exception 1: Incomplete DominanceIncomplete Dominance ii.Exception 2: Co-dominanceCo-dominance d.Two alleles for each trait segregate (separate) during gamete productionsegregate

Homologous pair of Chromosomes Locus for Flower Color Gene Allele for White Flowers (p) Allele for Purple Flowers (P) Law of Segregation: Factors for a particular trait occur in pairs Back to Mendel’s Conclusions Genes: The “factors” that control traits. Alleles: Different forms of a gene.

Each individual is diploid –Diploid: Containing a double-set of chromosomes (2n) Each gamete is haploid –Gamete: Reproductive cell (egg or sperm) –Haploid: Containing a single-set of chromosomes (n) Back to Mendel’s Conclusions Law of Segregation: One version of each gene is inherited from each parent Wwww W w or w w W = widow’s peak w = no widow’s peak

P (Parent) Generation True-breeding parents (PP x pp) F 1 (1 st Filial) Generation Hybrid Offspring (Pp) F 2 (2 nd Filial) Generation What is the F 2 ratio? 705 plants had purple flowers 224 plants had white flowers 705:224  3:1 What happened to the recessive traits? Dominant alleles mask recessive ones

Pp PP pp P p Pp PP Pp pp PP pp P Generation F 1 Generation F 2 Generation Phenotype (Appearance) Purple FlowersWhite Flowers Genotype (Genetic Makeup) Gametes Phenotype (Appearance) Gametes Purple Flowers Genotype (Genetic Makeup) 3:1 What is the Genotypic Ratio of the F2 Generation? PP:Pp:pp  1:2:1 What is the Phenotypic Ratio of the F2 Generation? Dominant alleles mask recessive ones Punnett Square

GenotypePhenotype Purple White PP Pp pp (homozygous) (heterozygous) (homozygous) Ratio = 1:2:1Ratio = 3:1 Dominant alleles mask recessive ones Homozygous: same alleles Heterozygous: different alleles

We will now play “Got Gametes?” in order to practice understanding the following new terms: alleles, genotype, phenotype, homozygous, heterozygous Vocabulary Practice

Each of you are haploid gametes carrying single alleles – for a trait. Your single letter can be combined with another single letter (i.e., Hh) to form a genotype in a diploid organism. You will observe a series of faces. Come to the front of the classroom if you think you have the right genotype to match the phenotype shown. Make sure to find the corresponding allele for the trait you represent!

How do you set up a Punnett square? Example 1: Heterozygous short hair _(____)_ X heterozygous short hair _(____)_ Hh HH hh HH Hh hh Genotypic Ratio HH:Hh:hh 1:2:1 Phenotypic Ratio Short hair:long hair 3:1 Now you try!

Punnett Square Exercise One-trait cross In a populations of Wisconsin fast plants, purple color is created by the pigment anthocyan. The gene that codes for this pigment is dominant (A), and without this pigment, the plant is green recessive (a). Predict the offspring from the cross of a purple homozygous plant and a green plant. What if the purple plant was heterozygous? How would the offspring be different? Green Purple Purple plant: AA Green plant: aa Aa a AAa 100% Purple Purple plant: Aa Green plant: aa Aa a aAa aa 50% Purple 50% Green

Punnett Square Exercise One-trait cross In a populations of hamsters, brown fur is dominant (B), and white fur is recessive (b). Predict the offspring from the cross of a white hamster and a brown hamster if the brown hamster's mother was white. Brown hampster: Bb White hampster: bb b Bb b bBb bb Offspring 50% brown mice 50% white mice

White board exercises One-trait cross The allele for the hairy trait is “H,” while the allele for the hairless trait is “h.” Make a cross between two homozygotes for each of these traits.

Punnett Square Exercise One-trait cross Henry VIII divorced six of his wives (two of whom were executed) for not bearing him any sons. Use Mendelian Genetics to prove to King Henry that it wasn’t his wives’ fault. Remember: Females are XX and Males are XY.

Dominate alleles mask recessive ones Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp If PP, then all offspring purple: If Pp, then ½ offspring purple and ½ offspring white: What would your hypothesis be if the genotype was PP? P p Ppp P p p Pp pp What would your hypothesis be if the genotype was Pp? This is called a test cross If you have a dominant phenotype (like purple flowers) how would you design an experiment to determine if it was homozygous (PP) or heterozygous (Pp)? Back to Mendel’s Conclusions

Exception to Dominant Alleles Masking Recessive Alleles Incomplete Dominance: Pink Snapdragons Use Root Letter “C” to show incomplete dominance interaction CRCRCRCR CRCRCRCR CWCWCWCW CWCWCWCW CWCWCWCW CWCWCWCW CRCRCRCR CRCRCRCR CRCWCRCWCRCWCRCW CRCWCRCWCRCWCRCW CRCRCRCRCRCRCRCR CWCWCWCWCWCWCWCW P Generation F 1 Generation F 2 Generation Phenotype(s): Red and White Phenotype: Pink Genotype(s): C R C R and C W C W Genotype: C R C W Gamete of Red flower: C R Gamete of White flower: C W Gametes: C R and C W Back to Mendel’s Conclusions

Exception to Dominant Alleles Masking Recessive Alleles: Co-dominance: Blood TypesBlood Types Use root letter “ I ” for dominant alleles of equal strength and “i” for recessive Back to Mendel’s Conclusions

This occurs in a process called meiosis: Specifically it is “crossing over,” which occurs very early during Prophase I of Meiosis and separation in Anaphase I and II Law of Segregation: Two alleles for each trait segregate (separate) during gamete production Back to Mendel’s Conclusions

Mendel’s Conclusions The law of segregation followed one single trait at a time, such as seed color. What if two traits were followed, such as seed color and seed shape? Are these genes somehow connected (linked) and inherited together?

Following two traits: Seed shape and seed color Hypothesis: If the genes for seed shape and color are connected in some way, then the dominant R and Y alleles and the recessive r and y alleles will be matched sets in the gametes. YR yr YyRr P Gametes F1F1 F2F2 YR yr OvaSperm For the traits of seed shape and color, this hypothesis is NOT supported by experimental evidence

What did the data support instead?

YYRRyyrr YR yr What are the genotypes of the P Generation? What does the P Generation pass on to F 1 ? F 1 Phenotype? YyRr F 1 Genotype? Gametes What does the F 1 Generation pass on to F 2 ? YR Yr yR yr SpermOva YYRR Combine the sperm and ova to produce the offspring in F 2 What do you expect the phenotype will be when YR and YR are combined? What will the Genotype be when YR and YR are combined? Now try to do it on your own! YYRr YyRR YyRr yyRr yyRRYyrr YYrr Yyrr yyrr Phenotypic Ratios Yellow & Round Green & Round Yellow & Wrinkled Green & Wrinkled 9/16 3/16 1/16 What are the phenotypic ratios? Law of Independent Assortment

Mendel’s Conclusion! Law of Independent Assortment: If the genes are not connected, then they should segregate independently. The alleles are randomly packaged into different gametes during meiosis (In other words, genes for seed shape and color were not inherited together.) YyRr  YRYryRyr FOIL

Significance of Mendel’s Research Punnett SquarePunnett Square was introduced as a tool to predict or determine the probability of an event

White-board exercises Two-trait cross Following two traits: Kernel shape: Plump vs. withered (P and p) and Kernel color Red vs. yellow (R and r) Predict the cross between a homozygous recessive corn plant and a homozygous dominant corn plant.

Punnett Square Exercise Two-trait cross Now take the offspring from that cross, and self-pollinate that plant. What is the phenotypic ratio of this cross?

Punnett Square Exercise Two-trait cross My pet guinea pigs (Joni and Chachi) are going to have little guinea pigs. What is the possibility that their offspring will have long hair, if I know that Joni and Chachi are heterozygous for the following traits? Black fur (B)White fur (b) Long hair (L)Short hair (l)