Gregor Mendel What is Genetics? the study of heredity Gregor Mendel s Peas Pollen: plant s sperm Egg Cells: plants reproductive cells Fertilization: joining of pollen + egg cells develops into embryo in a seed Born in 1822. His work with pea plants laid the foundation for Genetics.
Working with pea plants Self-pollinating: pollen fertilizes egg cells in the SAME flower (single parent reproduction) True-breeding: offspring genetically identical to parents due to self-pollination Cross-pollination: combining reproductive cells from 2 DIFFERENT parent plants Mendel could cross- breed a purple flower with a white flower What do you think is the color of the offspring?
Genes and Dominance TRAIT: specific characteristic (seed color, plant height, etc) What did Mendel do in his pea plant experiments? Studied 7 different traits each with contrasting characteristics (ex) Height, short or tall He crossed the plants (with contrasting characters) and looked at their offspring
P = parental generation = original pair of plants F 1 = first filial generation= first generation Hybrids: offspring from parents with different traits
Tracking Generations Parental generation mates to produce P First-generation offspring F 1 mate to produce Second-generation offspring F 2
CROSS-POLLINATION: Mendel cut the male parts of one flower and dusted the female parts with pollen from another flower. P GENERATION: purple x white flowers F 1 GENERATION: all purple flowers HYBRID PLANTS
What happened in Mendel s crosses? All the offspring only had one of the parent s characters the other parent s character disappeared! Mendel s Conclusions: 1. Inheritance is determined by factors that are passed down 2. GENES: the factors that determine traits 3. Contrasting characters are different forms of a gene called ALLELES
Mendel s Principle of Dominance some alleles are dominant, some are recessive DOMINANT ALLELE: form of trait that will always be exhibited; usually expressed in capitals RECESSIVE ALLELE: form of trait is only exhibited when the dominant allele is NOT present (ex) Allele for tall is dominant for and the allele for short is recessive
What happened to the recessive allele? Mendel wanted to know if the recessive allele disappeared from the F1 plants. F1 CROSS: He self-crossed the F 1 generations to make F 2 offspring
THE F 2 GENERATION The recessive traits reappeared!! ~¼ plants had white flowers, the recessive trait Summary of Crosses: tall plants X short plants tall plants P P F 1 tall plants self-pollinating ¼ short, ¾ tall F 1 Cross F 2
Explaining the F 1 Cross Why did the recessive allele reappear? At some point, the recessive allele had to separate from the dominant allele. This is called SEGREGATION: separation of alleles occurs during formation of gametes (eggs & sperm) in anaphase II of meiosis.
Explaining the F 1 Cross F 1 plants inherited 1 tall allele & 1 short allele from parents When gametes are formed, the two alleles segregate from each other each gamete has 1 copy of each gene So, 2 different types of gametes are formed (one w/ tall allele, one w/ short allele)
SEGREGATION
Punnett Squares What is a Punnett Square? A diagram showing the possible genetic combinations from a particular cross Can be used to predict and compare the genetic variations that will result from a cross
Punnett Squares What do the letters represent in a Punnett square? Letters represent alleles: capital = dominant lowercase = recessive Homozygous: has two identical alleles for a trait (ex) TT or tt Heterozygous: has two different alleles for the same trait (ex) Tt
Punnett Square for TT x Tt Punnett Square for YY x yy
Genotype vs Phenotype GENOTYPE: the genetic makeup of an organism (ex) TT PHENOTYPE: the physical characteristics exhibited (ex) tall plant
Probability and Segregation F 2 generation from Tall F 1 plants ¾ tall, ¼ short 3:1 ratio of tall to short plants Punnet squares work to predict outcomes, so Mendel s ideas about segregation are accurate!
Exploring Mendelian Genetics Mendel performed TWO-FACTOR CROSSES: crossing 2 different genes and following traits as they pass from one generation to the next
Two-Factor Cross: F 1 Two Genes: shape of pea & color of pea The Cross: Round yellow peas x wrinkled green peas RRYY x rryy What are the possible alleles parent 1 can pass? RY What are the possible alleles parent 2 can pass? ry All F 1 were RrYy (round and yellow) or HYBRIDS
Two-Factor Cross: F 2 F 1 Generation = RrYy How would these alleles segregate when F 1 selfpollinated? RrYy x RrYy Do the two dominant alleles stay together? What are the possible alleles each parent can pass on? There are 4 possible combinations: Ry, RY, ry, ry
INDEPENDENT ASSORTMENT The F1 Hybrid cross produces a 9:3:3:1 phenotype ratio Mendel found that the 2 alleles (seed shape & seed color) don t influence each other s inheritance This is called the principle of Independent Assortment: genes for different traits can segregate independently during the formation of gametes
Independent Assortment Metaphase I: OR A A a a A A a a B B b b b b B B Metaphase II: A A a a A A a a B B b b b b B B Gametes: A B A B a b b b b a A A a B a B 1/4 AB 1/4 ab 1/4 Ab 1/4 ab
Summary of Mendel s Principles A. Inheritance of characteristics is determined by genes which are passed to offspring B. If 2+ alleles of a trait exist, some alleles may be dominant, others may be recessive C. Sexually reproducing organisms have 2 copies of each gene which segregate during gamete formation D. Alleles for different genes segregate independently
Beyond Dominant & Recessive Alleles Genetics is more complicated Some alleles are neither dominant nor recessive Many traits are controlled by multiple alleles or multiple genes
Other Inheritance Patterns 1. Incomplete Dominance 2. Codominance 3. Multiple Alleles 4. Polygenic Traits
Incomplete Dominance Homozygous parent (RR) X Homozygous Parent (rr) When one allele is not completely dominant; recessive allele is not totally masked All F 1 are heterozygous Heterozygous phenotype is in between the two homozygous phenotypes X F 2 shows three phenotypes in 1:2:1 ratio
Incomplete Dominance Homozygous parent (RR) X Homozygous Parent (rr) (ex) Red snapdragon flowers (RR) X snapdragon white (rr) flowers pink hybrid flowers (Rr) X All F 1 are heterozygous F 2 shows three phenotypes in 1:2:1 ratio
Incomplete Dominance All F 1 offspring heterozygous for flower color: homozygous parent X homozygous parent Cross two of the F 1 plants and the F 2 offspring will show three phenotypes in a 1:2:1 ratio:
Codominance Both alleles contribute to the phenotype Heterozygous genotype expresses both phenotypes (ex) Feather colors in chickens: white feathers & black feathers speckled chicken (ex) Horse coats: red & white roan coat
Codominance:ABO Blood Types Alleles that controls blood type are codominant Two alleles A & B are both exhibited when paired, a third allele (i) is recessive to others AA or Ai = Type A Blood BB or Bi = Type B Blood AB = Type AB Blood ii = Type O Blood
Multiple Alleles > 2 possible alleles for a gene Individuals can still only have 2 alleles each but more than 2 alleles exist in a population (ex) coat color in rabbits lots of options due to 4 different alleles (ex) blood type is determined by multiple alleles
Polygenic Traits Traits controlled by the interaction of 2+ genes (ex) Fruit fly eye color (3+ different genes) (ex) Skin color in humans (4+ different genes), eye color, height, weight