Chapter 11 Introduction to Genetics

Chapter 11 Introduction to Genetics CPI BIOLOGY Hollison High School

11-1 The Work of 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 intos 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 characters(ex) Height, short or tall • He crossed the plants (with contrasting characters) and looked at their offspring

P = parental generation = original pair of plants • F1 = first filial generation= first generation • Hybrids: offspring from parents with different traits

Tracking Generations • Parental generation P mates to produce • First-generation offspring F1 mate to produce • Second-generation offspring F2

CROSS-POLLINATION: Mendel cut the male parts of one flower (ouch!) and dusted the female parts with pollen from another flower. P GENERATION: purple x white flowers F1 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: • Inheritance is determined by factors that are passed down • GENES: the factors that determine traits • 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 F1 generations to make F2 offspring

THE F2 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 F1 tall plants self-pollinating  ¼ short, ¾ tall F1 Cross F2

Explaining the F1 Cross • Why did the recessive allele reappear? At some point, the recessive allele had to separate from the dominante allele. This is called… • SEGREGATION: separation of alleles  occurs during formation of gametes (eggs & sperm) in anaphase II of meiosis • F1 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

11-2 Probability & Punnett Squares • Mendel realized that the principles of probability can explain the results of genetic crosses. • PROBABILITY: likelihood an event will occur (ex) Flip coin 3x in a row, 1/8 chance it will be heads all 3 times ( ½ x ½ x ½ ) • The pattern in which alleles segregate is random…just like a coin flip! So which ever allele gametes receive is also random.

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 What do the letters represent in a punnett square? • Letters represent alleles: capital = dominantlowercase = 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 Squarefor TT x Tt Punnett Squarefor YY x yy

Genotype vs Phenotype • GENOTYPE: the genetic makeup of an organism (ex) TT • PHENOTYPE: the physical characteristics exhibitied (ex) tall plant In the Punnett Square shown What is the genotype of the offspring? What is the phenotype?

Probability and Segregation • F2 generation from Tall F1 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!

11-3 Exploring Mendelian Genetics • Does the gene that determines flower color have anything to do with the gene for height? (ex) Do all tall plants have purple flowers? • Mendel performed TWO-FACTOR CROSSES: crossing 2 different genes and following traits as they pass from one generation to the next

Two-Factor Cross: F1 • 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 • Draw a Punnett Square for this cross. • All F1 were RrYy (round and yellow)or HYBRIDS This cross does not answer question, but provides hybrids for next cross

Two-Factor Cross: F2 • F1 Generation = RrYy • How would these alleles segregate when F1 self-pollinated? • 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 • Draw a Punnett Square for this cross.

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 OR Metaphase I: 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: B B b b b b B B A A a a A A a a 1/4 AB 1/4 ab 1/4 Ab 1/4 aB

Summary of Mendel’s Principles • Inheritance of characteristics is determined by genes which are passed to offspring • If 2+ alleles of a trait exist, some alleles may be dominant, others may be recessive • Sexually reproducing organisms have 2 copies of each gene which segregate during gamete formation • Alleles for different genes segregate independently

Beyond Dominant and 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… • Incomplete Dominance • Codominance • Multiple Alleles • Polygenic Traits

Incomplete Dominance Homozygous parent (RR) Homozygous Parent (rr) X • When one allele is not completely dominant; recessive allele is not totally masked • Heterozygous phenotype is in between the two homozygous phenotypes • (ex) Red snapdragon flowers (RR) X snapdragon white (rr) flowers  pink hybrid flowers (Rr) All F1 are heterozygous X F2 shows three phenotypes in 1:2:1 ratio

Incomplete Dominance homozygous parent X homozygous parent All F1 offspring heterozygous for flower color: Cross two of the F1 plants and the F2 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 X black feathers  speckled chicken • (ex) Horse coats: red X 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

Applying Mendel’s Principles • Early 1900’s Morgan used Mendel’s principles to study fruit flies  advanced study of genetics • Mendel’s Principles also apply to study the inheritance of human traits and to calculate the probability of traits appearing in the next generation.

11-1 & 11-2 Mini-Quiz (10pts) • Who worked with pea plants and came up with the fundamental principles of genetics? • After the P (parent) generation, what are the next two generations called? • Factors that determine traits are called _____ and contrasting characters of traits are called ______. • When two hybrid plants are crossed, what percentage exhibits the dominant phenotype? What percentage exhibits the recessive phenotype? • Draw the Punnett Square for a heterozygous tall plant and a short plant. What is the genotype ratio for the offspring? What is the phenotype ratio for the offspring?

11-1 & 11-2 Mini-Quiz Answer Key (10pts) • (1) MENDEL • (2) F1 & F2 • (2) GENES & ALLELES • (2) 75% DOMINANT 25% RECESSIVE • (3) Genotype ratio: 50% Tt, 50% tt Phenotype ratio: 50%Tall, 50%short

Chapter 11 Introduction to Genetics