Fundamentals of Genetics CHAPTER 9 Fundamentals of Genetics

Objectives Defend how Mendel was able to control how his pea plants were pollinated Analyze the steps in Mendel’s experiments on true-breeding garden peas Distinguish between dominant and recessive traits Analyze the two laws of heredity that were developed from Mendel’s work Describe how Mendel’s results can be explained by scientific knowledge of genes and chromosomes

Introduction: Breeding in plants and animals Examples: corn was developed from a wild grass in Mexico and Central America, probably about 5000 years ago dogs were domesticated by 12,000 years ago; selective crossing has created many breeds we have today some artificial pollination was done to increase yields in fruit Chromosomes - seen in the 1800s, but their role in heredity was not understood until the early 1900s

Genetics The science of heredity & the mechanisms by which traits are passed from parent to offspring

I. Gregor Mendel (1822-1884) History Austrian Monk who taught high school & worked in the garden 1851-Entered University of Vienna to study science & math He started breading pea plants (Pisum sativum) about 1855, studied heredity Mendel formulated 2 fundamental laws of heredity in the early 1860's

The “Father of Modern Genetics” Mendel’s experiments established the basic ideas & laws that predict the pattern of inheritance from generation to generation

Heredity Characteristic Trait (or strain) the transmission of characteristics from parents to offspring Characteristic An inheritable feature (flower color, height) Trait (or strain) A genetically determined variant of a characteristic (white or red flower)

Work with Peas: Mendel used statistics to study 7 Characteristics, each occurred in 2 Contrasting Traits:     Flower Color – purple or white Flower Position along Stem – axial or terminal Seed or Pea Color - yellow or green Seed Texture or Shape – round (smooth) or wrinkled Pod Color – green or yellow Pod Appearance or Shape – inflated or constricted Plant Height - long or short stems

Seven pea plant characteristics studied:

Mendel’s Methods Pollination: when pollen grains (from anther- male) are transferred to stigma (female) Self-pollination- occurs when pollen is transferred from flower anthers to stigma of either same flower or flowers on the same plant Cross-pollination- occurs between flowers of 2 plants

Pea plants usually reproduce through self-pollination

Pollination Methods Mendel first collected seeds that were true breeding (always produce offspring with that trait) for many generations for 14 traits Interrupted self-pollination & performed cross-pollination Removed all the anthers from flowers on a plant Manually pollinated the stigma of this plant with pollen from another plant

Cross section of a flower

Three Steps of Mendel’s Experiments Produce a true-breeding or PURE P generation By self-pollination for several generations “P” is for parent saw: 2 pure traits- 1 for each parent

Step Two Produce an F1 generation By cross-pollination of parents pure for different traits Example- purple flowers with white flowers “F1” is for first filial generation saw: only 1 trait (dominant - recessive trait disappeared)

Step Three Produce an F2 generation By allowing F1 generation to self-pollinate “F2” is for second filial generation saw reappearance of recessive trait in 3: 1 ratio (3 purple to 1 white)

Results of F2 generation- showed a 3:1 ratio Mendel’s Results & Conclusions Experiments were repeated carefully for hundreds of crosses Results of F1 generation- showed all offspring of crossed true-breeding plants were of one trait. Example: purple flowers x white flower parents Offspring all purple Results of F2 generation- showed a 3:1 ratio Example: 705 purple and 224 white flowers

Dominant & Recessive Traits When strains of plants pure for a trait were crossed, one trait always failed to appear in the F1 generation The absent trait reappeared in ¼ of the F2 generation Mendel concluded that each trait was inherited by means of a separate factor, so a pair of factors controlled each trait

Dominant- Term for a factor (allele) that masks the presence of another factor Recessive - Term for a factor (allele) that is masked by the presence of another allele.

Law of Segregation A pair of factors are separated during formation of gametes Each reproductive cell, or gamete, receives one factor of each pair Gametes combine during fertilization, offspring then have two factors for each characteristic

Law of Independent Assortment Different pairs of factors are passed to offspring independently New combinations of genes, present in neither parent, are possible.  Example: flower color & plant height not connected The genes controlling different traits are inherited independently of one another.

Independent assortment of these two homologous chromosomes (Pp and Yy) would result in gametes that contain these allele combinations

Mendel’s Conclusion Molecular genetics – study of the structure and function of chromosomes and genes Chromosomes made of DNA Genes on DNA control traits Chromosomes occur in pairs Genes occur in pairs Allele – each of two or more alternative forms of a gene

“alleles” represented by letters Dominant are CAPITAL letters Recessive are small letters Example 1: Brown eyes dominant: Blue recessive “B” for brown & “b” for blue Example 2: Red flower dominant-White recessive “R” for red and “r” for white REMEMBER- USE ONLY 1 LETTER PER PAIR: Bb or Rr for example - don’t mix!!! Never: Rw!!!

Section 1 Review Page 178 #1-5

Exit Slip What are the P, F1, and F2 generations that Mendel studied? What is the difference between dominant and recessive?

Objectives Differentiate between the genotype and the phenotype of an organism Explain how probability is used to predict the results of genetic crosses Use a Punnett square to predict the results of monohybrid and dihybrid genetic crosses Explain how a testcross is used to show the genotype of an individual whose phenotype expresses the dominant trait Differentiate a monohybrid cross from a dihybrid cross

Genetic Crosses Genotype - WHAT IS ON THE GENES the genetic make- up of an organism for a trait the alleles that the organism inherits from its parents Not always observable Example: white flower = pp ; purple flower = Pp or PP Phenotype - WHAT YOU SEE An organism’s appearance or observable characteristic the detectable expression that results from the organism’s genotype Example: pp = White ; PP or Pp = Purple

Homozygous & Heterozygous Homozygous (2 OF THE SAME) -When both alleles of a pair are alike Example: PP (Both alleles purple color) pp (both white color) Heterozygous (2 DIFFERENT) – When the alleles in the pair are different Examples: Pp (one Purple & one white) Or Tt (one tall & 1 short) ( REMEMBER not to use Ts or Rw !!!!!!!!)

Probability The likelihood that something will happen Predicted results more likely for more trials Example: flipping a coin Q1- What is the chance that it will be head or tails? Q2- Discuss what happens if you flip it 10 times? 100? Q3- If you flip 2 coins- Does that result have anything to do with the other- why or why not? REVIEW: Relate this back to the law of independent assortment.

Q1 What is your genotype for this trait? PROBLEM: Tongue Rolling is Dominant (R) You can roll your tongue but your spouse cannot Your first child also cannot roll his tongue Q1 What is your genotype for this trait? Q2 What is the probability of having a tongue roller child next time?

Predicting Monohybrid Cross Results Monohybrid Cross - A cross in which only 1 trait is tracked Punnett square – A graphic or diagram used to predict the results of a genetic cross. Draw a Punnett square - 4 small squares in the shape of a window. Write the possible gene(s) of one parent across the top and the gene(s) of the other parent along the side of the Punnett square.                                 

Fill in each box of the Punnett square by transferring the letter above and in front of each box into each appropriate box As a general rule, the capital letter goes first and a lowercase letter follows The letters inside the boxes indicate probable genotypes Predict the phenotype of these offspring

Monohybrid Cross Homozygous purple (PP) is crossed with homozygous white (pp) producing only heterozygous purple (Pp) offspring

Homozygous X Homozygous Black- BB homozygous dominant White- bb homozygous recessive All (100% probability) offspring are heterozygous (Bb) genotype & black phenotype

Homozygous X Heterozygous BB - homozygous dominant genotype, black phenotype Bb – heterozygous genotype, black phenotype Offspring: Genotype: 50% BB, 50% Bb Phenotype: 100% Black, 0% white

Heterozygous X Heterozygous Cross 2 heterozygous parents (Bb) x (Bb) (2 black rabbits) Offspring: Genotypic ratio: 1 BB: 2Bb: 1bb 25%BB, 50%Bb, 25% bb Phenotypic ratio: 3 black: 1 white 75% black, 25% white

Testcross Since you don’t know if a black rabbit might have a genotype of BB or Bb, you could perform a test cross A test cross is when an individual of unknown genotype is crossed with a homozygous recessive (bb) Try this to see what happens with BB, then Bb crosses

If a black rabbit is crossed with a brown rabbit and even one of the offspring is brown, the black rabbit is heterozygous

Monohybrid Practice Complete the following monohybrid crosses: Tt X Tt (T is dominant for tall, t is recessive for short) Dd X dd (D is dominant for dimples, d is recessive for no dimples) bb X BB (B is dominant for Brown eyes, b is recessive for blue eyes) What is the genotypic and phenotypic ratios for each cross?

Tt x Tt T t Genotypic Ratio: Phenotypic Ratio:

Dd x dd D d Genotypic Ratio: Phenotypic Ratio:

bb x BB B b Genotypic Ratio: Phenotypic Ratio:

Complete Dominance One allele is completely dominant over another (one trait masks another) Heterozygous & homozygous plants look alike Pea plants PP & Pp both have purple flowers

Incomplete Dominance Occurs when the trait of a heterozygote is intermediate between the phenotypes of the parents The dominant allele is unable to express itself fully Example: Japanese four o’clock flowers RR is red Rr is pink rr is white

Cross between 2 Pink Genotypic ratio: 1 RR:2Rr:1rr Phenotypic Ratio: 1 red: 2 pink:1 white

Codominance Occurs when both alleles for a gene are expressed in heterozygous offspring Neither allele is dominant or recessive Alleles do not blend Example: Human blood types A, B, AB, O Type of trait expressed by Multiple Alleles

Dihybrid Crosses A cross in which 2 characters are tracked More complicated, due to more possible combinations Examples: Peas can be round or wrinkled AND yellow or green Cats can have short or long tails AND brown or white fur Pea plants can be tall or short AND have purple or white flowers

Homozygous x Homozygous Homozygous for round, yellow seeds crossed with Homozygous for wrinkled, green seeds Round (R) is dominant over wrinkled (r) Yellow (Y) is dominant over green (y) Genotypic Ratio: 100% RrYy Phenotypic Ratio: 100% Round & Yellow

Heterozygous x Heterozygous Cross RrYy x RrYy results 9 genotypes and 4 phenotypes 9/16 have round, yellow seeds (RRYY, RRYy, RrYy, RrYY 3/16 have round, green seeds (RRyy, Rryy) 3/16 have wrinkled, yellow seeds (rrYY, rrYy) 1/16 have wrinkled, green seeds (rryy)

Heterozygous x Heterozygous

Extra Practice

What is the genotypic ratio in the F2 generation? What is the phenotypic ratio in the F2 generation?

Fill in the table to complete the cross between 2 heterozygous parents What is the genotypic ratio and phenotypic ratio?

Section 2 Review Page 186 #1-5