Gregor Johann Mendel Between 1856 and 1863 Mendel

Gregor Johann Mendel §Between 1856 and 1863, Mendel cultivated and tested some 28, 000 pea plants §He found that the plants' offspring retained traits of the parents 1

Gregor Mendel ¡ Called the “Father of Genetics ¡ Gregor Mendel (1860’s) discovered the fundamental principles of genetics by breeding garden peas.

Pea Garden (Pisum sativum)

Pea Garden (Pisum sativum) Easy to grow and can be grown in a small area Produce lots of offspring Produce pure plants when allowed to selfpollinate several generations (true breeding varieties) Clearly defined characteristics or traits Easy to be crossed between parents

Pea Characteristics

Mendel crosspollinated pea plants • Mendel probably chose to work with peas because they are available in many varieties. • The use of peas also gave Mendel strict control over which plants mated. • Fortunately, the pea traits are distinct and were clearly contrasting.

Mendel’s experimental design ¡ Statistical Worked with large numbers of plants counted all offspring made predictions and tested them ¡ Excellent analyses: experimentalist controlled growth conditions focused on traits that were easy to score chose to track only those characters that varied in an “either-or” manner

Mendel’s Work

Mendel’s Work

Typical breeding experiment P generation (parental generation) F 1 generation (first filial generation, the word filial from the Latin word for "son") are the hybrid offspring. Allowing these F 1 hybrids to self-pollinate produces: F 2 generation (second filial generation).

Mendel Conclusion Factors are passed from one generation to the next.

Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. All the offspring will be heterozygous and express only the dominant trait. RR x rr yields all Rr (round seeds) 12

The Principle of Dominance eye color locus B = brown eyes eye color locus b = blue eyes Paternal Maternal

Dominant and Recessive alleles Dominant alleles – upper-case (B) a. homozygous dominant (BB – Brown eyes) Recessive alleles – lower case (b) a. homozygous recessive (bb – blue eyes) b. heterozygous dominant (Bb – Brown eyes)

Phenotype vs. Genotype ¡ Outward ¡ Arrangement of genes that produces the phenotype ¡ Examples: ¡ Example: 1. TT, Tt 2. tt appearance ¡ Physical characteristics 1. Brown eyes 2. blue eyes

Segregation: Alleles separate during meiosis

Law of Segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring 17

The Law of Segregation 18

Punnett Squares ü ü Diagram used to predict genetic crosses Tool for calculating genetic probabilities A tool to predict the probability of certain traits in offspring that shows the different ways alleles can combine. Diagram showing the probabilities of the possible outcomes of a genetic cross

How to use Punnett Squares Choose a letter to represent the alleles in the cross. Write the genotypes of the parents. Determine the possible gametes (reproductive cells) that the parent can produce. Enter the possible gamete at the top and side of the Punnett square. Complete the Punnett square by writing the alleles from the gametes in the appropriate boxes. Determine the phenotypes of the offspring.

Punnet Square Process 1. 2. Determine alleles of each parent, these are given as TT, and tt respectively. Take each possible allele of each parent, separate them, and place each allele either along the top, or along the side of the punnett square.

Punnett Square Process ¡ Lastly, write the letter for each allele across each column or down each row. The resultant mix is the genotype for the offspring. In this case, each offspring has a Tt (heterozygous tall) genotype, and simply a "Tall" phenotype.

Punnett Square Process ¡ ¡ Lets take this a step further and cross these F 1 offspring (Tt) to see what genotypes and phenotypes we get. Since each parent can contribute a T and a t to the offspring, the punnett square should look like this….

Punnett Square Process ¡ Here we have some more interesting results: First we now have 3 genotypes (TT, Tt, & tt) in a 1: 2: 1 genotypic ratio. We now have 2 different phenotypes (Tall & short) in a 3: 1 Phenotypic ratio. This is the common outcome from such crosses. Monohybrid cross (cross with only 1 trait)

Testcross Cross the dominant phenotype (unknown genotype) with the recessive phenotype (known genotype).

Dihybrid cross The cross with a pure-breeding (homozygous) two loci. F 1 generation

Dihybrid cross ¡ Take the offspring and cross them since they are donating alleles for 2 traits, each parent in the f 1 generation can give 4 possible combination of alleles. TW, Tw, t. W, or tw. F 2 Generation

Dihybrid cross ¡ ¡ Note that there is a 9: 3: 3: 1 phenotypic ratio. 9/16 showing both dominant traits, 3/16 & 3/16 showing one of the recessive traits, and 1/16 showing both recessive traits. Also note that this also indicates that these alleles are separating independently of each other. This is evidence of Mendel's Law of independent assortment

Mendel’s Principles ¡ The inheritance of biological characteristics are determined by genes. ¡ For two or more forms of a gene, dominance and recessive forms may exist. ¡ Most sexually reproductive organisms have two sets of genes that separate during gamete formation. ¡ Alleles segregate independently.

Law of Independent Assortment ü ü Alleles for different traits are distributed to sex cells (& offspring) independently of one another. Different genes on different chromosomes segregate into gametes independently of each

Independent Assortment r r e e h ot ath M F E E e e n N N N n N e E e n e OR n n E Alignment of Homologs at Metaphase I Replication E e N E N N n Telophase II n

Segregation and Independent Assortment

Hypothetical example of independent Assortment Eye color Gene for brown eyes Hair color Gene for blue eyes Gene for black hair Gene for red hair

Independent Assortment OR Meiosis I & II Brown eyes Black hair Blue eyes Red hair Brown eyes Red hair Blue eyes Black hair

Three Conclusions of Mendel Experiment 1. Principle of Recessiveness Dominance and One allele in a pair may mask the effect of the other 2. Principle of Segregation The two alleles for a characteristic separate during the formation of eggs and sperm 3. Principle of Independent Assortment The alleles for different characteristics are distributed to reproductive cells independently.

Variations on Mendel’s Laws The relationship of genotype to phenotype is rarely simple Mendel’s principles are valid for all sexually reproducing species ¡But genotype often does not dictate phenotype in the simple way his laws describe ¡ There is an exceptional to Mendel Laws

Exceptions To Mendel’s Original Principles ¡ Incomplete ¡ dominance ¡ Codominance ¡ Multiple alleles ¡ Polygenic traits ¡ Epistasis ¡ ¡ ¡ Pleiotropy Environmental effects on gene expression Linkage Sex linkage

Incomplete dominance ¡ ¡ ¡ The phenotype of the heterozygote is intermediate between those of the two homozygotes. Neither allele is dominant and heterozygous individuals have an intermediate phenotype For example, in Japanese “Four o’clock”, plants with one red allele and one white allele have pink flowers: P Generation Red CR CR White CWCW Gametes CR CW Pink CR CW F 1 Generation Gametes Eggs F 2 Generation 1⁄ 1⁄ 2 C 1⁄ 2 1⁄ CR 2 C R 1⁄ 2 R C 1⁄ 2 C R R 2 C w CR CR CR CW CW CW Sperm

Incomplete Dominance Gametes CR CW CRCR CRCW CWCW CRCR CR Gametes CW F 1 generation All CRCW CWCW F 2 generation 1: 2: 1

Co-dominance Phenotype of both homozygotes are produced in heterozygotes individuals. Both alleles are expressed equally. Examples: Roan Cattle White-feathered birds are both homozygotes for both B and W alleles

Multiple Alleles Ø More than three alleles for a gene Found among all individuals in a population Diploid individuals only have two of the alleles Ø Phenotype depends on relationship between different pairs of alleles Still follows Mendel’s principles

Multiple Alleles Small differences in DNA sequences result in multiple alleles

Human ABO Blood Group Ø Antigens Ø Glycoproteins on surface of red blood cells IA allele produces A antigen (dominant) IB allele produces B antigen (dominant) i allele produces neither A nor B (recessive) Blood types (phenotypes) IA IA or IAi = type A blood IBIB or IBi = type B blood ii = type O blood IAIB = type AB blood

Universal donors Universal recipients

Epistasis ü Type of polygenic inheritance where the alleles at one gene locus can hide or prevent the expression of alleles at a second gene locus. ü Allele of one locus inhibits or masks effects of allele at a different locus ü Some expected phenotypes do not appear among offspring ü Labrador retrievers one gene locus affects coat color by controlling how densely the pigment eumelanin is deposited in the fur. ü A dominant allele (B) produces a black coat while the recessive allele (b) produces a brown coat ü However, a second gene locus controls whether any eumelanin at all is deposited in the fur. Dogs that are homozygous recessive at

Epistasis

Labrador Retrievers Ø Melanin pigment gene Ø Pigment deposition gene Ø B allele: black fur color (dominant) b allele: brown fur color (recessive) E allele: pigment deposition normal (dominant) e allele: pigment deposition blocked (recessive) Phenotypes Black fur: BB EE, BB Ee, Bb EE, Bb Ee Brown fur: bb EE, bb Ee Yellow fur: BB ee, Bb ee, bb ee

Labrador Retrievers

Polygenic Inheritance ü Most traits are not controlled by a single gene locus, but by the combined interaction of many gene loci. These are called polygenic traits. ü Several genes at different loci interact to control the same character ü Produces continuous variation ü Phenotypic distribution: Bell-shaped curve ü Often modified by environmental effects

Continuous Variation in Human Height

Continuous Variation in Plant Height

Pleiotropy Ø One gene affects more than one character Ø For example, in Labrador retrievers the gene locus that controls how dark the pigment in the hair will be also affects the color of the nose, lips, and eye rims.

Environmental Effects on Gene Expression ¡ The phenotype of an organism depends not only on which genes it has (genotype), but also on the environment under which it develops.

Environmental Effects ¡. hydrangea color – affected by soil (p. H, water, temperature)

Extranuclear inheritance Some genes are passed from parent to offspring without being part of nuclear chromatin Mitochondria (and chloroplasts in plants) are randomly assorted into gametes and daughter cells In animals, mitochondrial traits are maternally inherited Example: Leaf color in four o'clock plants Human mitochondrial disorders

Linked Genes that tend to be inherited together on the same chromosome due to their close proximity) ) Examples: Color blindness Hemophilia

Sex-linked traits A gene located on either sex chromosome (X in humans) Examples: Color blindness Hemophilia

Sex-limited traits 6. Autosomal gene is present in both sexes but expression depends on sex of individual (it’s dominant in one sex but recessive in the other) Example: Baldness in males: Man with one copy of gene will be bald Female needs two copies of gene to be bald Milk production in females Man with one copy does not lactate Female with one copy lactates

Probability ü The likelihood that a specific event will occur. ü The principles of probability can be used to predict the outcomes of genetic crosses.

Using probability in Mendelian genetics ¡ Segregation and random assortment are random events, and can thus be characterized by probability ¡ The two rules of probability state that: a. The probability of an outcome ranges from 0 to 1 b. The probabilities of all possible outcomes for an event sum to 1 ¡ The outcome of a random event is unaffected by

Laws of Probability Govern Mendelian Inheritance Mendel’s laws of segregation and independent assortment reflect the rules of probability The multiplication rule ¡ States that the probability that two or more independent events will occur together is the product of their individual probabilities The rule of addition ¡ States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities

Laws of Probability - Multiplication Rule The probability of two or more independent events occurring together is the product of the probabilities that each event will occur by itself ¡ Following the self-hybridization of a heterozygous purple pea plants (Pp), the probability of a homozygous offspring such as the production of white flowers (pp): a. Probability that a pollen seed will carry p: ½ b. Probability that an egg will carry p: ½ ¡ c. Probability that the offspring will be pp: 1/2 X 1/2 = 1/4

Laws of Probability - Addition Rule ¡ ¡ The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities Following the self-hybridization of a heterozygous purple pea plant (Pp), the probability of purple offspring: a. Probability of maternal P uniting with paternal P: 1/4 b. Probability of maternal p uniting with paternal P: 1/4 c. Probability of maternal P uniting with paternal p: 1/4 d. Probability that the offspring will be purple: 1/4 + 1/4 = 3/4

Probability in Mendel’s Crosses Purple-flowered × white-flowered (PP × pp) Probability of PP zygote = ½ × ½ = ¼ Probability of pp zygote = ½ × ½ = ¼

Probability in Mendel’s Crosses Purple-flowered × white-flowered (PP × pp) Probability of Pp zygote = ½ × ½ = ¼ Probability of p. P zygote = ½ × ½ = ¼ Total probability of heterozygote = ¼ + ¼ = ½

Probability in Mendel’s Crosses Heterozygous cross (Pp × Pp) Genotype probabilities ¡ PP zygote = ½ × ½ = ¼ ¡ pp zygote = ½ × ½ = ¼ ¡ Pp zygote = ¼ + ¼ = ½ Phenotype probabilities ¡ Purple flowers = PP + Pp = ¼ + ½ = ¾ ¡ White flowers = pp = ¼

Monohybrid Cross Rr Rr Segregation of alleles into eggs Segregation of alleles into sperm Sperm 1⁄ R 2 R 1⁄ R 2 1⁄ Eggs 1⁄ 2 R r r 1⁄ R 1⁄ 4 r 1⁄ 4 R r 2 r 4 r 1⁄ 4

Dihybrid Crosses

Statistical Testing Used by biologists to find out if observed results differ significantly from expected results. ¡ Biologists want more than 95% confidence which means the probability that the deviation of the observed from that expected is due to chance alone (no other forces acting). ¡ In a genetic experiment, it can be used to decide if observed data fits any of the expected Mendelian ratios or if data is too “far off” and should be rejected. ¡

Observed Values Expected Values 315 Round, Yellow Seed 108 Round, Green Seed 101 Wrinkled, Yellow Seed 32 Wrinkled, Green 5556 Total Seeds (9/16)(556) = 312. 75 Round, Yellow Seed (3/16)(556) = 104. 25 Round, Green Seed (3/16)(556) = 104. 25 Wrinkled, Yellow Seed (1/16)(556) = 34. 75 Wrinkled, Green Seed 556. 00 Total Seeds

• • • Xcalc 2 = 0. 47 (this is the answer, do not √ it) Find the correct critical value on the following table. Find the degrees of freedom (n-1) in your data. Xtab 2 = 7. 82 (Xcalc 2 <<<< Xtab 2 ) If calculated chi-square is lower than the critical value, this shows there is no significant difference between the expected and observed values and the results are within the range of acceptable deviation. • If it is above, the difference is too great and the results are outside the range of acceptable deviation and should be rejected!

Degrees of Freedom (df) 1 2 3 4 5 6 7 8 9 10 Probability (p) 0. 95 0. 90 0. 80 0. 70 0. 50 0. 30 0. 20 0. 10 0. 05 0. 01 0. 004 0. 02 0. 06 0. 15 0. 46 1. 07 1. 64 2. 71 3. 84 6. 64 10. 83 0. 10 0. 21 0. 45 0. 71 1. 39 2. 41 3. 22 4. 60 5. 99 9. 21 13. 82 0. 35 0. 58 1. 01 1. 42 2. 37 3. 66 4. 64 6. 25 7. 82 11. 34 16. 27 0. 71 1. 06 1. 65 2. 20 3. 36 4. 88 5. 99 7. 78 9. 49 13. 28 18. 47 1. 14 1. 61 2. 34 3. 00 4. 35 6. 06 7. 29 9. 24 11. 07 15. 09 20. 52 1. 63 2. 20 3. 07 3. 83 5. 35 7. 23 8. 56 10. 64 12. 59 16. 81 22. 46 2. 17 2. 83 3. 82 4. 67 6. 35 8. 38 9. 80 12. 02 14. 07 18. 48 24. 32 2. 73 3. 49 4. 59 5. 53 7. 34 9. 52 11. 03 13. 36 15. 51 20. 09 26. 12 3. 32 4. 17 5. 38 6. 39 8. 34 10. 66 12. 24 14. 68 16. 92 21. 67 27. 88 3. 94 4. 86 6. 18 7. 27 9. 34 11. 78 13. 44 15. 99 18. 31 23. 21 29. 59 Nonsignificant. The differences are due to acceptable chance. Do NOT reject without Significant. Reject! Differences are NOT due to chance