Genotype vs. phenotype Difference between how an organism “looks” & its genetics phenotype description of an organism’s trait the “physical” genotype description of an organism’s genetic makeup Explain Mendel’s results using …dominant & recessive …phenotype & genotype F1F1 P X purplewhite all purple
Making crosses Can represent alleles as letters flower color alleles P or p true-breeding purple-flower peas PP true-breeding white-flower peas pp PP x pp PpPp F1F1 P X purplewhite all purple
Notation A capital letter represents the dominant allele. A lower case letter represents the recessive allele. Example: Pea plant height – Tall: T Short: t
Allele: A variety of a gene A gene for hair color could have the allele: _______ for Brown hair or _______ for Blonde hair A gene for flower color could have allele: ______for blue flower or ________ for red flower A gene for height could have the alleles: _______ for tall or _______ for short B b B b Tt
Solving Genetics Problems Homozygous: organism with two identical alleles for a trait Heterozygous: organism with two different alleles for a trait
Homozygous dominant: organism with two dominant alleles Homozygous recessive: organism with two recessive alleles Solving Genetics Problems
Dominant: An allele that causes its phenotype in a heterozygous genotype. Examples: TT, Tt (Tall) Recessive: An allele that causes a phenotype only seen in a homozygous genotype. Example: tt (short) Remember…
A B C D E F G H Plant A: a. Phenotype: b. Genotype:
A B C D E F G H Plant B: a. Phenotype: b. Genotype:
A B C D E F G H Plant C: a. Phenotype: b. Genotype:
F 2 generation 3:1 75% purple-flower peas 25% white-flower peas ???? Looking closer at Mendel’s work P X true-breeding purple-flower peas true-breeding white-flower peas PPpp 100% F 1 generation (hybrids) 100% purple-flower peas PpPpPpPpPpPpPpPp phenotype genotype self-pollinate
Solving Genetics Problems We need a method to predict the traits of the offspring, and we have it, its called the Punnett Square!
Solving Genetics Problems Genetics Problems Goal: to predict the traits of offspring 1. Identify trait(s) and assign a letter to each (capital letter for dominant, lower case letter for recessive)
Solving Genetics Problems Genetics Problems Goal: to predict the traits of offspring 1. Identify trait(s) and assign a letter to each (capital letter for dominant, lower case letter for recessive) 2. Determine parents’ genotypes
Solving Genetics Problems Genetics Problems Goal: to predict the traits of offspring 1. Identify trait(s) and assign a letter to each (capital letter for dominant, lower case letter for recessive) 2. Determine parents’ genotypes 3. Draw Punnett square and fill in
Solving Genetics Problems Genetics Problems Goal: to predict the traits of offspring 1. Identify trait(s) and assign a letter to each (capital letter for dominant, lower case letter for recessive) 2. Determine parents’ genotypes 3. Draw Punnett square and fill in 4. Determine the probabilities for offspring of each genotype and phenotype
Solving Genetics Problems Example: In pea plants, the gene for tall height is dominant to the gene for short height. A short pea plant is cross pollinated with a true breeding tall pea plant. 1. Assign letters: tall = Tshort = t 2. Parents’ genotypes True breeding tall: TT True breeding short = tt
Solving Genetics Problems 3. Draw Punnett Square t t TTTT
Solving Genetics Problems 3. Draw Punnett Square Tt t t TTTT
Solving Genetics Problems 4. Calculate probabilities TT = _____ Tt = ________ tt = _________ Tall = _________ Short = _________ Tt t t TTTT
Solving Genetics Problems 4. Calculate probabilities 10) TT = 0/4 Tt = 4/4 tt = 0/4 11) Tall = 4/4 Short = 0/4 Tt t t TTTT
Punnett squares Pp x Pp Pp male / sperm P p female / eggs PP 75% 25% 3:1 25% 50% 25% 1:2:1 % genotype % phenotype PPPpPp PpPppp PpPp PpPp F 1 generation (hybrids) Aaaaah, phenotype & genotype can have different ratios
Genotypes Homozygous = same alleles = PP, pp Heterozygous = different alleles = Pp homozygous dominant homozygous recessive heterozygous
Phenotype vs. genotype 2 organisms can have the same phenotype but have different genotypes homozygous dominant PPpurplePpPp heterozygous purple How do you determine the genotype of an individual with with a dominant phenotype? Can’t tell by lookin’ at ya!
Test cross Breed the dominant phenotype — the unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele pp is it PP or Pp? x
PPpp How does a Test cross work? pp P P pp P p PpPppp xx PpPp PpPpPpPp PpPp 100% purple PpPp pp PpPp 50% purple:50% white or 1:1 pp Am I this? Or am I this?
Mendel’s 1 st law of heredity Law of segregation during meiosis, alleles segregate homologous chromosomes separate each allele for a trait is packaged into a separate gamete PP P P pp p p PpPp P p
Segregation of alleles and fertilization as chance events
DIHYBRID (2 FACTOR) CROSSES + INCOMPLETE AND CO- DOMINANCE
Vocabulary 15. Co-dominance 16. Incomplete dominance 17. Probability
Monohybrid cross Some of Mendel’s experiments followed the inheritance of single characters flower color seed color monohybrid crosses
Dihybrid (2 factor) cross Other of Mendel’s experiments followed the inheritance of 2 different characters seed color and seed shape dihybrid (2 factor) crosses Mendel was working out many of the genetic rules!
Dihybrid cross true-breeding yellow, round peas true-breeding green, wrinkled peas x YYRRyyrr P 100% F 1 generation (hybrids) yellow, round peas Y = yellow R = round y = green r = wrinkled self-pollinate 9:3:3:1 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas F 2 generation YyRr
What’s going on here? If genes are on different chromosomes… how do they assort in the gametes? together or independently? YyRr YRyr YyRr YryRYRyr Is it this?Or this? Which system explains the data?
9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled Is this the way it works? YyRr YRyr YR yr x YyRr YryRYR yr YyRr YRyr or YYRRYyRr yyrr
Dihybrid cross YyRr YRYryR yr YR Yr yR yr YYRR x YYRrYyRRYyRr YYRrYYrrYyRrYyrr YyRRYyRryyRRyyRr YyRrYyrryyRryyrr 9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled YyRr YryRYR yr YyRr YRyr or
The laws of probability govern Mendelian inheritance Rule of Multiplication: probability that 2+ independent events will occur together in a specific combination multiply probabilities of each event Ex. 1: probability of throwing 2 sixes 1/6 x 1/6 = 1/36 Ex. 2: probability of having 5 boys in a row ½ x ½ x ½ x ½ x ½ = 1/32 Ex. 3: If cross AABbCc x AaBbCc, probability of offspring with AaBbcc is: Answer: ½ x ½ x ¼ = 1/16
The laws of probability govern Mendelian inheritance Rule of Addition: Probability that 2+ mutually exclusive events will occur add together individual probabilities Ex. 1: chances of throwing a die that will land on 4 or 5? 1/6 + 1/6 = 1/3
Mendel’s 2 nd law of heredity round wrinkled Law of independent assortment different loci (genes) separate into gametes independently non-homologous chromosomes align independently classes of gametes produced in equal amounts YR = Yr = yR = yr only true for genes on separate chromosomes or on same chromosome but so far apart that crossing over happens frequently yellow green :11:1:1 Yr yR YR yr YyRr
Review: Mendel’s laws of heredity Law of segregation monohybrid cross single trait each allele segregates into separate gametes established by Metaphase 1 Law of independent assortment dihybrid (or more) cross 2 or more traits genes on separate chromosomes assort into gametes independently EXCEPTION linked genes
Beyond Mendel’s Laws of Inheritance
Extending Mendelian genetics Mendel worked with a simple system peas are genetically simple most traits are controlled by a single gene each gene has only 2 alleles, 1 of which is completely dominant to the other The relationship between genotype & phenotype is rarely that simple
Incomplete dominance Heterozygote shows an intermediate, blended phenotype example: RR = red flowers rr = white flowers Rr = pink flowers make 50% less color RR RR WW RW WWRW
Incomplete dominance true-breeding red flowers true-breeding white flowers X P 100% 100% pink flowers F 1 generation (hybrids) self-pollinate 25% white F 2 generation 25% red 1:2:1 50% pink It’s like flipping 2 pennies!
Solve the following crosses: Incomplete Dominance In flowers there is a gene for Purple (P) which has incomplete dominance to the recessive color white (p). The heterozygote plant produced by a white and purple flower is violet (light purple). a) What are the percentages of genotypes and phenotypes of the offspring of two violet colored plants (draw a punnett square)
Co-dominance 2 alleles affect the phenotype equally & separately Not blended phenotype Both alleles contribute to the phenotype Example is human ABO blood groups
Solve the following crosses: Co-Dominance In Cows the trait for coat (fur) color is expressed by R for Red fur and W for white fur. The hybrid of the two is called a Roan color (RW). a) Describe what you think the Roan cow’s fur will look like: b) What are the genotype and phenotype percentages of a cross between a Red and a White cow?
Solve the following crosses: Co-Dominance In mushrooms there is a gene for Purple Spots (P) and a gene for Green spots (G). Cross a Homozygous purple with a Homozygous Green. a) What do you think their offspring will look like? Genotype? Phenotype? b) What will the genotype and phenotype percentages be in the F2 have when you cross two from the F1 generation.
Pleiotropy Most genes are pleiotropic one gene affects more than one phenotypic character 1 gene affects more than 1 trait dwarfism (achondroplasia) gigantism (acromegaly)
Acromegaly: André the Giant
Epistasis B_C_ bbC_ _ _cc One gene completely masks another gene coat color in mice = 2 separate genes C,c: pigment (C) or no pigment (c) B,b: more pigment (black=B) or less (brown=b) cc = albino, no matter B allele 9:3:3:1 becomes 9:3:4
Polygenic inheritance Some phenotypes determined by additive effects of 2 or more genes on a single character phenotypes on a continuum human traits skin color height weight intelligence behaviors