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Genetics. Mendel and the Gene Idea. Multifactorial Inheritance. Genotype does not rigidly define the phenotype, but a range of possibilities over which the environment can influence genes + environment & diet phenotype. Phenotype depends on environment and genes.
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Genetics Mendel and the Gene Idea
Multifactorial Inheritance • Genotype does not rigidly define the phenotype, but a range of possibilities over which the environment can influence • genes + environment & diet phenotype
Phenotype depends on environment and genes • Tree: leaves that vary in size, shape, and greenness, depending on exposure to wind and sun. • Humans: nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests. • Identical twins: genetic equals, accumulate phenotypic differences as a result of their unique experiences.
Environment contributes to the phenotype acidic soil basic soil
Early Ideas about Heredity • Before Mendel • basic facts of heredity • studied garden pea plant • formed hybrids • found recessive traits in some offspring
Gregor Mendel • father of genetics • mentors - physics, botany • quantified results • subject - garden pea plant • 1843 – entered monastery • 1851-53 – studied at U of Vienna • 1857 – began breeding pea plants
Advantages of peas • Mendel had strict control over which plants mated with which • Each pea plant has male (stamens) and female (carpal) sexual organs. • Mendel could allow self- pollination or also move pollen from one plant to another to cross-pollinate
Mendel’s Experimental Design • self-pollinate • crossed alternative traits to produce hybrids (hybrids expressed only dominant traits) • hybrids were self-pollinated (produced 3:1 ratio)
Reginald Crundall Punnett Punnett square
Symbols • P = parental generation • F1 = first filial generation • F2 = second filial generation
Terminology • homozygous • homozygous dominant = RR • homozygous recessive= rr • heterozygous = Rr • genotype and phenotype • complete dominance
Monohybrid Cross • parental varieties differ in a single character • character = flower color (using letter P) • allelles purple, dominant (PP, Pp) white, recessive (pp)
Flower Color (Pea plant) topveg.com/wp-content/uploads/2009/03/pea-flower-ambience-300x286.jpg australianmajestictours.com/hovea.jpg
Monohybrid CrossF1 • purple (PP) X white (pp) • dominant x recessive • F1 are all purple (Pp) • all Pp = hybrid
Monohybrid Cross Ratio in F2 • phenotypic ratio 3:1 • 3 dominant: 1 recessive • PP & Pp are indistinguishable in complete dominance • genotypic ratio 1:2:1 • 1 homozygous dominant (PP) • 2 heterozygous (Pp) • 1 homozygous recessive (pp)
Testcross (Pedigree Analysis) • to determine unknown genotype • utilizes a homozygous recessive • phenotypic ratio of offspring determines genotype F2 F1 P
Example B_ B_ • F2 • 75% are black (B_) • 25% are brown (bb) B_ bb Bb BBBBb bBbbb bb B_
F1 • all black (Bb) B B b Bb Bb b Bb Bb
Parent generation (P) Black Labrador (BB) Brown Labrador (bb)
Mendel’s First Law"Law of Segregation" • allele pairs separate during gametogenesis then randomly reform pairs at fertilization
Dihybrid Cross • parental varieties differ in two characters characters: • seed color (Y) • seed shape (R)
seed color alleles • yellow: dominant (YY, Yy) • green: recessive (yy) • seed shape alleles • round: dominant (RR, Rr) • wrinkled: recessive (rr)
P1 • P: yellow/round X green/wrinkled • F1:dihybrid (YyRr) • F2 : offspring (9:3:3:1) 9 Y_R_ 3 Y_rr 3 yyR_ 1 yyrr F1 F2
How do I know my Punnett is done correctly? all four are homozygous on both genes • look at the diagonals! all four are heterozygous on both genes (YyRr)
Mendel’s Second Law"Law of Independent Assortment" • each allele pair assorts independently from one another during gamete formation
Patterns of Inheritance not described by Mendel • still follow Mendel’s laws • Incomplete dominance • Codominance • Pleiotropy • Epistasis • Polygenic inheritance
Incomplete Dominance • Blends are formed • examples: • Snapdragons • Tay-Sachs disease
Snapdragon • Intermediate phenotype • Red = CRCR • Pink = CRCW • White = CWCW
Codominance • Both alleles are expressed • examples: • Landsteiner blood groups • (A x B = AB) • shorthorn cattle • (white x red = roan) • calico cat • (orange-yellow x black)
Blood groups: Codominance & multiple alleles http://health.stateuniversity.com/article_images/gem_01_img0116.jpg
Blood type Test coagulates when it has the antibody for that type of blood same happens with the Rh group
Blood Genotypes • IAIAhomozygous • IAi heterozygous • IBIB homozygous • IBi heterozygous • IAIB • ii • Rh group functions independently by complete dominance rules: • DD or Dd is Rh positive • dd is Rh negative Blood type A Blood type B Blood type AB Blood type O Examples : IAiDd A+ IBIBdd B- iiDD O+
Class Activity • A X B = O is it possible? • What are the genotypes of the parents? • AB X O = O is it possible? • Give the genotypes of the possible children
Shorthorn Cattle Codominance Red White Roan
Calico Cat Codominance • B = black • R = red • XBXR • black XB • red XR • Barr body Barr body is an inactive strand of DNA
Pleiotropy • single gene has multiple effects • example: in Siamese cats, one gene causes abnormal fur pigmentation and esophoria (crossed-eyed) • Waardenburg syndrome • white forelock • pale iris • deafness http://dermatology.cdlib.org/123/case_presentations/waardenburg/1.jpg
Epistasis • one gene alters the expression of another that is independently inherited • example: albinism in animals (mice) • 9 B_ C_ = black • 3 B_ cc = albino • 3 bb C_ = brown • 1 bb cc = albino • 9:3:4 instead of 9:3:3:1
Polygenic Inheritance • An additive effect of 2 or more genes on one character example • skin pigmentation • at least 3 genes • hair color • at least 4 genes
Rule of Addition • The probability of an event that can occur in 2 or more different ways is the sum of the separate probabilities of those ways Rule of multiplication • Multiply individual probabilities to get overall probability
Rules of Addition and Multiplication Female Male roll tongue (Rr) no roll tongue (rr) long eyelashes (Ll) short eyelashes (ll) cleft chin (Cc) cleft chin (Cc) Probability of having a • daughter 2/4 (xx, xx, xy, xy) • roll tongue 2/4 (Rr, Rr, rr, rr) • long eyelashes 2/4 (Ll, Ll, ll, ll) • cleft chin 3/4 (CC, Cc, Cc, cc) 2/4 x 2/4 x 2/4 x 3/4 = 24/256 = 9.375%
Activity Eye color dark/light P_/pp Tongue rolling yes/no R_/rr small finger crooked/straight B_/bb widow's peak yes/no W_/ww hand clasping right/left C_/cc earlobe free/attached A_/aa Hitchhiker's thumb no/yes H_/hh PTC tasting yes/no D_/dd The End