Mendel & Genetics Chapter 11

Gregor Mendel Austrian Monk Researched inheritance using pea plants Teacher

Why pea plants? Variation Characters – heritable feature Ex: flower color Trait – variable Ex: purple or white Controlled plant “mating” by cutting stamens

Mendel's Experiments Established true breeding lines – offspring the same as the parent P generation - parent Hybridized true breeding lines P X P = F1 generation – first filial Hybridized F1 generation F1 x F1 = F2 generation – second filial generation P Generation (true-breeding parents) Purple flowers White  F1 Generation (hybrids) All plants had purple flowers F2 Generation

Alleles Allele – alternate version of a gene Accounts for variations in inherited characters Organisms inherit 2 alleles for each character One from each parent Figure 14.4 Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers

Dominant vs. Recessive Dominant allele – determines the organisms appearance if present Represented as a capital P = purple Recessive allele – masked if the dominant allele is present Appears if both alleles are recessive Represented as lower case p = white

1st law = law of segregation Two alleles for a heritable characteristic separate (segregate) during gamete formation and end up in different gametes Separate during anaphase I Egg or sperm only gets one of the alleles (haploid)

Punnett Square Predicts the possibilities or possible combinations of offspring from known parental genes.

Homozygous vs. Heterozygous Homozygous – same Homozygous dominant PP – genotype Purple - phenotype Homozygous recessive pp – genotype White - phenotype Heterozygous – different Pp – genotype

Testcross Possible genotypes for a purple flower? Pp or PP Impossible to know just by looking at the flower Cross with homozygous recessive pp Analyze results 100% purple – PP 50% purple 50% white - Pp

2nd law – law of independent assortment Each pair of alleles segregates independently of other allele pairs during gamete formation

Monohybrid vs. Dihybrid Monohybrid – one pair of contrasting characteristics Flower color – purple or white PP, Pp, or pp Dihybrid – two pairs of contrasting traits Demonstrates law of independent assortment Flower color and height – purple or white and tall or dwarf PPTT, PpTT, PPTt, PpTt, PPtt, Pptt, ppTT, ppTt

Sample Dihybrid PPTt x PpTt Possible Gametes PPTt – PT or Pt PpTt – PT, Pt, pT, pt PT Pt pT pt The size of the punnet square is determined by the number of traits being crossed. 3 traits: 2x = 23 or 8 gametes possible an 8x8 square 4 traits: 2x = 24 or 16 gametes possible a 16x16 square

Heterozygous dihybrid cross GgWw x GgWw G = green, g = yellow W = wrinkled, w = round Gametes for both the same GW, Gw, gW, gw Always ends up in a 9:3:3:1 phenotypic ratio 9 – dominant – dominant Green and wrinkled 3 – dominant – recessive Green and round 3 – recessive – dominant Yellow and wrinkled 1 – recessive – recessive Yellow and round

Probability # of times an event occurs # of opportunities it can occur  Rr Segregation of alleles into eggs alleles into sperm R r 1⁄2 1⁄4 Sperm Eggs Figure 14.9 Multiply individual allele probabilities to calculate overall probability Add probability of 2 or more mutually exclusive events (the heterozygotes can be produced in 2 different ways) PpTt x PpTt ¼ PP, ½ Pp, ¼ pp ¼ TT, ½ Tt, ¼ tt Probability of PPTt ? ¼ PP x ½ Tt = 1/8 chance of PPTt Probability of PpTt? ½ Pp x ½ Tt = ¼ chance of PpTt

Practice probability PpTtGg x PPTtgg What are the chances of producing recessive genotypes for 2 characteristics? PP = ½, Pp = ½, pp = 0 TT = ¼, Tt = ½, tt = ¼ GG = 0, Gg = ½, gg = ½ PPttgg = ½ x ¼ x ½ = 1/16 Ppttgg = ½ x ¼ x ½ = 1/16 ppTTgg = 0 x ¼ x ½ = 0 ppTtgg = 0 x ½ x ½ = 0 ppttGg = 0 x ¼ x ½ = 0 ppttGG = 0 x ¼ x 0 = 0 Total probability = 2/16 (remember the additional rule!) http://saifulislam.com/wp-content/uploads/2008/04/probability1.jpg

Incomplete Dominance Incomplete – traits are expressed together Ex: red (CRCR) + white (CWCW) snapdragon flowers = pink (CRCW) The heterozygote produces less red pigment so the flower appears pink. NOT BLENDING! F2 Phenotypic and genotypic ratios are the same.

Codominance Tay-Sachs Disease: codominant at the molecular level, and equal # of functional and dysfunctional enzymes are produced in the heterozygote, there is enough functional enzyme to break down the lipids in the brain, and the person is phenotypically normal. Codominance – both phenotypes show up – no recessive trait Blood Groups: M and N or A and B are Codominant, O is recessive over both MM: MN: NN A: Iai or IAIA B: IBi or IBIB O: ii AB: IAIB

Multiple Alleles More than two allele forms Ex: blood type A, B, O AB – codominant O - recessive Table 14.2

Pleiotropy One gene has more than one phenotypic effect Ex: Sickle cell anemia & cystic fibrosis

Epistasis One gene alters another gene at a different location BC bC Bc bc 1⁄4 BBCc BbCc BBcc Bbcc bbcc bbCc BbCC bbCC BBCC 9⁄16 3⁄16 4⁄16  Sperm Eggs One gene alters another gene at a different location B = black b = brown C = color c = albino

Polygenic Inheritance  AaBbCc aabbcc Aabbcc AaBbcc AABbCc AABBCc AABBCC 20⁄64 15⁄64 6⁄64 1⁄64 Fraction of progeny Quantitative characteristics Varied degrees of the characterstic Multiple genes affect one phenotype Ex: Skin Color, hair color, eye color

Multifactoral Impact Environment plays a role in gene expression – phenotype Ex: flower color Shades may vary based on acidity of the soil Ex: Height Actual height may vary based on childhood nutrition and sleep patterns. Figure 14.13

Pedigrees Track traits through generations Appearance of trait is shaded Absence of trait is unshaded

Recessively inherited disorders Must have both recessive alleles to have the disease Heterozygotes are carriers Increased probability of passing on a recessive disease when close relative mate Albinism Cystic Fibrosis Sickle Cell

Dominantly Inherited Dissorders Figure 14.15 Only need one dominant gene to have the disorder Achondroplasia Dwarfism – heterozygous Deadly when homozygous dominant Cartilage doesn’t form into bone during development0 Mild response to HGH Huntington’s Disease

Multifactoral Diseases Genetic component plus environmental factors Heart Disease Cancer Diabetes Alcoholism Mental Disorders

Genetic Testing and Counseling Fetal Testing Amniocentisis Chronic Villus Sampling – CVS