Mendel and the Gene Idea Blending hypothesis offspring

Mendel and the Gene Idea “Blending” hypothesis: offspring inherit mixture of traits; Implies that population would be uniform over many generations—not the case! “Particulate” hypothesis: discrete units of heredity that retain identity in offspring. Gregor Mendel—documented “particulate” hypothesis in abbey garden—esp. Pea plant; work done long before chromosomes were known; predated Darwin's, but not known at the time

Mendel and the Gene Idea Pea plants—at least 7 heritable traits studied character—feature that varies (e. g. flower color) trait—variant of character (e. g. purple vs. white flower) Pea flowers have both male and female parts. In nature, they self-fertilize, but Mendel could control crossing by removing stamens before maturity and dusting its carpel with pollen of another flower--crosspollination.

Mendel and the Gene Idea Mendel chose characteristics that showed two distinct alternate forms. Started with plants from true-breeding lines—no variation after many generations of self-fertilization Typical experiment has: True-breeding parents—parental (P) generation Hybrid offspring— F generation (first filial) 1 Offspring of hybrids— F generation (second filial) 2

Mendel and the Gene Idea Blending model would predict F 1 to be all pale purple —blending of purple and white But F 1 appeared identical to purple parents. White trait did not disappear—apparently masked in F 1 Appears in F 2— 1 out of 4 Mendel's data: F 2 had 705 purple and 224 white 0. 24111948 (ratio 3. 14732143: 1 purple: white) Mendel proposed that purple flower trait was dominant over white flower trait (recessive) Held true for six other characters.

Mendel and the Gene Idea Mendel's Law of Segregation: 1. Alternate versions of gene account for variation-alleles 2. Organism inherits two copies of gene—one from each parent 3. If alleles are different, dominant allele trait is shown, masking recessive one 4. Two alleles segregate into different gametes. Punnett square helps to predict results

Yellow (Y) dominant over green (y) True breeding genotypes? Yellow ____ Green ____

Yellow (Y) dominant over green (y) True breeding genotypes? Yellow YY Green yy Appearance of F 1?

Yellow (Y) dominant over green (y) True breeding genotypes? Yellow YY Green yy Appearance of F 1? Genotype of F 1? _______

Yellow (Y) dominant over green (y) True breeding genotypes? Yellow YY Green yy Appearance of F 1? Genotype of F 1? Yy

Possible gametes from F 1? ______ Yellow (Y) dominant over green (y) True breeding genotypes? Yellow YY Green yy Appearance of F 1? Genotype of F 1? Yy

Possible gametes from F 1? Y y Yellow (Y) dominant over green (y) True breeding genotypes? Yellow YY Green yy (1/2) sperm Appearance of F 1? ___ Yy eggs Genotype of F 1? (1/2) ___

Possible gametes from F 1? Y y (1/2) Yellow (Y) dominant over green (y) True breeding genotypes? Yellow YY Green yy sperm Appearance of F 1? Y Y Yy eggs Genotype of F 1? (1/2) y y

Possible gametes from F 1? Y (1/2) y (1/2) Yellow (Y) dominant over green (y) True breeding genotypes? Yellow YY Green yy sperm Appearance of F 1? Y y YY Yy Yy yy Y Yy eggs Genotype of F 1? y

Mendel and the Gene Idea Mendel's Law of Independent Assortment: 1. If we have two (or more) characters with distinct dominant and recessive alleles, these will be inherited independently of each other. *** 2. Have already seen how this happens during meiosis. 3. Can demonstrate with di-hybrid cross, where we cross individual homozygous for both dominant traits with individual homozygous for both recessive traits. ***Does not apply if genes are on same chromosome!

Mendel and the Gene Idea Laws of probability apply—multiplication rule: Probability that one of two possible alleles will become part of gamete is 0. 5. Probability that one of two alleles for another character becomes part of a gamete is also 0. 5. These two events are independent (if not on same chromosome). Multiply probabilities of each to get probability of both: 0. 5 x 0. 5 = 0. 25 (each square of a monohybrid cross) Probability of green seed color 0. 25 Probability of wrinkled seed is 0. 25 Probability of green, wrinkled seed is 0. 25 x 0. 25 = 0. 0625 (1/16)

Mendel and the Gene Idea Laws of probability apply—addition rule: If events are mutually exclusive—add the probabilities to probability of any one occurring. In monohybrid cross, three different ways to get yellow (dominant) trait: Y allele from both parents (homozygous) (prob. 0. 25) Y allele from egg, y allele from sperm (prob. 0. 25) y allele from egg, Y allele from sperm (prob. 0. 25) Add the probabilities for each of the three ways to get probability of any of the three occurring (0. 25 + 0. 25 = 0. 75)

Mendel and the Gene Idea Situations not explained by Mendelian laws: Incomplete dominance—e. g. Snapdragon flower color Codominance—both traits exhibited in heterzygote (e. g. human M, N bloodtypes) Multiple alleles--(e. g. Human AB bloodtypes: IA, IB, I) Pleiotropy—alleles have more than one phenotypic effect (e. g. pea flower color, cystic fibrosis, sickle-cell disease) Epistasis—expression of gene at one locus affects expression of gene at another (e. g. --black, chocoloate, golden Labrador retrievers) Polygenic—more than one gene controls characteristic. (hypothetical skin color example)

Mendel and the Gene Idea Human genetics Can't experimentally cross humans (ethically), but can learn from pedigree analysis → trace particular trait through family history Widow's peak

Most people with recessive disorders born to parents who are normal, but heterozygous--carriers.

Mendel and the Gene Idea Dominant disorders: Achondrioplasia—kind of dwarfism Lethal dominant disorders rare: Huntington's disease—no effect until 35 -45 yrs. (Woodie Guthrie)

Mendel and the Gene Idea Multifactorial disorders: Many disorders have inherited susceptibility, but expression of disease has additional factors: lifestyle, exposure to carcinogens, other genes Examples: heart disease, diabetes, cancer, alcoholism, some mental illnesses (e. g. schizophrenia, bipolar disorder)

Mendel and the Gene Idea Genetic testing and counseling: Can test fetus for inherited diseases—e. g. Tay-Sachs, cystic fibrosis Can test parents for likelihood of conceiving child with disorder. Example- Both man and woman have brothers with cystic fibrosis, but neither has it him(her)self Possible genotypes for each: FF or Ff or f. F Probability of each being a carrier is 2/3. If both are carriers, probability of child with disease is 1/4. Probability of child with disease is 2/3 x ¼ = 1/9 (Once we know both are carriers, prob. Is ¼)

Mendel and the Gene Idea Testing for fetal disorders: Amniocentesis (traditional)-th - 16 th week 14 Can get biochemical results w/i hours Must culture cells for weeks longer to karyotype and to get genetic results Chorionic villus sampling (CVS) th – 10 th week 8 Can get karyotype, genetic and biochemical tests w/i hours Chance of affecting pregnancy higher? Isolated fetal cells from mother's blood—new technique, probably wave of future