Download Chapter 14: Mendelian Genetics Early Inheritance Ideas

Chapter 14: Mendelian Genetics
Early Inheritance Ideas
 Preformationism – homunculus (little, fully formed human) inside either sperm or egg, fertilization
stimulates growth to full size
o Spermists vs Ovists
 Pangenesis – gemmules made by cells, accumulate info for traits through lifetime, passed on to offspring
via sex cells
 Blend Inheritance – kids blended genetic traits of parents, like paint colors mixing
Gregor Mendel
 Austrian (now Czech) Monk
 Father of Modern Genetics
 Used pea plants to systematically test blending hypothesis
o Chose 7 traits, each with two distinct varieties
o Removed immature stamens (no selfing), hand pollinated, covered w/ bags
 Monohybrid Cross
o A hybrid is a cross b/w two varieties of organisms
o Mendel started w/ true-breeding organisms to create ‘hybrid’ for one trait
 P (parent) generation crossed
 F1 (first filial) is offspring of P gen.
 F2 (second) filial is offspring of cross b/w F1 individuals
 F1 always 100% purple
 Purple males & females used to eliminate sex as a factor
 F2 a 3:1 ratio of purple to white
o Mendel’s Conclusions
 Unit Characters (Particulate Hypothesis)
 Traits are inherited in discrete units (genes) that come in pairs (one from each
parent)
 F1 must have a unit for both purple & white flowers since white flowers re-appear
in F2
 Law of Dominance
 When two contrasting units are present in an individual, one will always dominate
the other
o Unit that dominates is the Dominant trait (capital letter, P for purple)
o Unit that is dominated is Recessive trait (lower case, p for white)
 Law of Segregation
 During gamete formation, pairs of units are segregated from each other in such a
way that each gamete contains only one unit
o Meiosis separates homologous chromosomes
 Genetics Terminology
o Gene – unit of inheritance
o Allele – contrasting forms of a gene
 P & p for flower color (purple & white)
o Homozygote – individual w/ same alleles (PP/pp)
 True or pure breeding since always produce same type of offspring when self-pollinated
o Heterozygote - individual w/ different alleles (Pp)
o Genotype – actual combination of alleles (PP, Pp, pp)
o Phenotype – physical manifestation of allelic combination (purple or white flowers)
 Predictable Results
o For Monohybrid cross of homozygotes
o F1 generation 100% heterozygous
o F2 generation
 Phenotype – 3:1 dominant
 Genotype – 1:2:1
 Due to Law of Segregation
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Test Cross
o Used to figure out genotype for a dominant phenotype
o Cross individual with a recessive
o Purple (PP/Pp) w/ white (pp)
 PP = 100% purple
 Pp = 50:50 purple/white
 Dihybrid Cross
o Cross b/w individuals that are heterozygous for two traits
o Mendel wanted to see if traits where inherited independently of others
 Seed Color (Y=yellow, y=green)
 Seed Shape (R=round, r=smooth)
 P generation = YYRR x YYRR
 F1 = YyRr x YyRr
o Law Of Independent Assortment
 When alleles of two different genes segregate, they do so independently
 Genotype: YyRr
 Possible Gametes: YR, Yr, yR, yr
 Use ‘FOIL’ if needed
 Predictable 9:3:3:1 ratio in F2
 Genes close to each other tend to be inherited in pairs
Law of Probabilities
 Probability of two or more independent events occurring together is product of their independent
probabilities
o What is probability of tossing a coin three times & getting three heads?
o ½ x ½ x ½ = 1/8
 Dihybrid Cross & Higher
o Each trait can be treated separate
 Do a monohybrid cross for each
 Multiply probabilities for each desired combination
o Calculate phenotypic probabilities of dihybrid cross (YyRr: yellow/round)
o Parents: AaBBCc x AABbCc
 Probability of AABbCC?
Beyond Mendel
 Sex-Linked Traits
o Traits that are found on the X chromosome
o Usually disorders are recessive
 Red-green color blindness
 Hemophilia
o XX / XX+ & XY = Normal
o X+X+ & X+Y = Affected
o If dominant (very rare), one copy will affect female
 Incomplete Dominance
o Heterozygote is a ‘blend’ of the dominant & recessive alleles
o Snapdragons & Carnations
 Red (dominant), pink (heterozygous), white (recessive)
o Human hair
 Curly/wavy/straight
 Codominance
o Both alleles are expressed equally
o AB blood type (A & B alleles)
o Sickle Cell Anemia
o Tay-Sachs Disease
o Many flowers (2 different colors)
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Multiple Allelism
o Trait with 3+ alleles
o ABO Blood Group Alleles
 IA (A antigen), IB (B antigen), i (no antigens)
 IA & IB are codominant, but completely dominant over i
 IA IA / IAi = type A
 IB IB / IBi = type B
 IAIB = type AB & ii = type O
Pleiotropy
o Most genes will affect multiple phenotypic traits (one gene = multiple traits)
o Plants – flower color gene can affects seed coat color
o PKU
o Cystic Fibrosis
o Sickle-Cell
Epistasis
o Two or more genes interact in a way that can hide or mask traits
o Labrador Retrievers
 Black, brown, & yellow
 Gene (E) determines if pigment for fur color (B) will be deposited
o Sweet Peas
 Purple vs. White flowers
 Need dominant 2nd gene for purple
Polygenic Inheritance
o Quantitative characters are traits that fall on a continuum
o Caused by multiple genes interacting on one trait (phenotype)
 Human skin – 3+, more dominants = darker skin (plus environment)
 Human eye color – 2 main, ~16 total
 Human Hair – unknown, shades caused by varying levels of two chemicals
Nature vs. Nurture
o Plants
 Sunlight, soil nutrients, wind can affect color, height, weight
o Humans
 Diet, sun, altitude chemicals can affect all human systems
 Skin color, blood cell counts, hormone levels
o Expressivity & Penetrance
Disorders & Pedigrees
o Human disorders can be tracked by looking at pedigrees
o Family tree where squares are males, circles are females
o Shaded shape = affected
o ½ shade (when available) = carrier
o Use process of elimination for 4 options
o Autosomal dominant or recessive
o Sex-linked dominant or recessive