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OBJECTIVES• Understand Mendel’s principles governing genetics
• Understand meaning of relevant vocabulary
• Predict results of mono/dihybrid crosses using Punnett square
• Know exceptions to rules
MENDEL• Gregor Mendel
– Interested in basic patterns governing the transmission of traits from parent to offspring
– Mendel’s work precedes an understanding of meiosis/genes/chromosomes
HISTORICAL HYPOTHESES REGARDING INHERITANCE
Blending Inheritance Hypothesis:
Inheritance of Acquired Characteristics Hypothesis
MENDEL• Mendel performed crosses between pure bred
garden peas• Began studying inheritance of ONE trait
• Later, multiple traits studied together
MENDEL’S CROSSES• Mendel's plants had two observable “forms”
(alleles) of each “character” (gene)
– Green or Yellow seeds • Round or Wrinkled seeds
Trait Forms of Trait (Phenotype)
Seed shapeRound Wrinkled
Seed color
Pod shape
Pod color
Yellow Green
Inflated
Green
Constructed
Yellow
Round Wrinkled
Yellow Green
Smooth Constricted
Green Yellow
or
or
or
or
Trait Forms of Trait (Phenotype)
Flower color
Purple White
Axial (on stem) Terminal (at tip)
Flower andpod position
Stem length
Tall Dwarf
Purple White
Axial (on stem) Terminal (at tip)
Dwarf Tall
or
or
or
MENDEL
• Looked at traits exhibited by progeny of crosses
– Found observable evidence of how parents transmit traits to offspring
Stigma (receives pollen)
Anthers(produce pollengrains, whichcontain malegametes)
Ovules (produce female gametes)
Male parts = anthers–Pollen grain contain sperm
Female parts = pistil–Ovule = eggs
SELF FERTILIZATION
MENDEL’S PEA PLANTSControlled Mating:
– Cut off reproductive organ to stop self fertilization
– Cross fertilize plants in a controlled fashion– Plant that contributes pollen
– Plant that receives the pollen
CROSS-POLLINATIONCROSS-POLLINATION
1. Remove anthersfrom one plant.
2. Collect pollenfrom a different plant.
3. Transfer pollento stigma of the 1st plant(plant without anthers)
MENDEL’S CROSSESMendel Determines (Phenotypic) Ratio:
F2: 5474 Round : 1850 Wrinkled
– Similar pattern observed in other 6 traits
MENDEL’S SYMBOLS• Mendel assigned symbols to hereditary
characters
• Letters represent versions of a gene (alleles):– Ex: If Letter “r” represents a gene then:
PUNNETT SQUARE• R.C. Punnett
– Leading geneticist in early 1900s
– Invented technique called the “Punnett Square”– Place gamete possibilities for 2 parents along axes
– Internal boxes represent union of gametes
– Used to predict genotype and phenotype of potential zygote
MENDEL’S CROSS OF PURE LINE PEA PLANTS
PARENT: R RP
AR
EN
T:
r r
** hint: gametes along top, “whole people” on inside
R is roundr is wrinkled
MENDEL’S CROSS OF F1 PLANTS
• Phenotypic Ratio: • Genotypic Ratio
Crossing the F1s: Rr x Rr
R = roundr = wrinkled
POSSIBLE GENOTYPESThree possible genotypes for each gene in a diploid cell:• Homozygous:
• Homozygous Dominant:
• Homozygous Recessive:
• Heterozygous:
PROBABILITY THEORYIn Diploid Organisms,
• Each allele has 50% chance of being found in a particular gamete
• To calculate likelihood of two alleles combining (i.e. fertilization):
MENDEL’S CROSSES• Dihybrid Crosses:
– Mendel used pea plants to follow inheritance of TWO traits
• Another pattern began to emerge…
MENDEL’S SYMBOLS
• Seed Shape:• R is dominant allele (Round)
• r is recessive allele (Wrinkled)
• Seed Color:• Y is dominant allele (Yellow)
• y is recessive allele (Green)
F2 generationphenotype
Mendel’sNumbers
315 101 108 32 = 556
Fraction ofprogeny
9/16 3/16 3/16 1/16 = 1
F2 OFFSPRING FROM DIHYBRID CROSS
Note: New mixtures of traits are observed in F2!!
INDEPENDENT ASSORTMENT
OR
During meiosis I, homologs can line up two different ways before separating.
Brown eyes
Black hairRed hair
Blue eyes
MENDELWhy Mendel Was Successful:
• Unbiased• Selected good model organism• Used pure breeds as parents• Large sample size• Quantitative analysis• Controlled experiments• Studied obvious traits