Transcript Slide 1

Heredity
The Experiments of Gregor Mendel
• Genetics is the scientific study of
heredity
• Study of patterns of inheritance
& variations in organisms
History of Genetics
• Gregor Mendel
• Austrian monk
• Notorious for his work with pea
plants
• He was a priest
• He studied science & math at
University of Vienna
• He spent 14 years working in the
monastery & teaching high school
• He was in charge of the
monastery garden
Gregor Mendel’s
Experiment
• Fertilization produces a new cell
• Develops into tiny embryo encased in seed
• Pea plants are self-pollinating
• Pollen contains the male gametes and will
fertilize the eggs of the same plant
• Seeds produced by self-pollination inherit
all characteristics from the single plant
• Only have 1 parent
Gregor Mendel’s
Experiment
• Mendel’s garden had many pea plants:
• Peas were true-breeding:
• If allowed to self-pollinate, would produce
offspring identical to themselves
• Mendel wanted to produce seeds with 2
“parents”
• By joining male & female reproductive cells from 2
different plants
• Needed to prevent self-pollination
Gregor Mendel’s
Experiment
• To prevent plants from self-pollinating:
• He cut off pollen-bearing male parts
• Then dusted pollen from another plant onto
the flower
• This process is known as cross-pollination
• It produces seeds that have 2 different “parents”
Mendel Cross Pollinated
Gregor Mendel’s
Experiments
• Mendel studied 7 different plant traits
• Trait: specific characteristic that varies
from 1 individual to another
• Each trait had 2 contrasting characters
(p.264 in text)
• Mendel crossed plants with each of the
7 contrasting characters
• studied their offspring
Gregor Mendel’s
Experiments
• P (parental)
generation
• original pair of plants
• F1 (first filial) generation
• offspring
• FYI: Filius & filia: Latin words for “son” & “daughter”
• Hybrids
• offspring of crosses between parents
with different traits
Gregor Mendel’s Experiments
• After crossing:
• all offspring had character of only 1 parent
• Character from other parent “disappeared”
• Mendel came up with 2 conclusions
Mendel’s Conclusions
1. Biological inheritance determined
by factors passed from 1 generation
to next
•
Chemical factors that determine traits =genes
• Each trait is controlled by 1 gene
•
Each gene occurred in 2 contrasting forms
• The different forms of a gene are alleles
Mendel’s Conclusions
2. Law (Principle) of dominance:
• Some alleles are dominant, others are
recessive
• Dominant: the primary or strong allele
• Recessive: the secondary or weak allele
• In Mendel’s experiments:
•
•
Plant height: tall=Dominant, short=Recessive
Seed color: yellow-Dominant, green-Recessive
Mendel’s 2nd experiment
• Mendel’s question:
• “Did the recessive alleles disappear or were they
still present in F1 plants?”
• Mendel’s experimental design:
• allow the F1 plants to produce an F2 generation
by self-pollination
Mendel’s
nd
2
experiment
• Mendel’s results:
• Traits controlled by the recessive
alleles appeared!
•Approx. ¼ of F2 plants
showed traits controlled by
recessive alleles
Mendel’s Conclusion
• Alleles for tallness & shortness segregated
from each other during formation of gametes
• When each F1 plant produces gametes:
• 2 alleles separate
• Each gamete carries single copy of each gene
• Each F1 plant produces 2 types of gametes
• 1 with allele for tallness
• 1 with allele for shortness
Law (principle) of Segregation
Homozygous vs.
Heterozygous
• Homozygous
• Organism with 2 identical alleles for trait
• Both chromosomes have same form of gene
• Either both dominant or both recessive
• Ex. MM or mm
• Heterozygous
• Organism with 2 different alleles for trait
• Each chromosome has a different form of gene
• 1 dominant & 1 recessive
• Ex. Mm
An example:
Genotype vs. Phenotype
• Genotype:
• Genetic makeup
• Reveals type of alleles organism has
inherited
• Represented by a letter:
• Capital-dominant
• Lowercase-recessive
• Examples:
• TT = homozygous dominant genotype
• tt = homozygous recessive genotype
• Tt = heterozygous genotype
Genotype vs. Phenotype
• Phenotype:
• Physical characteristics
• Description of way trait is expressed in
organism
• The way organism LOOKS
• If genotype is:
• TT or Tt  phenotype = tall
• tt  phenotype = short
Phenotype vs. Genotype
Mendel’s 3rd experiment
• Mendel’s question:
• “Does the segregation of one pair of alleles affect
the segregation of another pair of alleles?”
• “Does the gene for seed shape have anything to
do with the seed for seed color?”
• Mendel’s experimental design:
• He performed a two-factor (dihybrid) cross
Mendel’s 3rd experiment
• Mendel’s results:
• F1: RRYY x rryy (all offspring were round and yellow)
• This wasn’t a surprise to Mendel
• The real question was: in the gametes of these offspring would
the dominant alleles stay together or would the segregate
independently?
• F2: RrYy x RrYy
• 315/556 were round & yellow
• 32/556 were wrinkled & green
• 209/556 were other combinations
Mendel’s Conclusion
• Segregation of alleles of 1 trait does not
affect segregation of alleles of another
trait
• Genes on separate chromosomes
separate independently during gamete
formation (meiosis)
Law (principle) of
Independent Assortment
Summary of Mendel’s Principles
• Inheritance of biological
characteristics is determined by
individual units called genes
• Genes are passed from parents to
offspring
Summary of Mendel’s Principles
• When 2 or more forms
for a single trait exist:
(alleles)
of the gene
• Some forms of gene may be dominant
• Others may be recessive
Law (Principle) of Dominance
Summary of Mendel’s Principles
• In most sexually producing organisms:
• Each adult has 2 copies of each gene
• 1 from each parent
• These genes are separated from each
other when gametes are formed
Law (Principle) of Segregation
Summary of Mendel’s Principles
• Alleles for different genes usually
segregate independently of one another
Law (Principle) of Independent Assortment