Patterns of Inheritance
Ch. 10.1-10.2
Objectives
1.
2.
3.
4.
Mendel’s methods
Explain the principal of segregation
Genotype vs. Phenotype
Principal of independent assortment
• Who is this guy? Any guesses?
• Mendel struggled with many
questions, such as:
• How do offspring receive the
traits that they have?
• Why do some offspring look
more like parents than others?
• Why may later generations
look more like grandparents
than parents?
• This all has to do with traits.
• What’s a trait?
• Trait:
– Variation of a character
• i.e., plant color; red vs. white
Gregor Mendel
1822-1884
Blending Hypothesis
• Early 1800’s
• Used to explain how offspring inherit
characteristics from parents
• Red plant with yellow plant --> orange plant
• This wasn’t always so
• Mendel decided to experiment . . .
• his work gave rise to . . .
• genetics:
– The study of heredity.
Plant Breeding Experiments
• Bred pea plants for seven
years
• Developed a particulate
hypothesis of inheritance:
– Parents pass on genes that are
responsible for inherited traits
– Genes retain their identity
generation after generation
– Genes are like marbles of
different colors, not like paints;
meaning that genes retain their
own identities
Mendel’s Garden
Plant Breeding Experiments
• Identified true-breeding pea plants
– All offspring identical every time
• Crossed true-breeding plants with different traits
• Purple pea plant fertilized with pollen from white pea plant:
• What color would the offspring be?
Mendel’s Principle of
Segregation
• Hybrids:
– offspring of two truebreeding varieties
• Monohybrid cross:
– testing ONE trait in a
cross (i.e., flower
color)
• What did Mendel
find?
?
Monohybrid Cross
• ALL plants in F1 generation
purple. Not a blend!
• Was the gene for white
flowers now lost?
• Let F1 (filial, greek for son)
generation fertilize
themselves
• Gene for white flowers was
present. 1/4 of plants had
white flowers!
• Investigated 6 other traits and found the same
thing: for each character, 1 of the 2 traits
disappeared in F1, and reappeared in F2
Mendel’s 4 Hypotheses
1.
2.
Alternative forms of genes (alleles)
Each gene has 2 alleles



3.
Dominant vs. Recessive alleles


4.
One from each parent
Homozygous: same (PP)
Heterozygous: different (Pp)
Allele affects trait --> dominant (P);
Alleles with no affect in a heterozygous individual are recessive (p).
The 2 alleles for a trait separate during meiosis (Principal of
Segregation) and reunite during fertilization.
Genotype vs. Phenotype
• Genotype:
– The genetic makeup
– i.e., Pp
• Phenotype:
– Observable trait
– i.e, purple flowers
• What’s your
genotype/phenotyp
e worksheet
Check your neighbor
• See if you can remember what Mendel’s 4
hypotheses are? Use your own words if possible.
1. Alternative forms of genes (alleles)
2. Each gene has 2 alleles
3. Dominant vs. Recessive alleles


Allele affects trait --> dominant (P);
Alleles with no affect in a heterozygous individual are recessive (p).
4. The 2 alleles for a trait separate during meiosis (Principal of
Segregation) and reunite during fertilization.
Probability Through Punnett
Squares
• Punnett Square:
– Shows possible genetic
outcomes
• Each individual square is an
individual offspring
• Each individual receives
ONE allele from each parent
• What’s the probability that
offspring will be purple for
match #1? #2?
Testcross
• What is the genotype of an organism that displays a
dominant phenotype?
• Cross of a plant with an unknown genotype (PP or Pp)
with a homozygous recessive individual, will yield one of
two possible results:
– pp x PP = 100% (Pp)
– pp x Pp = 50% (pp) : 50% (Pp)
• Do you see how this works?
Independent Assortment
• Dihybrid cross:
– crossing organisms differing in two characteristics
• Principle of Independent Assortment:
• A particular allele in one character can be paired with
either allele of another
– R can end up with y or Y and r can end up with Y or y
– Just because the seed is yellow, does NOT mean it will be
wrinkled.
Punnett Square Worksheet
• Let’s try a worksheet and see if you
understand. . .