Chapter 11 – Observable
Patterns of Inheritance
11.1 Mendel’s Insight into Inheritance
Patterns
Monk in Austria
-experimented with pea plants
to see how genes are
passed on to generation.
-found that sperm and egg
cells contain genetic
information that play a part
in the genes of the offspring.
Gregor Mendel
“Father of Genetics”
Terms to Know:
Gene locus – location of a specific gene
Allele – variations of a gene
Genes
• Units of information about specific traits.
When identical alleles are passed on
generation after generation this is called a
true-breeding lineage.
When offspring inherit a pair of non-identical
alleles for a trait this is called a hybrid
offspring.
Homozygous
-when both alleles are identical (TT)
Heterozygous
-when alleles are not identical (Tt)
Genotype and Phenotype
• Genotype – genetic makeup of an
individual.
• Phenotype – physical appearance of an
individual. Observable traits.
Tracking Inheritance
P = parental generation
F1 = first-generation offspring
F2 = second-generation offspring
11.2 Mendel’s Theory of
Segregation
• Mendel tested the idea that a plant inherits
two units (genes) of information about a
trait.
He performed a Monohybrid Cross
-cross of two parents that are homozygous
for different versions of a single trait.
F1 offspring will be hybrid, heterozygous.
Monohybrid Cross
P
Tall
DD
x
Dwarf
dd
Homozygous
Dominant
Homozygous
Recessive
All Tall
Dd
F1
Phenotype
Genotype
Clearly Tall is Inherited…
What happened to Dwarf?
Heterozygous
Tall is dominant to tall.
F1 x F1 = F2
Punnett Square:
F2
possible
gametes
possible gametes
D
d
D
Tall
DD
Tall
Dd
d
Tall
Dd
Dwarf
dd
3/
Tall
1/ Dwarf
4
4
3:1 ratio
Dwarf is not missing…just masked
as “recessive” in a diploid state…
Probability
The chance that an event will occur.
Punnett-square Method
• If half of a plant’s
sperm or eggs were a
and half were A, then
four outcomes would
be possible each time
a sperm fertilized an
egg.
Not precise
Possible event
Probable
outcome
Sperm A meets egg A
¼ AA offspring
Sperm A meets egg a
¼ Aa
Sperm a meets egg A
¼ Aa
Sperm a meets egg a
¼ aa
Testcrosses
• Fertilizing different genotype plants to see
results.
• Test results may reveal whether the
organism is homozygous dominant or
heterozygous.
Segregation
• Diploid cells have pairs of genes, on pairs
of homologous chromosomes.
• The two genes of each pair are separated
from each other during meiosis, so they
end up in different gametes.
• They do not effect eachother.
11.3 Independent Assortment
• Dihybrid Crosses – crossing two traits.
Mendel crosses pea plants with two pairs of
genes that were homozygous.
9:3:3:1 ratio