Download Chapter 11 Patterns of Inheritance

Observing Patterns in
Inherited Traits
Chapter 11
Updated Reading 11.1-11.3
Not 11.5-11.7
What you absolutely need to
know
• Punnett Square with monohybrid and
dihybrid cross
• Heterozygous, homozygous, alleles,
locus, gene
• Test cross, P, F1, F2
• Mendel and his work
Early Ideas about Heredity
• People knew that sperm and eggs
transmitted information about traits
• Blending theory
• Problem:
– Would expect variation to disappear
– Variation in traits persists
No Blending Involved
• We cannot say that a red flower crossed
with a what flower produces a pink
flower.
Name this man
Gregor Mendel
• Strong background in plant breeding and
mathematics
• Using pea plants, found indirect but
observable evidence of how parents
transmit genes to offspring
Genetic Terms
A pair of homologous
chromosomes
A gene locus
A pair of alleles
Three pairs of genes
The Garden Pea Plant
• Self-pollinating
• True breeding (different alleles
not normally introduced)
• Can be experimentally crosspollinated
Impact of Mendel’s Work
• Mendel presented his results in 1865
• Paper received little notice
• Mendel discontinued his experiments in
1871
• Paper rediscovered in 1900 and finally
appreciated
Genes
• Units of information about specific traits
• Passed from parents to offspring
• Each has a specific location (locus) on a
chromosome
Alleles
• Different molecular forms of a gene
• Arise by mutation
• Dominant allele masks a recessive
allele that is paired with it
Allele Combinations
• Homozygous
– having two identical alleles at a locus
– AA or aa
• Heterozygous
– having two different alleles at a locus
– Aa
Genotype & Phenotype
• Genotype refers to particular genes an
individual carries
• Phenotype refers to an individual’s
observable traits
• Cannot always determine genotype by
observing phenotype. Ex. A blond haired
person can produce a red-haired offspring
Tracking Generations
• Parental generation
mates to produce
P
• First-generation offspring
mate to produce
F1
• Second-generation offspring
F2
Monohybrid Crosses
• Use F1 offspring of parents that breed
true for different forms of a trait:
(AA x aa = Aa)
• The experiment itself is a cross between
two identical F1 heterozygotes, which
are the “monohybrids” (Aa x Aa)
F1 Results of One
Monohybrid Cross
Punnett Square of a
Monohybrid Cross
Female gametes
A
Male
gametes
A
AA
a
Aa
a
Aa
aa
Dominant
phenotype
can arise 3
ways,
recessive
only 1
F2 Results of
Monohybrid Cross
Testcross
• Individual that shows dominant
phenotype is crossed with individual
with recessive phenotype (AA x aa)
• Examining offspring allows you to
determine the genotype of the dominant
individual
Dominance Relations
• Complete dominance
• Incomplete dominance
– Heterozygote phenotype is somewhere
between that of two homozyotes
• Codominance
– Non-identical alleles specify two
phenotypes that are both expressed in
heterozygotes
F1 Results of Mendel’s
Dihybrid Crosses
• All plants displayed the dominant form
of both traits:
AaBb
• We now know:
– All plants inherited one allele for each trait
from each parent
– All plants were heterozygous (AaBb)
F1 Results of Mendel’s
Dihybrid Crosses
• All plants displayed the dominant form
of both traits:
AaBb
• We now know:
– All plants inherited one allele for each trait
from each parent
– All plants were heterozygous (AaBb)
Dihybrid Cross
Experimental cross between individuals
that are homozygous for different
versions of two traits
•
•
•
•
•
Now we want to try two traits
Purple AA
Tall BB
Parent with purple flowers and and long
stems = AABB
Purple AA
Tall BB
Parent with white flowers and short
stems
White aa and short stems bb
Dihybrid cross is AABB x aabb
Phenotypic Ratios in F2
AaBb
X
AaBb
Four Phenotypes:
– Tall, purple-flowered
(9/16)
– Tall, white-flowered
(3/16)
– Dwarf, purple-flowered (3/16)
– Dwarf, white-flowered
(1/16)
AaBb x AaBb produces the
following gametes
If the two
traits are
coded for by
genes
on separate
chromosome
s, sixteen
gamete
combinations
are possible