observing patterns in inherited...
TRANSCRIPT
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 cross-
pollinated
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 P
mates to produce
• First-generation offspring F1
mate to produce
• 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
Male
gametes
Dominant
phenotype
can arise 3
ways,
recessive
only 1
aA
aaAa
AaAAA
a
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
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
AaBb x AaBb produces the
following gametes
If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible