Chapter 14: Mendel and the Gene Idea chapter... · Chapter 14: Mendel and the Gene Idea 3. Human Genetics 2. Beyond Mendelian Genetics 1. The Experiments of Gregor Mendel
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Chapter 14:
Mendel and the Gene Idea
3. Human Genetics
2. Beyond Mendelian Genetics
1. The Experiments of Gregor Mendel
1. The Experiments of
Gregor Mendel
Chapter Reading – pp. 268-276
TECHNIQUE
RESULTS
Parentalgeneration(P) Stamens
Carpel
1
2
3
4
Firstfilialgener-
ationoffspring(F1)
5
Gregor Mendel
Deduced the basics of
inheritance in the 19th
century by analyzing
pea plant crosses:
• examined
several
characters
exhibiting
dominant &
recessive
traits or
phenotypes
EXPERIMENT
P Generation
(true-breeding
parents) Purpleflowers
Whiteflowers
F1 Generation
(hybrids)All plants hadpurple flowers
F2 Generation
705 purple-floweredplants
224 white-floweredplants
Mendel’s
Crosses
1) Cross plants
that breed true
for character
of interest
2) Cross the
resulting
hybrids
(a monohybrid
cross)
Results for
Various
Characters
For each character
Mendel saw the same
basic pattern:
• a 3:1 ratio of dominant
to recessive traits
following a
monohybrid cross
Phenotype
Purple
Purple3
Purple
Genotype
1 White
Ratio 3:1
(homozygous)
(homozygous)
(heterozygous)
(heterozygous)
PP
Pp
Pp
pp
Ratio 1:2:1
1
1
2
Phenotype vs Genotype
P Generation
Appearance:Genetic makeup:
Gametes:
Purple flowers White flowersPP
P
pp
p
F1 Generation
Gametes:
Genetic makeup:Appearance: Purple flowers
Pp
P p1/21/2
F2 Generation
Sperm
Eggs
P
P
PP Pp
p
p
Pp pp
3 1
Mendel’s Law of
Segregation
Each gamete ends up with
only one of the two copies
of each gene in the parent.
• e.g., either the purple flower
allele or the white flower allele
• 50% of gametes contain one
copy, 50% contain the other
This is due to the segregation of
chromosomes during meiosis.
TECHNIQUE
RESULTS
Dominant phenotype,unknown genotype:
PP or Pp?
Predictions
Recessive phenotype,known genotype:
pp
If PP If Ppor
Sperm Sperm
p p p p
P
P
P
p
Eggs Eggs
Pp
Pp Pp
Pp
Pp Pp
pp pp
or
All offspring purple 1/2 offspring purple and1/2 offspring white
Test Crosses
• cross the unknown with
a homozygous recessive
individual
Test crosses are used
to reveal an unknown
genotype:
• if all offspring display
dominant phenotype:
homozygous dominant (PP)
• if ~½ each of recessive and
dominant phenotypes:
heterozygous (Pp)
P? x pp
Allele for purple flowers
Homologous
pair ofchromosomes
Locus for flower-color gene
Allele for white flowers
Genetic Alleles & Genetic Loci
All genes have a specific chromosomal location
referred to as its locus.
Genes can have different versions known as alleles.
• e.g., purple flower vs white flower alleles
• homologous chromosomes have the same genetic loci
Important Genetic Terms
Character• physical characteristic (e.g., eye color)
• individual’s observable traits (e.g., brown eyes)
Phenotype
• the alleles an individual has for a given gene
(e.g., AA or Aa or aa)
Genotype
• different forms or versions of the same geneAllele
Dominant allele (designated by capital letter: A)
• 1 copy of the allele determines phenotype (A–)
Recessive allele (designated by lower-case letter: a)
• affects phenotype only when homozygous (aa)
• the 2 alleles for a gene are identical (AA or aa)
Homozygous
• the 2 alleles for a gene are different (Aa)
Heterozygous
Homozygous dominant = AA
Homozygous recessive = aa
Heterozygous = Aa
Rr Rr
Segregation ofalleles into eggs
Sperm
R
R
R R
R
R r
rr
r
r
r1/2
1/2
1/2
1/2
Segregation ofalleles into sperm
Eggs
1/41/4
1/41/4
Genetics & Probability
All genetic
inheritance is
based on the
probability of
inheriting specific
genetic alleles
from each parent.
The Nature of Probability
The probability of multiple events happening
simultaneously is the product of the probabilities of
each single event.
The probability of one event OR another is the sum
of the probabilities of each event.
EXPERIMENT
RESULTS
P Generation
F1 Generation
Predictions
Gametes
Hypothesis of
dependent
assortment
YYRR yyrr
YR yr
YyRr
Hypothesis of
independent
assortment
orPredicted
offspring of
F2 generation
Sperm
Sperm
YR
YR
yr
yr
Yr
YR
yR
Yr
yR
yr
YRYYRR
YYRR YyRr
YyRr
YyRr
YyRr
YyRr
YyRr
YYRr
YYRr
YyRR
YyRR
YYrr Yyrr
Yyrr
yyRR yyRr
yyRr yyrr
yyrr
Phenotypic ratio 3:1
EggsEggs
Phenotypic ratio 9:3:3:1
1/2 1/2
1/2
1/2
1/4
yr
1/41/4
1/4 1/4
1/4
1/4
1/4
1/43/4
9/163/16
3/161/16
Phenotypic ratio approximately 9:3:3:1315 108 101 32
Multiple Gene InheritanceIn a dihybrid cross,
the probability of
each combined
phenotype is the
product of each
individual
phenotype.
Expected Ratios:
9 Y–R–
3 Y–rr
3 yyR–
1 yyrr
Mendel’s Law of Independent
Assortment
“Each pair of genetic alleles segregates
independently of all other pairs of genetic
alleles during gamete formation (meiosis).”
• the distribution of the alleles of a given gene
into gametes has no connection to the
distribution of alleles for other genes
• applies only to genes on different chromosomes
2. Beyond Mendelian Genetics
Chapter Reading – pp. 276-281
Red
P Generation
Gametes
WhiteCRCR CWCW
CR CW
F1 GenerationPinkCRCW
CR CWGametes1/2
1/2
F2 Generation
Sperm
Eggs
CR
CR
CW
CW
CRCR CRCW
CRCW CWCW
1/21/2
1/2
1/2
Incomplete
Dominance
Dominant allele is
expressed to lesser
degree when only 1
copy is present
(heterozygous):
• heterozygotes have
an intermediate
phenotype (e.g., pink
vs red or white)
IA
IB
i
A
B
none
(a) The three alleles for the ABO blood groupsand their associated carbohydrates
Allele Carbohydrate
GenotypeRed blood cell
appearancePhenotype
(blood group)
IAIA or IA i A
BIBIB or IB i
IAIB AB
ii O
(b) Blood group genotypes and phenotypes
Codominance
• “A” & “B”
alleles are
codominant
(both alleles
expressed)
• “O” allele is
recessive
Two different alleles
when paired
together are
expressed equally(e.g., ABO blood type)
BbCc BbCc
Sperm
Eggs
BC bC Bc bc
BC
bC
Bc
bc
BBCC
1/41/4
1/41/4
1/4
1/4
1/4
1/4
BbCC BBCc BbCc
BbCC bbCC BbCc bbCc
BBCc BbCc
BbCc bbCc
BBcc Bbcc
Bbcc bbcc
9 : 3 : 4
Epistasis
The expression of
a genetic allele
from one locus
alters the
expression of
alleles at another
locus.
• in this example the
homozygous
recessive genotype
for one gene
overrides the
expression of the
other gene
Eggs
Sperm
Phenotypes:
Number ofdark-skin alleles: 0 1 2 3 4 5 6
1/646/64
15/6420/64
15/646/64
1/64
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/81/8
1/81/8
1/81/8
1/81/8
AaBbCc AaBbCc
Polygenic
Inheritance
Many phenotypes
are due to the
effects of multiple
genes (such as
skin color).
• in this example,
3 different genes
influence one
phenotype
resulting in 64
different variants
3. Human Genetics
Chapter Reading – pp. 282-285
Key
Male
Female
Affectedmale
Affectedfemale
Mating
Offspring, inbirth order(first-born on left)
1st generation(grandparents)
2nd generation(parents, aunts,and uncles)
3rd generation(two sisters)
Widow’s peak No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
Ww ww
Ww Wwww ww
ww
wwWw
Ww
wwWW
Wwor
Pedigrees
Diagrams
illustrating
family history
that allow
inheritance
patterns to be
deduced.
We can’t do
experimental
crosses on
humans, we
can only
analyze
existing family
pedigrees.
Attached earlobe
1st generation(grandparents)
2nd generation(parents, aunts,and uncles)
3rd generation(two sisters)
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?
Ff Ff
Ff Ff Ff
ff Ff
ff ff ff
ff
FF or
orFF
Ff
A Recessive Human Trait
Parents
Normal Normal
Sperm
Eggs
NormalNormal(carrier)
Normal(carrier) Albino
Aa Aa
A
AAA
Aa
a
Aaaa
a
Albinism is also Recessive
Eggs
Parents
Dwarf Normal
Normal
Normal
Dwarf
Dwarf
Sperm
Dd dd
dD
Dd dd
ddDd
d
d
Achondroplasia is Dominant
Key Terms for Chapter 14
• epistasis polygenic inheritance
• dominant, recessive, codominant
• test cross, Punnet square
• trait, phenotype, genotype, allele, locus
• true-breeding, monohybrid, dihybrid
• homozygous, heterozygous
• pedigree
Relevant
Chapter
Questions 1-14
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