Download Chapter 14: Mendel and the Gene Idea 2. Beyond Mendelian Genetics

Chapter 14:
Mendel and the Gene Idea
1. The Experiments of Gregor Mendel
2. Beyond Mendelian Genetics
3. Human Genetics
1. The Experiments of
Gregor Mendel
Chapter Reading – pp. 268-276
TECHNIQUE
Gregor Mendel
1
Deduced the basics of
inheritance in the 19th
century by analyzing
pea plant crosses:
2
Parental
generation
(P)
Stamens
Carpel
3
4
RESULTS
First
filial
generation
offspring
(F1)
5
• examined
several
characters
exhibiting
dominant &
recessive
traits or
phenotypes
Mendel’s
Crosses
1) Cross plants
that breed true
for character
of interest
2) Cross the
resulting
hybrids
(a monohybrid
cross)
EXPERIMENT
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)

Purple
flowers
White
flowers
All plants had
purple flowers
F2 Generation
705 purple-flowered
plants
224 white-flowered
plants
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 vs Genotype
3
Phenotype
Genotype
Purple
PP
(homozygous)
Purple
Pp
(heterozygous)
1
2
1
Purple
Pp
(heterozygous)
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
1
P Generation
Purple flowers
Appearance:
Genetic makeup:
PP
Gametes:
White flowers
pp
Mendel’s Law of
Segregation
p
P
Each gamete ends up with
only one of the two copies
of each gene in the parent.
F1 Generation
Appearance:
Genetic makeup:
Purple flowers
Pp
1/
2
Gametes:
1/
2
P
Sperm
F2 Generation
• e.g., either the purple flower
allele or the white flower allele
p
P
P
Eggs
p
PP
Pp
Pp
pp
• 50% of gametes contain one
copy, 50% contain the other
p
3
1
This is due to the segregation of
chromosomes during meiosis.
Test Crosses
TECHNIQUE
P? x pp

Test crosses are used
to reveal an unknown
genotype:
Dominant phenotype, Recessive phenotype,
unknown genotype:
known genotype:
PP or Pp?
pp
Predictions
• cross the unknown with
a homozygous recessive
individual
If PP
Sperm
p
p
If Pp
Sperm
p
p
or
P
• if all offspring display
dominant phenotype:
homozygous dominant (PP)
P
Pp
Eggs
Pp
Eggs
P
Pp
Pp
pp
pp
p
Pp
Pp
RESULTS
• if ~½ each of recessive and
dominant phenotypes:
heterozygous (Pp)
or
All offspring purple
1/2
offspring purple and
offspring white
1/2
Genetic Alleles & Genetic Loci
Genes can have different versions known as alleles.
• e.g., purple flower vs white flower alleles
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers
All genes have a specific chromosomal location
referred to as its locus.
• homologous chromosomes have the same genetic loci
Important Genetic Terms
Character
• physical characteristic (e.g., eye color)
Phenotype
• individual’s observable traits (e.g., brown eyes)
Allele
• different forms or versions of the same gene
Genotype
• the alleles an individual has for a given gene
(e.g., AA or Aa or aa)
Homozygous
• the 2 alleles for a gene are identical (AA or aa)
Heterozygous
• the 2 alleles for a gene are different (Aa)
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)
Homozygous dominant = AA
Homozygous recessive = aa
Heterozygous = Aa
Genetics & Probability

Rr
Segregation of
alleles into eggs
All genetic
inheritance is
based on the
probability of
inheriting specific
genetic alleles
from each parent.
Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
R
1/
2
r
R
R
Eggs
4
r
2
r
2
R
1/
1/
1/
1/
4
r
r
R
r
1/
4
1/
4
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.
Multiple Gene Inheritance
EXPERIMENT
YYRR
P Generation
yyrr
Gametes YR

F1 Generation
YyRr
Hypothesis of
dependent
assortment
Predictions
Hypothesis of
independent
assortment
Sperm
or
Predicted
offspring of
F2 generation
1/
2
yr
1/
4
Sperm
1/ YR 1/
2
2 yr
1/
4
YR
YR
1/
4
1/
4
Yr
yR 1/4 yr
YYRR YYRr
YyRR YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
YYRR YyRr
Eggs
1/
2
1/
4
Yr
Eggs
yr
YyRr
3/
4
yyrr
1/
4
yR
1/
4
Phenotypic ratio 3:1
1/
4
3/
16
3/
16
RESULTS
108
101
32
3 Y–rr
1/
16
Phenotypic ratio 9:3:3:1
315
Expected Ratios:
9 Y–R–
yr
9/
16
In a dihybrid cross,
the probability of
each combined
phenotype is the
product of each
individual
phenotype.
Phenotypic ratio approximately 9:3:3:1
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
P Generation
Red
CRCR
White
CWCW
CR
Gametes
CW
Pink
CRCW
F1 Generation
Gametes 1/2 CR
1/
2
CR
1/
2
CW
F2 Generation
1/
2
CR
Eggs
1/
2
Dominant allele is
expressed to lesser
degree when only 1
copy is present
(heterozygous):
CW
2
Sperm
1/
Incomplete
Dominance
CRCR
CRCW
CRCW
CWCW
CW
• heterozygotes have
an intermediate
phenotype (e.g., pink
vs red or white)
Allele
IA
Codominance
IB
Two different alleles
when paired
together are
expressed equally
(e.g., ABO blood type)
• “A” & “B”
alleles are
codominant
(both alleles
expressed)
• “O” allele is
recessive
Carbohydrate
A
B
i
none
(a) The three alleles for the ABO blood groups
and their associated carbohydrates
Genotype
Red blood cell
appearance
Phenotype
(blood group)
IAIA or IA i
A
IBIB or IB i
B
IAIB
AB
ii
O
(b) Blood group genotypes and phenotypes
BbCc

Epistasis
BbCc
Sperm
1/
4 BC
1/
4 bC
1/
4 Bc
1/
4 bc
Eggs
1/
1/
1/
1/
4 BC
BBCC
BbCC
BBCc
BbCc
BbCC
bbCC
BbCc
bbCc
4 bC
4 Bc
BBCc
BbCc
BBcc
Bbcc
BbCc
bbCc
Bbcc
bbcc
4 bc
9
: 3
: 4
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

Polygenic
Inheritance
AaBbCc
Sperm
1/
Many phenotypes
are due to the
effects of multiple
genes (such as
skin color).
Eggs
• in this example,
3 different genes
influence one
phenotype
resulting in 64
different variants
AaBbCc
1/
8
1/
8
1/
8
1/
8
1/
1/
8
1/
1/
8
8
1/
8
1/
64
15/
8
1/
1/
8
8
8
1/
8
1/
8
1/
8
8
Phenotypes:
Number of
dark-skin alleles:
1/
64
0
6/
64
1
15/
64
2
20/
3
64
4
6/
64
5
1/
64
6
3. Human Genetics
Chapter Reading – pp. 282-285
Key
Male
Affected
male
Female
Affected
female
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
Ww
Mating
Offspring, in
birth order
(first-born on left)
ww
Ww ww ww Ww
ww
Ww
Ww
ww
3rd generation
(two sisters)
WW
or
Ww
Widow’s peak
ww
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
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.
A Recessive Human Trait
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
Ff
FF
or Ff ff
Ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd generation
(two sisters)
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?
Albinism is also Recessive
Parents
Normal
Aa
Normal

Aa
Sperm
A
Eggs
a
A
AA
Normal
Aa
Normal
(carrier)
a
Aa
Normal
(carrier)
aa
Albino
Achondroplasia is Dominant
Parents
Dwarf
Dd
Normal

dd
Sperm
D
d
Eggs
d
d
Dd
Dwarf
Dd
Dwarf
dd
Normal
dd
Normal
Key Terms for Chapter 14
• trait, phenotype, genotype, allele, locus
• homozygous, heterozygous
• dominant, recessive, codominant
• true-breeding, monohybrid, dihybrid
• test cross, Punnet square
• epistasis polygenic inheritance
• pedigree
Relevant
Chapter
Questions
1-14