Lecture 18
GENETICS
Outline
Recombination – crossing over
Basic Genetic concepts
Genetic terms (Genotype, Phenotype, F1…)
Genetic Tools (Punnett Squares, Probabilities, Pedigrees)
Review
Review
Alleles – different versions of the same gene
Maternal Allele – the version of the gene from your mother
Paternal Allele – the version of the gene from your father
Independent Assortment
Homologous pairs of chromosomes orient randomly at metaphase I of meiosis
Each pair of chromosomes sorts maternal and paternal homologs into daughter cells
independently of every other pair
Independent Assortment
The number of combinations possible when chromosomes assort independently into gametes is
2n, where n is the haploid number
For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes
Figure 13.10-1
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Figure 13.10-2
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Figure 13.10-3
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1
Combination 2
Followed by Random Fertilization
Combination 3
Combination 4
Crossing Over
Crossing over produces recombinant chromosomes, which combine DNA inherited from each
parent
Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by
gene
Crossing Over
In crossing over, homologous portions of two nonsister chromatids trade places
Crossing over contributes to genetic variation by combining DNA from two parents into a single
chromosome
Figure 13.11-1
Prophase I
of meiosis
Pair of homologs
Nonsister chromatids
held together
during synapsis
Figure 13.11-2
Prophase I
of meiosis
Nonsister chromatids
held together
during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Figure 13.11-3
Prophase I
of meiosis
Nonsister chromatids
held together
during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Figure 13.11-4
Prophase I
of meiosis
Nonsister chromatids
held together
during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Figure 13.11-5
Prophase I
of meiosis
Nonsister chromatids
held together
during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughter
cells
Recombinant chromosomes
Summary of genetic variation
Three mechanisms contribute to genetic variation
◦ Independent assortment of chromosomes
◦ Crossing over
◦ Random fertilization
Figure 13.7-3
Interphase
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair
of homologous
chromosomes
Chromosomes
duplicate
Sister
chromatids
Diploid cell with
duplicated
chromosomes
Meiosis I
1 Homologous
chromosomes separate
Haploid cells with
duplicated chromosomes
Meiosis II
2 Sister chromatids
separate
Haploid cells with unduplicated chromosomes
Figure 14.2
TECHNIQUE
1
2
Parental
generation
(P)
3
Stamens
Carpel
4
RESULTS
First filial
generation
offspring
(F1)
5
Figure 14.3-1
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
Figure 14.3-2
EXPERIMENT
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)
Purple
flowers
White
flowers
All plants had purple flowers
Self- or cross-pollination
Figure 14.3-3
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had purple flowers
Self- or cross-pollination
F2 Generation
705 purpleflowered
plants
224 white
flowered
plants
Table 14.1
Terms
Trait/Phenotype/Genotype
Generations: Parental, F1, F2
Self pollination vs Cross pollination
True breeding
Hybrid
Mendel’s Model
Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2
offspring
Four related concepts make up this model
We now know the molecular explanation for this model
1st Concept To Explain 3:1 Pattern in F2
generation
First: alternative versions of genes account for variations in inherited characters
One Gene: Purple flower – White Flower
These alternative versions of a gene are alleles
Each gene resides at a specific locus on a specific chromosome
Figure 14.4
Allele for purple flowers
Locus for flower-color gene
Allele for white flowers
Pair of
homologous
chromosomes
2nd Concept To Explain 3:1 Pattern in F2
generation
Second: for each character (phenotype), an organism inherits two alleles, one from each parent
The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s
P generation
Alternatively, the two alleles at a locus may differ, as in the F1 hybrids
3rd Concept To Explain 3:1 Pattern in F2
generation
Third: if the two alleles at a locus differ, then one (the dominant allele) determines the
organism’s appearance, and the other (the recessive allele) has no noticeable effect on
appearance
In the flower-color example, the F1 plants had purple flowers because the allele for that trait is
dominant
4th Concept To Explain 3:1 Pattern in F2
generation
Fourth: The law of independent segregation: the two alleles for a heritable characteristic
(phenotype) separate (segregate) during gamete formation and end up in different gametes
An egg or a sperm get only one of the two alleles
Allele segregation is because homologous chromosomes segregate during meiosis
Figure 14.7
TECHNIQUE
Dominant phenotype,
unknown genotype:
PP or Pp?
Predictions
If purple-flowered
parent is PP
Sperm
p
p
Recessive phenotype,
known genotype:
pp
or
If purple-flowered
parent is Pp
Sperm
p
p
P
Pp
Eggs
P
Pp
Eggs
P
p
Pp
Pp
Pp
Pp
pp
pp
RESULTS
or
All offspring purple
1/
2
offspring purple and
offspring white
1/
2
Figure 14.9
Rr
Segregation of
alleles into eggs

Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
Eggs
4
r
2
r
R
R
1/
1/
r
2
R
R
1/
2
1/
1/
4
r
r
R
r
1/
4
1/
4
Figure 14.8
EXPERIMENT
YYRR
P Generation
yyrr
Gametes
yr
YR
F1 Generation
Predictions
YyRr
Hypothesis of
dependent assortment
Hypothesis of
independent assortment
Sperm
or
Predicted
offspring of
F2 generation
1/
Sperm
1/
2
YR
1/
2
2
YR
Eggs
1/
2
YYRR
4
YR
YyRr
1/
4
Yr
1/
4
yR
4
yr
Eggs
yr
YyRr
3/
4
YR
1/
4
Yr
1/
4
yR
1/
4
yr
yr
1/
1/
4
yyrr
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
1/
4
Phenotypic ratio 3:1
1/
9/
16
3/
16
3/
16
1/
16
Phenotypic ratio 9:3:3:1
RESULTS
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
Figure 14.UN02
(probability of pp)  1/2 (yy)  1/2 (Rr)
 1/16
 1/2  1/2
 1/16
2
 1/2  1/2
 2/16
1/
4
 1/2  1/2
 1/16
1/
4
 1/2  1/2
 1/16
ppyyRr
1/
ppYyrr
1/
Ppyyrr
1/
PPyyrr
ppyyrr
4
4
Chance of at least two recessive traits
 6/16 or 3/8
The ability to curl your tongue up on the sides (T, tongue rolling) is dominant to not being able to
roll your tongue. A woman who can roll her tongue marries a man who cannot. Their first child
has his father's phenotype. What are the genotypes of the mother, father, and child?
What is the probability that a second child won't be a tongue roller?
Often inheritance patterns are more
complicated
Many heritable characters are not determined by only one gene with two alleles
Basic principles of segregation and independent assortment apply even to more complex
patterns of inheritance
Examples of single gene not following
Mendelian patterns
Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the
following situations:
◦ When alleles are not completely dominant or recessive
◦ When a gene has more than two alleles
◦ When a gene produces multiple phenotypes
Degrees of Dominance
Complete dominance: phenotypes of the heterozygote and dominant homozygote are identical
Incomplete dominance, the phenotype of F1 hybrids is in between the phenotypes of the two
parental varieties
Codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
Figure 14.10-1
P Generation
White
CWCW
Red
CRCR
Gametes
CR
CW
Figure 14.10-2
P Generation
White
CWCW
Red
CRCR
Gametes
CR
F1 Generation
Gametes
CW
Pink
CRCW
1/
2
CR
1/
2 CW
Figure 14.10-3
P Generation
White
CWCW
Red
CRCR
CR
Gametes
CW
F1 Generation
Pink
CRCW
Gametes
1/
2
CR
1/
2
CW
Sperm
1/
2
F2 Generation
1/
2
CR
1/
2
CW
Eggs
CR
1/
2
CW
CRCR CRCW
CRCW CWCW
Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of
lipids in the brain
◦ At the organismal level, the allele is recessive
◦ At the biochemical level, the phenotype (i.e., the enzyme activity level) is
incompletely dominant
◦ At the molecular level, the alleles are codominant
Multiple Alleles
Most genes exist in populations in more than two allelic forms
The ABO blood group in humans are determined by three alleles
Single Gene codes for an enzyme that attaches a specific carbohydrate to the surface of the
RBC
◦
◦
◦
IA allele – The enzyme adds the A carbohydrate
IB allele – The enzyme adds the B carbohydrate
i allele – Adds neither
Figure 14.11
(a) The three alleles for the ABO blood groups and their
carbohydrates
IA
Allele
Carbohydrate
IB
i
none
B
A
(b) Blood group genotypes and phenotypes
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
A
B
AB
O
Red blood cell
appearance
Phenotype
(blood group)
Pleotrophy
Most genes have multiple phenotypic effects, a property called pleiotropy
Pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases,
such as cystic fibrosis and sickle-cell disease
Some traits may be determined by two or more genes
Epistasis
In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus
Labrador retrievers and many other mammals, coat color depends on two genes
One gene determines the pigment color (with alleles B for black and b for brown)
The other gene (with alleles C for color and c for no color) determines whether the pigment will
be deposited in the hair
Figure 14.12
BbEe
Eggs
1/
4 BE
1/ bE
4
1/
4 Be
1/
4
be
Sperm
1/
4 BE
BbEe
1/
4 bE
1/
4 Be
1/
4 be
BBEE
BbEE
BBEe
BbEe
BbEE
bbEE
BbEe
bbEe
BBEe
BbEe
BBee
Bbee
BbEe
bbEe
Bbee
bbee
9
: 3
: 4
Polygenic Inheritance
Quantitative characters are those that vary in the population along a continuum
Quantitative variation usually indicates polygenic inheritance, an additive effect of two or
more genes on a single phenotype
Skin color in humans is an example of polygenic inheritance
Nature vs. Nurture