Mendelian Genetics
Genetics: The Basics
• Allele- An alternative form of a gene
• Diploid organisms have one copy on each homologous chromosome
– Represented by letters:
• Capital letter = dominant form
• Lower case letter = recessive form
• Example= Eye Color
– Controlled by 2 alleles
– Blue Eyes = bb
– Brown eyes= Bb or BB
Dominant allele-fully
expressed in the
organism's
appearance
Recessive allele-no
noticeable effect on
the organism's
appearance
Genetics: The Basics
– Heterozygous: Have 2 different forms of the allele
• Example:
– Brown Eyes = Bb = heterozygous
– Homozygous: Have 2 of the same forms of the allele
• Example:
– Blue Eyes = bb = homozygous recessive
– Brown Eyes = BB = homozygous dominant
Genetics: The Basics
– Genotype = the genetic makeup of an organism
• Example = BB, Bb, bb
– Phenotype = the physical expression of genes
• Example =
– Brown Eyes = phenotype of either the BB or Bb
genotype
– Blue Eyes= phenotype of the bb genotype
Remember that phenotype is not necessarily an appearanceIt can be things like enzyme production, behavior, etc!!
It is ANY expression of a gene!!
Gregor Mendel
*1843 entered
monastery
*1851-53 studied at
Univ. of Vienna
*1857 started breeding
garden peas
* 1860 started forging
data!!
MENDEL'S MAIN QUESTION
Do units of inheritance retain
integrity (preserved) or
blend????
Sample Question: If you cross a purple flower with a white
flower are these flower colors retained in future crosses or
are they blended to form an intermediate color?
Law of Segregation-two alleles for a character are
packaged into separate gametes
2 plants
crossed
Selffertilized
Mendel's findings
1. Alternative version of genes (alleles) account for
variations in inherited characters
Homologous
chromosomes
Purple flowers
White flowers
Mendel's findings
2. For each character, an organism inherits
two alleles, one from each parent.
maternal
paternal
Purple flowers
White flowers
Homologous
chromosomes
Mendel's findings
3. If two alleles differ, the dominant allele is fully
expressed in the organism's appearance.
recessive
dominant
Purple flowers
White flowers
Mendel's findings
4. The two alleles for each character
segregate during gamete production.
dominant
recessive
PP
pp
P
p
Seed shape
Gametes
Punnett
Square
Predicts the
results of a
genetic cross
between
individuals
with known
genotypes
Rules for Genetic Problems
1. Identify traits (alleles) and assign letters to represent the various
traits: capital letters for dominant traits; lower case letters for
recessive traits.
2. Set up parental cross.
3. Draw individual gametes with corresponding letter for trait.
4. Identify F1 offspring phenotype and genotype.
5. Setup F1 cross.
6. Draw individual gametes with corresponding letter for trait.
7. Set up Punnett square to identify individual genotypes and
phenotypes for F2 offspring.
EXAMPLE: SEED COLOR
dominant
recessive
CC
cc
C
C
C
c
c
c
CC
Cc
C
c
Cc
cc
3
1
EXAMPLE: POD SHAPE
dominant
recessive
SS
ss
S
S
S
s
s
s
SS
Ss
S
s
Ss
ss
3
1
Monohybrid Cross Follows
single trait
Test Cross
Breeding a
homozygous
recessive with a
dominant
phenotype
(unknown
genotype) can
determine an
unknown allele.
In pea plants, spherical seeds (S) are dominant to dented
seeds (s). In a genetic cross of two plants that are
heterozygous for the seed shape trait, what fraction of
the offspring should have spherical seeds?
F1 generation, test cross:
Ss
Ss
What is the genotypic ratio?
What is the phenotypic ratio?
The test cross
To identify the genotype of yellow-seeded pea plants as
either homozygous dominant (YY) or heterozygous (Yy),
you could do a test cross with plants of genotype
_______.
A.
B.
C.
D.
E.
y
Y
yy
YY
Yy
Predicting the results of a test cross
A test cross is used to determine if the genotype of a
plant with the dominant phenotype is homozygous or
heterozygous. If the unknown is homozygous, all of the
offspring of the test cross have the __________
phenotype. If the unknown is heterozygous, half of the
offspring will have the __________ phenotype.
A. dominant, recessive
B. recessive, dominant
•Question: How are two traits inherited?
•DIHYBRID CROSS
•Experimental Approach: A cross involving two truebreeding traits.
System: Pea Plants; seed color (Y/y)
and seed shape (S/s).
F1 Generation
F1 Generation
F1 Generation
1. Each of the male gametes types (SY, Sy, sY, sy)
can fuse with each of the female gametes types (SY,
Sy, sY, sy).
2. 16 possible combinations of gametes are possible.
3. We will see that there are 9 possible genotypes and
4 possible phenotypes.
4. The two parental phenotypes, and
two new phenotypes were obtained.
Dihybrid Cross
Follows two traits
9:3:3:1 RATIO
The phenotypes of two
independent traits show a 9:3:3:1
ratio in the F2generation. coat
color is indicated by B (brown,
dominant) or b (white)
tail length is indicated
by S (short, dominant)
or s (long).
If the children mate with each
other, in the F2 generation all
combinations of coat color and
tail length occur: 9 are
brown/short (purple boxes), 3 are
white/short (pink boxes), 3 are
brown/long (blue boxes) and 1 is
white/long (green box).
Dihybrid Cross
In summer squash, white fruit color (W) is dominant over yellow fruit
color (w) and disk-shaped fruit (D) is dominant over sphere-shaped fruit
(d).. If a squash plant true-breeding for white, disk-shaped fruit is
crossed with a plant true-breeding for yellow, sphere-shaped fruit, what
will the phenotypic and genotypic ratios be for:
a. the F1 generation? b. the F2 generation?
Genotypic ratios:
1/16 will be homozygous dominant for
both traits (WWDD)
2/16 will be homozygous dominant for
color and heterozygous for shape
(WWDd)
2/16 will be heterozygous for color and
homozygous dominant for shape (WwDD)
1/16 will be homozygous dominant for
color and homozygous recessive for shape
(WWdd)
WD
4/16 will be heterozygous for both
traits (WwDd)
2/16 will be heteozygous for color and
homozygous recessive for shape (Wwdd) Wd
1/16 will be homozygous recessive for
color and homozygous dominant for shape
(wwDD)
wD
2/16 will be homozygous recessive for
color and heterozygous for shape (wwDd)
wd
1/16 will be homozygous recessive for
both traits (wwdd)
This is a 1:2:2:1:4:2:1:2:1 genotypic ratio
Phenotypic ratios:
9/16 will have white, disk-shaped fruit
3/16 will have white, sphere-shaped fruit
3/16 will have yellow, disk-shaped fruit
1/16 will have yellow, sphere-shaped fruit
This is a 9:3:3:1 phenotypic ratio
WD
Wd
wD
wd
WWDD
WWDd
WwDD
WwDd
WWDd
WWdd
WwDd
Wwdd
WwDD
WwDd
wwDD
wwDd
WwDd
Wwdd
wwDd
wwdd
Law of
SegregationEvery individual possesses a pair
of alleles for any particular trait and
that each parent passes a randomly
selected copy (allele) of only one of
these to its offspring.
Law of Independent AssortmentSeparate genes for separate traits are passed independently of one
another from parents to offspring. These allele pairs are then randomly
united at fertilization.
Inheritance that diverges from Mendel's
inheritance
GENE INTERACTIONS
The relationship between the
genotype and phenotype is
rarely simple.
* Each character is rarely controlled
by one gene
*Each gene usually has more than two
alleles, with one not always being
dominant over the other
Incomplete
Dominance
Heterozygotes
show a distinct
intermediate
phenotype, not
seen in
homozygotes
Traits are
separable in
further crosses
Not
BLENDED
In CODOMINANCE, both alleles are expressed
and functional, though they may be different.
Most genes have more than two alleles in a population.
(IA, IB, I)
Pleiotrophic
Most genes
affect more
than one
phenotypic
character.
Pleiotropy:Albinism
A single defect in one of the enzymes catalyzing tyrosine to melanin can
affect multiple phenotypic characters, from eye color to skin color to hair
color.
Epistasis
A gene at one
locus alters
the
phenotypic
expression of
a gene at a
second locus.
bb with dominant C allele
results in brown mouse
Polygenic
Inheritance
Additive effect of two or
more genes on a single
phenotypic character.
SKIN COLOR
Controlled by at least 4 different genes
Sex-linked traits
In humans, 2 of our 46 chromosomes are
classified as sex chromosomes
•Females = XX
•Carried on ova
•Males = XY
•Carried on sperm
In females, only 1 X chromosome is active
Sex linked traits usually aren’t expressed-
In males, their only X chromosome is active
•No other X chromosome to block sex linked trait
Sex-linked traits
In humans, the genes for colorblindness are both located on the X
chromosome with no corresponding gene on the Y.
Strawberries as they would appear to
someone who is red/green colorblind.
Sex Linked Traits
• Alleles are expressed on each of the sex chromosomes
• Female: XAXA or XAXa or XaXa
• Male: XAY or XaY
Setting up a punnet square for sex-linked traits:
Mom= XAXa Dad = XAY
XA
Xa
XA
Y
Mom is
carrier, dad
does not have
x-linked
recessive
disorder
Mom isn’t
carrier, dad
has x-linked
recessive
disorder
Sex Linked Traits
• Can a female end up with an X-linked trait????
– Example = Sex-linked baldness
• assume that baldness (b) is recessive
• Full head-o-hair (B) is dominant
.
Hemophilia is an X-linked
recessive disorder characterized by the
inability to properly form blood clots.
Y Linked Traits
Recessive Allele
Disorders
Dominant Allele Disorders
Achondroplasia
*Form of dwarfism (dominant
allele)
*Heterozygous/
Homozygous dominant
individuals have dwarf
phenotype
*99.99% of population are
homozygous recessive
Dominant Allele Disorders
Polydactyly
*Heterozygous/
Homozygous dominant
individuals have 6
finger phenotype
*399 out of 400 have
5 digits/appendage:
homozygous recessive
Pedigree Analysis
Information about presence/absence of
phenotypic trait is collected from
individuals in a family across generations.
Having the past help predict the future
DOMINANT TRAIT
RECESSIVE TRAIT
(Allelic to left column)
Blue eyes (more complex,
Brown eyes
simplified here)
PTC taster
PTC non taster
Widow's Peak
Lack Widow’s peak
Middigital hair
Hairless mid digits
Tongue roller
Cannot roll tongue
Detached earlobe
Attached earlobe
A and B blood type (codominant)
Type O blood type
Pattern baldness (dominant in
Pattern baldness (recessive in
males)
females)
Common Heritable Traits
Common Heritable Traits
Common Heritable Traits
Common Heritable Traits
Common Heritable Traits
Case Study: In Sickness and In Health
Greg and Olga’s Trip to the Genetic Counselor
Work in groups of 3-4
Write down answers to turn in
Part 1: Pedigree Construction
10 minutes
• What would the pedigree of Greg and Olga’s families look like?
Part 2: Autosomal Dominant Traits
10 minutes
•
What is an autosome???
•
Do autosomal dominant disorders skip generations?
•
Could Greg or his mother be a carrier of the gene that causes myotonic
dystropy (MD)? Why?
•
Is there a possibility that Greg’s aunt or uncle is homozygous for the MD
gene? Why?
•
Symptoms of MD sometimes don’t show up until after age 50. What is
the possibility that Greg’s cousin has inherited the MD gene?
•
What is the possibility that Greg and Olga’s children will inherit the MD
gene?
Part 3: Autosomal RecessiveTraits
10 minutes
• What are the hallmarks of an autosomal recessive trait (list
four)?
• What is it about the inheritance pattern of factor VIII
deficiency seen in Greg and Olga’s pedigree that point toward it
not being an autosomal recessive trait?
Part 4: Sex-Linked Inheritance
10 minutes
•
What are the characteristics of X-linked inheritance?
•
Why does a son never inherit his father’s defective X chromosome?
•
What is required for a female to display a sex-linked recessive trait?
•
Referring to the pedigree you drew in Part 1, mark the persons who are
carriers of the factor VIII deficiency gene.
•
What is the chance that Olga carries the gene for factor VIII
deficiency? Calculate the probability that she will pass it to her
offspring. Will male children be affected in a different way than female
children?
•
What is the chance that Greg carries the factor VIII gene? Can he pass
the gene on to his sons? His daughters? How will each be affected?