Mendelian Genetics

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Mendelian Genetics
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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

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Dominant allele-fully expressed in the organism's
appearance
Recessive allele-no noticeable effect on the
organism's appearance
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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

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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
appearance- It can be things like enzyme
production, behavior, etc!! It is ANY expression
of a gene!!
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Gregor Mendel
1843 entered monastery 1851-53 studied at Univ.
of Vienna 1857 started breeding garden peas
1860 started forging data!!
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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?
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Law of Segregation-two alleles for a character
are packaged into separate gametes
2 plants crossed
Self-fertilized
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Mendel's findings
1. Alternative version of genes (alleles)
account for variations in inherited characters
Homologous chromosomes
Purple flowers
White flowers
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Mendel's findings
2. For each character, an organism inherits two
alleles, one from each parent.
maternal
Homologous chromosomes
paternal
Purple flowers
White flowers
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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
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Mendel's findings
4. The two alleles for each character segregate
during gamete production.
dominant
recessive
Seed shape
PP
pp
Gametes
P
p
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Punnett Square
Predicts the results of a genetic cross between
individuals with known genotypes
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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.
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EXAMPLE SEED COLOR
dominant
recessive
CC
cc
c
C
C
CC
3
Cc
Cc
cc
1
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EXAMPLE POD SHAPE
dominant
recessive
SS
ss
s
S
S
SS
3
Ss
Ss
ss
1
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Monohybrid Cross
Follows single trait
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Test Cross
Breeding a homozygous recessive with a dominant
phenotype (unknown genotype) can determine an
unknown allele.
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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?
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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. y B. Y C. yy D. YY E. Yy
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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

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• Question How are two traits inherited?
• DIHYBRID CROSS
• Experimental Approach A cross involving two
  true-breeding traits.
• System Pea Plants seed color (Y/y)
• and seed shape (S/s).

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F1 Generation
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F1 Generation
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F1 Generation

• Each of the male gametes types (SY, Sy, sY, sy)
  can fuse with each of the female gametes types
  (SY, Sy, sY, sy).
• 16 possible combinations of gametes are
  possible.
• We will see that there are 9 possible genotypes
  and 4 possible phenotypes.
• 4. The two parental phenotypes, and
• two new phenotypes were obtained.

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Dihybrid Cross
Follows two traits
9331 RATIO
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Dihybrid Cross




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Law of Independent Assortment-separation of
alleles into separate gametes
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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 not always being dominant over the
other
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Incomplete Dominance
Heterozygotes show a distinct intermediate
phenotype, not seen in homozygotes
Traits are separable in further crosses
Not BLENDED
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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)
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Pleiotrophic
Most genes affect more than one phenotypic
character.
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PleiotropyAlbinism
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.
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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
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Pedigree Analysis
Information about presence/absence of phenotypic
trait is collected from individuals in a family
across generations.
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Having the past help predict the future
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DOMINANT TRAIT RECESSIVE TRAIT (Allelic to left column)
Brown eyes PTC taster Widow's Peak  Middigital hair Tongue roller Detached earlobe A and B blood type (codominant) Pattern baldness (dominant in males) Blue eyes (more complex, simplified here) PTC non taster Lack Widows peak Hairless mid digits Cannot roll tongue Attached earlobe Type O blood type Pattern baldness (recessive in females)
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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 arent expressed-
• In males, their only X chromosome is active
• No other X chromosome to block sex linked trait

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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.
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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
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Mom is carrier, dad does not have x-linked
recessive disorder
Mom isnt carrier, dad has x-linked recessive
disorder
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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

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.
Hemophilia is an X-linked recessive disorder
characterized by the inability to properly form
blood clots.
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Y Linked Traits
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Additive effect of two or more genes on a single
phenotypic character.
Polygenic Inheritance
SKIN COLOR
Controlled by at least 3 different genes
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SKIN COLOR
Gametes  ABC   ABc   AbC   Abc   aBC   aBc   abC   abc 
ABC 6 5 5 4 5 4 4 3
ABc 5 4 4 3 4 3 3 2
AbC 5 4 4 3 4 3 3 2
Abc 4 3 3 2 3 2 2 1
aBC 5 4 4 3 4 3 3 2
aBc 4 3 3 2 3 2 2 1
abC 4 3 3 2 3 2 2 1
abc 3 2 2 1 2 1 1 0
In the following cross between two mulatto
genotypes (AaBbCc x AaBbCc), each parent produces
eight different types of gametes and these
gametes combine with each other in 64 different
ways resulting in a total of seven skin colors.
The skin colors can be represented by the number
of capital letters, ranging from zero (no capital
letters) to six (all capital letters). The
approximate shades of skin color corresponding to
each genotype are shown in Table 1. Note Skin
color may involve at least four pairs of alleles
with nine (or more) shades of skin color.
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Recessive Allele Disorders
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Dominant Allele Disorders
Achondroplasia
Form of dwarfism (dominant allele) Heterozygous/
Homozygous dominant individuals have dwarf
phenotype 99.99 of population are homozygous
recessive
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Dominant Allele Disorders Polydactyly
Heterozygous/ Homozygous dominant individuals
have 6 finger phenotype 399 out of 400 have 5
digits/appendage homozygous recessive
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Common Heritable Traits
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Common Heritable Traits
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Common Heritable Traits
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Common Heritable Traits
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Common Heritable Traits
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Case Study In Sickness and In Health Greg and
Olgas Trip to the Genetic Counselor
Work in groups of 3-4 Write down answers to turn
in
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Part 1 Pedigree Construction10 minutes

• What would the pedigree of Greg and Olgas
  families look like?

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Part 2 Autosomal Dominant Traits10 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 Gregs aunt or uncle
  is homozygous for the MD gene? Why?
• Symptoms of MD sometimes dont show up until
  after age 50. What is the possibility that
  Gregs cousin has inherited the MD gene?
• What is the possibility that Greg and Olgas
  children will inherit the MD gene?

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Part 3 Autosomal RecessiveTraits10 minutes

• What are the hallmarks of an autosomal recessive
  trait?
• What is it about the inheritance pattern of
  factor VIII deficiency seen in Greg and Olgas
  pedigree that point toward it being an autosomal
  recessive trait?

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Part 4 Sex-Linked Inheritance10 minutes

• What are the characteristics of X-linked
  inheritance?
• Why does a son never inherit his fathers
  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?