Genotype

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Genotype
Activities of genes gene products
Environment development
Phenotype
Genotype collection of genes (and alleles) in
an organism Phenotype observable properties of
an organism
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• Mendelian Genetics
• Gregor Johann Mendel (1822-1884)
• Augustinian monk, Czech Republic.
• Foundation of modern genetics.
• Studied segregation of traits in the garden pea
  (Pisum sativum) beginning in 1854.
• Published his theory of inheritance in 1865.
• Versuche über Pflanzen-Hybriden
• Experiments in Plant Hybridization
• Mendel was rediscovered in 1902.

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• Mendels Experiments
• Began by self-fertilizing 34 different pea
  strains (phenotypes) so that they bred true
  (selfing, the opposite of cross-fertilization).
• Focused on 7 well-defined garden pea traits by
  crossing different phenotypes one at a time
• Flower/seed coat color purple vs. white
  flowers
• grey vs. white seed coats
• (controlled by single gene)
• Seed color yellow vs. green
• Seed shape smooth vs. wrinkled
• Pod color green vs. yellow
• Pod shape inflated vs. pinched
• Stem height tall vs. short
• Flower position axial vs. terminal
• Counted offspring of each phenotype and analyzed
  the results mathematically.

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Fig. 10.4, Mendels 7 garden pea characters.
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Some basic terminology Generations P
parental generation F1 1st filial generation,
progeny of the P generation F2 2nd filial
generation, progeny of the F1 generation (F3 and
so on) Crosses Monohybrid cross cross of two
different true-breeding strains (homozygotes)
that differ in a single trait. Reciprocal cross
sexes for the two strains are reversed (and if
the results are the same, trait is not
sex-linked). Dihybrid cross cross of two
different true-breeding strains (homozygotes)
that differ in two traits. Genetics etiquette -
female conventionally is written first
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Dominant recessive alleles (Fig. 10.7)
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Results of Mendels monohybrid parental
cross Mendels Principle of Uniformity in
F1 F1 offspring of a monohybrid cross of
true-breeding strains resemble only one of the
parents. Why? Smooth seeds (allele
S) are completely dominant to wrinkled seeds
(allele s).
Fig. 10.5
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• Fig. 10.8
• Smooth and wrinkled parental seed strains
  crossed.
• Punnett square
• F1 genotypes
• 4/4 Ss
• F1 phenotypes
• 4/4 smooth

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F1 x F1 crosses (Fig. 10.6) Mendel also
discovered that traits that disappear in the F1
generation reappear in the F2 generation in a 13
ratio.
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F1 x F1 Punnett square (Fig. 10.8) F2
genotypes 1/4 SS 1/2 Ss 1/4 ss F2
phenotypes 3/4 smooth 1/4 wrinkled
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• Confirming the Principle of Segregation with
  test-crosses
• SS x SS ? true breeding (100 homozygous
  dominant)
• ss x ss ? true breeding (100 homozygous
  recessive)
• How do you determine whether an individual with
  the dominant phenotype is homozygous or
  heterozygous?
• Cross it with homozygous recessive
• SS x ss ? 4/4 dominant trait
• Ss x ss ? 1/2 dominant trait 1/2 recessive
  trait

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Fig. 10.11, Test Crosses
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• 8.2.1 Calculate and predict the genotypic and
  phenotypic ratios of offspring of dihybrid
  crosses involving unlinked autosomal genes.

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• Mendels dihybrid crosses
• Mendel also performed crosses involving two pairs
  of traits, e.g., seed shape (smooth vs. wrinkled)
  and color (yellow vs. green).
• If alleles sort independently, four possible
  phenotypes (2n) appear in the F2 generation in a
  9331 ratio.
• Mendels Principle of Independent Assortment
• Alleles for different traits assort independently
  of one another.
• Modern formulation of independent assortment
• Genes on different chromosomes behave
  independently in gamete production.

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Fig. 10.12a Dihybrid cross F1 generation
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Fig. 10.12b Dihybrid cross F2
generationRatio9331
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• Summary of Mendels Principles
• Mendels Principle of Uniformity in F1
• F1 offspring of a monohybrid cross of
  true-breeding strains resemble only one of the
  parents.
• Why? Smooth seeds (allele S) are completely
  dominant to wrinkled seeds (allele s).
• Mendels Principle of Segregation
• Recessive characters masked in the F1 progeny of
  two true-breeding strains, reappear in a specific
  proportion of the F2 progeny.
• Two members of a gene pair segregate (separate)
  from each other during the formation of gametes.
  Inheritance is particulate, not blending as
  previously believed.
• Mendels Principle of Independent Assortment
• Alleles for different traits assort
  independently of one another.
• Genes on different chromosomes behave
  independently in gamete production.

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Non-Mendelian Inheritance

• Incomplete Dominance
• Snap Dragons
• Sickle-Cell Disease
• Codominance
• Blood type
• Polygenic Inheritance
• Eye color
• Pleiotropy
• Sex-determination
• Environmental influences
• Himalayan Rabbit fur color

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Incomplete Dominance
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Codominance
ABO Blood Typing
Range of genotypes
IA IA
IB IB
or
or
IA i
IA IB
IB i
ii
Blood types
A
AB
B
O
Three alleles IA, IB, i
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Polygenic inheritance

• Many genes affect one trait
• Lab Manual Exercise 5

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Pleiotropy

• One gene affects many traits
• SRY male characteristics
• Marfan syndrome

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Environmental Influences
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Continuous Variation
Height distribution for female students

• Example Height
• Why?
• Pleiotropy
• Environment

Number of individuals with some value of the trait
Range of values for the trait
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Sex Determination
female (XX)
male (XY)
eggs
sperm
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The X Chromosome The Y Chromosome

• Carries more than 2,300 genes
• Most genes deal with nonsexual traits
• Genes on X chromosome can be expressed in both
  males and females

• Fewer than two dozen genes identified
• One is the master gene for male sex determination
  
• SRY
• SRY present, testes form
• SRY absent, ovaries form

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Crossing Over

• Occurs during Prophase I
• of Meiosis
• Homologous chromosomes swap bits of genetic
  information
• You can learn how far apart two genes are by
  learning how often they cross-over

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Crossover Frequency
Proportional to the distance that separates genes
A
B
C
D
Crossing over will disrupt linkage between A and
B more often than C and D
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Full Linkage
x
Parents
AB
ab
All AaBb
F1 offspring
meiosis, gamete formation
Equal ratios of two types of gametes
50 AB
50 ab
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Incomplete Linkage
AC
ac
x
Parents
F1 offspring
All AaCc
meiosis, gamete formation
a
a
A
A
Unequal ratios of four types of gametes
C
c
C
c
parental genotypes
recombinant genotypes
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Human Genetic Analysis

• Karyotyping
• Pedigree Analysis

1 2 3 4 5
6 7 8 9 10
11 12
13 14 15 16 17 18
19 20 21 22 XX (or
XY)
offspring in order of birth, from left to right
male
Individual showing trait being studied
female
sex not specified
marriage/mating
generation
I, II, III, IV...
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• Changes occur.. Mutations
• Genetics - Animations

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Deletion

• Loss of some segment of a chromosome
• Most are lethal or cause serious disorder

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Duplication
normal chromosome
one segment repeated
three repeats
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Translocation
In-text figurePage 206
one chromosome
a nonhomologous chromosome
nonreciprocal translocation
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Inversion

• A linear stretch of DNA is reversed
• within the chromosome

segments G, H, I become inverted
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Changes in Chromosome NumberAneupolidy

• Trisomy
• Chromosome 21 Downs syndrome
• XXY Klinefelter syndrome
• YYX Jacobs syndrome
• XXX poly-X syndrome
• Monosomy
• XO Turners syndrome

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Downs Syndrome/ Trisomy 21
Three copies of chromosome 21 Symptoms developmen
tal delays variable mental retardation distincti
ve physical appearance loose muscles some heart
disease sociable
http//www.ds-health.com/
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Sex linked

• SEX-LINKED inheritance involves genes which are
  found only on the X chromosome.  The pattern of
  inheritance differs from somatic genes, because
  males have only one X chromosome.

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Sex-linked recessive hemophilia A
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Albinism / autosomal dominant

• single abnormal gene on one of the autosomal
  chromosomes (one of the first 22 "non-sex"
  chromosomes) from either parent can cause the
  disease.
• One of the parents will have the disease (since
  it is dominant) in this mode of inheritance and
  that person is called the CARRIER.
• Only one parent must be a carrier in order for
  the child to inherit the disease

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Autosomal dominant/ albinism and Huntington's
disease

• every affected child has an affected parent
• Males and females are affected equally
• 11 ratio of expected inheritance (50/50)

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X-linked recessive disorder? Glucose phosphate
dehydrogense deficiency

• error on the X chromosome
• XY no protection.. Only one X
• XX second good chromosome protection, but
  carrier status.

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Remember Heterozygous Advantage for malaria

• Teachers' Domain All Genetics Resources go to
  mutation story
• There are multiple resources on this page in the
  area of genetics.
• Ask me how to get in to the site..