Theoretical Genetics

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Theoretical Genetics
Gregor Mendel
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Objectives

• 4.3.1 Define dominant allele, recessive allele,
  codominant alleles, locus, homozygous,
  heterozygous, genotype, phenotype, carrier,
  and test cross.
• 4.3.2 Determine the genotypes and phenotypes of
  the offspring of a monohybrid cross using a
  Punnett grid.

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Review of DNA Structure

• Eukaryotic DNA is divided among
  several chromosomes (humans
  have 23 pairs of chromosomes
  one set from each parent).
• Chromosomes are divided into units called
  genes.
• Genes code for the bodys proteins eye pigment.
• Genes come in versions
  called alleles
• Eye color gene either brown or blue
  allele.
• Result of mutation.

4
Definitions

• Locus
• The particular position of a gene on homologous
  chromosomes.

Homologous codes for the same things.
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Definitions

• Homozygous vs. heterozygous alleles
• An organism with two identical alleles for a
  character is homozygous for that character.
• Organisms with 2 different alleles for
  a character are heterozygous.

Bb BB bb
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Definitions

• Dominant recessive allele (complete dominance)
• A dominant allele is strong and is expressed (or
  seen) in the phenotype. (Abbreviated with
  capitals)
• A recessive allele (if present in a
  heterozygous individual) is
  not expressed it is hidden because
  it is weak. Reces- sive alleles only
  show up if the in- dividual is
  homo- zygous. (Lowercase
  letters)

Bb BB bb
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Definitions

• Incompletely dominant alleles
• Sometimes, neither allele (when heterozygous)
  can overpower the other. They produce a blend.

allele for white flower color (Incomplete
dominance) allele for red flower color
(Incomplete dominance)
Pink Red
White flowers flowers flowers
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Definitions

• Codominant alleles
• Pairs of alleles that both affect the
  phenotype when present in a
  heterozygote.
• Ex Blood groups
• A, B, O are alleles that code
  for a sig- nal
  protein on the
  membrane surface. O is
  recessive, but A B
  codominant.

Punnet square
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Definitions

• Genotype vs. phenotype
• All the many alleles of an organism (for eye
  color, hair color, seed appearance, etc.) make
  up its genotype genetic make-up think type of
  genes.
• An organisms physical characteristics make a
  phenotype.

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The science of genetics

• Genetics - the study of heredity.
• Heredity a characteristic of life the passing
  of traits from parents to offspring.
• Traits - characteristics such as eye color, body
  size, sickle cell anemia, etc.
• The science of genetics be-
  gan with an Austrian monk named
  Gregor Mendel in the 1860s,
  working with pea plants
  (Pisum sativum).

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The science of genetics

• Mendel studied pea flower, seed pod color, and
  seed shape.
• Did careful pollination work counted offspring.
• Ex he mated plants with white flowers to plants
  with purple flowers, or plants with wrinkled
  seeds to plants with smooth seeds (the parental
  generation, or P).

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The science of genetics

• Mendels work with peas
• Mendel found that plants of the first
  generation (F1) were often identical.
• Ex all had purple flowers or
  all had yellow seeds. ??
• But mating the first generation (F1) plants
  gave a 31 ratio in the 2nd generation (F2).
• F stands
• for filial (son)
• F1 generation

F1 generation
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Mendels Law of Segregation

• A pair of allelic genes for a particular
  character, like eye color, separate (segregate)
  in equal ratios during gamete formation.
• During meiosis, alleles are separated on a 11
  ratio into sperm and eggs
• Ex if the genotype Gg, half the sperm must
  contain G and the other half must contain g.

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Punnet grids

• To make a Punnet grid
• Determine the geno- type of the
  parents.
• Make every possible
  combination of gamete.
• Combine the sperm
  and the egg.
• Determine the phenotypes from
  the genotypes of
  the offspring.

A monohybrid cross one locus is considered
Male parent Female parent
Sperm
Eggs
P represents the gene that makes the purple
flower pigment. The allele p is mutated, and the
protein is defective. All F1 offspring are Pp and
purple.
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Punnet grids
F1 generation

• To make a Punnet grid
• Determine the geno- type of the
  parents.
• Make every possible
  combination of gamete.
• Combine the sperm
  and the egg.
• Determine the phenotypes from
  the genotypes of
  the offspring.

F2 generation
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Punnet grids

• Incomplete dominance
• Ex red and white snapdragon flowers
• Blending of color
• A 121 ratio of
  offspring

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Test cross

• To test an unknown individual
• Testing a suspected heterozygote by
  crossing it with a known homozygous
  recessive individual.
• 1 of 2 possible outcomes
  shows the genotype of
  the unknown
  parent.

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Objectives

• 4.3.3 State that some genes have more than two
  alleles (multiple alleles).
• 4.3.4 Describe ABO blood groups as an example
  of codominance and multiple alleles.

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Mendels Law of Independent Assortment

• Alleles of genes on nonhomologous chromosomes
  assort independently during meiosis.
• All blonds do not have blue eyes.
• Hair color eye color are on different
  chromosomes.

Chromosomes are shuffled.
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Punnet grids

• To make a dihybrid cross
• (crossing 2 pairs of genes)
• From parents genotypes,
  determine gametes.

every possible combination
Make every possible combination of
gametes. (independent assortment)
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Punnet grids

• To make a dihybrid cross
• From parents genotypes, determine gametes.
• Combine the sperm and eggs, and determine
  phenotypes.

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Multiple alleles

• Codominant alleles
• Pairs of alleles that both affect the
  phenotype when present in a
  heterozygote.
• Ex Blood groups
• A, B, O are alleles that code
  for a sig- nal
  protein on the
  membrane surface. O is
  recessive, but A B
  codominant.

Punnet square
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Multiple alleles

• Some genes have more than two alleles.
• ABO blood group shows codominance of multiple
  alleles (IA and IB are codominant i is
  recessive).

O (ii) is the universal donor AB accepts any
blood.
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Punnet square

• Punnet square for blood groups
• I is dominant, but there are 2 types IA IB
• i is recessive.

Here, a mother with blood group O mates with a
father whos AB.
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Punnet square

• Punnet square for sex-linked traits
• For genes found on a sex chromosome, X or Y.
• Remember, boys have only one X chromosome, so
  they are more likely to get these diseases, like
  color-blindness or hemophilia.

Notice, 50 of babies are boys, and 50 are
girls. Girls get 2 Xs, so they are likely to
have a good back-up copy and dont get affected.
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Multiple alleles

• Polygenic inheritance
• Two alleles on each of three genes have
  
  an additive effect
  on skin
  color.

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Genetic diseases

• Sickle-cell anemia
• The gene for hemoglobin is mutated.
• An individual with two copies of the recessive
  allele cant move oxygen around the body well.

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Genetic diseases

• Sickle-cell anemia
• A carrier is an individual that has one copy of a
  recessive allele that causes a genetic disease in
  individuals that are homozygous for this allele.
• Carrier genotype Hhs - resistant to malaria

The gene for hemo- globin (H) is mutated.
Both Hhs
HH
hshs
Hhs
Homozygous recessive individuals often die young.
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Sex-linkage and Pedigrees
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Diseases linked to sex chromosomes

• Sex-linked traits genes are on the sex
  chromosomes
• XX individuals are female
• XY individuals are male.

Theoretically 5050 malefemale. Actually 5149
malefemale at birth then 5050 by age 3 (boys
weaker).
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Diseases linked to sex chromosomes

• Color-blindness is caused by a defective gene for
  a pig- ment receptor in the eye. Gene is on X
  chromosome.
• Boys only get 1 X, so they are more likely than
  girls to get this problem.

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Diseases linked to sex chromosomes

• Color-blindness is caused by a defective gene for
  a pig- ment receptor in the eye. Gene is on X
  chromosome.
• Boys only get 1 X, so they are more likely than
  girls to get this problem.

Retina of the eye
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Diseases linked to sex chromosomes

• Hemophilia a genetic disease in which the body
  does not produce sufficient amounts of a clotting
  factor. As a result, fibrin (necessary to
  maintain the blood clot) does not form, so the
  individual is more likely
  to bleed long- er from a
  wound, and to bleed
  internally.
• The gene for this clotting factor is on the
  X chromo- some.

Boys have no back-up copy of the X.
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Genetic diseases - sex linkage

• Hemophilia genotypic phenotypic ratios

The female carrier is heterozygous
Note the daughters can be either
heterozygous for sex-linked diseases or
homozygous if a carrier mother marries a diseased
man. 1 in 10,000 males is affected with
hemophilia, and 1 in 100,000,000
females.
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Genetic diseases

• Hemophilia A case study
• Alexei, tsarevich of Russia inherited the
  disease from his mother.

The original mutation in this lineage
occurred in Queen Victoria.
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Pedigrees

• X-linked dominant trait passed from the father
• Only his daughters get the disease.

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Pedigrees

• Y-linked trait (carried on the Y chromosome).
• Only boys get the disease.

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Karyotype

• A karyotype is a picture of the bodys
  chromosomes
• It shows abnormalities, also the individuals sex.

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Karyotype

• A karyotype is a picture of the bodys
  chromosomes
• It shows abnormalities, also the individuals
  sex.
• Down Syndrome three copies of chromosome 21.