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Theoretical Genetics

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Theoretical Genetics Gregor Mendel Genetic diseases - sex linkage Hemophilia: genotypic & phenotypic ratios *Note: the daughters can be either heterozygous for sex ... – PowerPoint PPT presentation

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Title: Theoretical Genetics


1
Theoretical Genetics
Gregor Mendel
2
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.

3
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.
5
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
6
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
7
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
8
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
9
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.

10
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).

11
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).

12
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
13
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.

14
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.
15
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
16
Punnet grids
  • Incomplete dominance
  • Ex red and white snapdragon flowers
  • Blending of color
  • A 121 ratio of
    offspring

17
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.

18
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.

19
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.
20
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)
21
Punnet grids
  • To make a dihybrid cross
  • From parents genotypes, determine gametes.
  • Combine the sperm and eggs, and determine
    phenotypes.

22
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
23
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.
24
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.
25
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.
26
Multiple alleles
  • Polygenic inheritance
  • Two alleles on each of three genes have

    an additive effect
    on skin
    color.

27
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.

28
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.
29
Sex-linkage and Pedigrees
30
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).
31
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.

32
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
33
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.
34
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.
35
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.
36
Pedigrees
  • X-linked dominant trait passed from the father
  • Only his daughters get the disease.

37
Pedigrees
  • Y-linked trait (carried on the Y chromosome).
  • Only boys get the disease.

38
Karyotype
  • A karyotype is a picture of the bodys
    chromosomes
  • It shows abnormalities, also the individuals sex.

39
Karyotype
  • A karyotype is a picture of the bodys
    chromosomes
  • It shows abnormalities, also the individuals
    sex.
  • Down Syndrome three copies of chromosome 21.
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