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Patterns of Inheritance Chapter 14, 15

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Patterns of Inheritance Chapter 14, 15 Mendelian Genetics and its Extensions Mendel,Garden Peas, and Heredity Pre-Mendelian theory of hereditary: Blending theory ... – PowerPoint PPT presentation

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Title: Patterns of Inheritance Chapter 14, 15


1
Patterns of Inheritance Chapter 14, 15
  • Mendelian Genetics and its Extensions

2
Mendel,Garden Peas, and Heredity
  • Pre-Mendelian theory of hereditary Blending
    theory
  • Mendelian theory of hereditary Particulate
    theory
  • Law of Segregation/Law of Dominance
  • Law of Independent Assortment

3
Monohybrid Cross on Flower Color
4
Genotype versus phenotype
5
The Results of Mendels F1 Crosses for Seven
Characters in Pea Plants
6
Alleles, alternative versions of a gene
7
Mendels Law of Segregation/Law of Dominance
Alternative forms of genes (alleles) are
responsible for variations in phenotypes For
each character, an organism inherits two alleles,
one from each parent (maternal and paternal) If
the two alleles differ, one is fully expressed
(dominant allele) the other is completely masked
(recessive allele) The two alleles for each
character segregate during gamete production
(meiosis)
8
Testcross
  • To determine whether an organism with a dominant
    phenotype is homozygous or heterozygous, you use
    a testcross

9
Testing Two Hypotheses for Segregation in a
Dihybrid Cross
10
Mendels Lucky Choices of Characters in Garden
Peas
1) Each character is determined by one gene 2)
Each gene has only two alleles 3) One allele is
completely dominant over the other 4) In dihybrid
crosses, the two genes (seed color and seed
shape) are located on different pairs of
chromosomes
11
Theory of Inheritance
Blending Theory
Particulate Theory (discrete heritable factor)
  • Chromosomal Theory
  • Heritable factors are located on chromosomes
  • It is the chromosomes that segregate and
    independently assort

12
1860s Mendel proposed that discrete inherited
factors segregate and assort independently during
gamete formation
1875 cytologists worked out process of
mitosis 1890 cytologists worked out process of
meiosis
  • 1902 Cytology and genetics converged as Sutton,
    Boveri and others noticed parallels between the
    behavior of Mendels factors and the behavior of
    chromosomes
  • chromosomes and genes are both paired in diploid
    cells
  • homologous chromosomes separate and allele pairs
    segregate during meiosis
  • fertilization restores the paired condition for
    both chromosomes and genes

Genes are located on Chromosomes
13
Comparing chromosome segregation and allele pairs
segregation
14
Chromosomal Theory of Inheritance
  • Mendelian genes have specific loci on
    chromosomes, and it is the chromosomes that
    undergo segregation and independent assortment
  • Theory of segregation
  • diploid cells have pairs of genes, on pairs of
    homologous chromosomes. During meiosis, the two
    genes of each pair segregates from each other,
    and end up in different gamete
  • Theory of independent assortment
  • by the end of meiosis, genes on different pairs
    of homologous chromosomes are independently
    distributed to gametes

15
Extending Mendelian Genetics
  • Incomplete dominance
  • Codominance
  • Multiple alleles for one gene
  • Polygenic inheritance multiple gene effect
  • Pleiotropy multiple effects of a single gene
  • Environmental impact
  • Linked genes
  • genetic recombination between linked genes
  • sex determination and sex-linked genes

16
Incomplete Dominance
  • One allele is not completely dominant over the
    other, so the heterozygote has a phenotype that
    is intermediate between the phenotypes of the two
    homozygotes
  • It is not support for the blending theory of
    inheritance, because

17
Human ABO Blood Types
  • Multiple Alleles
  • more than two alternative forms of a gene (IA,
    IB, i)
  • Codominance
  • Inheritance characterized by full expression of
    both alleles in the heterozygote (IAIB)

18
Complete dominance
Expression of dominant allele masks the
expression of recessive allele
Incomplete Dominance
Intermediate phenotype in heterozygote
Full expression of both alleles in heterozygote
Codominance
19
Pleiotropic effects of the sickle-cell allele in
a homozygote
20
Polygenic Inheritance
  • Two or more genes determine a single phenotypic
    character
  • commonly seen in quantitative characters, e.g.
    skin color
  • Environmental effects could also affect the
    phenotype

21
Flower Color depends on acidity
22
Extending Mendelian Genetics
  • Incomplete dominance
  • Codominance
  • Multiple alleles for one gene
  • Polygenic inheritance continuous variation
  • Pleiotropy multiple effects of a single gene
  • Environmental impact
  • Linked genes
  • genetic recombination through crossing over
  • sex determination and sex-linked genes

23
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24
Recombination due to crossing over
25
Recombination Frequency and Genetic Mapping
26
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27
An example of Sex Linked Genes
XRXr
XRY
Carrier
XrY
XRXR or XRXr
28
Human Genetics Follows Mendelian Principles
  • Recessively Inherited Disorders
  • cystic fibrosis
  • sickle-cell disease
  • Tay-Sachs disease
  • Dominantly Inherited Disorders
  • Huntingtons disease
  • Multifactorial Disorders
  • cancer
  • diabetes
  • heart disease
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