Patterns of Inheritance

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Patterns of Inheritance

• Chapter 9

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Gregor Mendel

• Deduced the fundamental principles of genetics
• Cross-fertilization

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Whats with the Peas?

• Little spontaneous variation between generations
• Can self-fertilize
• Easy to control pollination
• Possessed several easily observable traits
• Pea form
• Pea color
• Flower location
• Flower color
• Stem size

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What do we get??

• Genetic cross
• Bb x Bb
• P generation
• F1 generation
• F2 generation

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Monohybrid Cross

• Cross between parent plants that differ in only
  one characteristic
• Mendel developed four hypotheses from the
  monohybrid cross
• There are alternative forms of genes
• Alleles
• For each characteristic, an organism inherits two
  alleles
• One from each parent
• Alleles can be dominant or recessive
• Gametes carry only one allele for each inherited
  characteristic

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Mendels Laws

• Genes
• Set of instructions that determine
  characteristics of an organism
• Segments of nucleic acid that specifies a trait
• Found at designated place on chromosomes
• Locus
• Not all copies of a gene are identical

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Mendels Laws

• Alternative forms of a gene lead to the
  alternative form of a trait
• Alleles
• way of identifying the two members of a gene pair
  which produce opposite contrasting phenotypes
• Chromosomes that are homologous are members of a
  pair and carry genes for the same traits in the
  same order

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Genes v. alleles

• Genes
• Basic instruction
• Sequence of DNA
• General
• Hair color

• Alleles
• Variations of that instruction
• Specifics
• Brown hair

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Alternative alleles of genes are located on
homologous chromosomes
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Genotype verse Phenotype

• Genotype
• the alleles an individual receives at
  fertilization
• Homozygous
• an organism has two identical alleles at a gene
  locus
• Heterozygous
• an organism has two different alleles at a gene
  locus
• Phenotype
• the physical appearance of the individual

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The journey from DNA to phenotype
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Mendel Proposes a Theory

• genetic traits are assigned a letter symbol
  referring to their more common form
• Dominant traits are capitalized while a
  lower-case letter is reserved for the recessive
  trait
• P signifies purple
• p signifies white

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Describing Genotypes

• Homozygous Dominant
• when both alleles are dominant
• BB
• Homozygous Recessive
• when both alleles are recessive
• bb
• Heterozygous
• when one allele is dominant and one is recessive
• Bb

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Punnet Square..


Genetic cross determines arrangement
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Variations on Mendels Laws

1. Incomplete dominance
2. Multiple allelism codominance
3. Pleiotropy
4. Polygenic inheritance
5. Environmental factors

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Why Some Traits Dont Show Mendelian Inheritance

• Incomplete dominance
• Alleles have combined (equal) effect on phenotype
  of heterozygote
• Phenotype is intermediate

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Multiple Allelism existence of more than 2
alleles of gene
Example Blood type (A, B, O)
Remember Each person still only has 2 alleles
for that trait, but more than 2 exist
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Multiple Allelism Blood typing
ABO Blood Type in Humans exhibits multiple
allelism
Phenotype Genotype O OO
A AA or AO B BB or BO AB
AB
How many ALLELES are there?
3 ( A, B, O)
How many Phenotypes are there?
4 (A, B, AB, O)
How many Genotypes are there?
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Question If a woman with blood type O marries a
man with blood type B, can they have a child with
blood type A?
Phenotype Genotype O OO
A AA or AO B BB or BO AB AB
No. The mothers genotype must be OO and the
fathers either BB or BO. Their child will either
be type B (BO) or type O (OO)
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Why Some Traits Dont Show Mendelian Inheritance

• Codominance
• A gene may have more than two alleles in a
  population
• in heterozygotes, there is not a dominant allele
• Both alleles are expressed
• Relationship between the A B alleles in blood
  typing

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Why Some Traits Dont Show Mendelian Inheritance

• Pleiotropic effects
• Allele that has more than one effect on a
  phenotype
• these effects are characteristic of many
  inherited disorders
• Sickle-cell anemia
• Must be homozygous for sickle cell allele

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Why Some Traits Dont Show Mendelian Inheritance

• Continuous variation
• Characters can show a range of small differences
  when multiple genes act jointly to influence a
  character
• Polygenic

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Phenotypes are not always a direct translation of
genotype
Phenotypes may also be influenced by the
environment

• Examples?
• skin color influenced by sun
• height/weight influenced by nutrition
• animal coat influenced by climate

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Remember.. P G E
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Sex-linked Traits
Female XX
Male XY
Genes located on the X or Y chromosome are
sex-linked
X and Y chromosomes are not homologous, they
contain different genes
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Sex-linked traits

• Sex chromosomes
• Are designated X and Y
• Determine an individuals sex
• Influence the inheritance of certain traits
• Sex-linked genes
• Are any genes located on a sex chromosome

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Sex-Linked Traits
Females (XX) have 2 copies of each gene on the X
chromosome Males (XY) have only 1 copy of each
gene on the X chromosome
Females can show a dominant condition if present
on 1 or both X chromosomes
Females can only show a recessive condition if
present on both X chromosomes
Males ALWAYS show X-linked alleles, regardless of
dominance
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Sex-Linked Disorders in Humans

• number of human conditions result from sex-linked
  (X-linked) genes
• Red-green color blindness
• characterized by a malfunction of light-sensitive
  cells in the eyes

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Question..

• Will a mother that is colorblind automatically
  have a son that is colorblind?

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Mutations

• Changes to the nucleotide sequence of the genetic
  material of an organism
• Can be caused by
• copying errors in the genetic material during
  cell division
• exposure to UV light or chemical mutagens
• Viruses
• can be induced by the organism itself
• Create variety within gene pool
• Less favorable verse more favorable

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Human Heredity

• To study human heredity, scientists examine
  crosses that have already been made
• Pedigree
• Determine whether a trait is sex-linked or
  autosomal and whether the traits phenotype is
  dominant or recessive

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Huntingtons disease is a dominant genetic
disorder
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Recessive Disorders

• Most human genetic disorders are recessive
• Individuals can be carriers of these diseases