Patterns of Inheritance

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Patterns of Inheritance

• Chapter 16

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Why Study Mendel?

• Mendel was the first to demonstrate the
  principles of inheritance
• He used a systematic scheme based upon
  mathematics and statistics before formal
  statistics was developed
• He applied his knowledge of mathematics with an
  excellent scientific method

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Mendel Disproved Blending

• The age-old way of thinking of inheritance was
  the concept of the blending of characters
  (characteristics)
• We look somewhat like our parents
• Breeders would combine desired characteristics in
  crosses of domestic animals
• The thinking was that characteristics were merely
  added together blending in the new
  generation.
• But, blending of characters is incorrect
• No one studied the problem systematically until
  Gregor Mendel (1822-1884)

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What Did Mendel Show?

• He applied the scientific method combined with
  mathematical analysis to demonstrate
• Single characteristics
• Segregation of those characteristics
• Dominance of some characteristics
• Independent assortment of characteristics
• Today, we substitute the term gene for
  characteristics

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Mendel Carefully Chose His Organism

• Pisum sativum the garden pea
• Very productive produces many peas (large N
  good statistics)
• Short life cycle produce many generations in a
  short time
• Easy to control pollination (fertilization)

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• Has 7 distinct phenotypic characteristics
• Yellow versus green seeds
• Round versus wrinkled seeds
• Green versus yellow pods
• Tall versus short plants
• Fat versus tight pods
• White versus grey seed coats
• Flowers terminal vs. axial

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Crosses Produce Progeny

• Definitions. We use specific symbolic notation
  to represent specific crosses
• P represents the parental generation of true
  breeders
• F1 represents the the 1st filial generation the
  progeny of crossing true-breeding P generation
  individuals that had varied in one character.
• F2 represents the the 2nd filial generation,
  derived from crossing the F1 generation to itself
  at random

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Mendels Crosses

• Mendel knew nothing of the chemical basis for
  inheritance
• He had to work very carefully and examine ONLY
  the outward appearance of the plants
• He compared what we call today the phenotype,
  which is the outward expression of the genes

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Dominant and Recessive Traits

• Tall X Short
• All of the F1 generation are tall.
• Demonstrates the concept of dominance
• The dwarf plant has the recessive trait

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• The recessive (Dwarf) trait reappears in the F2
  generation
• It was masked or hidden in the F1 generation
• Mendel recognized that the only way to achieve
  the masking of the characteristics was to
  segregate (separate) them somehow in the gametes.
• The characters must have been separated during
  formation of the pollen (male) and the ova
  (female).
• This is the Principle of Segregation.

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Genotypes Genes and Alleles

• The genetic make-up of an organism is its
  genotype
• Every organism is diploid
• Each trait is designated by a different letter
• DOMINANT alleles ARE CAPITAL LETTERS
• Recessive alleles are small caps.

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An Allele is a variation of a Gene
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Homozygous and Heterozygous Describe the Genotype
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The Principle of Segregation

• Diploid chromosomes of parents undergo meiosis
• Alleles separate as they become haploid egg or
  sperm
• They are combined again during fertilization to
  produce a diploid offspring

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The Law of Segregation

• Mendel recognized that factors were transmitted
  from parent to offspring.
• The idea that the two copies of a gene separate
  from each other during transmission from parent
  to offspring is known as the
• Law of Segregation
• Today we know that the factors are genes and the
  different versions of genes are called alleles

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Chromosomal Basis of Segregation

• Segregation occurs in Meiosis I
• As a result, the ova or sperm contribute
  different chromosomes to the progeny

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Punnet Squares Can be Used to Predict the Outcome
of Genetic Cross
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Two-factor or Dihybrid Crosses

• If we consider what happens to two genes on
  different chromosomes at the same time, we are
  describing a dihybrid cross
• We discover that the two traits separate from
  each other, resulting in offspring with
  combinations of traits.
• The dihybrid cross is strong evidence for
  independent assortment (i.e. the genes recombine
  without influence from each other).

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• Independent Assortment
• Analyzing a 2-factor cross demonstrated the law
  of independent assortment
• Meiosis I results in the independent assortment
  of chromosomes and genes into the gametes.
• The homologous chromosomes are arranged at random
  at the equatorial plane in Metaphase I
• This results in four different possible
  combinations of chromosomes and genes in the
  gametes, in a 1111 ratio

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The Chromosomal Basis of Independent Assortment
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Pedigree Analysis Examines the Inheritance of
Human Traits

• Pedigrees are records of phenotypes of
  individuals in each generation
• Trace inheritance patterns through multiple
  generations
• Useful for single gene traits and genetic
  diseases due to single genes
• Good for predicting the type of inheritance of
  the trait in question
• Useful to clinicians attempting to provide
  genetic counseling

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• Its important to establish how a trait is
  inherited
• Pregnancy outcome
• Adult onset disorders
• Recurrence risks in future offspring
• Based on the Principles of Segregation and
  Independent Assortment in humans
• Can determine whether traits are
• Dominant or recessive
• Autosomal or sex-linked
• Trait controlled by a single gene or many genes

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Rules for Generating a Pedigree

• Successive generations marked by Roman numerals
• Individuals in each generation are marked by
  Arabic numerals
• Male/female
• Unaffected individual
• Affected individual

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Cystic Fibrosis is an Autosomal Recessive Trait

• Individuals with two copies of the faulty
  chloride ion channel gene have the disease
• Individuals can be carriers carry only one copy
  of the faulty gene
• The recessive trait can be hidden by carriers
  and show up in the next generation

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Pedigree of an Autosomal Dominant Trait

• Huntingtons disease is an autosomal dominant
  trait
• It only take one copy of the Huntington gene to
  have the disease
• Some individuals in every generation are affected
  regardless of sex

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Sex Chromosomes and X-Linked Inheritance

• The pattern of inheritance still follows Mendels
  predictions
• Genes that are found on the X chromosome.
• Hemophilia
• Color blindness
• Males are more often affected than females
  because they inherit only one X chromosome

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Sex Chromosome Patterns in Animals FYI
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Variations on a theme Non-Mendelian Traits

• When the pattern of inheritance is different from
  expected
• Incomplete dominance
• Co-dominance
• Polygenic traits

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• Incomplete Dominance
• Heterozygotes express an intermediate phenotype
• Still NOT blending inheritance
• Codominance
• Heterozygote displays characteristics of two
  dominant alleles
• ABO blood typing several red blood cell antigens
  can occur on cell surface

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• Incomplete Dominance
• Four OClock flowers
• Heterozygotes express an intermediate phenotype
• Flower color traits segregate and can be seen in
  the F2 generation
• Still NOT blending inheritance

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Co-Dominance

• The inheritance of ABO blood groups demonstrates
  co-dominant inheritance
• Two alleles are dominant (Type A and Type B)
• Both phenotypes are expressed in the heterozygote
  (Type AB)

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3. Polygenic Traits

• Polygenic inheritance refers to the control of
  one trait by more than one gene
• Skin color is controlled by more than 2 genes
• The hallmark of a polygenic trait is
• A bell curve distribution
• A continuous distribution

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Skin Color Distribution

• Follows expected frequency distribution for a
  polygenic trait
• There are at least 6 alleles or genes that
  determine skin color
• Each gene product is additive to other gene
  products

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Height in Humans

• gt10 loci are involved in height inheritance in
  humans
• As expected, the distribution of height follows a
  normal distribution

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The Bell Curve