Mendelian Genetics

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Mendelian Genetics
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Blending Hypothesis

• Used in early 1800s to explain how offspring
  inherit traits from parents.
• A yellow flower plant crossed with a red flower
  plant will produce plants that have orange
  flowers
• Orange flower plants will produce orange flower
  plants
• But there were many exceptions like a red flower
  plant that produced yellow flowered offspring

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Gregor Mendel

• Gregor Mendel was an Austrian monk who is often
  called the "father of genetics" for his study of
  the inheritance of traits in pea plants.
• Between 1856 and 1863 Mendel cultivated and
  tested some 28,000 pea plants.
• He was the first person to predict how traits are
  transferred from one generation to the next.
• Applied an experimental approach to studying
  genetics.
• He studied only one trait at a time to control
  variables, analyzed data mathematically.
  Statistics.

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Why Pea plants?

• Pea Plants have a short life cycle.
• Pea plants self fertilized their own flowers.
• They are easy to see and recognize the traits.

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Mendels 7 pea plant traits

• Flower color
• Flower position
• Seed color
• Seed shape
• Pod shape
• Pod color
• Stem length

Trait a specific inherited characteristic that
can vary in individuals
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True breeding Pea plants

• Because pea plants are self pollinating they
  inherit all their characteristics from their
  single parent. Mendel noticed that tall plants
  always produced seeds that grew to be tall and
  that short plants always produced seeds that
  produced short plants. All yellow seed plants
  produced plants with yellow seeds. In other
  words all his plants were true-breeding
  
  
  
  

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Mendels experiment

• Mendel cut off the male flower reproductive parts
  to prevent self pollination.
• He then dusted pollen from another plant onto the
  flower cross pollination.
• Produced seeds that had 2 different parents.

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• http//www.cactus-art.biz/note-book/Dictionary/aaa
  _Dictionary_pictures/cross_pollination.jpg

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• Each of the seven pea plant traits had two
  contrasting characters.
• Flower color purple or white
• Flower position axial or terminal
• Seed color yellow or green
• Seed shape round or wrinkled
• Pod shape inflated or constricted
• Pod color yellow or green
• Stem length tall or short (dwarf)
• Mendel crossed plants with the contrasting
  traits.
• Monohybrid crosses study one trait at a time

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• When Mendel crossed a purple and a white flower
  he always got a purple flower for the offspring

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• Mendel called each of the original pairs of
  plants the parental (P) generation.
• The offspring from these crosses were known as
  the F1 generation. (F for filial, Latin for son
  or daughter)
• Mendel noticed that all the offspring only had
  the character of one of the parents. The
  character of the other parent seemed to disappear.

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Mendels 1st Principle

• The law of unit factors - each organism has two
  factors for each of its traits
• We now know these factors are genes located on
  the chromosomes.
• These different gene forms are called alleles.
• Alleles are located in different copies of a
  chromosome one from ? parent, one from ? parent

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• Gene is the chemical factor that determines
  traits. Flower color gene
• Alleles are the different forms of a gene.
• purple flower allele or white flower allele

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Mendels 2nd Principle
The law of dominance - only one factor was
visible in the F1 generation, the dominant
trait The dominant trait is the visible or
observable trait. The recessive trait is the
hidden trait, masked by the dominant trait.

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• Mendel allowed the F1 generation to self
  pollinate and planted the resulting seeds
• When the F2 plants grew he observed that about
  three-fourths of the F2 plants had the dominant
  trait and one-fourth had the recessive trait

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Mendels 3rd Principle

• Mendel concluded that the paired factors separate
  during the formation of gametes and that when 2
  gametes combine during fertilization the
  offspring have two factors for each trait. This
  is called the Law of Segregation a pair of
  factors is separated or segregated during the
  formation of gametes

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• Mendel crossed plants that differed in two traits
  such as flower color and seed color. His data
  showed that the dominant traits do not have to
  appear together. A yellow seed (dominant) could
  appear in a plant with wrinkled seeds
  (recessive). Mendel concluded that factors for
  individual characteristics are not connected.

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Mendels 4th Principle

• Factors for different traits separate
  independently of one another during the formation
  of gametes. This is called the Law of
  independent assortment.

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Mendel and Modern Genetics

• Most of what Mendel concluded agrees with what we
  now know about molecular genetics. Molecular
  genetics is the study and function of chromosomes
  and genes.
• Mendels Law of independent assortment is
  supported by the independent assortment of
  chromosomes to gametes during meiosis.

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Genotype

• An organisms genetic makeup is its genotype.
  The genotype consists of alleles that the
  organism inherits from both parents.
• Letters are used to represent alleles. Capital
  letters refer to dominant alleles and lowercase
  letters refer to recessive alleles.
• The actual letter used to selected to represent
  an allele is typically the first letter of the
  dominant trait. Like P for purple flower and p
  for white flower.

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Genotype vs. Phenotype

• The genotype for a plant with purple flowers
  maybe PP or Pp because the P allele is dominant.
• The genotype for a plant with white flowers an
  only be pp.
• An organisms appearance is its phenotype. The
  phenotype of a PP or a Pp plant is purple
  flowers.
• The phenotype of a plant with pp is white flowers.

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Homozygous Vs Heterozygous

• When both alleles of a pair are alike the
  organism is homozygous.
• PP homozygous dominant
• pp homozygous recessive
• When both alleles of a pair are different the
  organism is heterozygous
• like Pp

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Probability

• Probability is the likelihood of that a specific
  event will occur.

Probability can be expressed a a percentage, a
decimal or a fraction eg. 50, 0.50 or 1/2
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Probability

• In Mendels experiment the dominant trait of
  yellow seed color appeared in the F2generation
  6,022 times and the green seed color appeared
  2,001 times. The total number of individuals was
  8,023 (6,022 2,001).
• The probability that the dominant yellow seed
  color will appear is

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Punnett Squares

• The gene combinations that might result from a
  genetic cross can be determined by drawing a
  diagram known as a Punnett square. Punnett
  squares can be used to predict and compare the
  genetic variations that will result from a cross.
• The types of gametes produced by each parent are
  shown along the top (for males) and along the
  left side (for females). The possible gene
  combinations for the offspring, appear in the
  four boxes that make up the square

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Homozygous dominant vs Homozygous recessive TT
(tall) plant vs tt (short) plant 100
heterozygous (Tt) offspring
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Homozygous dominant vs Heterozygous TT (tall)
plant vs Tt (tall) plant 50 heterozygous (Tt)
offspring 50 homozygous dominant (TT)
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Heterozgous vs HeterozygousTt (tall) plant vs Tt
(tall) plant 25 TT (tall), 50 Tt (tall), and
25 tt (short) 3 tall 1 short phenotypic ratio
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Test cross

• How can you determine whether on organism with a
  dominant trait is homozygous dominant or
  heterozygous? Perform a test cross with a
  homozygous recessive organism.

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TestcrossIf the organism is homozygous dominant
then a test cross with a homozygous recessive
will produce all heterozygous offspring, 100
tall
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TestcrossIf the organism is heterozygous then a
test cross with a homozygous recessive will
produce 50 heterozygous dominant and 50
homozygous recessive heterozygous offspring, 50
tall 50 short phenotypes
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Dihybrid crosses

• Crossing organisms with 2 different traits
• The principle of independent assortment states
  that during gamete formation any allele for one
  trait can be paired with either allele for
  another trait
• Punnett squares with 16 blocks can be used to
  determine the probability of the offspring.

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Dihybrid Crossheterozygous round yellow seed
(RrYy) plant with a another heterozygous round
yellow seed (RrYy) plant
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Intermediate Inheritanceincomplete dominance

• Seen in many flower species
• four oclocks
• impatients
• snapdragons
• Also seen in Andalusian chickens

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Intermediate Inheritance

• Neither allele is dominant
• Andalusian chickens have 3 phenotypes Black,
  White, Blues
• A C for color is used because neither the black
  or white allele is dominant
• CB is the black allele
• CW is the white allele

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All F1 offspring have blue phenotype and are
heterozygous CBCW
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50 of F2 offspring are heterozygous and have the
blue phenotype 25 are homozygous black 25
are homozygous white
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CRCR (red) x CWCW (white) ? CRCW (pink)
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Multiple Alleles

• For many genes several alleles exist. Like blood
  type. A carbohydrate chain seen on the membrane
  of Red Blood cells.
• IA-allele carbohydrate A on red blood cells.
• IB-allele carbohydrate B on red blood cells
• i-neither allele recessive to both IA IB
• Both IA IB are dominant and are referred to as
  codominant because both traits are expressed in
  individuals with bothe A and B alleles

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Blood Types

• Type A blood can have the possible genotypes IAIA
  homozygous or IAi heterozygous.
• Type B blood IBIB or IBi
• Type AB blood IAIB
• Type O blood ii

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Type B
Type A
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Polygenetic Inheritance

• Many traits are affected by multiple genes
• Like height, skin color, intelligence, and
  personality
• Example suppose genes A, B C which are dominant
  control dark skin pigmentation. Genes a, b c
  are recessive and control light pigmentation.
  There would be 64 possible combinations. AABBCC
  has the maximum amount of melanin and very dark
  skin. aabbcc has the lowest amount of melanin and
  very light skin. A person with AaBbCc would have
  an intermediate skin color

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Poylgenetic Inheritance

• When there is a lot of variation in a trait like
  skin color, or height than it is usually
  polygenetic Inheritance and multiple genes are
  responsible for the trait.

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Genes and the Environment

• An individuals genotype can depend on the
  environment and their genes.
• Nutrition, exercise, sunlight, illness can result
  in changes in phenotype.
• Siamese cats and Himalayan rabbits coat color is
  affected by temperature. Colder regions of their
  bodies have darker coat color.
• Bothe genotype and environmental factors can play
  a role in phenotype

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Meiosis and Mendel

• Chromosome theory of Inheritance states that
  genes are on chromosomes and the behavior of
  chromosomes during meiosis and fertilization
  accounts for inheritance patterns.
• Chromosomes undergo segregation and independent
  assortment during meiosis and support Mendels
  two principles of the same name.

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Crossing over

• During meiosis and prophase I homologous
  chromosomes pair up and genetic information can
  be exchanged between chromatids. The alleles for
  a gene for a particular trait reside at the same
  location called the gene loci. If an individual
  has the same alleles on both homologous
  chromosomes at the gene loci then they are
  considered homozygous and heterozygous if the
  alleles at a gene loci are different.

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Genetic linkage and crossing over

• The only way alleles on the same chromosome could
  assort themselves independently is if crossing
  over occurs in meiosis and separates them.
• The tendency for alleles on one chromosome to be
  inherited together is called genetic linkage.
  The closer two genes are on a chromosome the
  greater the genetic linkage. The farther apart
  the genes are on the chromosome the more likely
  that a crossover event will separate them.

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Sex-Linked Traits

• Genes located on sex chromosomes are sex-linked
  genes.
• Thomas Hunt Morgan studied fruit flies. Most
  fruit flies have red eyes, white eyes are very
  rare.
• He crossed a white eyed male with a red eyed
  female all offspring were red eyed ? red eyes
  were dominant.

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• When Morgan bred the F1 offspring he got a 31
  ratio for phenotype red eyes white eyes but all
  the white eyes were always male.

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Sex linked disorders

• Hemophilia
• Re-Green Color blindness
• Duchennes Muscular Dystrophy
• All are more common in males because they are
  carried on the X chromosome and not on the Y
  chromosome. Because males only get one X
  chromosome if they get they recessive gene they
  dont have another X chromosome.

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Hemophilianormal father carrier mother