Patterns of Inheritance

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Chapter 9

• Patterns of Inheritance

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Heritable Variations and Patterns of Inheritance

• 1860 Gregor Mendel analyzed inheritable patterns
  and deduced the fundamental laws of genetics
  using garden peas
• studied flower color and seed shape
• Inheritance follows a few simple rules
• Parents pass on to offspring to discrete
  heritable factors which we now know as genes
• retain individual identities generation to
  generation no matter what

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Peas

• Easy to grow with distinguishable varieties
• Could control matings
• The egg is within the carpel and the sperm is the
  stamen
• peas are self-fertilizing
• sperm falls on carpel and move to the egg and
  fertilize it
• Mendel controlled by placing bag over flower

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Cross-Pollination

• Mendel knew the parentage of his plants because
  he controlled the breeding
• Choose traits that had 2 distinct forms
• Worked until he had true breeding varieties
• when cross you get off-spring identical to parents

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Genetic Terms

• Hybrids offspring of 2 true-breeding parents
• Cross-fertilization of one parent by another
  parent is referred to as a cross
• Parents are P generation
• Hybrid offspring is F1 generation (F for filial)
• When F1 self-fertilize or cross-fertilize the
  offspring are F2 generation

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Traits Studied
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Mono-Hybrid Crosses

• Parents only differ in one trait

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Cross Between F1 Generation

• White flower crossed with purple flower F1 was
  all purple Was the ability to make a white
  flower lost
• F1 x F1 showed that 1 in 4 had white flowers
• White didnt disappear but purple flower was
  affecting F1 offspring
• each carried 2 factors for flower color 1 white
  and 1 purple

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4 Hypotheses Developed by Mendel

• There are alternative forms of genes that
  determine heritable traits called alleles
• Alleles are inherited 1 from each parent
• if the same (2 purple) they are homozygous
• if different (1 purple, 1 white) they are
  heterozygous
• If alleles are different and 1 determines
  appearance that allele is dominant (use capital
  letter to ID) and the other is the recessive
  allele (use lowercase letter)
• Alleles separate during meiosis so eggs or sperm
  have only one allele Law of Segregation
• fertilization reunites the alleles

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F1 Generation Cross

• Egg has a Pp genotype as F1
• Sperm has a Pp genotype as F1
• Offspring from a P egg with a P sperm or p sperm
  both will be purple flowered (PP or Pp)
• Offspring from p egg with a P sperm or p sperm
  one will be purple (Pp) and one white (pp)
• Combinations of equal likelihood
• Phenotype is 3 purple 1 white genotype is 1 PP
  2 Pp 1 pp

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More Terms

• Phenotype the physical appearance of an
  organism
• purple or white
• Genotype the genetic make-up of the organism
• PP, Pp or pp
• Law of Segregation applies to all sexually
  reproducing organisms

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Homologous Chromosomes

• Carry alleles for the same gene (alleles
  alternative forms of gene) one from each parent
• Each band is a loci specific location of genes
  on chromosomes
• genes are on the same spot on both homologues

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Dihybrid Crosses

• Also studied seed shape R round, r wrinkled
  and seed color Y yellow, y green
• Could do dihybrid cross mating of parental
  varieties differing in 2 characteristics
• Do they come together or are they independent of
  one another?

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F1 Followed by F2 Cross

• F1 cross resulted in all yellow round seeds
• Crossing the F1 offspring should have resulted in
  a 31 phenotype if they came together but that is
  not what was seen

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F2 Offspring

• Showed 4 phenotypes (YR, Yr, yR, yr) with 9
  genotypes (9331)
• Saw 2 new phenotypes wrinkled yellow and round
  green as well as parent types
• Each characteristic is like a monohybid cross
• 124 for green is same as 31 if done alone

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Law of Independent Assortment

• Each pair of alleles assorts independently of
  other pairs of alleles during gamete formation
• Inheritance of one trait has no effect on
  inheritance of another trait
• must be on separate chromosomes

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Rules of Probability

• Same as flipping a coin
• Heads vs tails is essentially dominant vs
  recessive
• What happens on one toss isnt affected by
  previous tosses
• Probability of tossing 2 heads
• 1 of 2 for first and 1 of 2 on the second
• ½ x ½ ¼

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Dominant/Recessive Traits

• Mendels laws apply to many human characteristics
  that are determined by simple dominant recessive
  inheritance
• Dominant does not mean normal or more common
• wild-type traits most common in nature is not
  necessarily specified by dominant allele
• recessive is more common in nature most dont
  have freckles

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Pedigrees

• Cant cross test humans so we go backwards and
  look at family history
• Example of inherited disease such as deafness
• People with copy of recessive allele but without
  symptoms are called carriers

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Disease Controlled by Single Gene
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Recessive Diseases

• Relatively harmless to very severe
• Most are born to parents that are carriers but
  normal phenotype
• Use the Punnett square to determine likelihood to
  have child with recessive disorder
• ¼ as ½ from each parent
• 2/3 are carriers as the one that is DD is neither
  a carrier or affected
• Most disorders are not evenly distributed among
  ethnic groups

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Not Many Affected

• Relatively unlikely that 2 people with recessive
  harmful alleles will meet and mate to make a baby
• See more often when close relatives mate
  inbreeding
• see in animal populations when trying to get
  certain characteristics but also get weak hips
  and eye problems

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Dominant Disorders

• Some are non-lethal extra fingers or toes or
  webbing between toes
• Some are more serious such as achondroplasia a
  form of dwarfism that has normal body but
  shortened arms and legs
• homozygous of this allele is usually lethal and
  so only heterozyogotes have chance of passing on
  allele
• gt99.9 of population has recessive allele

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Dominant Alleles

• Dominant alleles that cause a lethal disorder are
  really much rarer than recessive lethal
• usually results in spontaneous abortion or an
  individual that doesnt live to reproduce
• this results in elimination of the allele from
  the population

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Escape of Elimination

• Some diseases such as Huntingtons disease is not
  evident until an advanced age
• death occurs 10-20 years from onset
• by the time symptoms arise they already have
  children and 50 of them will have the disease

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Variations on Mendels Laws

• Mendels laws require discrete factors many
  characteristics cannot be explained by Mendel
• Incomplete dominance phenotype is between that
  of the parents
• red x white snapdragons yield pink flowered
  offspring
• 1 red 2 pink 1 white

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Hypercholesterolemia

• Caused by recessive allele
• Normal is HH normal cholesterol
• Heterozygous Hh 2x normal cholesterol
• Recessive hh 5x normal cholesterol
• heart attack by age 2
• Allele H is a receptor for taking up cholesterol,
  heterozygous has half as many and so increase
  cholesterol level, hh have no receptors

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ABO Blood Type

• Most genes occur in more than 2 forms multiple
  alleles
• Blood type has 3 alleles and co-dominance in
  making up blood type
• 4 phenotypes A, B, AB and O
• alleles are responsible for sugars on the surface
  of red blood cells

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Determining Blood Type

• Have antibodies to sugar not on your blood in
  your serum
• 3 alleles are IA, IB and i so have 6 possible
  genotypes
• Different alleles are co-dominant both are
  expressed in type AB
• See in cattle and horses with roan coloring (red
  and white hairs to make up roan)

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• Dont confuse co-dominance (both alleles
  expressed) with incomplete dominance (express one
  intermediate trait)

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Pleiotropy

• Many cases have 1 gene that influences several
  characteristics
• See in sickle cell disease
• make abnormal hemoglobin because of 1 amino acid
  exchanged in the protein
• many side effects in patient
• only homozygous for allele have disease
• heterozygous will make both normal and abnormal
  hemoglobin co-dominant

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Sickle Cell

• 1 in 500 African-American are homozygous, 1 in 10
  are heterozygous
• Most common in tropical Africa, in high malaria
  areas
• Malaria has part cycle in the RBCs and when have
  a sickle crisis, removed from circulation
• heterozygotes live longer with malaria and
  therefore can pass the gene on to children

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Polygenic Inheritance

• Additive effect of 2 or more genes on a single
  characteristic (opposite of pleiotropy)
• Skin color uses at least 3 genes A, B, C dark
  and a, b, c light
• AABBCC would be very dark and aabbcc would be
  very light
• AaBbCc would be in between above but the same
  color as AABbcc
• 7 levels of pigmentation

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Effects of Environment

• In real world can see more than 7 shades of
  skin color
• Based on environmental factors such as sun
  exposure
• influenced by genes and environment

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Chromosomal Basis of Inheritance

• Mitosis and meiosis were figured out after Mendel
  died
• Then made the connection between heritable
  factors and chromosomes which was the gene
• Genes are at specific spots on chromosomes and
  behavior of chromosomes during meiosis and
  fertilization
• Chromosomal Theory of Inheritance

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Mendel and Chromosomes
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Linked Genes

• Genes located near each other on same chromosomes
• tend to be inherited together and dont follow
  Mendel's Law of Independent Assortment
• Used the fruit fly to study genetics and
  discovered this

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Fly Studies

• Made observations of gray body with long wings x
  black body with short wings thought we would
  see Mendelian distribution 25 of each
  offspring
• Of 2300 flies only 17 where the non-wild type or
  non-parent
• Believed that the genes were on 2 chromosomes
  rather than 4

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Crossing Over Review
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Genetic Recombination Crossing Over

• Gray/short and black/long come from crossing over
  because if linked you wouldnt see these 2
  phenotypes new combinations of alleles
• Explains the non-parental offspring

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Recombination Frequencies

• Can calculate by adding the offspring that comes
  from recombination and divide by total offspring
• (206 185)/2300
• Can also be done with parental like offspring as
  well but not called recombinant frequency

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• All the inheritance patterns we have talked about
  deal with autosomal chromosomes but some diseases
  are linked to the sex chromosomes

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Review of Sex Determination

• Each gamete has 1 sex chromosomes and 22
  autosomes
• All eggs have 1 X and half the sperm have X and
  half have Y
• XX girl XY boy
• SRY gene on the Y chromosome triggers the
  development of testes without developing ovary
  other genes on Y play a role in sperm formation

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Sex-Linked Genes

• Also have genes that have nothing to do with
  maleness or femaleness
• Genes on X or Y are sex-linked most not related
  to gender are on X-chromosomes
• In flies red eyes are wild type and white are
  rare and on X chromosome (X-linked)

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Fly Eyes

• Mate homozygous red eye female to white eye male
• All offspring are red eyed because eye color is
  determined by X chromosome

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Additional Crosses

• Most white eyed flies are male only see a white
  eyed female when you cross a heterozygous female
  mating with white eyed male
• 22 phenotype and 1111 genotype as they are
  all have different alleles

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Color Blindness

• Red-green color blindness is more prominent in
  males than females
• Allele is on X chromosomes so males do not have
  another one to counteract the bad allele on the X

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Hemophilia

• Bleeding disorder that causes bleeding with only
  minor injury
• See in the royal families of Europe Queen
  Victoria was a carrier and so was her daughter to
  married and passed on to her daughter who
  married and gave to her son

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Muscular Dystropy

• A disease that causes weakening and loss of
  muscle tissue
• The type called Duchenne is sex-linked to the X
  chromosome (Note not all types are sex-linked)
• Almost all affected are male 1 in 3500 male
  births
• missing protein found in normal muscle tissue