Mendelian patterns of Inheritance

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Mendelian patterns of Inheritance

• Chapter 11

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Heredity

• The first scientists to study the laws of
  heredity had some difficult initial problems to
  work with
• Two parents have to contribute equally to make
  one child
• Offspring show similar traits to parents OR they
  show traits that havent appeared in a long time
• Mixed breeds two different species can sometimes
  produce offspring
• Laws of heredity must explain not only the
  stability but the variation

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Darwin and mendel

• Darwins Origin of Species was an attempt to
  explain how species evolve from generation to
  generation
• He had to explain how traits are passed for this
  to make sense
• Darwin proposed a blending theory, which says
  that parent genes (called particles at the time)
  blend traits to produce an offspring
• The first to explain heredity using experimental
  data however was an Austrian monk named Gregor
  Mendel
• Note genes as we know them wont be officially
  explained for another 100 years, but Mendel
  explained how they are passed from parent to
  offspring

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Mendels experiment

• Mendel was a mathematician, so he attempted to
  explain heredity using statistics and data
• He used the common garden pea for his experiments
• The pea is easy to cultivate, has a short
  generation time, and can be self-pollinated or
  cross-pollinated
• The first experiment involved crossing (mating)
  gametes of a tall pea plant with gametes of a
  short pea plant.
• The first generation is called the P generation
• If the blending theory is correct, all offspring
  should be medium length
• The first offspring generation is called the F1
  generation

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Mendels Experiment

• Mendel crossed plants thousands of times to
  ensure the accuracy of his data
• When the F1 generation grew, his results were
  contrary to his hypothesis 100 of the plants
  were tall plants. Only one parental trait was
  passed on.
• Did the short parental genes disappear?
• Mendel then crossed members of the F1 generation
  with each other to produce the F2 generation.
• The results 787 tall plants and 277 short plants
• The ratio was pretty close to 31 (74 26)
• The short trait had disappeared for a
  generation, but reappeared later

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Mendels Experiment

• Perhaps, however, this was just a trait with the
  height of plants
• Mendel repeated the same experiments over the
  next few months with the following traits pea
  pod shape, seed shape, pod color, flower color,
  seed color, flower position.
• Each time, the same results 100 of one trait in
  the F1 generation, a 31 ratio is the F2.
• Note this ratio represents the simplest type of
  gene. Only a small percentage of all genes in all
  organisms are actually this simple and easy to
  calculate.
• In other wordsMendel got really lucky.

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Mendels Explanation

• After analyzing all possible mathematical
  explanations for his results, Mendel wrote his
  first of two laws The Law of Segregation
• Each organism has two factors for each trait
• During meiosis (formation of gametes) the factors
  separate into different cells
• (From diploid to haploid)
• Each gamete contains only one factor for each
  trait
• When gametes fertilize, each new organism
  contains one factor from each parent for each
  trait

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Modern Genetics

• We now know that these factors are the strands
  of DNA that contain our genes
• Each gene has a minimum of two possible alleles
• An allele is an alternate form of the same gene
• Gene plant size. Alleles tall and short
• One of these genes is a dominant allele and the
  other is recessive
• Dominant alleles means the trait they code for
  will always appear in an organism
• Recessive alleles can be masked (covered, but not
  absent) by the dominant allele

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Modern Genetics

• Because you receive a set of genes from each
  parent, eukaryotic organisms all have two alleles
  for each gene (one from mom, one from dad)
• The combination of alleles an organism has is
  called their genotype
• Each allele in a genotype is given a
  single-letter label
• Capital letters are dominant, lower-case are
  recessive
• The actual trait that appears in an organism is
  called a phenotype
• Ttall plants, tshort plants
• TT or Tt genotypes tall phenotypes
• tt is only genotype that codes for a short
  phenotype
• Genotypes with the same allele are called
  homozygous. Different alleles are called
  heterozygous

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Punnett Squares

• When you understand the vocabulary and the
  process behind genetics, youre not only able to
  calculate what genes an organism likely has, but
  what genes its offspring are likely to have
• In other words, you can calculate the likelihood
  of what your kids will look like.
• We use a tool called Punnett Squares to calculate
  these odds.
• Punnett Squares are especially helpful for humans
  since we cant make lots of humans just to see
  what traits come up

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Punnett Squares

• Monohybrid Crosses
• Use a monohybrid cross if you want to calculate
  the odds of only one gene
• On the top, put the two alleles of the female
  parent
• On the left side, put the two alleles of the male
• Then fill in the boxes according to the allele
  from each parent

T
t
TT Tt
Tt tt
T
t
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Punnett Squares

• Monohybrid Crosses
• Each of the squares represent the potential
  sequence of genes in the offspring of the parents
• Once the squares are filled in, you can start
  calculating ratios
• Genotypic Ratios
• 121
• TT Tt tt
• Phenotypic Ratio
• 31
• Tall Short

T
t
TT Tt
Tt tt
T
t
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Punnett Squares

• Dihybrid Crosses
• What if you want to calculate the possibility of
  two genes at once?
• Dihybrid crosses are larger, but the overall
  concept is the same.
• For this example, we will use Mendels
  experiment The shape of peas AND the color of
  peas
• Round (R) is dominant over Wrinkled (r)
• Yellow (Y) is dominant over Green (y)

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Punnett Squares

• First, find the four possible allele combinations
  for each parent.
• Assign one combo for each row and column.
• Fill in the squares as with the monohybrid cross

Round R Wrinkled r Yellow Y Green y
RY
Ry
rY
ry




RY
Ry
rY
ry
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Punnett Squares

• When you calculate the different squares, you end
  up with a 9331 ratio

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Mendels experiment

• Mendel noticed that in his dihybrid crosses all
  possible combinations occurred.
• This must mean that genes are not connected to
  each other.
• This led to Mendels second law, the Law of
  Independent Assortment
• Each gene separates independently from itself
• Every theoretical combination of genes is
  possible within an individual organism

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Non-Mendelian Traits

• Theres lots of other sets of rules that genes
  follow
• Incomplete dominance
• In incomplete dominance, the heterozygote
  phenotype actually is a blending of the two
  alleles
• Snapdragons show incomplete dominance
• Rred flowers
• Rwhite flowers
• RR red flowers
• RR white flowers
• RR pink flowers
• Hair styles (straight, wavy, curly) is an
  incomplete dominance trait in humans

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Non-Mendelian Traits

• Codominance
• Codominant traits are traits that show both
  alleles equally in the genotype
• Dairy cow fur has two alleles and is codominant
• B Black fur
• W White fur
• BB All black
• WW All white
• BW Black and White spotted
• A human example of codominance is sickle blood
  cells
• The two traits are round blood cells and
  sickle-shaped blood cells

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Non-Mendelian Traits

• Multiple Allele Traits
• Multiple allele traits are traits that have three
  or more alleles
• The alleles all have an order of dominance
• Labrador fur shows multiple alleles
• YYellow
• Y1Black
• Y2Chocolate
• YY2 Yellow
• Y1Y2 Black
• For humans, a multiple allele trait is blood type

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Multiple Alleles in Humans

• Your immune system must recognize the difference
  between foreign substances and your own blood
• To do this, your blood has specific proteins
  called antigens on its plasma membrane.
• Antigens are glycolipids
• Your immune system recognizes these proteins and
  knows that the blood cell belongs to you and
  isnt an intruder

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Multiple Alleles in Humans

• The different antigens are labeled A and B
• Alleles IAA-Type Blood IBB-Type Blood
  iNeither type
• There are 4 possible phenotypes of blood, arising
  from 6 possible genotypes

Genotype Antigens Present Phenotype (Blood Type)
IAIA (AA) IAi (AO) A-Type only Type A
IBIB (BB) IBi (BO) B-Type only Type B
IAIB (AB) Both A and B Types Type AB
ii (O) Neither A or B Types Type O
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Multiple Alleles in Humans

• It is important for you to know what your blood
  type is BEFORE you get a blood transfusion
• If you get blood with a different protein than
  what your immune system is used to, it will
  attack the blood
• This results in blood clots and, usually, is
  deadly
• (Because O type blood has NO proteins on it,
  your cells wont recognize the WRONG proteins.)

If you have this blood type You can receive these blood types
Type A Type A or Type O
Type B Type B or Type O
Type AB Type A or Type B or Type O
Type O Type O only
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Non-Mendelian Traits

• Polygenic Traits
• Polygenic traits follow all normal Mendelian
  rules, but are combinations of multiple different
  genes
• Seed color in wheat three different genes
• Human height and skin color an unknown number of
  genes, but we think its between 5 and 12.
• Polygenic traits tend to result in what appears
  to be multiple or infinite different phenotypes

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Non-Mendelian Traits

• Epistasis
• Epistasis is when one gene masks the effect of
  another gene.
• Albinism in humans is an example of epistasis
• Humans have multiple genes for what their skin
  color will be.
• The different tones of color are controlled by
  how much melanin is produced by skin cells
• The more melanin, the darker the skin
• They have another gene elsewhere that controls
  whether or not melanin will be produced
• If no melanin is produced, then the organism will
  be an albino and their skin color genes no longer
  matter

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Autosomal recessive disorders

• Three diseases that are recessive alleles that
  can be passed from parents to offspring
• 1. Tay-Sachs disease
• Neurological impairment at 7-8 months old
• Blindness, seizures, paralysis possible
• 2. Cystic Fibrosis
• Mucus forms in the bronchial tubes and prevents
  lungs from working properly
• CF children develop more slowly and only live to
  20-30 years
• Phenylketonuria
• Cannot digest the amino acid phenylalanine
• Results in severe mental retardation

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Autosomal Dominant disorders

• Neurofibromatosis
• Tumors covering the nerve endings may cause
  deformations in bone and tissue structure
• Huntingtons Disease
• Brain cells begin to deteriorate at age 40-50
• Victims lose motor and cognitive function as well
• You can be tested to see if you have the gene for
  Huntingtons, but there is no cure
• Question If these diseases are dominant, how
  come hardly anyone ever gets them?

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Chromosomal patterns of inheritance

• Chapter 12

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Chromosomes

• Humans have a total of 46 chromosomes (23 from
  mom, 23 from dad
• Homologous chromosomes 1-22 are all called
  autosomes. These chromosomes contain the same
  genes no matter which parent they came from
• Not necessarily the same alleles.
• There is one set of homologous chromosomes that
  may be different. These are the sex chromosomes
• Two X chromosomes (Female)
• One X chromosome, one Y chromosome (Male)
• These chromosomes also contain genes. Any trait
  controlled by one of these chromosomes is called
  a sex-linked trait.

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Sex-Linked Traits in Humans

• Females have two X chromosomes. Therefore,
  mothers can only donate an X chromosome to their
  offspring
• Males have both an X and Y chromosome.
• If Dad donates an X chromosome, the offspring
  will be female. If Dad donates a Y chromosome,
  the offspring will be male.
• A father cannot pass a sex-linked Y-chromosome
  trait to his daughter OR an X-chromosome trait to
  his son.

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Sex-Linked Traits in Humans

• A female will not have any gene located on a Y
  chromosome
• What if a gene is located on the X chromosome?
• Males only have one X chromosome. They will only
  have one allele for that trait, dominant or
  recessive.
• Females have two X chromosomes, so they will have
  two alleles for the trait.
• Because of this, it is harder for females to have
  a recessive X-linked phenotype than it is for
  males.
• Phenotype ratios for sex-linked traits are
  different depending on the gender of the
  offspring

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• In punnett squares, sex-linked traits are
  expressed as an X or a Y.
• There is also a superscript to describe which
  allele is being represented
• Fruit flies XRRed eyes. Xrwhite eyes. YY
  chromosome, so no gene is present on the
  chromosome

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Recessive sex-linked trait
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Human X-linked disorders

• Muscular Dystrophy
• Muscles are weak, to the point where the victim
  loses almost all use of their muscles
• Death usually results by age 20
• Color Blindness
• Color-blind people have difficulty distinguishing
  colors, particularly in the red/green spectrum
• Hemophilia
• Hemophilia is an absence of the ability to clot
  blood.
• Fragile X syndrome
• A form of mental retardation, but victims are
  able to live to become a grandfather

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Sex-Determined Traits in Humans

• Sometimes organisms have a gene for a specific
  trait, but the trait is not expressed because of
  the organisms gender.
• A sex-determined trait is when a trait only
  appears in a certain gender
• Hormones produced by other genes block
  sex-determined genes from expressing in an
  organism
• Reason Parents of one gender may not express the
  same traits as children of the opposite gender.
  However, parents still need to pass all necessary
  genes to their child regardless of age.

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Sex-Determined Trait in Humans

• Both men and women produce testosterone and
  estrogen.
• Around puberty, the body begins to produce higher
  amounts of one or the other
• Your gender determines which structures your body
  will form (testes or ovaries), and these
  structures produce high quantities of
  testosterone and estrogen, respectively
• Therefore, even though you have the gene to
  produce both hormones, your gender decides which
  you will produce more of.

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Changes in Chromosome number

• Chromosomal mutations occur when the gametes
  formed during meiosis contain unusual numbers of
  chromosomes
• After meiosis I, you should have two cells, each
  with two copies of each chromosome
• After meiosis II, you should have four cells,
  each with one copy of each chromosome
• Its always possible that homologous chromosomes
  fail to separate or sister chromatids fail to
  break

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Polyploidy

• Polyploidy is a chromosomal mutation where a cell
  has more than two copies of each chromosome
• Triploid 3n
• Tetraploids 4n
• Pentaploids 5n
• Polyploidy occurs often in plants, and is one
  reason for huge evolutionary diversity of flower
  designs and plant species
• A set of gametes may go through meiosis and form
  polyploids, but only 1 out of these 4 gametes
  will be fertilized.

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Monosomy and Trisomy

• If an organism has one less or one more of a
  specific chromosome, it is referred to as
  monosomy or trisomy
• Monosomy and Trisomy is a result of
  nondisjunction
• Both members of a homologous chromosome enter the
  same daughter cell during meiosis I
• Both sister chromatids fail to separate in
  meiosis II
• Down syndrome is the most common set of human
  trisomy, when the individual has a third 21
  chromosome.

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Sex Chromosome number

• It is possible to inherit an abnormal number of
  sex-chromosomes as well.
• XOTurner syndrome, female
• Short, broad chested and extra folds of skin.
• Do not undergo puberty, menstruation, but are
  capable of giving birth and can lead fairly
  normal lives
• XXYKlinefelter syndrome, male
• The male sexual organs are underdeveloped, but
  the female organs (breasts, in particular,) are
  slightly overdeveloped
• Very large hands, feet and arms
• XXXPoly-X female
• Usually, poly-x females lead normal lives. Only
  slight physical abnormalities are evident
• XYYJacobs syndrome, male
• Taller than average, persistent acne, and
  communication problems

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Chromosome structure

• Some chromosomes are more fragile than others,
  and damage is always possible
• Most of the time damage is repaired by the cell.
  If not, it can result in more chromosome
  mutations
• Deletion
• In a deletion, a chromosome breaks, usually at
  the end of the chromosome
• Williams syndrome is a loss of the end of 7
  chromosome
• Williams children are academically and physically
  awkward, but show excellent communication and
  musical skills
• Cru de Chats syndrome is a loss of the 5
  chromosome end
• Individuals are mentally retarded with facial
  abnormalities, and an enlarged larynx results in
  an infant cry similar to a cats

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Chromosome Structure

• A translocation is when a chromosome section
  moves from one chromosome to another
• Down syndrome (Trisomy 21) is a result of a
  section of the 21 chromosome breaking and
  attaching to a 14 chromosome, resulting in three
  21 chromosomes
• Alagille syndrome, when a translocation occurs
  between 20 and 2 chromosomes
• The missing section of chromosome 20 results in
  severe itching and irritation of eyes, skin, and
  genital areas

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Chromosome structure

• Other types of chromosome break mutations
• Duplication
• A chromosome section breaks and reattaches to
  its sister chromatid, resulting in a repeated
  section
• Inversion
• A chromosome section breaks and reattaches, but
  backwards

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Gene Linkage

• Because crossing over and/or chromosome structure
  mutations are so common, geneticists study the
  locations of genes on chromosomes
• The centromere is the structure of a chromosome
  where two sister chromatids are held to each
  other
• The closer a gene is to a centromere, the less
  likely it will crossover or break from the
  chromosome
• To study this, geneticists make gene-linkage maps

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Gene Linkage

• There is a direct relationship between the
  frequency of crossing over (the percentage of
  recombinant phenotypes) and the distance between
  alleles.
• A recombinant is a section of offspring DNA that
  contains multiple, recessive genes
• The further apart genes are, the more likely a
  crossing over will occur between these two genes.
• If 6 of organisms are recombinant organisms,
  then the rate of crossing over is 6.
• A map unit is the unit used for measuring
  relative distance on a chromosome
• 1 of crossing over rate 1 map unit

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Gene Linkage

• Example An experiment is conducted on fruit
  flies to find the of recombinants for certain
  traits.
• Black-body and purple-eyes recombinants occurred
  6 of the time, so these genes were 6 map units
  apart.
• Purple-eyed and vestigial-wing recombinants
  occurred 12.5 of the time, so these genes were
  12.5 map units apart.
• Black-body and vestigial-wing recombinants
  occurred 18.5 of the time, so these genes were
  18.5 map units apart.
• What is the order of alleles on a chromosome, and
  how far apart is each one from each other?

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Gene Linkage