Basic Genetics by cuiliqing


   In the 1800s
   Austrian Monk
   Studied garden peas
   Noticed that over time certain patterns appeared in the
   For many traits, the pea plants would have two contrasting
     Flowers could be purple or white
     The seeds could be wrinkled or smooth
     The height of the plant could be tall or short
   He also noticed that some of the plants were true-breeding
    for certain characteristics…this means that the parent
    ALWAYS produced offspring with traits that were identical
    to the parents
   He began to experiment with the pea plants and discovered
    many of the basics that are still used in the study of
   Genetics - the study of heredity
       Heredity – the passing of traits from parent to offspring (from one
        generation to the next)
   The process of meiosis results in the formation of gametes (sex
    cells; or egg and sperm)
     Gametes are Haploid (having half the number of chromosomes of the
      parent cell)
     When a haploid egg cell is fertilized by a haploid sperm cell, the
      resulting offspring gets half its genetic information from its mother
      and half from its father
   Each chromosome is made of a long strand of DNA
   Sections of DNA contain Genes that determine specific traits
   You get one set of genes from your mother and another set of
    genes from your father…
       For example, if your mother has blue eyes and your father has brown
        eyes, your mother may give you the gene for blue eyes and your father
        may give you the gene for brown eyes…you now have two different
        forms of the same gene…these different forms of a gene are called
   In the studying of pea plants, Gregor Mendel
    noticed that one trait would show up more often
    than another and said that the gene for that trait
    was dominant over the other gene. The gene
    that was not dominant was called recessive.
     Mendel crossed pure purple plants with pure white
      plants and the offspring ALWAYS had purple flowers
     He crossed pure tall plants with pure short plants
      and all of the offspring were tall
     Pure yellow-seeded plants and pure green-seeded
      plants produced yellow-seeded plants every time
     Mendel determined that the traits tall, yellow, and
      purple flowers were dominant over the other traits
   If there is a dominant allele present, the
    dominant trait will always show up (even if the
    recessive allele is also present)…the recessive
    trait only shows up when there is NO dominant
    allele present
     Capital letters are used for dominant alleles and
      lower case letters are used for recessive alleles
     Since both are forms of the same gene, the same
      letter is used…for example, W might represent the
      allele for purple flower color, and w would represent
      the allele for white flower color
     Different letters are usually used for different traits
   Genotype is the combination of alleles an
    organism has for a particular trait…a pair of
    letters is used for each trait in a genotype (TT, tt,
    or Tt) because one allele (one letter) comes from
    the mother organism and one allele (one letter)
    comes from the father organism…so each baby
    organism has two alleles (letters) for each trait
       For most traits, there are three possible genotypes:
         HH – Homozygous Dominant (or pure dominant)
         hh – Homozygous Recessive (or pure recessive)

         Hh – Heterozygous (sometimes also called
          HYBRID…the alleles for this trait are different)
 Phenotype is the physical characteristic
  expressed in an organism…the phenotype for a
  trait is the EXPRESSION of the gene
  combination or genotype for that trait
 Examples of phenotypes:
       Brown hair, Blonde hair, Blue Eyes, Freckles, No
        freckles, Yellow flowers, Red flowers, etc…
   Another way to explain the relationship between
    genotype and phenotype is to say that Genotype
    codes for Phentotype
 GENOTYPE                DEFINITION              PHENOTYPE
 BB                      Homozygous              Brown fur
                         dominant (or pure
 bb                      Homozygous              White Fur
                         recessive (or pure
 Bb                      Heterozygous (or        Brown Fur

From the chart, you can see that there are three different possible genotypes.
There are only two possible phenotypes. Two of the genotypes (BB and Bb)
Give the same phenotype…Brown fur.
 Each offspring produced gets half of it’s genetic
  information from the mother and half from the
  father. If an offspring gets a dominant gene from
  one parent and a recessive gene for the same
  trait from the other trait, it is said to be
  “heterozygous” for that trait…however, being
  heterozygous refers to the genotype…the
  phenotype will ALWAYS result in the expression
  of the dominant trait in this situation.
 Mendel’s Law of Segregation says that when
  an individual forms sex cells (sperm or egg), the
  different forms of the gene will separate.
   Examples of Mendel’s Law of Segregation:
     If one pea plant parent has a heterozygous genotype
      for flower color (Ww) the phenotype for this gene is
      purple but during meiosis, the different forms of the
      gene will separate…only one can be passed to an
      offspring but not both:
     When meiosis ends, there will be 4 gametes…two will
      carry the W gene (dominant), and the other two will
      carry the w gene (recessive)…the allele passed on to
      the next generation depends on which gamete
      combines with a gamete from the opposite sex during
     Mendel’s Law of Segregation states that only one of
      the forms can be passed on, but not a combination of
      more than one form.
 When the inheritance patterns for only one trait
  are studied, the cross between two parents is
  called a monohybrid cross
 In a monohybrid cross, one allele from a female
  egg is joined with one allele from a male sperm to
  determine the probability of each genotype and
  phenotype showing up in the next generation
 Because alleles for a trait separate independently
  of each other during meiosis, we can use the laws
  of probability to predict genetic outcomes.
 Probability is the chance that a certain event
  will occur.
   Three things you should know:
     Probability predicts what is most likely to occur, but
      what actually occurs may be different.
     Each outcome is NOT dependent on other outcomes
      before it.
           Example: if humans have a 50/50 chance of having a
            boy…and two parents have three boys in a row, what is the
            probability that the next child will be a boy?
              The probability is still 50% because each time, the
               probability of producing a male offspring is
               INDEPENDENT of previous results.
       The actual results will be closer to the predicted
        results for a large number of outcomes.
           If, in fact humans have a 50/50 chance of having a girl, the
            more children they have, the greater the probability that
            they will have an equal number of sons and daughters.
 Monohybrid crosses are demonstrated by using a
  model called a punnett square
 A monohybrid cross that is only predicting the
  outcome for one trait uses a Punnett square with
  four compartments.
The next step is to take the
genotype for one parent and put
one letter outside the punnett
square on the left side…ONE
LETTER beside each of the two
chambers on that side.

This example shows a punnett
square with the genotype for
spherical (round) pea shape of the
male parent only.

The father’s genotype is Ss
(heterozygous) and so one letter
(the upper case S) is placed beside
the first compartment to the left,
and the other letter (the lower
case s) is placed beside the second
compartment to the left.
Next, the genotype for the other
parent (in this case, the female)
should be added across the
top…again ONE LETTER for
each of the two compartments.

This punnet square shows the
male’s genotype for pea shape in
blue and the female’s genotype
for pea shape in red…by setting
up this punnett square, we can
predict the probability of the
genotype (and phenotype for pea
shape)for offspring produced
when these two plants reproduce
The next step is to fill in the
possible ways that the alleles
could be passed on from one

Each compartment represents
the gene being passed on from
one of the four gametes
produced during meiosis.

In this example, the male
parent is heterozygous for
pear shape…half of the
gametes produced will have
the Dominant allele, and the
other half will have the
recessive allele as shown.
Finally we can fill
in the squares for
parent 2 (in this
case the female
parent: red)

Again, the parent is
heterozygous for
pea shape and ½ of
the offspring will
receive the
dominant allele and
the other half will
receive the recessive
The results of the punnet

                               INTERPRETING THE RESULTS OF A PUNNETT
square show that the
probability of the offspring
being heterozygous for pea
shape is 50%
The probability that the
offspring will be
homozygous dominant for
pea shape is 25%, and the
probability that the
offspring will have a
homozygous recessive
genotype for pea shape is

Hence we can also predict
phenotype…the probability
that the offspring’s
phenotype will be Spherical
shaped peas (round) is
75%...the probability that
the offspring will express
the phenotype of wrinkled
 Probability can be given as a ration, a fraction, or
  a percentage.
 In genetics, probability is often expressed in ratio
  form…see the following table for an example

Description   Ratio        Fraction      Percent

1 to 3 or     1:3          1/4           25%
1 out of 4
2 to 2 or     2:2          1/2           50%
2 out of 4
3 to 1 or     3:1          3/4           75%
3 out of 4
4 out of 4                               100%
   Draw a punnett square and perform the following
    monohybrid crosses to predict the genetic
    probability for the following:
       TT x tt
       Pp x pp
       bb x bb
       QQ x Qq
       Dd x Dd

           One more note…when writing GENOTYPE…if there are
            two different alleles for a gene…the Upper case or
            dominant allele is always written FIRST
   The same principles of pea plant genetics apply to
    humans…a punnett square can be used to predict the
    probability of a child being born with a genetic disease.
   Remember that genetic diseases are caused by mutations
    in genes or chromosomes…the abnormalities of the
    mutation are present in all of the individual’s body cells,
    and they are present for the rest of the individual’s life.
   Some genetic diseases are rare and only affect one person
    out of millions…others are more common disorders
    affecting up to one in twenty people
   Three main types of genetic disorders:
    1.   Autosomal gene dieases
    2.   Sex-linked gene diseases
    3.   Missing or extra chromosome diseases
         1.   (we will only be looking at autosomal disorders in this part of the
              course since we have already discussed missing and extra
              chromosome diseases and we will look at sex-linked disorders
   Autosomes are all of the chromosomes EXCEPT the
    X and Y chromosomes that determine the sex of the
   Autosomal diseases are diseases carried on
   Most autosomal diseases are recessive…if the
    disease is recessive, the offspring must inherit a copy
    of the gene from BOTH parents in order to “inherit”
    the disease.
   If only one parent passes the gene on to the offspring,
    it will NOT actually have the disorder, but is called a
    carrier of the disorder
       Carriers will not have symptoms of the disease but they
        can pass the gene on to their children
   Autosomal recessive disorder
   In the US, about 5% of the human population carries
    the gene for cystic fibrosis
   The defective gene is found on chromosome 7 and
    causes the production of a protein that makes the
    person produce unusually sticky, thick mucus
   The mucus can lead to severe lung infections and
    prevents enzymes from the pancreas from helping the
    body break down food and absorb nutrients
   People with cystic fibrosis tend to have a persistent
    cough, frequent lung infections, poor growth and
    weight loss even when he/she eats well, and shortness
    of breath
   Currently no known cure exists, but medical
    treatments are available to help people with cystic
    fibrosis live longer and better lives than in the past.
   Autosomal recessive disorder…also called PKU for short
   People with PKU are missing an enzyme called
    phenylalanine hydroxylase that breaks down and allows
    the body to use phenylalanine (one of the 8 essential amino
    acids found in protein)
   Without the enzyme, phenylalanine builds up in the body
    and the build-up affects the central nervous system and
    causes brain damage
   PKU is treatable and easily detected with a blood test (all
    50 states currently test EVERY newborn born in the US for
   The treatment (if an infant is diagnosed) simply requires a
    diet that avoids all food containing phenylalanine…by the
    time the child is 5-6 years old, a regular diet can generally
    be resumed.
   If the special diet is not followed during the first few years
    of the child’s life, the result is usually severe mental
   Another recessive autosomal genetic disorder
   Affects people of Eastern European Jewish decent
    more often than any other ethnic group (although
    other CAN be affected)
   Babies with Tay-Sachs seem normal for the first few
    months of their lives but fatty material begins to
    build up around the nerve cells and the brain after a
    few months
   Babies with this disorder will become deaf, blind, and
    unable to swallow
   Eventually Tay-Sachs victims become paralyzed and
    may need a feeding tube
   These children very rarely live past the age of 4
   At this time, there is no cure for Tay-Sachs
   Recessive Autosomal Genetic Disease
   About 1 in 1000 people in the US have this disorder
   Causes the body to make “sickle-shaped” (C shaped red-blood cells)
     Normal red blood cells are disc-shaped and look like doughnuts without holes
      in the center…the shape of these normal red blood cells helps them to move
      easily through blood vessels.
     Red blood cells contain the protein hemoglobin which is rich in iron, gives
      blood it’s red color, and carries oxygen from the lungs to the rest of the body.
   People with sickle cell anemia have abnormal hemoglobin (this
    causes the abnormal sickle-shape)…these blood cells do not move
    easily. They are stiff and often form clumps…getting stuck in the
    blood vessels
   These “clumps block blood-flow in the blood vessels leading to the
    limbs and organs…blocked vessels cause severe pain, serious
    infections, and organ damage.
   Sickle-cell is only one form of anemia (a disorder that results in a
    lower than normal red blood cell count
   No cure currently exists for sickle cell anemia, but there are
    treatments for the symptoms and complications of the disorder…with
    treatment, individuals with this disease are living into and past their
    40s and 50s…

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