Document Sample
Heredity Powered By Docstoc
Genetics is how genes control what
             we inherit
                          Inheriting Traits

• Heredity: the passing of traits from parents to offspring
• What is Genetics?
   – Genes control an organism’s form (shape, size, etc.) and
     function (how everything works)‫‏‬
   – Different forms of a gene are Alleles
      example: a gene will code for dimples or no dimples
        Mendel: Father of Genetics
                    Science + Math

•   Began experimenting with pea plants in 1856
•   1st to trace one gene through several generations
•   1st to use mathematics of probability to explain heredity
•   Not validated until 1900
Genetics: the study of how traits are inherited through
  interactions of alleles

   – Example: gene in female sex cell may code for dimples,
     gene in male sex cell may code for no dimples

   – the interaction of these genes determines if you will have
     dimples or not
D is dimples and d is no dimples. A pair of chromosomes
with the alleles DD separate during meiosis. After sister
chromatids separate during Meiosis II, two male sex cells
(sperm) have the alleles D and D. The two female sex cells
(eggs) have the alleles d and d.

                  DD                dd

   male       D         D       d        d    female
  Fertilization results in a zygote inheriting one allele from each sex cell
  (male and female). How these alleles interact determines which trait
    will be expressed in the offspring (dimples, D, or no dimples, d?)‫‏‬

One allele is dominant, and that is the allele that is expressed (that
physically occurs in the organism). Dimples is represented by an
uppercase letter, so it is dominant, and if an organism carries Dd,
then you will observe dimples.
             Genetics in a Garden: purebred
• An organism that always produces the same traits generation after
  generation is called a purebred, which means that it only carries
  alleles for one trait (like DD for dimples). Therefore, there is only one
  trait that can possibly be expressed, and only one trait that can be
  passed on to offspring. (If you cross two plants purebred for green
  seeds, then the 100% of offspring will have green seeds.) It takes many
  generations of breeding to produce a purebred.
• Purebred green: GG Purebred yellow: yy
                     Dominant and Recessive
• When two purebred plants are crossed, the dominant gene is
  expressed (it dominates, or covers up, the recessive gene). G is
  the allele for green, and y is yellow.

                      gg                  YY

    male        g           g         Y         Y   female
                      Yg         Yg
    Both offspring look yellow. Do they still carry the green gene?
        Genetics in a Garden

• Each time Mendel studied a trait, he crossed two plants with
  different expressions of the trait and found that the new
  plants all looked like one of the two parents.
   (ex: a plant with smooth seeds crossed with wrinkled seeds)

• He called these new plants hybrids (HI brudz) because they
  received different genetic information, or different alleles,
  for a trait from each parent.
                            Purebred Yellow: (after
                            many generations)‫‏‬

GG        yy                   Cross-pollination:
                               breeding two plants
                               purebred for different
                               traits on the same gene

               Hybrid: has characteristic
     Gy        from one of two different
                  Alleles determine traits

Most cells have two alleles for every trait (one from each

   – Homozygous cell: has two alleles that are the same GG

   – Heterozygous cell: has two alleles that are different Gy
Genotype: genetic makeup
Phenotype: the way an organism looks and behaves as a
result of the genotype

               GG                   yy

male       G         G          y         y    female

                Gy         Gy
         Genotype: green and yellow
         Phenotype: green
        Dominant and Recessive Factors
• Mendel called the tall form the dominant factor because it
  dominated, or covered up, the short form.

                                                   Dominant (W)‫‏‬
• He called the form that
  seemed to disappear
  the recessive factor.
                                                    Recessive (s)‫‏‬

What are the possible genotype(s) for the phenotype “widow’s
What are the possible genotype(s) for the phenotype “straight
        Punnett Squares
• How could you predict what the offspring would look like
  without making the cross?
• A handy tool used to predict results in Mendelian
  genetics is the Punnett square.

• The Punnett square shows the mathematical
  probability that specific allele combinations will be
  inherited; and therefore, that specific traits will be
        Punnett Squares

• In a Punnett square,
  letters represent
  dominant and
  recessive alleles.
• An uppercase letter
  stands for a dominant
• A lowercase letter
  stands for a recessive
 If Y is dark yellow and y is light yellow, then what is the %
 chance of an offspring being dark yellow? Light yellow?
                  Section Check
       Question 2
According to this diagram, if meiosis proceeds
correctly, how many alleles of a particular gene
can a female pass on to her offspring?
                  Section Check
Although she has two alleles of each gene, a
mother can pass only one allele to her
offspring. Meiosis separates alleles so that eggs
have only one allele for each gene. The new
individual then gets one allele from the mother
and the other from the father.
                 Section Check
       Question 3
Mendel crossed pea plants that were pure-bred
for yellow seeds with plants that were pure-
bred for green seeds. All the offspring of this
cross had yellow seeds. Based on these results,
which form of color was recessive and which
was dominant?
                 Section Check
Green seed color was recessive and yellow
seed color was dominant. Mendel called the
form that seemed to disappear (green in this
case) recessive and the form that covered up
(yellow in this case) dominant.
              Genetics Since Mendel
      Incomplete Dominance
• When the offspring of two homozygous
  parents show an intermediate phenotype, this
  inheritance is called incomplete dominance.
• Examples of
  incomplete dominance
  include the flower
  color of some plant
  breeds and the coat
  color of some horse
                    Genetics Since Mendel
        Multiple Alleles
• Many traits are controlled by more than two alleles.

• A trait that is controlled by more than two alleles is said to
  be controlled by multiple alleles.
                   Genetics Since Mendel
        Multiple Alleles

• Traits controlled by multiple alleles produce more than three
  phenotypes of that trait.

• Blood type in humans is an example of multiple alleles that
  produce only four phenotypes.

• The alleles for blood types are called A, B, and O.
                  Genetics Since Mendel
       Multiple Alleles
• When a person inherits one A
  allele and one B allele for
  blood type, both are
  expressedphenotype AB.

• A person with phenotype A
  blood has the genetic makeup,
  or genotypeAA or AO.
                 Genetics Since Mendel
       Multiple Alleles
• Someone with phenotype B
  blood has the genotype BB or

• Finally, a person with
  phenotype O blood has the
  genotype OO.
           Polygenic Inheritance

• A group of gene pairs works together to control one trait.
  Your height and the color of your eyes and skin are just
  some of the many human traits controlled by polygenic

• It is estimated that three to six gene pairs control your skin

• The environment also plays an
  important role in the expression
  of traits controlled by polygenic
        Impact of the Environment

• Although genes determine many of your traits, you might be
  able to influence their expression by the decisions you

• For instance, if some people at risk for skin cancer limit their
  exposure to the Sun and take care of their skin, they might
  never develop cancer.
        Human Genes and Mutations

• Occasionally errors occur in the DNA when it is copied inside
  of a cell.

• Such changes and errors are called mutations.

• Not all mutations are harmful. They might be helpful or
  have no effect on an organism.
        Recessive Genetic Disorders

• Many human genetic disorders, such as cystic fibrosis, are
  caused by recessive genes.

• Some recessive genes are the result of a mutation within
  the gene.

• Many of these alleles are rare.
        Recessive Genetic Disorders

• Such genetic disorders occur when both parents have a
  recessive allele responsible for this disorder.

• Because the parents are heterozygous, they don’t show any
  symptoms. The parents are carriers of the disorder.
        Recessive Genetic Disorders

• If each parent passes the recessive allele to the child, the
  child inherits both recessive alleles and will have a recessive
  genetic disorder.
        Recessive Genetic Disorders

• Cystic fibrosis is the most common genetic disorder that can
  lead to death among Caucasian Americans.
• In most people, a thin fluid is produced that lubricates the
  lungs and intestinal tract.
• People with cystic fibrosis produce thick mucus instead of
  this thin fluid.

• This buildup often results in repeated bacterial respiratory

• The thick mucus builds up in the lungs and makes it hard to
        Sex Determination

• Each egg produced by a female normally contains one X

• Males produce sperm that normally have either an X or a Y
        Sex Determination

• When a sperm with an X chromosome fertilizes an egg, the
  offspring is a female, XX.

• A male offspring, XY is the result of a Y-containing sperm
  fertilizing an egg.
        Sex-Linked Disorders
• An allele inherited on a sex chromosome is called a sex-
  linked gene.

• Color blindness is a sex-
  linked disorder in which
  people cannot distinguish
  between certain colors,
  particularly red and green.
        Sex-Linked Disorders

• This trait is a recessive allele on the X chromosome.

• Because males have only one X chromosome, a male
  with this allele on his X chromosome is color-blind.

• A color-blind female occurs only when both of her X
  chromosomes have the allele for this trait.
         Pedigrees Trace Traits

• A pedigree is a visual tool for following a trait through
  generations of a family.

• Males are
  represented by
  squares and
  females by

 Why is it important to distinguish between males and females when
 tracing color blindness through a family?
                Pedigrees Trace Traits
• A completely filled circle or square shows that the trait is
  seen in that person.

• Half-colored circles or squares indicate carriers.

• People represented by
  empty circles or squares
  do not have the trait
  and are not carriers.

How do you describe the genotype of an individual who is a carrier of a trait?
        Using Pedigrees

• A pedigree is a useful tool for a geneticist.

• When geneticists understand how a trait is inherited, they
  can predict the probability that a baby will be born with a
  specific trait.
            Using Pedigrees
• Pedigrees also are important in breeding animals or plants.

• These organisms are bred to increase their
   yield and
        Chromosome Disorders

• Every organism has a specific number of chromosomes.

• However, mistakes in the process of meiosis can result in a
  new organism with more or fewer chromosomes than
        Chromosome Disorders

• If three copies of chromosome 21 are produced in the
  fertilized human egg, Down’s syndrome results.

• Individuals with Down’s syndrome can be short, exhibit
  learning disabilities, and have heart problems.
• How do two alleles of a gene result in one expressed
  trait? What determines which alleles are inherited?

• If you cross a green pea plant and a yellow pea plant, what are
  the possible alleles that the offspring carries?

• If the green pea plant and the yellow pea plant produce 500
  offspring and they are all green, what hypothesis can you
  make about the dominance, or strength, of the green and
  yellow alleles?

Shared By: