Genetics After Mendel
Are all traits this simple?
  Some phenotypes are
   due to the action of a
   single gene

  MOST, however, are
    Height
    Skin color
    Disease predisposition
         Heart disease
         High blood pressure
         Diabetes
         Cancer
1) Concept of Polygene
 =Traits are
  determined by
  several genes at
  different loci.
 e.g. Human height
2) Concept of Multiple Alleles
 Many traits have multiple alleles not just two
  such as T or t.

e.g. ABO blood type there are 3 alleles = A, B, O

      AA, AB, AO, BB, BO, OO = 6 Genotypes
3) Concept of Pleiotropy
= Multiple effects caused by
a single gene
  e.g. Marfan’s
4) Chromosomes Exist
When do you see
  First of all you have to
   use stains for DNA
  Chromosomes only in
   condensed form when
   preparing for nuclear
  Otherwise present as
   chromatin (unraveled)
5) Chromosome Numbers
Vary Among Species
    Ant         2
    Mosquito    6
    Orange      18, 27, 36
    Frog        26
    Coffee      44
    Spanish Butterfly 380
    Stalked adders tongue 1020
Human Karyotype
= Chromosome Pattern
      Come in Pairs

                CELL   Cell Nucleus
6 Chromosomes
    3 Pairs
= Chromosome Pattern
Humans Have 46 Chromosomes
         23 Pairs

 Diploid Number = 46 (2n)

 Haploid Number = 23 (n)

 Polyploid Numbers = 3n, 4n, 5n,…….etc
Humans Have 46 Chromosomes
         23 Pairs

 One set of 23 from father

 One set of 23 from mother
Humans Have 46 Chromosomes
         23 Pairs

 22 Pairs are autosomes

 1 Pair are sex chromosomes
    ♀ = xx            ♂ = xy
Human Karyotype
= Chromosome Pattern
   Is this from a woman or a man?

A) Woman
B) Man
C) I don’t know
7) Mendel’s Factors (Genes)
  Occur on Chromosomes
    in particular places= locus
                  2 Alleles
             Homozygous Dominant


              Homozygous Recessive
8) Many Genes Exist on
   Each Chromosome
  Viruses = 5  10 genes

  Bacteria = 1,000  6,000 genes

  Human = 25,000 genes
9) Concept of Linkage
 Genes on the same chromosome are linked
Do you remember this slide?
What if RY are linked together &ry are linked
together ?

Non Independent Assortment = REDUCED VARIABILITY
    9) Concept of sex-linked
    traits = Traits carried on the
        sex chromosomes
   Muscular dystrophy
   Color blindness
   Pattern baldness
   Hemophilia
   All are carried on the X chromosome
        and are displayed in males
Because ♂ only have one X chromosome.
So if they have the allele for the trait,
          they will show it.
10) Cell Division Occurs
     Step 1: The chromosome duplicate
  There are two identical arms on each one.

                             Each arm is called a
     Cell Division in Body Cells
      Step 2: The Chromatids Divide Equally
       During Cell Division
            Division in body (somatic) cells

      46            92

46            46
The chromosomes are lined up

The chromatids are pulled apart

46 chromosomes in each cell
      Division in Sex Organs
         (In the Testes)

        46              92

 23           23

23 23    23        23

  4 Sperm
      Division in Sex Organs
          (In the Testes)

        46                 Meiosis =
                        Daughter cells with
                        ½ # Chromosomes
23 23    23        23   As parent cell

  4 Sperm
Stages of
 Meiosis (in animals)
  produces 4 haploid
  gametes from 1
  diploid cell
 At the end of the first
  division (Meiosis I)
  the 2 cells are
  already haploid
 The second division
  (Meiosis II) splits the
   2 chromatids
  Division in Sex Organs
     (In the Ovary)


Meiosis               23      23

                                   3 Polar
                      Egg +        Bodies
         Division in Sex Organs
              (Testes & Ovaries)

        46                          46

 23           23

23 23    23        23
                                     3 Polar
                            Egg +
  4 Sperm                            Bodies
   Distribution of
Chromosomes in gametes
The gametes end up with only one set of
 chromosomes. It is random which
 chromosome of a pair goes into which

 SO, each gamete has a mixture of the
  mother & father’s chromosomes.

 This produces enormous gamete variability
How many combinations of maternal
chromosomes are possible in a
human egg due to independent
assortment during meiosis?

   A: 23 combinations
   B: 46 combinations
   C: 232 = 529 combinations
   D: 223 = ~ 8 million combinations
The Fate of Genetic Material
During Sexual Reproduction
 You contain
  genetic material
  from both your
  Mom and Dad.
 You have 46
  and so will your
9) Concept of Cross-over
=Sister chromosomes swap
pieces during meiosis
Now for a Story
Santhi Soundarajan
 Santhi Soundararajan won
   the silver medal for 800
   meters at the 2006 Asian
   Games in Doha, Qatar.

 Following her silver medal
   performance, her sex
   was officially questioned.

18 Dec. 2006: Indian athlete Santhi
Soundararajan fails gender test
 Sports writer KP Mohan said that a team of
  doctors, including a gynecologist,
  endocrinologist and psychologist, normally
  examines athletes, and puts them through
  physical and clinical examinations during a
  gender test.

 Santhi Soundararajan's test was done soon after
  Soundararajan came second in the women's
  800m race on 9 December but is not clear how
  she failed the test at the Asian Games in Doha.
How is sex determined in
 Anatomy: Primary and secondary sexual
  characteristics - genitalia, body hair, pelvis, etc.
 Physiology: Function and interaction of the sex
  organs including concentrations of sex
  hormones such as estrogen, progesterone, and
 Imagine that you are a member of the
committee assigned to determine whether
Santhi is female. Here are possible results of
the initial tests (we don’t know the real results):

  Female genitalia: Yes
  Breasts and pubic hair: Yes
  Regular menstrual cycle: Never

From this information, you conclude
  that Santhi is
A: Male                B: Female
Is there another way that sex is
characterized in humans?
 Chromosomes:
   Females possess two X chromosomes in
    each of their cells, whereas males have one X
    and one Y chromosome.
    How is Sex Determined in
    Sex is determined by
                                    X           Y
     the sex chromosome
     carried by the sperm.
                                    XX      XY
                               X    XX      XY

                                   girls   boys
    Suppose this was Santhi’s
   Would you
   A) Disqualify her
   B) Let her keep her medal
   C) Do more tests
How is Sex Characterized in
 In addition to anatomy, physiology, and
   chromosomes, there is a 4th answer:

  Genes: Specific genes determine
  whether an embryo will develop as a
  male or female.
SRY (Sex- Determining Region
of the Y chromosome) Gene
 Early in development, the
  immature gonads of males and
  females are indistinguishable.

 Males: In the 7th week of
  development, the SRY gene on the
  Y chromosome activates a number
  of genes, and the gonads develop
  as testes.

 Females: With no SRY gene,
  gonads develop as ovaries by
Could Santhi have an
XX karyotype and be

A: No, an XX individual is always female.
B: Yes, this is common.
C: Yes, if a male-determining control gene is
   carried on one of her X chromosomes.
What about crossing over ?

 Crossing over can occur anywhere along
  the autosomes.

 In males, the sex chromosomes (X and Y
   chromosomes) normally cross over only
   at their tips.

 BUT sometimes the SRY gene is involved
Special note:

It may take a little while for the
next slide to load.
Can you have an XX male or
XY female?

 What sex is XX (SRY+)?
 What sex is XY (SRY-)?

 Sex reversal occurs in 1 in 20,000 births!
If you were a member of the Asian
Games medal committee, and
Santhi’s karyotype revealed that she
is XY and SRY-, what would you do?
A: She has female genitalia, allow her to
  keep her medal
B: She is genetically male, take her
medal away
C: She has male genitalia , take
  her medal away
What do you think about requiring
gender testing for female athletes
in international competition?
A: It should be banned because gender
  determination is so complex
B: It is necessary to ensure an even playing field
C: It is necessary but needs to include a large
  number of genetic tests to ensure fairness
D: It should be required for all athletes, both male
  and female
What about Santhi
             Santhi was stripped
             of her silver medal.
             An anonymous official in
             the know has reported that
             Ms. Soundarajan's tests
             revealed more Y
             chromosomes than are
             usually present in the
             genetic make-up of a
             female. No official
             statement has been made.
What about Santhi
             In September 2007, Santhi
             Soundarajan was reported
             to have attempted suicide,
             reportedly by consuming
             pesticide at her residence.
             The attempt was blamed on
             gender, economic, and
             sports pressure in India.

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