F'Morgans Experiments with fruit flies

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F. Morgans Experiments with fruit flies
1. Most fruit flies have red eyes but some males
have white eyes
2. Crossed a white-eyed male (XrY) with a
red-eyed female (XRXR)
a. Notice the chromosomes for the male are still
X and Y but a superscript of an r is used to
indicated a recessive allele for white eyes
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b. Notice the chromosomes for the female are
still X and X but a superscript of an R is used
to indicate a dominant allele for red eyes
i. 50 probability of having a heterozygous
dominant female for red eyes in the F1
generation ii. 50 probability of having a male
with a dominant allele for red eyes in the F1
generation
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c. Crossed the F1 generation resulting in
i. 25 probability of having a homozygous
dominant female for red eyes ii. 25 probability
of having a heterozygous female for red
eyes iii. 25 probability of having a male with
a dominant allele for red eyes iv 25
probability of having a male with a recessive
allele for white eyes
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3. Morgan concluded that eye color in fruit
flies is an X-linked trait
a. Notice that no allele for eye color is
located on the Y chromosome
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What are Linkage Groups?
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A. Linkage Groups
1. Each chromosome carries many genes since
there are 1000s more genes than chromosomes
2. Genes located on one chromosome form a
linkage group.
a. 2 genes located on one chromosome are said
to be linked
i. tend to be inherited together
B. Morgan conducted a dihybrid cross with
homozygous gray, long-winged fruit flies (GGLL)
with homozygous black, short-winged fruit flies
(ggll)
1. Resulted in 100 probability of heterozygous
gray, long-winged fruit flies
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C. Crossed 2 heterozygous individuals from the F1
generation (GgLl x GgLl)
1. Knew that if the allele for body color and
the allele for wing length were on separate
chromosomes, he would get a 9331 ratio
(typical dihybrid cross)
a. Allele for body color would independently
assort from allele for wing length
b. What the results would have looked like if
the alleles were on different chromosomes
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2. Predicted that these two alleles were on the
same chromosome and would therefore obtain
results of a 31 ratio instead
a. Allele for body color would not independently
assort from allele for wing length (aka less
mixing)
b. The heterozygous cross produced offspring
close to his 31 ratio
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i. 50 probability of having a heterozygous
gray, long-winged fruit fly ii. 25 probability
of having a homozygous gray, long-winged fruit
fly iii. 25 probability of having a homozygous
black, short-winged fruit fly
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c However, several gray, short-winged and black
long-winged flies also appeared
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d. If the alleles were on the same chromosome,
how could they separate to create the other 2
genotypes that showed up (Ggll ggLl)?
i. Not a mutation because there wouldnt have
been several similar offspring
ii. is believed to be
the cause of the unexpected results the alleles
were exchanged among the homologous pairs
Crossing Over
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What does crossing over have to do with this?
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A. Crossing over only occurs during meiosis.
1. During meiosis, one cell will eventually
divide to form 4 entirely different cells.
2. Each cell is different due to this crossing
over or swapping of genetic material.
a. During Prophase I, homologous pairs line up
close together...so close that sometimes they
swap genetic information causing variations in
offspring
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3. This causes genetic variation in offspring or
genetic recombination.
a. If dad is AA and Mom is aa...crossing over
can create Aa , a new combination.
b. If dad is Aa and Mom is AA, depending on what
alleles are found in the sperm/egg, crossing over
can create AA or Aa possibilities.
c. If dad is aa and Mom is Aa, depeding on what
alleles are found in the sperm/egg, crossing over
can create Aa or aa possibilities
d. If dad is Aa and Mom is Aa, depending on what
alleles are found in the sperm/egg, crossing over
can create Aa, AA or aa.
e. Same thing for the Bs or any other trait.
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Meiosis I
Section 11-4
Interphase I
Prophase I
Metaphase I
Anaphase I
Cells undergo a round of DNA replication, forming
duplicate Chromosomes.
Each chromosome pairs with its corresponding
homologous chromosome to form a tetrad. Crossing
over occurs during this phase.
Homologous chromosomes line up and share a
spindle fiber on the metaphase plate.
The fibers pull the homologous chromosomes toward
the opposite ends of the cell.
Go to Section
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Meiosis II
!
Telophase I Cytokinesis
Prophase II
Metaphase II
Anaphase II
Telophase II Cytokinesis
Each chromosome lines up on a single spindle
fiber at the metaphase plate.
The sister chromatids separate and move toward
opposite ends of the cell.
Chromosomes reappear, nuclear membrane disappears.
Nuclear membrane reforms around chromosomes.
Cytoplasm divides and new cell membrane forms.
Nuclear membrane reforms around remaining
chromosomes. Cytoplasm divides. Cell membrane
forms. Meiosis II results in four haploid (N)
daughter cells.
Interkinesis
Chromosomes relax back into chromatin for a
temporary resting period between divisions.
Go to Section
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What are Chromosomes?
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A. The DNA in a eukaryotic cell's nucleus is
coiled intoo very compact structures called
chromosomes
1. Each chromosome is a rod-shaped structure
made up of a single, tightly coiled DNA
molecule associated with proteins
a. The DNA in eukaryotic cells wraps tightly
around proteins called histones
i. histones-help maintain the shape of the
chromosome and aid in the tight packing of the
DNA molecules
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2. Consists of 2 identical halves called
chromatids connected by the centromere
a. Chromatids-form as the DNA makes a copy of
itself before cell division
b. Centromere-the constricted area that holds 2
chromatids together until they separate during
cell division
i. especially important for the movement of
chromosomes during cell division
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Chromatid
Centromere
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3. When the cell divides, the two new cells each
receive one chromatid from each chromosome
B. Each species has a characteristic number of
chromosomes in each cell.
1. Humans have 46
2. Cats have 32
3. Dogs have 78
4. Adder's Tongue Fern has 1,262!
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C. Sex Chromosomes and Autosomes
1. Sex Chromosomes
a. Chromosomes that determine the sex of an
organism and carry genes for other
characteristics
i. humans have X and Y chromosomes
- normal females have 2 X chromosomes (XX)
- normal males have 1 X and 1 Y chromosome (XY)
2. Autosomes
a. All other chromosomes (that are not sex
chromosomes) in an organism
i. humans have 2 sex chromosomes (XX or XY) and
the remaining 44 chromosomes are autosomes
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b. Every cell of an organism produced by sexual
reproduction (not asexual) has 2 copies of
each autosome called homologous chromosomes or
homologues
i. organism receives one copy of each autosome
from each parent
ii. homologous chromosomes are the same size and
shape and carry genes for the same trait
- example if one chromosome in a pair contains
a gene for eye color, so will its paired
chromosome
iii. there are 22 homologous pairs of
chromosomes (22 x 2 44)
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D. Karyotype
1. A photomicrograph of the chromosomes in a
dividing cell found in a normal human
a. Shows all 22 pairs of autosomes and the 1
pair of sex chromosomes
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Autosomes
Sex Chromosomes
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Chromosome Number
Diploid

• Diploid (2N) - term used to refer to a cell that
  contains both sets of homologous chromosomes
• Haploid (N) - term used to refer to a cell that
  contains only a single set of chromosomes and
  therefore only a single set of genes

Haploid
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Know your terminology!

• Somatic cells- regular body cells (diploid)
• Undergo mitosis forming identical diploid cells
• Germ cells- cells used for sexual recombination
  (haploid)
• Specialized diploid cells undergo meiosis to form
  haploid cells with ½ the number of the original
  diploid cells chromosomes

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Gametogenesis
Spermatogenesis- Process by which sperm gametes
are produced during meiosis from a diploid (2n)
cell to 4 viable haploid (1n) sperm cells
(spermatozoids).
Oogenesis- Process by which egg gametes are
produced during meiosis from a diploid (2n) cell
to 1 viable haploid (1n) egg cell (oocyte).
Animations http//wps.aw.com/bc_martini_eap_4/40
/10469/2680298.cw/content/index.html
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Goal of Meiosis

• To reduce the number of chromosomes from a
  diploid number to a haploid number. The new
  cells that result are called gametes, and can
  join together with their counterparts (egg with
  sperm, sperm with egg) to produce a new diploid
  organism.
• Meiosis is also called reduction division

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Figure 13.7 The stages of meiotic cell division
Meiosis I
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Figure 13.7 The stages of meiotic cell division
Meiosis II
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Figure 13.8 A comparison of mitosis and meiosis
summary
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Gene Expression
A. Only a fraction of the genes in a cell are
expressed at any given time (genes are either
turned on or off)
1. An expressed gene is a gene that is
transcribed into RNA
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Gene Expression
2. Regulatory elements found in DNA (what
controls the production of certain chemicals in
the body)
a. Structural Genes- actual genes that code for
particular proteins
b. Promoters- segment of DNA that recognizes the
RNA enzyme and where transcription is to begin
c. Operators- segment of DNA that allows special
proteins (called repressors) to bind and stop
transcription from occurring (turns off a
gene)
d. Regulator Genes- segment of DNA that codes
for the production of repressors
e. Inducers- chemicals that bind to repressors
and allow transcription to occur
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OFF
Repressor protein prevents translation from
occurring. This is when the gene is off.
ON
When a proteins is needed to be synthesized, the
repressor is removed so translation can occur.
Direction of Transcription
Gene DNA can be copied by mRNAremember what I
told you about introns and exons (junk DNA), it
applies here.
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Chromosomal Mutations

• Mistakes in shapes of chromosomes
• Mistakes in number of chromosomes

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Chromosomal Mutations

• 1. Mistakes in shape of chromosomes
• a. deletion part of chromo is broken off and
  lost completely
• b. duplication broken fragment of chromo
  attaches to sister chromatid so section is
  repeated on that chromatid
• c. inversion when fragment reattaches to
  original chromo but in reverse order
• d. translocation broken fragment attaches to
  a nonhomologous chromosome

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Chromosomal Mutations (mistakes in shape of
chromosome)
Deletion
These are chromosomal segments not alleles
Duplication
Inversion
Translocation
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Chromosomal Mutations (mistakes in number of
chromosome)
2. Mistakes in numbers of chromosomes nondisjunc
tion -- members of a pair of homologous chromos
do not move apart properly
?result in offspring that have Aneuploidy
abnormal chromo number Trisomy or Monosomy or
Polyploidy
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Normal Karyotype (22 pairs and a 23rd sex set)

• Karyotype- A standard arrangement of the
  chromosome complement done for chromosome
  analysis.
• Autosomes- 22 non-sex
• pairs of chromosomes
• Sex chromosome- placed after the 22nd chromosome
  in a karyotype
• (XX female)
• (XY male)

Compare this to the next slide
This is the sex chromosome a boy (X and Y)
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Trisomy 21 is a form of aneuploidy
Trisomy 21 or downs syndrome
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Trisomy 16 cause of most miscarriages in first
trimester of pregnancy
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Monosomy X Turners syndrome
Turner's Syndrome is a rare chromosomal disorder
of females (12500) characterized by short
stature and the lack of sexual development at
puberty. One X chromosome is missing from the
cell