Beyond Mendel Genetic changes and Human heredity

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Beyond Mendel Genetic changes and Human heredity

• Last part of Chapter 11 and Chapter 12 for gifted
  book

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• 1879 Walther Flemming German biologist first
  sees the CHROMOSOMES!!!
• also observed and described MITOSIS and noted
  that a full set of chromosomes was being passed
  on to each daughter cell.
• Sixteen years after Mendels death,
  scientists realize that the chromosomes are the
  carriers of heredity Mendels FACTORS are
  ensuring the passing of traits from parents to
  offspring.
• 1902 Walter Sutton American biologist who
  supports idea that factors are located on
  chromosomes

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• 1905 E.B. Wilson and Nettie Stevens Americans
  studying insect chromosomes
• Saw that male insects always showed a
• chromosome that did not seem to have a match
  (females always had a perfect matching set of
  chromosomes.) Thus, they referred to the
  non-matching chromosomes as Sex Chromosomes.
• In females the sex chromosomes do match
• XX
• In males, one of the chromosomes looked as if it
  were missing a part, so called it a Y
• XY

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• 1909 Wilhelm Johannsen Danish biologist who
  coined the term gene to define the physical
  units of heredity
• GENE segment of DNA molecules that carries the
  instructions for producing a specific trait

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• 1912 Thomas Hunt Morgan Showed evidence that
  the presence of white eye color in fruit flies
  was associated with a particular gene on a
  particular chromosome. (Sex Linked traits)

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Inheritance
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What is Sex Determination?
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A. Begins with the differentiation of the sex
chromosomes in males and females
1. These characteristics were observed by Thomas
Hunt Morgan in the early 1900s while studying
fruit flies (Drosophilia)
2. Hypothesized that these chromosomes
determined the organisms sex
B. Sex chromosomes (in pairs) separated during
meiosis I
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Punnett Squares
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C. Sex Linkage
1. The presence of a gene on a sex chromosome
a. Some genes on sex chromosomes play a role in
sex determination, but these chromosomes also
contain genes for other traits (see Linkage
Groups)
2. Morgan hypothesized that more genes could be
carried on the X chromosome than the Y chromosome
a. Genes found on the X chromosome are
considered X-linked genes
b. Genes found on the Y chromosome are
considered Y-linked genes
c. There are more X-linked than Y-linked traits
due to the size of the X chromosome
D. X-Linked Traits
1. A gene located on the X chromosome codes for
that trait
a. Example Red-Green Color Blindness
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E. Sex-Influenced Traits
1. The presence of male or female sex hormones
influences the expression of certain human traits

2. Males and females have different phenotypes
but can have the same genotypes.
a. A trait may only be expressed in one sex
Example Male Pattern Baldness
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.