Download X Chromosome

CH 14
HUMAN
GENETICS
The Role of Chromosomes
Genome – the full set of genetic
information that an organism carries
in its DNA.
Karyotype – shows a picture of
chromosomes
- Chromosomes are always in pairs.
- Arranged in order of decreasing size.
- The last pair of chromosomes, or the 23rd
pair are called the sex chromosomes.
- Males (XY)
- Females (XX)
- Autosomes do not determine a
person’s gender. They are body
chromosomes. They determine ones
characteristics!
- Chromosome #1-22
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Karyotype
- Amniocentesis, a sampling
of the fetal fluid which then
can help determine if there
are any genetic disorders of
the developing fetus.
- Geneticist will look at the
chromosomes of the fetus.
- Chromosomes are taken
during mitosis because they
are fully condensed and
visible.
Does any of the following fit with Mendel’s
Conclusions?
•Cross red flower and white 4 flower??
•Result: 4 pink flowers
•Cross a black chicken with a white chicken??
•Result: 4 black & white speckled chickens
•Ending up with 4 different colored rabbits as
possible offspring??
•Why are there many shades of skin color and hair
color in humans as opposed to just black and
white?
Transmission of Human Traits
A. Incomplete dominance, neither allele/gene is
completely dominant nor completely recessive.
- Heterozygous phenotype is in between the two
homozygous phenotypes
- individual is a blend
Snap dragons (flowers)
RR= Red
WW= White
If a red snap dragon were crossed
with white snapdragon, the
heterozygous plant would be PINK!
RR x WW
Offspring = RW = pink
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B. Codominance
- Codominant alleles/genes are both fully and
separately expressed.
- Both phenotypes are shown in the Heterozygous
individual.
- Black hamster crossed with a white hamster
B B
- BB X WW
W BW BW
- The heterozygous
individual BW, would
be both black and white
in color.
W BW BW
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Co-dominance and Multiple Alleles in Blood
Types
Example: 4 Blood Types
Blood groups: 3 alleles = A (IA), B (IB), O (i)
The four blood types are
Type A, Type B, Type AB, Type O
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Blood Typing
• Type A Genotypes = AA (IA IA), AO (IAi)
• Type B Genotypes = BB (IB IB), BO (IBi)
• Type AB Genotype = AB (IA IB)
• Type O Genotype = OO (ii)
A and B are always dominant to O.
AO (IAi) = A type blood
BO (IBi) = B type blood
• A and B are NOT dominant to each
other, they are Codominant
So remember…
There are 3 alleles (A, B, O), however there
are 4 blood types. (A, B, AB, O)
Punnett Square
Cross a woman Heterozygous for A-blood and
a man with AB-blood
•Parent genotypes: AO x AB
•Genotypes: 1IAIA :1IAIB :1IAi :1IBi
•Phenotypes:
i
IA
1: AB type
2: A-type
1: B-type
IA
IAIA
IAi
IB
IAIB
IBi
More Codominance
Normal Red
Blood Cell
Sickled Red
Blood Cell
•African American
Population
•Resistance to
Malaria
•
•
•
Sickle Cell
NN= Normal Blood Cells
SS= Sickled Blood Cells
NS = Both Normal and Sickled Cells
•Cross a heterozygous mom with a heterozygous
dad for blood cells.
Parent genotypes:
Genotypes:
Phenotypes:
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Multiple Alleles & Polygenic Traits
• Multiple Alleles
• Genes with > 2 ALLELES
• Ex) Rabbits fur color
• C: wild type
• Cch: chinchilla
• Ch; himalyan
• c: albino
• Ex) Human blood types
• A, B, AB, O
• Polygenic Traits
• Traits controlled by 2 or
more GENES
• Ex) hair color
• Ex) eye color
• Ex) skin color
• More than 4 genes
Codominance: Erminette chickens
Multiple Alleles: Rabbit Fur Color
Incomplete Dominance:
4 o’ clock flowers
Genes and the Environment
•Your phenotype is not just dependent on your
genes, but also the environment
•Examples
•Sea turtles gender –determined by the temp of
sand
•Gender of alligators – determined by temp.
•Height of people- influenced by nutrition
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C. Sex-linked traits - Genes on the X
chromosome
1. Only the X Chromosome carries genes
so if a gene is on the X chromosome, the
female would have TWO of those genes
and the male would have only ONE.
Female that has the disorder
Female carrier for a disorder
X
Y
Male that has
the disorder
X
X
• X & Y Chromosomes
& Traits
• X chromosome much
larger than the Y. Has
over 1,200 genes on it.
• Y chromosome only
has about 140 genes
on it, most which are
associated with male
sex determination and
sperm development.
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C. Sex-linked traits
-
A genetic disorder that is found or linked to the X
chromosome ONLY
Females can carry a sex-linked genetic disorder and
not have the disorder.
-
This is known as being a carrier of the disorder.
In order for the female to have the disorder both X
chromosomes must have the gene for the disorder
- Males (XY) express all of their sex linked genes because
they only have one X chromosome.
- If the X chromosome has the gene, the male has the
disorder.
- Males can not be carriers because they only have one X
chromosome
- Examples: Hemophilia and Colorblindness
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1. Colorblindness
a sex-linked recessive trait.
(C- normal vision; c- colorblindness)
• In order for a female to be color blind, she
would have to have two colorblind genes.
• A male needs to only have one colorblind
gene.
X
Y
X
X
What do you see?
A woman is heterozygous for Normal vision.
She marries a man who is colorblind.
What is the predicted colorblindness outcome for
their children?
LET … N = normal vision and
n = color blind
Parent genotypes:
N
n
X X
x
n
XY
Mom
Dad
FIND THE:
Genotypic Ratio:
Phenotypic Ratio:
X
X
n
Y
N
X
n
X Chromosome Inactivation
• Males and females can differ in sex linked traits.
• The expression of genes on the sex chromosomes differs from the
expression of autosomal genes.
• Genes located on the sex chromosomes are called sex- linked
genes or X-linked genes.
• Males express all of the alleles on both sex chromosomes.
• In females one of the two X chromosomes is randomly turned off
by a process called X chromosome inactivation.
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Human Traits
• Pedigree – a chart which shows the relationships within a family.
• Phenotypes are used to infer genotypes on a pedigree.
• You can often determine if an allele for a trait is dominant or
recessive or sex-linked.
A circle
represents
a female.
A horizontal line
connecting a male
and a female
represents a
marriage.
A shaded circle or
square indicates
that a person
expresses the
trait.
A square
represents
a male.
A vertical line and a
bracket connect the
parents to their
children.
A circle or square that
is not shaded
indicates that a
person does not
express the trait.
Studying the Human Genome
1. Manipulating DNA
• Because DNA is so large, in order to work with it, it must be cut into smaller
pieces.
• Many bacteria produce enzymes that can cut DNA
• Restriction enzymes – Cut DNA into precise pieces at specific locations that are
several hundred bases in length (known as restriction fragments).
• When the restriction enzyme cuts the DNA, the ends are called “sticky ends” so
they can bond with the complementary base sequence.
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Studying the Human Genome
2. Separating the DNA
• Gel electrophoresis – a technique used to separate and analyze the differently
sized cut fragments of DNA.
• The fragments of DNA are put into wells on a gel.
• An electric voltage moves them down the gel.
• DNA is negatively charged so they move toward the positive charge
• Shorter fragments of DNA move faster than longer fragments.
• Fragments appear as bands on the gel when specific stains are applied.
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Studying the Human Genome
3. Reading DNA
• The single-stranded DNA fragments are placed in a test tube containing DNA
polymerase and the four nucleotide bases along with a chemical dye.
• Each time a dye-labeled base is added to a new DNA strand, the synthesis of
that strand stops.
• The result is a series of color-coded DNA fragments of different lengths.
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Genetic Fingerprinting
Gel Electrophoresis
The Human Genome Project
• Launched in 1990.
• A 13 year, international effort with the main goal of sequencing all 3 billion
base pairs of human DNA and identifying all human genes.
Known as Shotgun sequencing
This method rapidly sorts DNS fragments
by overlapping base sequences.
Locating a gene
A typical gene, such as insulin, has several
DNA sequences that can serve as locators.
These include the promotor, sequences
between introns and exons, and start and
stop codons.
Bioinformatics
A new field that combines molecular
biology with information science. It is
critical to studying and understanding the
human genome.