Download X Chromosome

Human Genetics
Chapter 7
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The Role of Chromosomes
A. Chromosome number
1. Each human sperm/egg
has 23 chromosomes
2. Each human body cell has
23 pairs of chromosomes
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23 PAIRS
3. Each type of organism has a different number
of chromosomes.
Humans - 46 chromosomes
Fly – 8 chromosomes
Cat – 38 chromosomes
The Role of Chromosomes
Chromosome number
- The last pair of chromosomes,
or the 23rd pair are called the
sex chromosomes.
- Males have an X and a Y
chromosome (XY)
- Females have two X
chromosomes (XX)
- Autosomes do not determine a
person’s gender. They are body
chromosomes. They determine
ones characteristics!
- Chromosome #1-22
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• X&Y
Chromosomes
& Traits
• X chromosome
much larger
than the Y
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A way to tell chromosome number
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.
Karyotype
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
- BB X WW
B
B
W BW BW
- The heterozygous
W BW BW
individual BW, would
be both black and white
in color.
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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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Co-dominance and Multiple
Alleles in Blood Types
Example: Blood Types
The four blood types are
Type A, Type B, Type AB, Type O
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• Codominant alleles will both be completely
expressed.
– Codominant
alleles are
neither dominant
nor recessive.
– The ABO blood
types result from
codominant
alleles.
• The 3 multiple alleles are A, B, and O
• A and B are always dominant to O (i).
–
IAi = _____ type blood
IAIA = _____ type blood
–
IBIB = _____ type blood
Ibi = ____ type blood
• A and B are NOT dominant to each
other, they are Codominant
• AB genotype would look like this… IAIB
So remember…
There are 3 alleles (A, B, O), however there
are 4 blood types. (A, B, AB, O)
Genes
ii
IAIB
IAIA or IAi
IBIB or Ibi
Blood type
O type
AB type
A type
B type
IA
i
IA
IAIA
IAi
IB
IAIB
IBi
Cross a woman Heterozygous for A-blood
and a man with AB-blood
–Parent genotypes: IAi x IAIB
–Genotypes: 1IAIA:1IAIB:1IAi:1IBi
–Phenotypes: 1: AB type
2: A-type
1: B-type
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NOW YOU TRY: Bb x Bb as
Complete, Incomplete, & Co- Dominance
Say that:
B = Black
b = White
W = White
Complete
Dominance
Incomplete
Dominance
Co-Dominance
Genotype
Ratio
1:BB
2:Bb
1:bb
1:BB
2:BW
1:WW
1:BB
2:BW
1:WW
Phenotype
Ratio
3:Black
1:White
1:Black
2:Grey
1:White
1:Black
2:Black &
White
1:White
Polygenic Traits
• Polygenic traits are produced by two or more
genes.
• This occurs when many genes interact to produce
one trait.
• Order of dominance: brown > green > blue.
• Example: Eye Color, skin color, height
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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
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
Colorblindness - is a sex-linked recessive trait. (Cnormal vision; c- colorblindness)
•
In order for a female to be color blind, she would
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have to have this many colorblind genes. _______
•
1
A male needs to have this many _______
•
It is more likely to have a colorblind male than a
colorblind female because
The male only needs one gene
______________________________________
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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
N
n
Parent genotypes:
N n x
n
X X
XY
Mom
Dad
FIND THE:
Genotypic Ratio:
Phenotypic Ratio:
X
X
Y
n
X
Answer:
LET … N = normal vision and n = color blind
N n
MOM = heterozygous normal vision. X X
n
DAD = colorblind. X Y
Genotypic Ratio
n n
N n
1X X : 1X X :
n
N
1X Y:1X Y
Phenotypic Ratio
1 normal female carrier :
1 colorblind female :
1 Normal male :
1 colorblind male
X
X
Y
n
X
N
X
N
N
X
n
n
n
X X X
n
n
Y X Y
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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 sexlinked 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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Environment + Genes
• 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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A pedigree is a chart for tracing
genes in a family.
• Phenotypes are used to infer genotypes on a
pedigree.
• Autosomal genes show different patterns on a
pedigree than sex-linked genes.
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