Transcript xCh 20 genetics W11

Bio11 – Announcements
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TODAY – Genetics
Test 2 AVG = 73 pts
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Test scores posted
Next week’s lab: Case study
presentations
Extra credit due Thur Mar 3
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15 pts available, see my website
Ch. 20 Patterns of Inheritance
How traits are passed from
generation to generation
Human Chromosomes
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46 chromosomes in
pairs
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22 pairs of matching
chromosomes
Plus 1 pair of sex
chromosomes
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XX or XY
This is a human karyotype
Male or female?
DNA packing in chromosomes
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Chromosomes contain
DNA and protein
The long strands of
DNA are condensed
The DNA is packed into
an elaborate, multilevel
system of coiling and
folding.
Chromosomes and genes
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Each chromosome
contains one very long
DNA molecule
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Typically bears thousands
of genes
Genes carry our traits
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Genes are segments of
DNA
What is heredity?
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Heredity – passing traits
from parent to offspring
The genes for certain traits are
passed down in families from
parents to children.
For example,
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parents with black hair will have
kids with black hair
Tall parents will have tall kids
Genetics 101: Where do your genes come from? (4:14)
http://www.youtube.com/watch?v=lJzZ7p-47P8&NR=1
Genes carry our traits
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Genes
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found at specific locations on a chromosome
Alleles are different versions of a gene
Genotype and phenotype
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Genotype – an individual’s
genes
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Gene “F” codes for freckles
Phenotype – what an
individual looks like
F
F
Genotype and phenotype
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What is her genotype?
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The gene for freckles has 2 alleles
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The dominant allele F codes for freckles
The recessive allele f does not
She has two copies of each
chromosome
Possible genotypes: FF or Ff
F
F
F
f
Examples of traits controlled
by a single gene in humans
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These 3 traits are
determined by simple
dominant-recessive
inheritance
Possible genotypes for
someone with
freckles? FF and Ff
Without freckles? ff
More about alleles
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They are alternative
versions of the same
gene
The gene for hairline has
2 alleles
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P for Widow’s peak
p for straight hairline
Widow’s peak
Straight hairline
More about alleles
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For each trait, we inherit two
alleles, one from each
parent.
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The alleles can be the same or
different
If they are different, the
organism is heterozygous – Pp
If they are the same, the
organism is homozygous for
that gene – PP, pp, YY
Widow’s peak
Straight hairline
More about alleles
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Alleles can be dominant
or recessive.
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The dominant allele
determines the
organism’s appearance
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Phenotypes:
Use upper case: P
The recessive allele has
no noticeable effect
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Use lower case: p
Genotypes?
PP or Pp
pp
Inheriting a trait
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If a man with short fingers marries a woman
with long fingers, what genotypes and
phenotypes will their children have?
Dad
Dominant trait
SS or Ss
Mom
Recessive trait
ss
Forming the gametes
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Gametes carry only one allele for each
inherited characteristic.
Dad
S
S
S
S
meiosis
Genotype = SS
S
Gametes only carry
one copy of each
chromosome
Forming the gametes
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Gametes carry only one allele for each
inherited characteristic.
Mom
s
s
s
meiosis
Genotype = ss
Fertilization
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When sperm and egg unite at fertilization,
each contributes its allele.
S
s
Fertilized egg
S
s
Genotype = Ss
Phenotype?
Forming the gametes
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What if dad is Ss?
Dad
S
S
s
s
meiosis
Genotype = Ss
S
Then after fertilization …
Dad
S
Gametes:
Fertilized egg:
genotype?
phenotype?
Mom
s
S or s
Ss or ss
s
s
All s
Crosses (one-trait inheritance)
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If neither parent has freckles, what will
be the genotype of their offspring?
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Parents
no freckles
no freckles
ff
ff
meiosis
gametes
Offspring
f
Offspring
f
ffff
no freckles
Crosses (more complicated)
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If both parents are heterozygous for
freckles, what will be the genotype of
their offspring?
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Parents
no freckles
Freckles
Ff
ff
no freckles
Freckles
ffFf
meiosis
gametes
f
Gametes?
f
Gametes?
Offspring
ff
no freckles
Punnett square analysis
Used to predict the genotypes of the offspring
#1 Decide what gametes would be produced by each parent.
F
Genotype? Gametes?
Mom
Ff
F
f
F
Dad
Ff
F
f
f
f
Punnett square analysis
What are the genotypes of the offspring of two
heterozygous freckled parents (Ff)?
F
F
f
FF
Ff
Ratio: 1FF, 2 Ff, 1 ff
f
Ff
ff
What are the phenotypes?
Human Disorders Controlled
by a Single Gene
Recessive disorders
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Most human genetic
disorders are recessive.
Albinism
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Albino alligator
lack of melanin pigment in
the eyes, skin and hair
affects mammals (including
humans), fish, birds, reptiles
and amphibians
Recessive disorders
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Cystic fibrosis
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Most common lethal
genetic disease in US
Symptoms: excessive
secretion of a very thick
mucus which interferes
with breathing
Symptoms usually
appear shortly after birth.
Dominant Disorders
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Some human genetic
disorders are dominant.
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Achondroplasia is a
common form of
dwarfism.
Cause: abnormal bone
and cartilage formation
Huntington’s disease:
a dominant genetic disorder
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Caused by a dominant
allele–every individual who
carries the allele gets the
disorder
Fatal: causes progressive
deterioration of the brain
Late age of onset: most
people do not know they
are affected until they are
more than 30 years old
Many neurons in
normal brain.
Loss of neurons in
Huntington brain.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Some traits are controlled by
multiple genes
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At least 180 genes
control how tall a
person will grow
Beyond simple inheritance
Incomplete Dominance
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The heterozygotes have an
intermediate phenotype
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appearance is between the phenotypes of
the two parents.
Alleles are not fully dominant or fully
recessive
Incomplete Dominance:
Hypercholesterolemia
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Characterized by dangerously high levels of
cholesterol in the blood
Multiple Alleles:
ABO blood groups
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Three alleles for the
same gene
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IA = A antigen on RBC
IB = B antigen on RBC
i = neither A or B antigen
The IA and IB alleles
exhibit codominance.
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Both alleles are
expressed in the
heterozygote.
Sex-linked genes
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Genes located on a sex
chromosome, usually
the X chromosome.
Their pattern of
inheritance reflects the
fact that females have
two X chromosomes,
but males have only
one.
Sex-Linked Disorders in
Humans
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Sex-linked disorders
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due to recessive alleles
seen mostly in males
Example
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Red-green color
blindness
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characterized by a
malfunction of lightsensitive cells in the
eyes.
Inheritance of colorblindness
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An X-linked, recessive disorder
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XB = normal vision
Xb = color blindness
Write the following genotypes
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Normal man
Color-blind man
Normal woman
Woman carrier
Inheritance of colorblindness
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A man with normal vision and a woman
carrier have children. What is the
chance that the couple will have a colorblind daughter? A color-blind son?
Man’s genotype
Man’s gametes
Woman’s genotype
Woman’s gametes
Inheritance of colorblindness
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Set up a Punnett square
XB
Xb
Mom = XB , Xb
Dad = XB , Y
XB
Y
Possible genotypes and
phenotypes of
Sons?
Daughters?
Sex-linked disorders:
Duchenne muscular dystrophy
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X-linked recessive disorder
Symptoms: Wasting away of
the muscles
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Eventually confined to
wheelchair
Death by age 20
Due to absence of protein
involved in release of Ca
from ER in muscles
Sex-linked disorders:
Hemophilia
Hemophilia: an X-linked, recessive trait
Two genes that encode blood-clotting
proteins are on the X chromosome
Any male (XY) who inherits the mutant gene
will develop hemophilia
Rare: occurs in 1/10,000 Caucasian males
Family pedigrees
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Used to find out if a particular
human trait is inherited
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geneticists can’t control the
mating of their subjects, so they
analyze the results of matings
that have already occurred
Assemble info into a family tree
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represent males,
females
Colored symbols = person has
trait being studied
Practice problems
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Prepare for the quiz on Thur
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Questions at end of Chapter 20:
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#1, 3, 7, 11, 13, 15, 16, 19, 20, 26, 27
and 30