Download xCh9 genetics F11

What is heredity?
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Ch. 9 Genes and Inheritance
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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
Father of genetics
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Gregor Mendel solved the
puzzle of heredity 150 yrs
ago
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Mendel’s one-trait experiments
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before the discovery of DNA
and chromosomes
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Mendel was the first person
to analyze patterns of
inheritance
Deduced the fundamental
principles of genetics.
Characteristic – flower color
Crossed true-breeding strains
1st generation: all purple flowers
Mendel allowed these plants to
self-pollinate.
2nd generation: ratio of 3 plants
with purple flowers to 1 with
white flowers
Where did the
white flower
come from?
Mendel developed 4 hypotheses
Hypothesis #1
 There are alternative
forms of genes, called
alleles.
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Gene for flower color
Purple (P)
The gene for flower color
has 2 alleles – one for
purple color (P), the other
for white color (p)
Mendel’s 2nd hypothesis
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Each individual has 2
alleles for each trait
purple
The alleles can be the same
or different
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White (p)
Gene for flower color
If they are different, the
organism is heterozygous –
Pp
If they are the same, the
organism is homozygous –
PP, pp, YY
white
Mendel’s 3rd hypothesis
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Mendel’s 4th hypothesis
Alleles can be dominant
or recessive.
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The dominant allele
determines the
organism’s appearance
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Gametes carry only one allele for each trait
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PP or Pp
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pp
Mendel’s law of segregation
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Use upper case letters: P
When sperm and egg unite at fertilization, each
contributes its allele.
When an individual reproduces, the two alleles
split up (segregate) and go to separate gametes
The recessive allele has
no noticeable effect
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P
Use lower case letters: p
p
Mendel’s theory of heredity
Where did that
white flower
come from?
Terms used in modern genetics
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Genes
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Mendel explains his
theory on YouTube
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Carry the instructions for
making proteins
Each gene is found at a
specific site on a chromosome
Parent’s genes are inherited by
offspring
Alleles are different versions
of a gene
Diploid cells (2n) have pairs
of genes on homologous
chromosomes
"Thread of Life" -Mendel's Gene Theory Explained. (4 min)
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Genotype and phenotype
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Genotype – an
individual’s genes
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These separate in meiosis
http://www.youtube.com/watch?v=pkYxEy9gF1w&feature=related
Allele “F” codes for
freckles
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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F
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The dominant allele = F
The recessive allele = f
Chromosomes are found in pairs
Possible genotypes:
Phenotype – what an
individual looks like
F
F
F
f
Examples of traits controlled
by a single gene in humans
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Inheriting a trait
These traits are
determined by simple
dominant-recessive
inheritance
Possible genotypes for
someone with
freckles? FF and Ff
Without freckles? ff
If a man with short fingers marries a woman
with long fingers, what genotypes and
phenotypes will their children have?
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Dad
Mom
Dominant trait
SS or Ss
Forming the gametes
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Recessive trait
ss
Forming the gametes
Gametes carry only one allele for each trait.
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Gametes carry only one allele for each trait.
Mom
Dad
S
meiosis
s
s
S
S
s
meiosis
S
S
Genotype = ss
Genotype = SS
Gametes are
haploid–only carry
one of each
chromosome
Fertilization
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Crosses
When sperm and egg unite at fertilization,
each contributes its allele.
S
s
Fertilized egg
S
s
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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.
no freckles
Freckles
ffFf
Parents
no
freckles
Freckles
Ff
ff
meiosis
Genotype = Ss
Phenotype?
gametes
Offspring
f
Gametes?
F
f
Gametes?
f
ff
no freckles
f
F
Punnett square analysis
Grid used to predict the genotypes of the offspring
Shows all possible combinations of egg and sperm!
Punnett square analysis
What are the genotypes of the offspring of two
heterozygous freckled parents (Ff)?
#1 Decide what gametes would be produced by each parent.
F
Genotype? Gametes?
Mom
Ff
Dad
Ff
F
F
f
F
FF
Ff
f
Ff
f
f
Ratio: 1FF, 2 Ff, 1 ff
F
F
f
ff
What are the phenotypes?
f
Recessive disorders
Mendel: single-gene
inheritance
Most human genetic disorders
are recessive.
Albinism
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1 gene → 1 trait
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lack of melanin pigment in the
eyes, skin and hair
affects mammals (including
humans), fish, birds, reptiles and
amphibians
Need 2 copies of mutated gene
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Genotype = mm
Albino alligator
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.
Caused by a mutation in
the FGFR3 gene
→ abnormal bone and
cartilage formation
Just need 1 copy of
mutated gene
Huntington disease:
a dominant genetic disorder
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Caused by mutations in the
HTT gene.
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
Incomplete Dominance
RR
Many neurons in
normal brain.
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Rr
rr
The heterozygotes have an intermediate
phenotype
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Alleles R an r are not fully dominant or fully recessive
Loss of neurons in
Huntington brain.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Co-dominance
both alleles are expressed
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The ABO blood groups
in humans are an
example of multiple
alleles:
 A, B and O
The A and B alleles
show co-dominance
 Both alleles are
expressed
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When a single gene
has more than 1
effect
AB blood type.
Marfan syndrome–
an example of pleiotropy
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A mutation in the FBN1
gene
Traits controlled by multiple
genes
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Height: At least 180 genes control how
tall a person grows
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Connective tissue
defects
Number of Men
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Pleiotropy
most
are
this
height
few
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Disproportionately long
hands, a weak aorta,
caved in breastbone,
and other symptoms
62
short
64
Courtesy University of Connecticut/Peter Morenus, photographer;
few
66
68
Height in Inches
70
72
74
tall
Sex-linked inheritance
Involves genes located on
a sex chromosome, usually
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Any gene located on a sex
chromosome
Most are on the X
chromosome
Disorders caused by sexlinked recessive alleles:
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the X chromosome.
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Sex-linked genes
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X chromosomes contains
nearly 2000 genes
Y chromosomes contains only
80 genes
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Females are XX, but males
have only 1 X chromosome
(XY)
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Affects the pattern of
inheritance
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Inheriting colorblindness
Red-green colorblindness
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Sex-linked disorder
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An X-linked, recessive disorder
due to recessive allele
seen mostly in males
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Red-green color
blindness
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The light-sensitive cells
in the eyes don’t
function properly.
Prevalence: 5-10% of
males
Red-green colorblindness
Hemophilia
Male-pattern baldness
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Write the following genotypes
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A test for red-green
colorblindness
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Inheriting 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?
XB Y
Man’s genotype
XB and Y
Man’s gametes
Woman’s genotype XBXb
Woman’s gametes XB and X b
XB = normal vision
Xb = color blindness
Normal man
XBY
Color-blind man
XbY
XBXB
Normal woman
XBXb
Woman carrier
Color-blind woman XbXb
Inheriting colorblindness
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Set up a Punnett square
XB
XB
Y
Xb
Mom = XB , X b
Dad = X B , Y
Possible genotypes and
phenotypes of
Sons? XBY or XbY
Daughters?
XBXB or Xb X B
The son’s phenotype results
from the sex-linked gene on
his single X chromosome