Transcript Unit 7 Genetics

• “The Father of Genetics”
• Monk during the 19th
century (1822-1884)
• Studied Pea plants
•
Peas were good choice.
– Readily available
– Easy to self-pollinate and cross-pollinate
•
Good experimental choices.
– Only chose “either-or” traits (purple OR white)
– Started with true-breeding (purebred) plants
– Followed for 3 generations (P, F1, F2)
•
Kept good quantitative data.
– Very large sample sizes


Cross-pollinate 2 purebred
plants
(P generation)
Resulting offspring
(F1 generation)
were all with dominant
trait
So where did the
“white” go?
Mendel allowed F1
plants to self-pollinate
to see if they really
had “lost” the white
Approximately ¾
of F1 plants
produced seeds
that grew into
purple flower
plants
The remaining ¼
made white
flower plants
•
Alternate versions of hereditary “factors” account
for variation in inherited traits
•
For each trait, an organism inherits 2 “factors”
(one from each parent)
 If
the “factors” differ, one is dominant and
other is recessive
 The
2 “factors” for each trait separate
during gamete production (meiosis)
• Law of segregation - when sex cells are made…
the 2 factors separate…1 per gamete
Mathematically
proven through
both generations






Genotype
• type of genes (ex: Tt)
Phenotype
• traits (ex: tall)
Homozygous
• Two of same allele (ex: TT or tt)
Heterozygous
• One of each allele (ex: Tt)
Dominant
• Gets expressed; use capital letter
Recessive
• Gets covered; use lowercase letter
 Where
are the “factors” that Mendel
discovered?
• On our chromosomes
 How
do these “factors” get passed on to
offspring?
• Through the gametes during fertilization
 What
do we call these “factors” now?
• Alleles (different forms of the same genes)
 Why
can’t we use mitosis to make
gametes?
• Mitosis makes diploid cells with two sets of
chromosomes (2n = diploid)
* gametes must be haploid (n) having only one
set of chromosomes
 What is the main goal of meiosis?
• Meiosis produces cells with only one set of
chromosomes which are haploid gametes
Fertilization – fusing
of sperm & egg
Zygote – fertilized
egg (diploid) which
develops into an
embryo
Meiosis – type of cell
division that produces
egg & sperm; occurs
in ovaries & testes
MITOSIS

cell division that produces
2 genetically identical
diploid daughter cells
MEIOSIS

ex. Somatic or body
cells

This type of cell division
produces identical
daughter cells which leads
to the development of
tissues and organs
cell division thatproduces 4
genetically different
haploid daughter cells
ex. Gametes or sex
cells

This type of cell division
produces gametes which
are all different and unique.
Meiosis is process to split
chromosome # in half
Result: 4 cells each with 1
of each type of
chromosome
Meiosis I – halves the
chromosome #
Meiosis II – reduces
amount of DNA by half
•
Homologous chromosomes
– Carry same type of genes (though not necessarily
the same version of that gene)
– Ex: chromosome pair #1…both have gene for eye
color in same spot…one codes for blue, other for
brown
KEY
TERM: Synapsis Homologous chromosomes pair up
(prophase I)
KEY
TERM: Tetrad Group of 4 chromatids together during
synapsis
KEY
TERM: Chiasma (chiasmata) Crossing of non-sister chromatids
(see crossing over)
Metaphase I: tetrads line up
Anaphase I: homologous chromosomes separate
Works just like mitosis
The
positioning of
tetrads in
metaphase
determines
variability of
resulting
gametes
 If
diploid # is 4 chromosomes
• 2 x 2 = 4 possible gametes
 If
diploid # is 6 chromosomes
• 2 x 2 x 2 = 8 possible gametes
 If
diploid # is 46 chromosomes (like us!)
• 2 x 2 x 2 x …x 2 = 8 million possible gametes
And possibility after fertilization…
8 million x 8 million = 64 trillion possible individuals
Crossing over
during meiosis I,
nonsister chromatids of
homologous
chromosomes switch
places
Results in even more
genetic variability



Parent genotypes listed
on edges
Fill in spaces…big letter
listed first
List genotype (G) and
phenotype (P) including
fractions or percents
G: 4/4 Aa
P: 4/4 red
To
determine
genotype of
a dominant
phenotype
organism
 What
happens if you test 2 traits at the
same time? (dihybrid cross)
 What
if you cross purebred yellow-round
with purebred green-wrinkled?
• Will traits “stick” to each other?
• Will traits “split up” from each other?
Alleles are segregated (and inherited) separately
(T=tall, t=short, P=purple, p=white)
(T=tall, t=short, P=purple, p=white)
TP
TP
Tp
tP
tp
Tp
tP
tp
(T=tall, t=short, P=purple, p=white)
TP
Tp
tP
tp
TP
TTPP
TTPp
TtPP
TtPp
Tp
TTPp
TTpp
TtPp
Ttpp
tP
TtPP
TtPp
ttPP
ttPp
tp
TtPp
Ttpp
ttPp
ttpp
How many different phenotypes is that?
(T=tall, t=short, P=purple, p=white)
TP
Tp
tP
tp
TP
TTPP
TTPp
TtPP
TtPp
Tp
TTPp
TTpp
TtPp
Ttpp
tP
TtPP
TtPp
ttPP
ttPp
tp
TtPp
Ttpp
ttPp
ttpp
P: 9/16 tall, purple
3/16 tall, white
3/16 short, purple
1/16 short, white
 Intro
to genetics
Meiosis
Punnett square basics
Monohybrid/Dihybrid crosses
Mendelian Genetics
Mendel’s experiments
Mendel’s Laws
 Mendel’s
laws still apply, but many traits
due to more complicated relationships
between alleles


A single dominant allele
inherited from one
parent is all that is
needed for a person to
show the dominant trait.
Ex:
-Earlobes attached
is recessive trait
- Flower color in
peas

Dominant partially covers
recessive; heterozygotes will
have an in-between
phenotype
Ex:
curly-wavy-straight hair

Sample Problem #1
RR – red flower
WW – white flower
 Both
alleles dominant…
both expressed (no
blending in hetero’s)
Ex:
human blood grps
Sample Problem #
Type AB – IAIB
Some traits have more than 2 possible alleles
Ex: Human blood has A, B, and O
Which other
pattern
does this
reflect?
codominance
• In humans, sex-linked genes are the ones on the
X chromosome
• Fathers pass these on to their daughters only and
mothers pass these on to both sons & daughters
• Males more likely to have recessive sex-linked
traits
Sex-linkage – Sample Problem #1XX - female
XY - male
 Ex: male-pattern
baldness; hemophilia;
color-blindness
Due to more than one
gene controlling a trait
Has an “additive effect”
Ex: human eye color,
skin color, hair color,
height
•
Ranges from complete dominance to
incomplete dominance to codominance
•
Reflects expression of alleles, NOT one
allele “covering up” another
•
Does not reflect prevalence in population
– Recessive allele may be more common
Ex: Flower color differs based
on pH of soil
 Phenotype
depends on environment &
genes
 Ex: nutrition, physical activity, education,etc
 Norm of reaction = range of phenotype
governed by a gene
• Some traits have no range (blood type)
• Some traits have large range (esp. polygenic)
 Not
easy to study
• Generations too long
• Not enough offspring
• Cannot selectively breed
 Must
find alternative methods to figure
out human inheritance patterns
Traces traits through a family
Used to determine genotypes & phenotypes
Used to predict probability of certain traits in future offspring
Purple = has
disease
alkaptonuria
Is this trait due to a dominant or recessive gene?
What are the genotypes for each individual?
•
Cystic fibrosis (recessive)
– 1/2500 whites of European descent
– 4% of whites are carriers (heterozygous)
– Cl- transport is abnormal…thick mucus
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
+ = wild type
allele
cf = cystic fibrosis
allele
• Recessive Inheritance
• 3:1 Phenotype
• 1:2:1 Genotype
+ cf
+ cf
Carrier parents
For each child conceived:
+
+ +
25% chance
unaffected
noncarrier
(a)
cf
+
cf
50% chance unaffected
carrier (cf allele inherited
from either parent)
cf
cf
25% chance
affected
cf
I
cf
cf
cf
Joe
Mary
cf cf
II
(b)
Punnett Square
(c)
Bill Sue Tina
50
 Tay-Sachs
Disease (recessive)
• 1/3600 of Ashkenazic (European) Jews
• Dysfunctional enzyme that does not break down
brain lipids
• Seizures, blindness, motor & mental degeneration
 Duchenne’s
Muscular Dystrophy
(sex-linked
recessive)
• Muscles atrophy
• Gene carried on X
chromosome
 Recessives
should be rare so chance that
2 people will have exact same recessives
are low
 Chances
increase if the 2 people are
related
 Lethal
recessive traits much more
common than lethal dominant traits…
 Sickle-Cell
Disease (codominance)
• 1/400 African Americans
• Substitution of 1 amino acid in hemoglobin
• Abnormal cell shape = less oxygen = many other
symptoms (pleitropic)
• Heterozygotes may/may not have symptoms
 Codominance – both hemoglobins made
 Increases resistance to malaria
 Sickle-Cell
Disease
 Achondroplasia
• Type of dwarfism
• 1/10,000 people
 Huntington’s
disease
• Degenerative disease of
nervous system
• starts ~35-45 yrs of age
(after reproductive age)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Dominant Inheritance
• 1:1 Phenotype
• 1:1 Genotype
+ = wild type
allele
HD = Huntington
disease
allele
+ HD
Affected parent
++
Unaffected parent
For each
Individual
conceived:
50% chance
unaffected
50% chance
affected
++
+ HD
(a)
I
HD HD
Dan
Ann
Pam
Eric
HD
II
(b)
Punnett Square
(c)
57
 Heart
disease
 Diabetes
 Cancer
 Alcoholism
 Schizophrenia
 Manic-depression
Male but often sterile; often with feminine characteristics
Male; perhaps taller than normal
 XXX
• female; nondistinguishable from XX
 X0
• Turner’s syndrome
• Female; typically sterile
 0Y
• Not viable; would not be born