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Chapter 14
Mendel and the Gene Idea
Mendelian genetics
• Gregor Mendel –
father of genetics
Austrian monk
pioneer in the field
gardener at
monastery
Mendel’s Laws
1) Law of Dominance and Recessiveness –
when 2 different alleles are present, one
masks, or covers up, another
2) Law of Segregation – alleles separate when
gametes form
3) Law of Independent Assortment – one allele
does not influence another ex: tall does not
influence yellow
Probability
• Rule of Multiplication –
probability that 2 or more independent events
will occur simultaneously in some specific
combination
-take probability of each event and multiply
together
-ex: both coins landing heads up
½x½=¼
or Pp x Pp = pp
Probability
• Rule of Addition – probability of an event that
can occur in 2 or more different ways
- add separate probabilities
- ex: heterozygote from Pp x Pp
¼+¼=½
Law of Incomplete Dominance –
(Intermediate inheritance)
- When 2 different alleles are present
(heterozygous), an intermediate trait is
expressed
ex: red flowers x white flowers = pink flowers
Multiple Alleles
- 3 or more alleles for 1 gene
- ex: human blood groups
phenotype
A
B
AB
O
genotype
IAIA or IAIa
IBIB or IBIb
IAIB
ii
antigens antibodies
A
anti–B
B
anti–A
A or B
neither A
nor B
anti–A &
anti-B
Test cross
Pleitropy
• One gene = many effects
ex: sickle cell anemia
Penetrance
• Proportion of individuals who show
expected phenotype from their genotype
- ex: neuroblastomas
Polygenic inheritance
• Many genes = one trait
-ex: human skin color
Pedigree analysis
Human Genetic Disorders
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Cystic fibrosis
Tay-Sachs
Sickle-cell anemia
Huntington’s chorea
Duchenne’s Muscular Dystrophy
Down Syndrome
Achondroplasia
Preventive
Testing for genetic disorders
• usu. done when risk is high
1) pedigree determination
2) fetal testing:
a) amniocentesis – 14th – 16th wk. of pregnancy; needle
inserted into uterus; 10 ml fluid extracted & karyotype done
b) chorionic villi sampling – sm. Amt. of fetal tissue is
suctioned off from embryonic membrane villi (chorion)
which forms part of placenta, then karyotype (results in 24
hrs.)
advantages: 24 hr results vs. several weeks ;
8-10 wks of pregnancy
Preventive
c) ultrasound – soundwaves
(noninvasive; no risk)
d) fetoscopy – tube with viewing scope
directly examines fetus
3) newborn screening –
PKU test
Chapter 15
The Chromosomal Basis
of Inheritance
• Genes are located on
chromosomes, the
structures that
undergo segregation
&
independent
assortment
Thomas Hunt Morgan – 1st one to associate
specific genes with specific chromosomes
- studies with fruit flies, Drosophila
melanogaster
1) grow rapidly
2) require small amt. of space
3) few chromosomes & these are large
• 1st to discover a sex-linked gene (white eyes)
X-linked
Sex-linked traits
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•
•
•
Carried on sex chromosomes
May be X-linked or Y-linked
No Y-linked found thus far
ex: red-green color blindness in humans
Recombination
• In unlinked genes, when 2 organisms produce
offspring, the end result could be:
parental types or recombinants (unlike either
parent)
Frequency of recombination – if ½ have
different phenotype than the parent, we say there
is a 50% frequency of recombination (maximum)
Recombination frequency
Frequency of = # of recombinants
recombination
total # of offspring x 100
Gene mapping
• map units - number assigned to show
relative distance between genes on
chromosomes
• recombination frequency = # of map units
ex: recombination frequency of 25%
translates to 25 map units
Sex determination systems
a) X-Y system
-humans, mammals, some insects
-sperm (X or Y) determines sex
b) X-O system
-grasshoppers, crickets, roaches, some
insects (only 1 sex chromosome)
-female XX male XO
-sperm either contains X or O
Sex determination systems
c) Z-W system
-birds, some fishes, some insects (moths,
butterflies)
-Z & W used to avoid confusion with X-Y
-female ZW male ZZ
-egg determines sex
d) haplo-diploid system – most bees, ants
-no sex chromosome
-females develop from fertilized eggs (2n)
-males develop from unfertilized eggs (1n);
X-inactivation (in mammals)
• fur color in calico cats determined by X
chromosome 1X orange fur, 1X black fur
• calico cats almost always female
• 2 X chromosomes inherited, but in embryonic
development, 1 is almost completely inactivated
(inactive X condenses to Barr body)
• selection of which X occurs ramdomly
• ex: in humans dev. of sweat glands (mosaicism)
heterozygous female have patches of normal
skin & patches lacking sweat glands
Aneuploidies
• abnormal number of chromosomes
• due to nondisjunction – failure of chromosomes
to separate in anaphase
1) trisomy – 2n+1
having 3 chromosomes in a pair
ex: Trisomy 21 (Down Syndrome)
2) monosomy – 2n-1
having only 1 chromosome in a pair
ex: Turner Syndrome
Polyploidy
• having extra sets of chromosomes
• Triploidy (3n)
ex: diploid egg fertilized
• Quatraploidy (4n)
ex: 2n zygote may not divide
-fairly common in plants; almost nonexistent in
animals (appear more normal than aneuploids)
Karyotype
Chromosomal mutations
1) deletion – piece of chromosome is lost
*most serious
2) duplication – extra piece of chromosome
3) inversion – piece of chromosome breaks off
& reattaches in a different orientation
4) translocation – piece of chromosome breaks
off & reattaches to a nonhomologous
chromosome
Chromosome mutations
Genomic imprinting