chapter 14
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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 - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 14Mendel and the Gene Idea
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Mendelian genetics
• Gregor Mendel – father of genetics
Austrian monk
pioneer in the field
gardener at monastery
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Mendel’s Laws1) 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
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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
½ x ½ = ¼
or Pp x Pp = pp
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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
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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
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Multiple Alleles
- 3 or more alleles for 1 gene- ex: human blood groups
phenotype genotype antigens antibodies A IAIA or IAIa A anti–B B IBIB or IBIb B anti–A AB IAIB A or B O ii neither A anti–A
& nor B anti-B
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Test cross
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Pleitropy
• One gene = many effects
ex: sickle cell anemia
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Penetrance
• Proportion of individuals who show expected phenotype from their genotype
- ex: neuroblastomas
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Polygenic inheritance
• Many genes = one trait
-ex: human skin color
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Pedigree analysis
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Human Genetic Disorders
• Cystic fibrosis
• Tay-Sachs
• Sickle-cell anemia
• Huntington’s chorea
• Duchenne’s Muscular Dystrophy
• Down Syndrome
• Achondroplasia
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Preventive Testing for genetic disorders
• usu. done when risk is high1) pedigree determination2) 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
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Preventive
c) ultrasound – soundwaves
(noninvasive; no risk)
d) fetoscopy – tube with viewing scope
directly examines fetus
3) newborn screening –
PKU test
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Chapter 15
The Chromosomal Basis of Inheritance
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• Genes are located on chromosomes, the
structures that undergo segregation &
independent assortment
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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
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Sex-linked traits
• Carried on sex chromosomes
• May be X-linked or Y-linked
• No Y-linked found thus far
• ex: red-green color blindness in humans
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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)
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Recombination frequency
Frequency of = # of recombinants
recombination total # of offspring x 100
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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
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Sex determination systemsa) 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
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Sex determination systemsc) 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);
fatherless
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Sex-linked traits
• carried on sex chromosomes
• may be X-linked or Y-linked
• example: red-green colorblindness, hemophilia
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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
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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
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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)
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Karyotype
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Chromosomal mutations1) 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
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Chromosome mutations
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Genomic imprinting
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