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Chapter 14Mendel and the Gene Idea
Mendelian genetics
• Gregor Mendel – father of genetics
Austrian monk
pioneer in the field
gardener at monastery
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
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
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 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
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
• 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 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
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
• 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 havedifferent phenotype than the parent, we say thereis 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 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
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 sexd) haplo-diploid system – most bees, ants -no sex chromosome -females develop from fertilized eggs (2n) -males develop from unfertilized eggs (1n); fatherless
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 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
Chromosome mutations
Genomic imprinting