Chapter 3 - Genetics I.Structure Chromosomes in cell nuclei - 23 pairs/46 per cell 22 pairs = autosomes 1 pair = sex chromosomes.
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Chapter 3 - Genetics
I. Structure
• Chromosomes in cell nuclei
- 23 pairs/46 per cell
22 pairs = autosomes
1 pair = sex chromosomes
• DNA - 1 long molecule
- double-helix
• Gene - segment of DNA
- gene pairs determine traits
• Allele - 1 member of gene pair
- each parent gives 1 allele to child
II. Genetic Transmission of Characteristics
• 1 Gene/allele from each parent
- 1 Brown or 1 blue from each parent
• Traits = Dominant v. Recessive
Brown = dominant
blue = recessive
• Gene pairs = Homozygous or Heterozygous
“same cell” “different cell”
BB or bb Bb
• Phenotype vs. Genotype
Appearance vs. Genetic code
III. Transmission Types
• 1 Gene pair, Dominant trait
- if even 1 allele for dominant trait, that trait will be exhibited (BB or Bb)
- if parent is heterozygous, kids might not have trait
• 1 Gene pair, Recessive trait- to exhibit trait,
must have both recessive alleles (bb)- both parents must contribute recessive trait- if both parents exhibit (bb), all kids will- if both parents carry (Bb)
1 child doesn’t have (BB), 2 carry (Bb),1 exhibits (bb)
• Sex-linked traits- X determines more characteristics than Y - recessive x overridden by dominant X, not by Y- skews sex distribution of characteristics from recessive genes- so more boys exhibit, more girls carry- girl exhibits only if both parents have recessive x
Note:
• Sex-limited gene = on autosome, not sex chromosome
- exerts effects more strongly in 1 sex than the other
- hormones
IV. Polygenetic Research
• Polygenetic traits
- most traits involve multiple genes + the environment
- so most are on a continuum
Three types of studies – all are consanguinity/concordance
• Family
• Twin
• Adoption
• Family Study- the more closely related people are
the more genes they share,the more similar they will be
- 50% shared between siblings &parent/child
- 25% between grandparents, aunts/uncles- 0% between unrelated people
Problem
- those who are more closely related are more likely to share environment
• Twin Study - identical vs. fraternal twins
MZ DZ
- special environment of twinness
- difference = amount of genes shared
• MZ/identical = 100% shared genes
• DZ/fraternal = 50% shared genes
• If MZ = more similar than DZ, genetic evidence
Problem
- MZ also more likely to share environment& be treated the same
• MZ together vs. MZ apart
• Adoption Study
- adopted children share only genes with biological parents
- share only environment with adoptive parents
- if more similar to biological parents, evidence for genetic influence
• Problem
- Adoptive homes often similar to biological homes
V. Chromosomal Abnormalities
Types: Abnormal number vs. structure
Abnormal number
- problems with more or less than 46
• Down syndrome (Trisomy 21)
- 1 in 600 births
- additional 21st autosome
- genetic but not inherited
• Turner’s syndrome- 1 in 3000 female births XO- females with only 1 X chromosome
• Klinefelter’s syndrome- 1 in 500 male births- males with 1+ extra X chromosome(s) XXY
• XYY complement - Supermale Syndrome- 1 in 1000 male births- males with 1+ extra Y chromosome(s) XYY
• Superfemale Syndrome- 1 in 1000 female births- females with 1+ extra X chromosome(s) XXX
Abnormal structure
• Fragile X syndrome- 1 in 2000 births- long arm of X breaks- recessive- males- MR- prevention: folic acid
VI. Genetic Testing
5 Methods
• Preimplantation - one cell from embryo
• Amniocentesis
- extracts amniotic fluid
- after 16th week
• Chorionic Villus Sampling (CVS)
- extract tissue from chorion
(membrane surrounding fetus)
- 7th/8th week
• Quadruple screen- test mother’s blood for 4 proteins/hormones
(Alpha-feto protein, HCG, Estriol & Inhibin)- high = neural tube defect or multiple fetuses- low = Down syndrome
• Targeted ultrasound- gross abnormalities- heart & neural tube abnormalities- proportions can indicate Down
What can you screen for?
• Diseases
• Fragile X
• Down Syndrome
• Pharmacogenomics
• Behavioral traits?
• Genetics and Ethics
• Genetic determinism
- polygenetic nature of most traits
- role of environment
- too complex to isolate single gene
• Cloning
- identical twins
- clones … original
(brain connections determined by experience)
• Eugenics (“good genes”)
- from evolution
- in Britain and U.S.
- laws preventing marriage of “feebleminded”
- immigration restrictions
- included forced sterilization
- Carrie Buck (1928)
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