Download Chapter 3 - Genetics

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)