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)