PSYC 3102: Introduction to Behavioral Genetics
Lecture 14
DNA Chips
1. Target tissue
2. get mRNA from tissue
3. Put mRNA on chip
4. mRNA hybridizes with complimentary strand of DNA (Each well of the chip has a certain
section of single stranded DNA)
5. If hybridization occurs the wells ‘light up’ (signals)
6. Chip is optically scanned and data is recorded

More and more relevant in research

Different types

Most common assays for gene expression


Used in cancer research frequently

In neuroscience –
o use control group of animals and experimental group
o take brain tissues
o have a chip for control group and chip for experimental group
o compare chips, differential expression is observed

Companies make chips specifically for areas of research

Have to know gene sequence in order to have a specific DNA strand put on chip
Mendelian Genetics

Mendel bred pea plants -- inbred plants which are homozygous at all loci
Example 2
Example 1
Parental Strains
F1 generation
F2 generation
(derived from
crossing F1x F1)
F1 generation has information for green or wrinkled phenotypes, even if they don’t show it
Mendel’s Laws of Inheritance
1. Law of Dominance

Some hereditary factors dominate others
o yellow dominates green
o cross fertilization of this generation
o resulting generation ¼ green and ¾ yellow
o somehow has the ability to pass on previous information for green
2. Law of Segregation

Hereditary factors are discrete and do not blend

Each parent has 2 hereditary factors and passes 1 at random to offspring
Two green peas with GY genotypes crossed:
G (green)
Probability = .5
GG
.25
GY
.25
G (green)
Probability = .5
Y (yellow)
Probability = .5
Genotypes:
Phenotypes:
Y (yellow)
Probability = .5
GY
.25
YY
.25
25 GG; .5 GY; .25 YY
.75 green; .25 yellow
3. Law of Independent Assortment

Hereditary factors for one trait (eg. color) are independent of those for another trait (eg.
shape)
PUNNETT SQUARE
Gamete
s
Probabilities
A .5
a .5
A
.5
AA
.25
Aa
.25
a
.5
Aa
.25
aa
.25
Phenotypes – have to know relationship between genotypes and phenotypes (dominance, etc.)
Put down:
Genotypes:
AA = .25
Aa = .5
Aa = .25
Phenotypes: (flower color)
If A is dominant:
Red = .75; White = .25
BUT if A and a are co-dominant:
Red = .25; Pink =.5; White = .25
X-linked genes

Genes that are found on the X chromosome

follow the same rules but keep track of the chromosomes

gene for hemophelia is on the X chromosome

2 alleles: A normal gene, a hemophelia

Y chromosome does not have a comparable gene
Look at father with hemophelia, mother who is a carrier (Aa – hemophelia is recessive)
Xa1/2
Y1/2
XA1/2
XAXa ¼
XAY ¼
Xa1/2
XaXa ¼
XaY ¼
Genotypes
Phenotypes
Frequencies
XAXa
normal ♀ (carrier)
¼
XAY
normal ♂
¼
XaXa
hemopheliac ♀
¼
XaY
hemopheliac ♂
¼
ABO Blood System
Genotype
Phenotype
AA
A
AO
A
BB
B
BO
B
AB
AB
OO
O
1. gametes from mother
2. gametes from father
3. label rows and columns with these gametes and probabilities
A
.5
AB
.25
AO
.25
B .5
O .5
Genotypes
Phenotypes
Frequency
AB
AB
¼
BO
B
¼
OA
A
¼
OO
O
¼
O
.5
BO
.25
OO
.25
Mendel’s laws are not 100% true

Co-Dominance can occur: AB blood type; Pink petunias

Segregation almost always applies, but is not always the case
o By dumb luck, Mendel looked at 7 traits, all on different chromosomes
o Independent assortment is violated when 2 genes are close together on the same
chromosome
Linkage
Central mathematical observation that underlies linkage:

Mathematically considered, the chromosome is a long string with beads at intervals that
represent genes

When genes are close together, they don’t independently assort and are inherited as a unit

Genes that are far away or are on different chromosomes follow independent assortment
We have homologous chromosomes – maternal and paternal versions of each chromosome (2
homologs of #1, #2, etc.)

In the process of generating sperm or eggs, recombination (crossing over) occurs

Homologous chromosomes join together (mom’s and dad’s)

They exchange genetic material

Resulting chromosomes that get put into sperm and eggs (that you pass on to your children)
contain part of your dad’s chromosome plus part of your mom’s chromosome

The probability of recombination occurring between 2 loci on the same chromosome is a
function of the distance between them

The further the 2 loci are from each other, the more likely the homologous chromosomes will
recombine somewhere between them

The closer the 2 loci are to each other, the less likely there will be recombination and the
more likely they will be transmitted together
Calculating Alleles for Gametes in Linked Loci

θ Theta = recombination fraction (probability that a recombinant gamete comes from 2
linked loci)

1 – θ = probability that 2 linked alleles are transmitted as a unit

Get value of theta (by means we won’t go into), then figure out:
D
d
a
A
D .5
d .5
Recombination?
No
1–θ
Da
.5 (1 – θ)
dA
.5 (1 – θ)
Yes
θ
DA
.5 θ
da
.5 θ

If theta = .1 then:
Da = ½ (1-.1) = .45
dA = .45
DA = ½ (.1) = .05
da = .05

When theta is close to zero, alleles are adjacent to one another

If theta reaches the max of .5, then independent assortment holds true
Linkage Analysis

Consists of tracing co-segregation of a trait allele (usually a disease) with one or more
marker genes within a pedigree

Purpose is to find the approximate location of the trait allele