Codominant alleles
– Ex: hair color in cats (calico), horses (roan)
– Ex: Human blood type and the sickle cell trait
• 2 dominant and 1 recessive alleles
for a trait (vs. 1 dominant, 1
Blood
Phenotype
Matching
Genotype
Type A IA IA
IA i
Type B IBIB
IB i
Type AB IA IB
Blood typing:
the ‘A’ and ‘B’ alleles are both dominant, and will both
show up if they’re inherited from parents. The ‘O’ is
recessive, and will only show if both parent donate
an ‘O’ to their offspring.
• Do these squares…
•
I
AI
A•
I
B•
i
•
I
Ai
•
I
BIncomplete dominance
• When 2 alleles blend, producing a third
• Example: snapdragon flowers
• Difference between incomplete and codominance?
– Neither allele is dominant, no 3rd
Pedigree
chart = family tree.
This pedigree is tracking familial
blood type. Determine the genotype
of each member.
Key:
Square = male Circle = female Red = Type A Blue = Type B Open = Type O
Linked genes
• Geneticists use 2 parallel bars to indicate
genes located on the same chromosome
• Exception to independent assortment law
Ex: AaBb
x
aabb
=
A B
x
a b
a b
a b
…Sex-linked traits
• Sex-linked traits are always linked to their
sex chromosome
Remember,
XX = girls XY = boys
• Includes
Lethal alleles
• Affect survival of organism
– Changes Mendelian F2 genotype frequency from 1:2:1 to 2:1
– Also, conditional (environmental) lethal, sublethal (hemophilia)
alleles
1. Recessive : only active in homozygous form
– Cystic fibrosis (mucosal disorder)
– Sickle-cell anemia (hemoglobin disorder)
2. Dominant: usually culls itself from population
(unless l.a. expression is after reproduction age)
– Achondroplasia (bone disorder that causes dwarfism) – Huntington’s (CNS)
The #1 problem most scientists have is disseminating their
observations clearly to the public, so present your data in a way that’s obvious to everyone…
Figure 1: Castle's experimental results (1910). The experiment is a recreation of Cuinot (1905).
These results suggest that some alleles are lethal. Copyright American Association for the Advancement of Science, Castle, W. L., et. al., On a modified
mendelian ratio among yellow mice, Science 32, 868 - 870
Figure 2: Lethal allele segregation.
A 2:1 ratio among the progeny of a cross results from the segregation of a lethal allele.
W. H. Freeman and Company (2005).
Nondisjunction
• During meiosis,
chromosomes may not split
• Can occur in Meiosis I or Meiosis II • Most nondisjunction events end in
spontaneous abortion
• Some end in specific genetic disorders
– Turner’s syndrome – female missing X chromosome from dad
Using Karyotyping to identify
chromosomal abnormalities
• Amniocentesis = analysis of amniotic fluid during fetal
development
• Chorionic villus sampling = placental tissue sample
– Both are used in conjunction with…
• Karyotype = snapshot of
Parents choose
amniocentesis to test for…
• Recessive disorders
– Albinism: no skin/hair/eye pigment – Cystic fibrosis: mucus buildup in
organs
• Dominant disorders
– Huntington disease: mid-life mental and muscular degeneration
• Codominant disorders
Recall Mutations’ main ideas
• What are genes?
• How do genetic mutations occur?
• Can we pass on mutations like skin cancer from too much sun exposure?
– what types of cells must be affected by genetic
mutations in order for mutations to affect offspring?
• How do we identify mutations from normal genes?
• How do mutations affect a species? Population?
What is a gene?
Causes of gene changes
• Environment
• Chemicals
• Natural selection
What happens to the genes to
cause these disorders?
• Mutations within the chromosomes of sex
cells can be passed on to offspring
• Somatic cell mutations not inherited
Types of mutations
• Point: where only one nucleotide is
changed
→
may (or may not) cause an
amino acid to change
→
may (or may not)
cause a protein to change
12-4 Mutations
Another gene “mutation”
• A form of translocation, Transposons were identified by B. McClintock in 1948
• Transposon = Jumping gene
• Indian corn – pigment genes jump into and out of each kernel's
chromosomes, and make colorful ears
Genetic Engineering
• Making new species
• Selecting ideal traits of existing species
• Creating new drug delivery systems
Making new species
• Tigers
+
Lions
=
Ligers
Selecting ideal traits of existing
species
• Say you want a good tempered dog, but without all the shedding…
• Lab + Poodle = Labradoodle
• All dogs belong to the same species, Canis
familiaris. You can mix gametes from any 2
Creating new drug delivery systems
• Once scientists isolate a gene (section of
DNA) that codes for some medicine, they
have to make lots of copies
• Bacterial DNA (plasmid) get infused with
medicinal DNA, and reproduce to make
millions of copies in a short time =
recombinant DNA
• Currently used for producing human
• Bacterial DNA in
plasmid is easy to manipulate
• Cut it using restriction enzymes
(endonuclease) • Insertion can only
happen at specific sites (sticky ends) using DNA ligase
• Grow recombinant
(non-parent) plasmid culture • Scientists harvest meds
from bacteria, then
Treating existing genetic disorders
• Gene Therapy (AKA
therapeutic cloning
)
• Recognizes defective gene and tries to
replace it using cloning techniques
• 2 types
– Adult (differentiated) stem cell
Homework
• Create a presentation based on one real example of any of the 4 goals of genetic engineering
– Making new species (ex: history of the glowing cat)
– Selecting ideal traits of existing species (ex: the evolution of the labradoodle)
– Creating new drug delivery systems (ex: antibiotics and bacterial pathogens)
– Treating existing genetic disorders (ex: the potential cure for ALS)
• Utilize terms such as genetically modified organisms,
transgenic, DNA, genome, and Human Genome Project, DNA extraction, PCR, gel electrophoresis, gene transfer,
others.
• Cite at least 3 relevant sources in APA.
