Download Modified Mendelian Ratios

Modified Mendelian Ratios Ch. 13
1. Multiple alleles (more than 2 alleles for gene in
population)
Chromosome 9 I gene

Example: Blood Groups
Karl Landsteiner 1900’s
ABO blood system = polymorphic I gene
Blood type
A
B
AB
O
genotype
IAIA or IAi
?
IAIB
ii
What is the mechanism of inheritance of A, B, AB, O?
Autosomal or sex chromosome?
Which two alleles are codominant?
Which allele is recessive?
Example


A child has type O blood. The mother of the child
has Type B blood. What could the blood types of
the f
IBi
ii
A,B,O carbohydrate antigens
Bombay phenotype

A man IAIA (Type A) and a woman of type IAIB have
a child with type O blood. How??

I gene encodes glycotransferases
 Glycotransferases

add sugar group to glycolipid
H gene encodes the glycolipid antigen
HH IAI A
Hh IAIi
2. Incomplete dominance
Example: snapdragons
Allelic symbols do not specify dominance
CR = red color
Cw = white color
CR CR
X Cw C w
3. Codominance
Each allele encodes separate gene
product distinct in heterozygote
L gene for human blood cell surface protein
LM = M antigen
LMLN x LMLN
LN = N antigen
1M
A man with the M bloodtype has a child with
a woman of the MN bloodtype
 Expected ratio of offspring?

4. Lethal alleles
MM = normal spine
MM’ = manx cat (no tail)
M’M’ = lethal
Cross two manx, what is ratio of phenotypes in offspring?
How do breeders obtain manx cats?
Lethal alleles in humans

Tay Sach disease tt is fatal

Huntington disease HH is fetal lethal Hh
causes death ~ age 50.
Polydactyly, dominant
5. Penetrance

% individuals that exhibit
phenotype corresponding to
genotype
Pp
5,5
pp
6, 5
6, 6
6. Expressivity =the extent to which
a trait is exhibited
Piebald spotting
Both sisters have
same genotype
Human piebald trait
autosomal dominant defect in melanocyte development mutation in the KITgene,
which encodes cell surface receptor tyrosine kinase
http://dermatlas.med.jhmi.edu/derm/indexDisplay.cfm?ImageID=386927278
Incomplete penetrance AND expressivity

NF-1 = Neurofibromatosis1

(1/4000, 17q11.2)
 (350


kb gene, 60 exons)
Autosomal dominant trait NN and Nn with 50- 80%
penetrance
Variable expressivity
Mild form
tumors on nerve CT coverings, skin, eyes, organs, face
speech, blood pressure, spine curvature, headaches
Gene expression also affected by:




Sex (baldness)
Temperature (melanin in Siamese cats)
Chemicals (PKU)
Diet (height, cancer)
7. Epistasis- gene product interactions
A product of one gene influences, or
masks, the expression of another gene(s)
 Modification of dihybrid cross ratio

AaBb
X
AaBb
9:3:3:1
Epistasis in labrador retrievers

B and E color genes (labs)
B black
E color
b brown
e no color (yellow)
ee is epistatic
Cross two double heterozygotes
Phenotypes of parents?
Phenotypes of offspring? ratio?
The genotype rrpp gives roosters a single comb.
R-P- gives a walnut comb. rrP- gives a
pea comb, R-pp gives a rose comb.
Determine the ratio of phenotypes in the
F2 generation of the following cross
P
?
F1
?
ROSE
SINGLE
F2
?
PEA
WALNUT
Epistasis in Cats
W = white
w = not white
 B = black
b = brown
Mate 2 heterozygous cats
What is the expected ratio?

8. Polygenic traits

Vary continuously
 Weight,
height, IQ
Example: CAD is a multifactorial
trait affected by
 Obesity,
diabetes, high blood pressure, high
levels of LDL-cholesterol, and gum disease
 Although CAD runs in families, does not show
Mendelian inheritance patterns
 CAD occurs more often in men
 Risk higher among US African Americans than
Caucasians or Asians
 >200 genes identified
 LDLR and familial hypercholesterolemia
(dominant)

Influenced by many genes and environmental
factors
 Predisposition genes + environment

Cleft palate and identical twins, colon cancer may
run in families
4. Dominance series – C series/ rabbits
c+ = full color
cch = chinchilla (hypomorphic)
ch = himalayan (hypomorphic)
c = albino (apomorphic allele = nonfunctional)
Chinchilla
Himalayan
Albino
Genotype
cch cch
cch ch
ch c
c+ cch
c+ = full color
cch = chinchilla (hypomorphic)
ch = himalayan (hypomorphic)
c = albino (apomorphic allele = nonfunctional)
phenotype?