1.  Describe cases in which simple principles of Mendelian inheritance do not apply

2.  Predict phenotypic and genotypic outcomes resulting from cases where Mendelian inheritance does not apply.

3.  Describe the origin of variation as it applies to the source of new alleles (and, ultimately, variation among organisms).

Objectives, 10/06/08:

Case 2: Co-Dominance

•  Alleles for trait are equally dominant – Both alleles are expressed

However: •  Both alleles are expressed in the

heterozygous phenotype

ABO Blood Types in Humans

3 Alleles:

IA = dominant (carbohydrate A on RBC)

IB = dominant (B carbohydrate on RBC)

i=recessive (no carbohydrate on RBC)

What are possible genotypes for each of the ABO Blood Types?

Phenotype (blood type) Genotype O A B AB

3 Alleles:

IA = enzyme that attaches “A” carbohydrate to RBC

IB = enzyme that attaches “B” carbohydrate to RBC

i= recessive (no enzyme)

Genotype(s) that result in type AB blood include:

A.  IA IB B.  IA i or IB i C.  iA iB

D. A and B E. All of the above

CQ

ABO Blood Types in Humans

Phenotype (blood type) Genotype O ii A IAIA or IAi B IBIB or IBi AB IAIB

3 Alleles:

i=recessive

IA, IB = codominant

Case 3: Epistasis

•  “Standing upon”

•  One gene affects the action of another gene at a different locus.

•  However, the epistatic allele (E) must be present in order for melanin to be deposited in the hair shaft.

In dogs, the pigment, melanin, is responsible for coat color. Black (B) is dominant to brown (b).

Two dogs with genotypes BBEE and bbee are mated to produce pups. What

are the phenotypes of the parents?

A. Black and brown B. Black and yellow C. Brown and yellow D. Both black E. Both yellow

CQ

•  What are the ratios of genotypes and phenotypes of the F1 and F2 generations?

BBEE bbee

B = Black b = brown

E - must be present for ANY melanin deposition

Case 4: Pleiotropy

Gene Protein Trait 2 Trait 1

Trait 3

One gene determines many traits

Example: sweet peas

flower color 1 Gene seed color spotting of leaf axils

In humans: sickle cell trait, cystic fibrosis, etc.

Case 5: Polygenic Inheritance

Multiple genes determine a single trait.

Examples: height, weight, longevity, risk for disease, etc.

Gene 1 Gene 2 Gene 3

Protein Protein Protein

Trait

Case 6: Environmental Interactions

Phenotype = Genes + Environment Sun Shade

Case 7: Linked Genes

•  Genes that occur on the same chromosome will not assort independently –  Independent assortment applies to

whole chromosomes

t

f F

TTt

F f

QQq q How many unique genotypes of gametes will form?

R R R R

A A a a

Special case of linkage: Sex-Linked Genes

•  Genes that occur on the sex chromosomes

•  X-linked traits - more common in men –  In males, inheritance is from mother only

– X-linked Recessive: color-blindness, male-patterned baldness, hemophilia

– X-linked Dominant: incontinentia pigmenti, Coffin-Lowry Syndrome

Color-blindness is caused by a recessive gene (b) on the X chromosome. Predict phenotypes for children of a female carrier with a normal male.

Y- Chromosome

•  A “genetic wasteland”???

•  Gene: SRY (Sex-determining Region Y) – Produces protein =

“testis determining factor”

–  Inheritance: Father to Son (only!)

Case 8: Crossing

Over

Case 8: Crossing Over

•  Homologous chromosomes exchange genetic info on “non-sister” chromatids –  Meiosis 1 –  Chiasma = point

of contact and exchange

Pair 2 gametes to form a zygote (fertilized egg).

What will the sister chromatids look like when DNA replicates?

t

f F

TTt

F f

QQq q Which crossing-over event(s) will change the genotype of resulting gametes?

R R R R

A A a a

D dd D

A)

B)

C)

D) B and C

E) All

Case 9: Mutation

•  Gene level - change in base sequence of an allele – Recall function of a gene/allele – What are the consequences of change in

base sequence?

Parkinson’s Disease - associated with change in base sequence and resulting protein produced