Lecture 3 Non Mendelian Genetics

Question 1

Incomplete dominance (ex - four o’clock plant)

Answer 1

Incomplete heterozygote has a phenotype intermediate phenotypes (think of red and white flower offspring. Instead of only having red and white flower, there is a pink flower)

Question 2

Codominance (ex - human MN blood group)

Answer 2

both alleles are expressed

both and phenotypes are exhibited by heterozygotes, since both molecules are present (think of blood groups and sickle cell)
slide 5

Question 3

Multiple Alleles (blood types)

Answer 3

More than two alleles are present within a group of individuals.

• The ABO blood type is an example of a trait that has more than just 2 different alleles (A, B, and O)
• Most genes have multiple alleles (different forms of the same gene
  -hair, eyes, and fur color genes all have many different alleles
  -multiple alleles usually exhibit some sort of incomplete dominance

ex: slide 9 and 10

Question 4

Pleiotropy

Answer 4

● Mendelian inheritance - each gene affects only one phenotypic character
● Most genes - have multiple phenotypic effects

slide 11

Question 5

Pleiotropic effects of sickle cell anemia

Answer 5

● Mutations in the beta-globin gene in RBC -
● Formation of sickle cells.
● Irregular in shape of blood cells, clustering together, blocking the flow of blood in the veins.
● Results: damages to many vital human organs such as the
heart, brain, and lungs.

Question 6

Pleiotropic effects of the cystic fibrosis gene

Answer 6

slide 13

Question 7

Epistasis

Answer 7

● The phenotypic expression of a gene at one locus alters that of a gene at a
second locus
● Coat color in Labrador retriever
○ Gene 1 - Black (B) coat color is dominant to Brown (b)
○ Gene 2 - determines whether or not pigment will be deposited in the hair
E deposit pigment; e no deposit of pigment
● Gene for pigment deposition is said to be epistatic to the gene that codes for
black or brown pigment

Epistasis is a circumstance where the expression of one gene is modified (e.g., masked, inhibited or suppressed) by the expression of one or more other genes.

Question 8

Polygenic (when a gene has different phenotypes in different people) Inheritance

Answer 8

● Mendelian characters - either-or basis
● Quantitative characters - vary in the population in gradations along a
continuum (additive effect of two or more genes on a single phenotypic
character)
Ex - skin color, height

slide 17

Question 9

Effect of the environment

Answer 9

● Phenotype for a character depends on environment as well as genotype

slide 18, 19, 20

Question 10

Reading a pedigree

Answer 10

slide 24-30

Question 11

Lethal Alleles

Answer 11

Causes death at an early stage
of development – some
genotypes do not appear
● Lethal dominant
● Lethal recessive

● E.g Yellow coat color in mice
● Always 2:1 ratio in heterozygous
cross, instead of 3:1
● may produce ratios that
seemingly deviate from
Mendelian ratios

Question 12

The Manx cat

Answer 12

● Carries a dominant mutation (M) that affects the spine
● This mutation shortens the tail (Tail-lessness)
● abnormal development of the coccygeal and sacral vertebrae
● This allele is lethal as a homozygote
● All Manx cats are heterozygous for the Manx gene (i.e. Mm),

Question 13

In humans: Huntington’s disease (HD)

Answer 13

● Lethal Dominant allele (called HTT or huntingtin )
● caused by a stretch of the letters C-A-G in the huntingtin gene which
repeat over and over, too many times…CAGCAGCAGCAGCAG.
This is known as a CAG repeat expansion
● No effect until individual is about 35 to 45 years old.

slide 34-36

Question 14

Penetrance and Expressivity

Answer 14

● So far, assumption is - every individual organism with a genotype
expresses the expected phenotype
Rr - always round
rr – always wrinkled

Complete penetrance
● a dominant allele does not influence the outcome of a trait in a heterozygote
individual
● dominant allele does not always “penetrate” into the phenotype of the
individual
● If 60% of heterozygotes carrying a dominant allele exhibit the trait allele, the
trait is 60% penetrant

Question 15

Polydactyly - incomplete penetrance

Answer 15

-Genotype does not always produce expected phenotype

Polydactyly
-Dominant allele
-Individual s with dominant allele can occasionally have normal number of digits, but have affected children

Penetrance
-% of individuals with a particular genotype that express expected phenotype
-42 individuals have polydacylous allele; 38 express polydactyly
-38/42=90% penetrance

slide 38

Question 16

Expressivity

Answer 16

● The degree to which the trait is expressed

slide 39

Question 17

Fetal Testing

Answer 17

● Screening tests - non invasive
● Diagnostic test - invasive screening

slide 40