19. Multiple alleles & other extensions to Mendel

Question 1

Ratios of monohybrid cross with complete dominance - 1 gene

Answer 1

genotype ratio: 1:2:1

phenotype ratio: 1:3

Question 2

Incomplete dominance

Answer 2

A form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele

Question 3

Phenotypic ratio of incomplete dominance

Answer 3

1:2:1 (same for genotypic ratio)

Question 4

What is co-dominance

Answer 4

The full effect of both alleles can be seen in the phenotype of the heterozygote

Question 5

Monohybrid ratio with codominance

Answer 5

For both genotype and phenotype - 1:2:1

Question 6

Example of co-dominance in humans

Answer 6

sickle cell anemia
• HbA - normal beta-globin
•HbS - abnormal beta-globin

HbA and HbA - normal
HbA and HbS - normal and sickled
HbS and HbS - sickled

Question 7

Define pleiotropy

Answer 7

Pleiotropy occurs when one gene will code and control the phenotype or expression of several different and unrelated traits

Question 8

Pleiotropy example - sickle cell anemia

Answer 8

•genotype: HbS/HbS
•abnormal haemoglobin - sickling of RBCs
• clumping of cells causing interference with blood circulation
•local failures in blood supply can lead to:
- heart failure
- pneumonia
- brain; paralysis etc.

Question 9

Example of pleiotropy in cats

Answer 9

Allele that decides fur colour also influences the eye colour and hearing
- most cats with blue eyes and white fur are deaf

Question 10

Pleiotropy example - Marfan syndrome

Answer 10

• Marfan syndrome (autosomal dominant) - mutation in fibrillin gene important for connective tissue
• skeleton: loose joints, curved spine
• problems with heart
• eyes: short sighted

Question 11

Define phenocopy

Answer 11

• a copy of an inherited condition but results from an environmental factor
• occurs when the environment modifies the phenotype to mimic the effect of a specific genotype
• a phenocopy is NOT inherited

Question 12

Example of phenocopy - meromelia

Answer 12

• meromelia - mutation that affects limb development
• limb deformity causes by taking thalidomide during pregnancy for morning sickness (this is the phenocopy because it is not inherited - due to consumption of thalidomide)

Question 13

Vitamin D phenocopy example

Answer 13

• inherited trait: vitamin D-resistant rickets

* phenocopy: low Vitamin D in diet and lifestyle

Question 14

Define expressivity

Answer 14

The degree or range in which a particular phenotype is expressed by individuals
e.g. mild, moderate, severe

Question 15

Example of expressivity in dogs

Answer 15

• beagle shows variable expression fur colour and pattern

* same genotype but different distributions of black and white fur

Question 16

What is penetrance?

Answer 16

The proportion of genotypes that show the expected phenotype

Question 17

What is reduced penetrance?

Answer 17

When a portion of those with the genotype do not show the trait
ie. Polydactyly - everyone with genotype Pp may not have extra digits

Question 18

Example of completely penetrant trait

Answer 18

Huntington’s disease - all of those with the mutation will develop the disease

Question 19

Example of condition that demonstrates both variable expression and reduced penetrance

Answer 19

Polydactyly - extra digits

variable - both Pp genotype, but someone can get 1 extra finger while someone has 2 extra

Reduced penetrance - both Pp but one shows trait and other doesn’t

Question 20

What factor can contribute to penetrance and expressivity?

Answer 20

Phenotype also influenced by environmental factor

•ie. temperature (Flower patterns in Zinnias, siamese cats)

Question 21

What is lethality

Answer 21

An allele, genotype or mutation which results in death

Question 22

When are ‘recessive lethals’ and ‘dominant lethals’ lethal?

Answer 22

Recessive lethal
• only lethal as a homozygote (ie. aa, or BB)

Dominant lethal
• Both HH and Hh are lethal
•late onset so allele (H) is passed to next generation