Topic G: Extensions to Mendelian Inheritance

Question 1

What is an allelic interaction?

Answer 1

the interaction between two alleles of the same gene in a diploid. This interaction determines phenotype.

Question 2

what are the three types of allelic interactions in heterozygotes?

Answer 2

• dominance/complete dominance
• incomplete dominance
• codominance

Question 3

What is complete dominance?

Answer 3

• Complete dominance is when a single allele (P) is sufficient for dictating the full phenotype in the heterozygote (Pp).
• This is also described as allele dominance! If PP (Purple) and pp (white) produced a Pp individual, the phenotype would be purple (so P is dominant/ has complete dominance)

Question 4

What is incomplete dominance?

Answer 4

• When we see that the phenotype of the heterozygous is intermediate to the two homozygous phenotypes (like a mix). A single allele is not sufficient to fully dictate the phenotype
• so If PP (Purple) and pp (white) produced a Pp individual, the phenotype would be light purple.

Question 5

what is haplosufficiency?

Answer 5

when a single allele is sufficient for dictating the full phenotype in a heterozygote (complete dominance)

Question 6

What is haploinsufficiency?

Answer 6

When a single allele is not sufficient to fully dictate the phenotype of a heterozygote (incomplete dominance)

Question 7

What would be the phenotypic ratio of the F2 generation in incomplete dominance?

Answer 7

1:2:2 (1 Purple: 2 light purple: 1 white)

Question 8

What is codominance?

Answer 8

• When a heterozygous individual expresses each of the two phenotypes observed in the homozygotes (not a mix)
• so If PP (Purple) and pp (white) produced a Pp individual, the phenotype would be purple and white petals.

Question 9

What would be the phenotypic ratio of the F2 generation in codominance?

Answer 9

1:2:2 (1 Purple: 2 purple and white petals: 1 white)

Question 10

What would be the phenotypic ratio of the F2 generation in complete dominance?

Answer 10

3:1 (3 purple:1 white)

this is the same as the mendelian ratio for F2

Question 11

Most genes have ____ alleles, rather than ___, but because we are discussing diploid individuals, a person can only have a max of ____ alleles.

Answer 11

Most genes have MULTIPLE alleles, rather than 2 but because we are discussing diploid individuals, a person can only have a max of 2 alleles.

Question 12

What is pleiotropic?

Answer 12

When a single gene influences more than one unrelated phenotype.

Question 13

How do pleiotropy and allelic interactions complicate phenotypes? give an example

Answer 13

• If a gene is pleiotropic- and thus influences more than one unrelated phenotype, the alleles for that gene may have different interactions with each other for each phenotype.
• ex. If a gene is responsible for two phenotypes; pea shape and starch accumulation,
• the pea shape phenotype may be a complete dominance interaction ( RR-round, Rr-round, rr-wrinkled).
• The starch phenotype may be an incomplete dominant interaction, (RR-high starch, Rr- intermediate starch, rr- low starch)

Question 14

Two alleles of a single gene can demonstrate different _____ _________ depending on which phenotype you consider

Answer 14

Two alleles of a single gene can demonstrate different allelic interactions depending on which phenotype you consider

Question 15

Allelic interactions must always be defined on the basis of a _____ _____ ______

Answer 15

Allelic interactions must always be defined on the basis of a SINGLE SPECIFIC PHENOTYPE

Question 16

What is epistasis?

Answer 16

• The interaction between alleles of different genes, which influences a phenotype
• When the allele of alleles of one gene mask the phenotypic effects of alleles of another gene

Question 17

What is the difference between epistatic and hypostatic?

Answer 17

Epistatic: alleles that ‘mask’ other alleles

Hypostatic: the alleles that are ‘masked’

Question 18

What is recessive epistasis? What is the phenotypic ratio from a dihybrid double heterozygous cross?

Answer 18

When the recessive allele of one gene ‘masks’ the phenotype of another gene.

(Ex. so any genotype that has ‘cc’, will be masked with the ‘cc’ phenotype, no matter what the genotype of the other gene is)

The F2 genotype ratio produced from a heterozygous cross will still be 9:3:3:1, but the phenotypic ratio will be 9:4:3

Question 19

What is dominant epistasis? What is the phenotypic ratio from a double heterozygous cross?

Answer 19

When the dominant allele of one gene ‘masks’ the phenotype of another gene.

(Ex. so any genotype that has ‘B’, will be masked with the ‘B’ phenotype, no matter what the genotype of the other gene is)

The F2 genotype ratio produced from a heterozygous cross will still be 9:3:3:1, but the phenotypic ratio will be 12:3:1

Question 20

What is reciprocal, recessive epistasis? What is the phenotypic ratio from a double heterozygous cross?

Answer 20

In a dihybrid cross, if both recessive alleles of two genes block the others dominant allele, this is reciprocal recessive epistasis.

(Ex. Cc and Pp, cc will mask P, pp will mask C.)

The F2 genotype ratio produced from a heterozygous cross will still be 9:3:3:1, but the phenotypic ratio will be 9:7, with only 2 phenotypes

Question 21

What is reciprocal, dominant epistasis? What is the phenotypic ratio from a double heterozygous cross?

Answer 21

In a dihybrid cross, if both dominant alleles of two genes block the others recessive allele, this is reciprocal dominant epistasis.

(Ex. Tt, and Vv. T is epistatic to vv, V is epistatic to tt.)

The F2 genotype ratio produced from a heterozygous cross will still be 9:3:3:1, but the phenotypic ratio will be 15:1, with only 2 phenotypes

Question 22

What is a variable phenotype/ variable expressivity?

Answer 22

When multiple individuals with a particular genotype express varying degrees of the corresponding phenotype

(all the people have the condition, but it is expressed in DIFFERENT ways)

Question 23

What is reduced/incomplete penetrance?

Answer 23

an individual who appears to have the genotype for a condition, but does not express the corresponding phenotype.