Lecture 16: Gene Interactions Flashcards
Genes can exhibit two levels of independence…
1. Inheritance
2. Phenotype
- Inheritance
Genes segregate independently at meiosis because they are in different linkage groups.
This depends on genetic recombination. - Phenotype
Genes affect different processes in the organism: they are independent in their phenotypic expression.
This depends on the function of individual gene products.
Two genes that are independent at both levels will generate the familiar 9:3:3:1 phenotypic ratio in a dihybrid intercross, as shown by Mendel
Gene interaction occurs when genes at multiple loci contribute to a single phenotype
- What is “gene interaction”?
- To infer interaction between two genes:
- What is “gene interaction”?
– When the phenotypic expression of one gene is influenced by alleles at another locus: their expression is not independent - To infer interaction between two genes:
– Perform a dihybrid self-cross (Aa Bb x Aa Bb) – If the two genes interact, you will obtain: - a modified 9:3:3:1 phenotypic ratio AND/OR
- a new phenotype not observed in the original homozygous parents.
Gene interaction to create new phenotypes:
Where did these new phenotypes come from?
**Gene interaction with epistasis = 5
usage?
1 * One form of gene interaction is called EPISTASIS. Epistatic genes may be dominant or recessive in their effects.
2 * Epistasis is a form of gene interaction where the phenotypic effect of one allele is MASKED by the presence of another allele at a second locus.
3 * The gene or allele that is doing the masking is the epistatic allele (“epi-” = “on top of, above”; “-stasis” = stand)
4 * The gene or allele that is being masked is the hypostatic allele (“hypo-” = below, underneath)
- Usage: allele a is epistatic to allele b
or: allele b is hypostatic to allele a
Both say the same thing: the presence of allele a prevents the phenotypic expression of allele b.
EXPLAIN Recessive epistasis: Labradors
- Coat colour determined by interaction of two loci:
- B locus determines pigment in skin (black/brown)
- E locus determines deposition of pigment in hair
- BB or Bb (often written “B_”) – black pigment (B is dominant)
- bb – brown pigment
- EE or Ee (E_) – pigment deposited in hair (E is dominant)
- ee – no pigmentation in hair (yellow)
ee masks expression of B phenotype: e is epistatic to B and b
- e isaRECESSIVE epistatic allele because two copies of e are needed to mask the expression of the alleles at the second locus (B)
Predict the outcome of these crosses:
1. Bb ee x bb EE
2. BB ee x Bb ee
- Bbee x bbEE* Bb ee is yellow (because ee)
* bb EE is brown
* Progeny will be 50% Bb and 50% bb (monohybrid test cross Bb x bb)
* Progeny will be 100% Ee, so all will be pigmented
* Therefore 50% black (Bb Ee) and 50% brown (bb Ee) - BBee x Bbee
* Immediately note that both parents are ee (yellow)
* ee will mask any effect of B or b
* Therefore 100% yellow (genotypes 50% BB ee, 50% Bb ee)
Duplicate recessive epistasis: snails
- Two recessive alleles at two loci both suppress a phenotype e.g. albinism in snails
- Cross two albino snails, F1 are all pigmented
- Intercross the F2, 9/16 pigmented, 7/16 albino
Compound 9:3:3:1 ratio - Loss of either functional enzyme results in albinism. In this case, both a and b are recessive epistatic alleles because either one will mask the other locus.
In sweet peas, the synthesis of purple anthocyanin pigment in the petals is controlled by two genes, B and D.
The pathway is
gene B
- white intermediate—-ENZYME B —> blue intermediate
gene D
enzyme D —–anthocyanin (purple)
a) What colour petals would you expect in a pure-breeding plant unable to catalyse the first reaction?
b) What colour petals would you expect in a pure-breeding plant unable to catalyse the second reaction?
c) If the plants in parts a) and b) are crossed, what colour petals will the F1 have?
d)What ratio of purple: blue: white plants would you expect in the F2?
e) Why is this recessive epistasis?
a) bb DD, white
b) BB dd, blue
c) Bb Dd, purple
d) 9:3:4
e) b is epistatic to d and D,but two copies of
the recessive allele b is needed to mask the function of d or D.
Dominant epistasis: fruit colour in squash
A single copy of an allele at one locus masks the expression of an allele at a second locus.
Cross-homozygous white with homozygous green squash
—- F1 are ALL WHITE (what does this tell us?)
Self the F1 and observe F2 phenotypes:
12/16 white, 3/16 yellow, 1/16 green
Or: 3⁄4 are white, 1⁄4 are coloured. Looks like a 3:1 ratio where white (W) is dominant over coloured (w).
Genotype W_ inhibits pigmentation, ww allows pigmentation
Among the coloured fruit, 3⁄4 are yellow, and 1⁄4 are green:
Therefore dominant Y_ allows yellow pigmentation, while yy remains green
3/16 yellow :1/16 green
3:1
12/16 white 3/16 yellow 1/16 green
W_ Y_ & W_ yy ww Y_ ww yy
12 : 3 : 1
Another modified 9:3:3:1 dihybrid ratio
EXPLAIN Modified dihybrid ratios result from gene interaction = 3
- The inheritance of genes giving interacting phenotypes is the same as for simple traits: each locus has two alleles that sort independently.
- But the products of the two genes interact.
Why are phenotypes in 16ths? - Probability of inheriting 1 of 2 alleles = 1⁄2. With 2 loci = 4 alleles in total, the probability of any one combination = (1/2)4 = 1/16.
A number of white cats are intercrossed and produce the three types of progeny in the ratio 12/16 white, 3/16 black, 1/16 grey.
What is the genotype of the black progeny?
Break the problem down:
a) Does this ratio indicate dominant or recessive epistasis?
b) What colour is the double homozygous recessive?
c) Assume one locus (A/a) controls presence/absence of pigment, and another (B/b) controls pigment intensity. Which allele is epistatic to which?
d) What is the genotype of the black progeny?
a) The ratio 12:3:1 indicates dominant epistasis.
b) The double homozygous recessive is grey.
c) The A allele is epistatic to the B allele. This means that the presence of at least one dominant A allele masks the expression of the B/b locus, resulting in a white phenotype regardless of the genotype at the B/b locus.
d) The genotype of the black progeny is aaBB or aaBb.
How do we know if mutations affect different loci?
- If multiple genes affect the same process, then mutations in multiple genes can give rise to the same phenotype (e.g. albino snails). How do we know that the mutations are in different genes, and not simply different alleles at the same locus?
–> Perform a complementation test.
Let’s say we have two mutants, homozygous recessive: aa and bb
Cross these two mutants and observe phenotype in F1
E.W. Lindstrom crossed two corn plants with green seedlings and obtained the following progeny: 3583 green seedlings, 853 virescent-white seedlings, and 260 yellow seedlings
a) Give genotypes for the green, virescent-white, and yellow progeny.
b) Explain how colour is determined in these seedlings.
c) Is there epistasis among the genes that determine colour in corn seedlings? If so, which gene is epistatic, and which is hypostatic?
Hint: what ratio are the phenotypes in? Are they multiples of 1/16? Which alleles are dominant?
a) The ratio of green:virescent-white:yellow seedlings is approximately 9:7:4. This suggests that two genes are involved in determining the color of the seedlings. Let’s call these genes A/a and B/b. The green seedlings have the genotype A-B- (where “-“ represents either the dominant or recessive allele), the virescent-white seedlings have the genotype A-bb, and the yellow seedlings have the genotype aaB-.
b) The color of these seedlings is determined by two genes, A/a and B/b. The dominant A allele masks the expression of the B/b locus, resulting in a green phenotype regardless of the genotype at the B/b locus. When the recessive aa genotype is present, the expression of the B/b locus determines whether the seedling is yellow (B-) or virescent-white (bb).
c) Yes, there is epistasis among the genes that determine color in corn seedlings. The A/a gene is epistatic to the B/b gene, meaning that it masks its expression. The B/b gene is hypostatic to the A/a gene.
