Penetrance & Expressivity

Question 1

What is expressivity?

Answer 1

-the degree to which a phenotype is expressed (mild to severe) variation in the individual
-not whether or not it is expressed different shades of allele and other genes influence one allele

Question 2

What is Penetrance?

Answer 2

-the proportion of individual organisms having a particular genotype that express the expected phenotype (whether or not gene is expressed)-variation in the population

Question 3

What is complete penetrance?

Answer 3

-identical known genotypes yield 100% expected phenotype

Question 4

What is incomplete penetrance?

Answer 4

-identical known genotypes yield <100% expected phenotype
-example: polydactyly extra fingers and toes

Question 5

What is constant expressivity?

Answer 5

-identical known genotypes with no expressivity effect yield 100% expected phenotype

Question 6

What is variable expressivity?

Answer 6

-identical known genotypes with an expressivity effect yield a range of phenotype

Question 7

What is incomplete penetrance with variable expressivity?

Answer 7

Identical known genotypes produce a broad range of phenotypes due to varying degrees of gene activation and expression
-mixture of colours

Question 8

What is split-hand-foot syndrome?

Answer 8

-a rare autosomal dominant disorder that shows variable expressivity
-single gene/mutation
-involves the deficiency of absence of one or more central digits of the hand or foot

Question 9

What is piebaldism?

Answer 9

-a rare autosomal dominant disorder that shows variable expressivity
-absence of cells called melanocytes in certain areas of the skin and hair
-patch of colour which comes in different sizes

Question 10

What is Huntington disease?

Answer 10

-a rare autosomal dominant disorder that shows variable expressivity in the time onset of the disease
-occurs in different times in life
-a neuro-degenerative disease that causes loss of muscle coordination cognitive decline and dementia

Question 11

What causes incomplete penetrance and expressivity?

Answer 11

-due to the effects of other genes and to environmental factors that can alter or completely suppress the effect of a particular gene

Question 12

What are the environmental factors that can affect phenotypic expression?

Answer 12

1. Age
2. Sex
3. Temperature
4. Chemicals (exposure or consumed)

Question 13

What is the norm of reaction?

Answer 13

-the range of phenotypes expressed by a single genotype under different environmental conditions
-all different phenotypes you can get from the same mutation

Question 14

What are the environmental effects on the phenotype?

Answer 14

All cells express the enzyme
-core body temp causes protein to DE synthesize/unfold
-affects how the protein works and affects the phenotype

Question 15

What is the temperature-sensitive allele?

Answer 15

-an example is the siamese allele c subscript s in the Tyrosinase gene
-the protein is inactive at the higher temperature near the cat’s core body temp leaving a light brown background
-

Question 16

What is phenocopy?

Answer 16

-a change in phenotype arising from environmental factors that mimic the effects of a mutation in a gene
-occasionally, environmental factors alone produce a phenotype that is the same as the phenotype produced by a genotype
-thalidomide can produce a phenocopy of a rare dominant trait called phocomelia
-causes children to be deformed in the embryo but they don’t actually have the gene for it

Question 17

How can we control our environment to influence the appearance of mutant phenotype of a disease?

Answer 17

-having a good diet and exercising can bring the disease to be mild rather than deathly
-effects the occurrence (penetrance) and seriousness (expressivity) of the disease

Question 18

What does genetic interaction mean?

Answer 18

-different combinations of alleles from two or more genes can result in different phenotypes because of interactions between their products at the cellular or biochemical level
-part of the same pathway leading to a phenotype

Question 19

What happens when multiple genes contribute to a single characteristic?

Answer 19

-interactions between genes
-f1 cross A/a B/b X A/a B/b (dihybrids)
-would both have brown eyes because of A and B gene
-if there is complete dominance at two distinct traits Phenotypes= 9:3:3:1
-9 have at least two dominant alleles 3 dominant at green three dominant B grey eyes and 1 recessive blue eyes
-Functioning A changes blue to green, functioning B changes from green to brown and in absence of green takes blue to make it gray (slightly lighter than brown)

Question 20

What occurs for Complete dominance?

Answer 20

-two gene pairs contributing to a single trait four distinct phenotypes which helps visualize independent assortment
-3/4 B/-
-also 9:3:3:1

Question 21

What occurs for complete dominance in cats?

Answer 21

-D/D this cat does not carry the dilute Gene
-D/d this cat is a carrier of the dilute Gene
-d/d this cat carries two copies of the dilute Gene the color is diluted
-black dilutes to Blue chocolate dilutes to Lilac

Question 22

What is the molecular explanation for the complete dominance of cat colouration?

Answer 22

-the dilute gene affects the distribution of melanin granules of the cat’s hair
-dominant or dense (D) produces dense pigmentation
-recessive (d) dilute allele: results in pigment clumping for me areas in the hair with no granules this causes the cat coat to dilute or lighten
-the dilute effect is autosomal recessive hence a cat requires two copies of the d allele for the coat to dilute

Question 23

What is complementation?

Answer 23

-occurs when two strands humans with different genotypes of an organism with different homozygous recessive mutations that produce the same phenotype produce offspring of the wild-type phenotype when mated or crossed
-will only occur if the mutations are in different genes
-the other genome supplies the wild type allele to complement the mutated allele
-will not occur if the mutations are in the same gene
-can occur for two different albinos as well
-9:7
-refer to slide for when black progeny are crossed

Question 24

What is a heterogeneous trait

Answer 24

-a mutation in any one of a number of genes can rise to the same phenotype
-an example is two parents are deaf but all children can hear therefore they are deaf for different genetic reasons (complementation)
-if the two parents are deaf all of their children are deaf for the same genetic reasons (no complementation)

Question 25

What is genetic epistatis?

Answer 25

-genetic epistasis the masking of the impression of one gene by another no no phenotypes are produced
-the epistatic gene does the masking the hypostatic gene is masked
F2 phenotypic ratio 9:3:4 recessive epistasis
-homozygous recessive at one pair mask expression from the other Gene B / B cancels out what Adoes to give the same phenotype
-F2 phenotypic ratio 12:3:1 dominant epistasis one dominant allele at 1 Gene masks expression from the other Gene

Question 26

What is recessive epistasis?

Answer 26

-review slide but know it has ratio of (9:3:4)
-precursor molecule (colorless) synthesizes black pigment generates pattern for agouti (A/-) pattern
-no pigment is synthesized therefore mice are white regardless of genotype at the A Locus this is recessive epistasis
-the genotype of the A locus determines how the pigment is deposited either black (a/a) or agouti (A/-)

Question 27

What is dominant epistasis?

Answer 27

-phenotypic ratio = 12:3:1
-genotype is W the cat is white regardless of the genotype at the other locus
-only the w/w genotype B/black and b/b is brown
-look at slide

Question 28

What are the F2 ratios for monohybrid crosses?

Answer 28

-3:1 complete dominance
-1:21 incomplete dominance or codominance
2:1 recessive lethal allele (two heterozygotes allele is lethal (lose one in the embryo of the homozygotes)

Question 29

What is Pleiotropy?

Answer 29

-a single gene can be responsible for a number of distinct and seemingly unrelated phenotypic effects

Question 30

What is Sickle Cell Disease?

Answer 30

-respiratory problems, sickled cells, chronic infections (problem with immune system), joint pain, enlarged spleen, stroke

Question 31

What is Cystic Fibrosis?

Answer 31

-single mutation in a single gene
-mucus clogs the lungs and leads to infections
-obstructs the pancreatic ducts creates digestion problems (inherited mutation is in all your genes)

Question 32

What is inbreeding Depression?

Answer 32

-the norm in agriculture
-inbred lines of experimental species are often less vigorous than hybrid lines
-not genetically diverse as fungal can pop up and pesticides can cause it to go extinct
-inbred lines of self-fertilized plants are homozygous for alleles that were present in the founding line
-buying seeds and known genotypes is better for crops to kill diseases
-homozygous for different genes-cross gives a stronger more robust plant
-*increases the frequency of homozygotes and decreases the frequency of heterozygotes

Question 33

What is Heterosis?

Answer 33

-when two different inbred lines are crossed the hybrids are heterozygous for many genes
-these heterozygotes display heterosis or hybrid vigor

Question 34

What is the Hardy Weinburg Principle?

Answer 34

-predicting genotypes through allele frequencies in a population
-random mating will produce genotypes of the next generation in proportions p2(AA), 2pq (Aa), and q2 (aa0
-p2+2pq+q2
-if there is two alleles you can do 100- given # to get the other frequency

Question 35

What 4 factors do not allow the Hardy-Weinberg principle to work?

Answer 35

-1. Nonrandom Mating: cannot prefer a certain genotype as mate
2. Unequal survival: homozygous recessives are killed
3. Population subdivision: population is split
4. Migration: species move away from each other
-alleles are lost

Question 36

What is the Tay-Sachs Disease?

Answer 36

-rare recessive disease, lethal allele
- all parents and children are not affected but one who died