Chapter 5 - Extensions/Modifications of Basic Principles Flashcards
(62 cards)
1
Q
What are complete dominance/recessiveness?
A
- Extremes of a range
- BB=Bb is different from bb
2
Q
What is incomplete dominance?
A
- BB, Bb and bb all differ phenotypically
- Bb is intermediate between homozygous phenotypes (e.g. red, white, pink)
3
Q
What is codominance?
A
- BB, Bb and bb all differ phenotypically
- Bb exhibits phenotypes of both homozygotes (ex. Blood type)
4
Q
How many alleles can coexist in each cell?
A
2 (diploid)
5
Q
Are some alleles associated with certain breeds/varieties of plants/animals?
A
Yes
6
Q
What is a dominance series/allelic series?
A
- Relationship between alleles
- Ex. C > cb = cs > c > ca
- ’>’ indicates the dominance and ‘=’ indicates incomplete dominance or codominance
- Describes the dominance hierarchy of multiple alleles
7
Q
What does the superscript ‘+’ represent?
A
- Dominant/wildtype allele
8
Q
What enzyme is coded for by the gene for blood phenotype?
A
- Transferase
9
Q
What does I^A encode for?
A
- Transferase which adds acetylgalactosamine
10
Q
What does I^B encode for?
A
- Transferase which adds galactose
11
Q
What does i encode for?
A
- Non-functional transferase
12
Q
What is type AB blood?
A
- Has acetylgalactosamine and galactose
- Alleles are codominant (both are fully expressed)
13
Q
What is the wildtype allele?
A
A functional enzyme or other protein is produced
- Sometimes used to refer to most common phenotype/genotype in a natural population
- Often dominant over loss of function
14
Q
What is a loss of function allele?
A
An enzyme or other protein is no longer being produced, is produced at lower levels, or is nonfunctional
15
Q
What is haplosufficiency?
A
- Half as much protein is synthesized in heterozygote yet this is often sufficient to achieve wildtype phenotype
16
Q
What is haploinsufficiency?
A
- Dominant alleles can be loss of function alleles
- In heterozygote, half as much protein is synthesized and this is not sufficient for normal phenotype
17
Q
What are recessive lethal alleles?
A
- Essential genes
- When mutated lead to lethal phenotype
- Death only in homozygotes
- Can still have dominant effect on other phenotypes (i.e. colour)
- Ex. Tay Sachs disease
18
Q
What are dominant lethal alleles?
A
- Expressed in homozygote and heterozygote
- Ex. Huntingdon disease
19
Q
What is an amorphic allele? What phenotype is produced?
A
- Recessive
- Loss of function
- Does not produce functional polypeptide
- Severe mutant phenotype
20
Q
What is a hypomorphic allele? What phenotype is produced?
A
- Recessive
- Loss of function
- Partially functional polypeptide
- Mild mutant phenotype
21
Q
What is a dominant-negative allele? What phenotype is produced?
A
- Produces polypeptide that interferes with wildtype
- Severe mutant phenotype
22
Q
What is penetrance?
A
- The proportion of individual organisms having a particular genotype that express the expected phenotype
- Variation in the population
- Confounding issues
23
Q
What is expressivity?
A
- The degree to which a phenotype is expressed (mild to severe)
- Variation in the individual
- Ex. shades of brown hair
24
Q
What is complete penetrance?
A
Identical known genotype yield 100% expected phenotype
25
What is incomplete penetrance?
Identical known genotypes yield <100% expected phenotype
| - Ex. polydactyly
26
What is constant expressivity?
Identical known genotypes with no expressivity effect yield 100% expected phenotype
27
What is variable expressivity?
Identical known genotypes with an expressivity effect yield a range of phenotypes
28
What is incomplete penetrance with variable expressivity?
Identical known genotypes produce a broad range of phenotypes, due to varying degrees of gene activation and expression
29
Assume that long fingers are inherited as a recessive trait with 80% penetrance. Two people heterozygous for long fingers mate. What is the probability that their first child will have long fingers?
1/4 x 80% = 20%
30
What are some human disorders that show variable expressivity?
- Split Hand-foot Syndrome
- Piebaldism
- Huntingdon Disease
31
What causes incomplete penetrance and expressivity?
- Effects of other genes
| - Environmental factors that can alter/suppress effect of a particular gene
32
What are some environmental factors that can affect phenotypic expression?
- Age
- Sex
- Temperature (Ex. siamese allele)
- Chemicals
33
What is the norm of reaction?
Range of phenotypes expressed by a single phenotype under different environmental conditions
34
What is a phenocopy?
A change in phenotype arising from environmental factors that mimic the effects of a mutation in a gene (e.g. thalidomide can produce phenocopy of phocomelia)
35
Can controlling environment influence appearance of mutant phenotype/disease?
- Yes
| - Ex. heart disease (regulate diet/exercise)
36
What is Mendel's Law of Independent Assortment? How did he discover this?
- The inheritance pattern of one trait will not affect the inheritance pattern of another trait
- Examined dihybrid crosses
37
What is genetic interaction?
Different combinations of alleles from 2 or more genes can result in different phenotypes, because of interactions b/n products at cellular/biochemical level
38
What happens when multiple genes often contribute to a single characteristic?
- Interactions between genes
- Still dihybrid cross so we have to consider Mendelian principles
- Still 9:3:3:1 (if there is complete dominance at 2 distinct traits)
39
What is the molecular explanation for complete dominance of cat colouration?
- Dilute gene affects distribution of melanin
- D: dense pigmentation
- d: pigment clumping, areas with no granules (lighten)
- Dilute effect is autosomal recessive, so cat requires 2 copies of d allele for coat to dilute
40
What is complementation and when does it occur?
- 2 strains of an organism with different homozygous recessive mutations that produce same phenotype produce offspring of wildtype phenotype when crossed
- Occurs if mutations are in different genes
- Other genome supplies wildtype to complement mutated allele
41
What is a heterogeneous trait?
A mutation in any one of a number of genes can give rise to the same phenotype
Ex. about 50 genes have recessive mutant alleles that can cause deafness in humans
42
What is epistasis?
- The masking of the expression of one gene by another
| - No new phenotypes are produced
43
What is the hypostatic gene?
The masked gene in epistasis
44
What is recessive epistasis?
F2 Ratio = 9:3:4
| Homozygous recessives at one gene pair mask expression from other gene
45
What is dominant epistasis?
F2 Ratio = 12:3:1
| One dominant allele at one gene masks expression from another gene
46
What is pleiotropy?
A single gene can be responsible for a number of distinct and seemingly unrelated phenotypic effects (Ex. sickle cell disease, cystic fibrosis)
47
What is inbreeding depression?
- Inbred lines of experimental species are often less vigorous than hybrid lines
- Inbred lines of self-fertilized plants are homozygous for alleles present in founding line
48
What is heterosis?
- When 2 different inbred lines are crossed, hybrids are heterozygous for many genes
- Hybrids display heterosis, or hybrid vigor
- Hybrids are much larger than parents
49
What is the Hardy Weinberg principle?
- Predicts genotypes through allele frequencies in a population
p^2 + 2pq +q^2 = 1
p = dominant
q = recessive
50
What are 4 evolutionary influences that make the Hardy Weinberg principle inaccurate?
- Nonrandom mating
- Unequal survival
- Population subdivision
- Migration
51
What are allele frequencies useful in?
- Genetic counselling
| - Find risk of child being affected
52
Tay Sachs is an autosomal recessive mutation with a frequency of 0.017 in this population. Therefore, ts = 0.017 and TS = 0.983. What are the chances somebody is a carrier (TS ts) in this population?
2pq
= 2(0.017)(0.983)
= 0.033 or 1/30
53
What is the expected frequency of heterozygotes in a population with allelic frequency x and y that is in Hardy-Weinberg equilibrium?
2xy
54
What is dosage compensation?
A way of equalizing gene expression in the face of different gene dosage
- Extra copy of gene/chromosome can affect development (ex. Females have 2 copies of X)
55
What are pseudoautosomal regions?
- Homologous
| - Ex. in sex chromosomes, essential for X-Y chromosome pairing in meiosis in the male
56
What is the XX-XO system for sex chromosomes?
XX - Female
XO - Male
Ex. Grasshoppers
57
What is the XX-XY system for sex chromosomes?
```
XX - Female
XY - Male
XO - Female
XXY - Male
Ex. Mammals
**Males need Y chromosome
```
58
What is a Barr body?
- Condensed mass in nucleus of female cells in cats, not males
- It was proposed that this was the X-chromosome (inactive)
59
What is the Lyon Hypothesis/X-inactivation?
- Mary Lyon proposed random inactivation of one female X chromosome
- If a cell contains more than 2 X chromosomes, all but one are inactive
- Females are functionally hemizygous (expressing from only 1 gene) for X-linked genes at cellular level
- 50% will express one allele and other 50% will express the other
60
What happens if there are genes on the X-chromosome?
- Some cells are orange and others are black
- Allele is present, but not contributing to phenotype
- Ex. Calico cats
61
When does X-inactivation occur?
- Early in development
| - One X chromosome is inactivated in each cell at random
62
What is an example of a genetic mosaic?
- Females that are heterozygous for X-linked traits
| - Ex. one eye may be colourblind while other isn't
