Final Review Flashcards
Before Midterm
1
Q
Classical Genetics
A
Understanding the inheritance of phenotypes
2
Q
Molecular genetics
A
Understanding the mechanisms of genetic regulation.
3
Q
Assumed blending inheritance
A
average of parental phenotypes.
4
Q
Most of our understanding came from plant/animal breeding
A
artificial selection
5
Q
Gene
A
DNA sequence for a specific protein.
Generally, a specific region of the genome that codes for a particular protein; a specific locus.
6
Q
Allele
A
A specific sequence of a gene, generally to compare between alleles of a single gene
7
Q
Homozygote
A
In diploids, where both copies of a gene are the same allele-a genotype
8
Q
Heterozygote
A
In diploids, where both copies of a gene are different alleles- a genotype
9
Q
Dominant
A
Allele’s phenotype appears in both homozygotes and heterozygotes
Capital letters
10
Q
Recessive
A
Allele’s phenotype only evident in homozygotes.
Lower case letters
11
Q
Genotype
A
The genetic sequence of interest
12
Q
Phenotype
A
The physical/cellular/protein results of a genotype and environmental effects.
Characteristics produced by different genotypes, also a response to the environment.
13
Q
Features of the phenotype
A
dominant
recessive
14
Q
If you cross 2 homozygotes with different alleles you will get
A
a heterozygote
15
Q
P/P0
A
Parental cross (homozygote)
‘true breeding’
16
Q
F1
A
First generation offspring/hybrid (heterozygotes).
Shows dominant phenotype, genotypes hidden.
17
Q
F2
A
Second generation offspring/hybrids (not all heterozygotes).
Shows all possible phenotypes, can infer all genotypes.
18
Q
Monohybrid cross
A
Looking at a single trait/gene.
Crosses of two varieties of true-breeding plants that differed in one character.
19
Q
Dihybrid cross
A
Looking at 2 traits/genes
Crosses of two varieties of true-breeding plants that differed in two characters.
For every chracter, one trait is dominant and one trait is recessive.
20
Q
Test cross
A
Determining unknown genotype by crossing an unknown individual with a homozygous recessive
21
Q
Mendel’s peas
A
High number of progeny, easy to grow, short life cycle, easy to control mating (pollination)
Different varieties were readily available.
Two easily distinguishable states (tall or short)
No linkage
Traits controlled by single genes.
22
Q
The fate of mendel’s genes
A
Identification of the mutations/genes for 4/7 traits so far.
23
Q
Mendel’s approach 1
A
Designed crosses carefully and kept detailed records of each.
Counted the number of offspring that had the traits he was following.
Kept track of generations and followed inheritance over several generations.
Asked very specific questions and made testable predictions from results.
24
Q
Mendel’s approach II
A
Key to his success:
Mendel created tru breeding lineages; plants were crossed repeatedly until all offspring looked like parentals-homozygotes. Mostly done through selfing.
Followed a key rule of the scientific method: good controls
25
Selfing
easy to make true breeding lineages amimals are too much work.
26
Mendel's Experiment #1
Monohybrid cross: crosses of two varieties of true-bredding plants that differed in only one character. Only 1 phenotype appears in the F1 (dominant, no info about genotype). F2 shows all possible phenotypes from combinations of genotypes. VAst majority of the peas were still round.
Wrinkled information was not lost.
27
Mendel's Conclusions I
There is a difference between a trait (phenotype) and the information for the trait (genotype).
Round seeds is the dominant phenotype- genotype is homozygous dominant RR or heterozygous Rr.
Wrinkled seeds is recessive- genotype is homozygous recessive rr.
28
Gene notation for round peas
Homozygote- RR
Heterozygote- Dominant allele listed first Rr
Dominant- Capital letters RR
Recessive- small letters rr
29
Gene notation for vestigial winged flies
Homozygote- +/+ vg/vg
Heterozygote- dominant allele listed first +/vg
+ indicates wildtype, not dominance.
Most of the time wildtype will be dominant.
30
Mendel's conclusions II- modern terminology
Alternative versions of genes (different alleles) account for variation in inherited chracteristics.
Diploid organisms inherit two alleles, one from each parent.
If two alleles are different, one may be dominant.
Each haploid gamete carries only one allele of a given trait because they segregate from one during meiosis.
31
The First Mendelian Law
Law of segregation
32
Law of segregation
Alleles of a gene separate independently (randomly) from each other during transmission from parent to offspring (=meiosis).
The dominant phenotype appears at 100% in the F1 (hybrid genotype).
The phenotypic frequencies in F2 conforms to: 3:1 (dominant:recessive)
33
How can you determine the genotype of an unknown?
Test cross
34
The frequency of the hybrid recessive allele=
Frequency of the recessive Phenotype
35
The Frequency of the hybrid dominant allele=
Frequency of the dominant phenotype
36
In a Mendelian cross, the F1 generation
Will consist exclusively of heterozygotes displaying the dominant phenotype for the allele combination in the locus in being examined.
37
F2 Generations shows
Shows the outcome of the segregation of alleles in the F1 Gametes.
F2 will show all of the possible phenotypes.
38
9:3:3:1
Phenotypic proportions in F2 of a dihybrid cross.
9(double dominance)
3(single dominance)
3(single dominance)
1(recessive)
39
Parental phenotypes
Phenotypes seen in P0
40
Recombinant Phenotypes
Phenotypes not seen in parents or F1
41
The Second Mendelian Law
Law of Independent Assortment.
Simply the first law applied to alleles of two or more genes: simply multiplying the monohybrid ratios.
Applies not only to dihybrid crosses, but for any number of genes.
42
Law of Independent Assortment
Alleles of two (or more) genes (loci) segregate independently during transmission from parent to offspring.
The two dominant phenotypes appear at 100% in the F1 (hybrid genotype).
In F2, 4 phenotypes are present.
The expected F2 frequency for phenotypes generated by alleles of two loci is: 9:3:3:1 (double dominant:dominant recessive:recessive dominant:double recessive
43
In the F2, 4 phenotypes are present:
Two parental phenotypes present in the P0 generation.
Two new phenotypes (recombinants) absent from P0 or F1.
44
Does the arrangement of alleles in the P0 affect the F1?
no
45
Does the arrangement of alleles in the P0 affect the F2?
no
46
Hypothesis
an idea to test
47
Data
Collection of observations or experimental data specifically planned to test the hypothesis
48
The Chi-Square (X^2) test
Tests whether the sample collected (peas counted) can be used to support a hypothesis. (expected mendelian ratio).
Often this will test against the hypothesis (null hypothesis)
49
Null Hypothesis
There is no difference between the observed phenotypic ratio and the expected ratio.
50
Steps for Chi-square test
1: do the experiment and count the observed numbers.
2:predict expected numbers based on hypothesis.
3: calculate how well the data fit our hypothesis.
4: determine the degrees of freedom.
5: accept or reject our hypothesis. Compare your X2 to the critical value. Compare P-value to significance level.
51
Expected number
Total # offspring counted x expected frequency
52
X2
=Sum (observed-expected)^2/expected
53
Degrees of freedom (df)
Number of phenotypic classes e.g. 4-1
A measure of how many 'types' of data are being used relative to how much information you are trying to learn.
Bigger df means your results are more likely to be representative.
54
X2
not significant
55
Reject null hypothesis
There is a significant difference between the observed phenotypic ratio and the expected ratio.
56
The Multiplicative Rule
If the events A and B are independent, the probability that they will occur together, P(A and B) is: P(A) x P(B)=
57
The Additive Rule
If the events A and B are independent, the probability that only one of them occurs, denoted P(A or B), is: P(A)+P(B)- [P(A) xP(B)]
If the two events do not overlap in the sample space, they are said to be mutually exclusive.
58
Mendel's Law create predictions
If alleles segregate at random during gamete formation, and fertilization is also random, offspring ratios should follow rules of probability.
59
probability of two independent events happening together (A and B)
Probability of 1st event multiplied by the probability of the 2nd event.
P(A) x P(B)
60
Probability of a particular genotype (in offspring)(Y and y)
Probability of obtaining a particular allele in the male gamete X probability of obtaining a particular allele in the female gamete.
P(Y) x P(y)
61
Human genome
23 pairs of chromosomes (2n=46)
22 pairs of autosomes (homologous chromosomal pairs); expected diploid mendelian inheritance.
1 pair of sex chromosomes (X and Y), which are not allele; causes very different inheritance patterns (non-mendelian)
62
Obstacles to Human Genetic Analysis
Incomplete family records
Small number of progeny
Uncontrolled environment- eg Phenotypic plasticity
63
mendelian Principles in human genetics
Wildtype=normal=healthy
Affected=Diseased=mutant
Not all low frequency human alleles are bad- i.e. red hair.
This is not a judgement on the condition
These terms are relative to the majority of humans
64
Pedigrees
Are diagrams that show the relationships among the members of a family.
They represent the inheritance pattern of a specific character/condition
65
Pedigree analysis predicts genotypes
Parents aren't affected so trait must be recessive.
Homozygous would be mean affected parents.
Parents are heterozygous
66
Inheritance of a Recessive Trait
Recessive traits may occur in individual whose parents are not affected. (not true for dominant traits, 'skipping' generations)
Recessive traits often occur rarely in a pedigree.
Recessive allele hidden in heterozygotes.('carrier' notation)
Rare recessive traits are most likely to appear in a pedigree when spouses are related to each other.
67
Recessive Mutations
Most common causes of human genetic diseases.
Individuals with the disease are homozygous for the recessive allele.
Recessive alleles usually lack function= NULL (illegible DNA 'instructions', fail to make a product/protein- or make a non-functional product)
68
Interpretation of pedigrees
often a pedigree is consistent with only a particular mode of inheritance (dominant or recessive).
69
Rules for interpretation of pedigrees
dominant trait- unaffected parents can't have an affected child.
recessive trait- unaffected parents can have an affected child
70
Guidelines of interpretation of pedigrees
Dominant trait- tends to appear in every generation
Recessive trait- tends to skip generations.
71
If a genetic condition is rare
It will be present in very few families in a population.
Unaffected individuals that marry into the family with the conditions, are likely homozygous for the normal allele rather than heterozygous.
72
Human Example: Albinism
recessive
Albinism- deficient pigmentation- inability to produce melanin (most often due to lack of active tyrosinase)
A= normal allele- active tyrosinase
a=mutant allele- inactive tyrosinase
A/A=makes tyrosinase- melanin
a/a= no active tyrosinase-no melanin- albine
A/a=same phenotype as A/A genotype; one A allele is sufficient to produce adequate melanin
73
Cystic Fibrosis
A common human recessive disease.
Serious disease caused by a nonfunctional CFTR gene. (cystic fibrosis transmembrane conductance regulator)
Defective Cl transport causes thick mucus to build up in lungs.
1 in 2500 affected among Caucasians.
Individuals who have the disease (cc) rarely reproduce, but heterozygous carriers are common among Caucasians (1:20)
Recessive
74
Autosomal Dominant Traits
Are caused by a mutant allele that is dominant over the normal allele.
Such mutations are not as common as recessive, lack-of-function mutations.
74
Inheritance of a dominant trait
Every individual who carries the dominant allele manifests the trait. (unaffected individuals (dd) cannot be carriers)
Every affected individual is expected to have at least one affected parent. (the trait tends to show up in every generation).
If both parents are heterozygotes (Dd), they will be affected, but they can have normal children.
74
Cystic Fibrosis Probability
Cc x Cc= 1/4 probability of CC (unaffected), 1/2 probability of Cc (unaffected), 1/4 probability of cc (affected)
75
Autosomal Dominant Traits- Due to
Production of too much of a normal protein.
One functional copy of a gene does not make enough gene product.
Production of abnormal variant of a protein that interacts incorrectly with the normal protein or with some other structure in the cell.
Production of protein with some entirely new function or normal function in new place (gain-of function)
76
Determine Likely inheritance
By exclusion: is it recessive? When a trait is rare, you assume other families do not cary it. Can't be recessive.
Is it dominant? Yes appears in every generation.
Is it sex linked? no, equally in males and females.
77
Achondroplasia
A dominant form of dwarfism.
Wild type gene (d) encodes a fibroblast growth factor receptor (FGFR-3), regulates chondrocyte proliferation and long bone growth.
Mutant allele (D) produces to much FGFR-3 which causes reduced growth of long bones.
78
Achondroplasia Genotypes
Dd genotype- dwarfism phenotype
dd genotype- normal bone growth
DD- lethal
79
Chromosome
Single strand of DNA in the nucleus
80
Chromatid
1/2 of replicated chromosome, attached together at the centromere in X shape
81
Sister Chromatids
Identical copies (diploid)
Same genes in same order
Same alleles in same order
82
Homologous chromosomes
Same chromosomes from different parents.
Same genes in same order
Homozygous- same alleles in same order
Heterozygous- different alleles in same order
83
What biological processes explain Mendelian Laws?
First law: meiosis
Second law: meiosis
84
Mitosis
1 diploid cell- 2 identical diploid cells
85
Meiosis
1 diploid cell- 4 different haploid cells (gametes)
86
Chromosomal basis for mendel's first law
Different alleles of the same gene segregate independently when homologous chromosomes seperate in Meiosis I.
87
Chromosomal basis for mendel's second law
Different genes segregate independently when different chromosomes seperate in Meiosis I.
4 possible gametes: Ab, AB, ab, aB
88
Drosophila melanogaster
the canonical genetic model organism
89
Cross WT x white eye (w)
100% dominant phenotype
90
Sex influences expression of the mutant eye phenotype
Sex is associated with eye: not independent segregation.
w gene is on the X chromosome
Males are hemizygous (1X and 1Y chromosome)
Only 1 copy of w is required for white eyes in males.
91
Genes for eye colour and sex
are co-segregating.
Defies mendel's law
92
Unit of segregation
Chromosomes which carry genes
93
SRY
is a sex determining gene
Transcription factor
Activates other genes to develop testes
Part of a cascade of several genes
94
Androgen insensitivity syndrome
X-linked dominant- gene present on X chromosome
Dysfunctional androgen receptor:cells cannot respond to androgens
Affected XY people may have female external genitalia or present as intersex
XX people are carriers with generally no phenotype
95
Sex in Drosophila
Sex determination is based on the ratio of X chromosomes to sets of autosomal (A) chromosomes
96
X-linked recessive rules (in humans)
(hemophilia, red-green colorblindness)
Nearly all affected people are male: all his daughters are carriers, no sons will be affected.
Carrier females are usually phenotypically normal but are heterozygous: half her sons are affected, half her daughters are carriers
All sons of an affected females will be affected
97
The chromosome theory of inheritance
A cytological explanation to Mendel's ratios.
Introduces the concept of sex dtermination and sex linkage
Marks the birth of cytogenetics
98
Chromosome
single strand of DNA in the nucleus
99
Chromatid
1/2 of replicated chromosome, attached together at the centromere of X in shape
100
Sister chromatids
identical copies (diploid)
Same genes in same order
Same alleles in same order
101
Homologous chromosomes
Same chromosomes from different parents
Same genes in same order
102
Mendelian First law Biological process
Alleles of a single gene segregate independently from each other.
Why? Meiosis
103
Mendelian Second Law biological process
Alleles of different genes segregate idependently from each other.
Why? Meiosis
104
Mitosis
1 diploid cell- 2 identical diploid cells
105
Meiosis
1 diploid cell- 4 different haploid cells (gametes)
106
Chromosomal Basis for Mendel's first law:
Different alleles of the same gene segregate independetly when homologous chromosomes seperate in Meiosis I
107
Chromosomal Basis for Mendel's Second Law
Different genes segregate independently when different chromosomes seperate in Meiosis I
4 possible gametes: Ab, AB, ab, aB
108
Humans
23 pairs of homologs, 2n=46
109
Drsophila melanogaster
The canonical genetic model organisms
Fruit fly
Thomas H. Morgan
110
What type of inheritance does the white=eye allele have?
Prediction: the white-eye phenotype follows Mendel's laws
111
Cross WT x white eye (w)
get the red eye
100% dominant phenotype
112
Female red eye (WT) x white eye male
Red eye offspring (WT)
100% dominant phenotype
113
Male red eye (WT) x female white eye
offspring are female red eye (WT) and male white eye
1:1 eye ratio
sex linked
114
How does sex influence expression of the mutant eye phenotype?
Sex is associated with eye colour: not independent segregation.
w gene is on the X chromosome
Males are hemizygous (1X and 1Y chromosome)
Only 1 copy of w is required for white eyes in males
115
The white eye gene is not 'assorting'
eye color is physically linked to sex
116
Unit of segragation
Chromosomes which carry genes
117
Red-green color blindness
Recessive X-linked
Mainly found in men, common (10% of male population)
118
Duchenne Muscular Dystrophy (DMD)
A rare X-linked recessive muscle disorder where patients get progressively weaker
119
X-linked dominant (Vitamin D-resistant rickets)
Affected males transmit the trait to all of their daughters but to none of their sons.
Affected heterozygous females transmit the trait to 1/2 of their children, regardless of sex (like autosomal dominant)
Affected homozygous females transmit the trait to all their children.
Because females can be heterozygous or homozygous, more females have the trait than males
120
KAryotype
A collection all chromosomes (with specific number and structure) of a species/individual in a condensed state
121
Cytogenetics-using karyotypes
Staines identify specific chromosomes and allow analysis
Done while chromosomes are bound up for mitosis
122
Diploid
2n
Normal for most eukaryotes
123
Monoploid
1n
The number of unique chromosomes in a set
124
Haploid
1n
Only 1 set of chromosomes, no homologous chromosomes
E.g. Male bees, gametes
125
Hemizygous
only for haploid organisms or sex chromosomes
126
Euploid
Having a 'normal' number of each chromosome
127
Polyploid
extra copies of each chromosome
Common in plants
common in certain tissues
Causes larger cells and increase cellular metabolism=transcription
128
Triploid
3n
129
Tetraploid
4n
130
Polyploid organisms
Possible because relative gene dosage is preserved
Widespread in plants
Less common in animals- flatworms, leeches, shrimp, lizards, snails
131
Advantages of Polyploidy
Lowers chance of inbreeding risk-buffered against deleterious alleles
Lessen selection on individual gene copies- gene function diversification
Associated with asexual reproduction-mechanism unclear
132
Allopolyploid
Mixing two uneven genomes via hybridization
Example: brassica hybrids
Combination of allopolyploidy and chromosome loss or gain creates new species
133
Autopolyploid
Multiplication of chromosomes from the same organism
1) bigger plants=bigger fruit and flowers
2) seedless plants-sterile
134
'Polyploidization' events
Underlined evolution of flowering plants- whole genome duplication
135
How are autopolyploids generated?
Natural
Artificial
136
Natural autopolyploids generated
Mistakes during meiosis
A failure of one of the meiotic divisions produces a gamete with twice as many chromosomes (2n gametes)
Can be identified via karyotyping
137
Artificial Autopolyploids generated
Treat meiotic or mitotic cells with microtubule inhibitor (i.e. colchicine) to arrest cells in metaphase.
Look for viable progeny (gametes) or plant tissue (for vegetative growth=cuttings)
138
How are allopolyploids generated?
Naturally or induced
Usually sterile
139
Naturally or induced Allopolyploids generated
Are formed by hybridization: crosses between two related species
140
Usually sterile allopolyploids generated
Horse x Donkey= sterile mule
Number and type of chromosomes matter
141
Polyploid tissues are important for normal physiology
In some tissues, cells replicate the DNA (s-phase) but do not divide: polyploid tissue in an otherwise euploid organism
Polyploidy can be a mechanism of selectively increasing cell volume and/or cell metabolism
142
Polyploidy tissues in humans
Liver- hepatocytes, 16n
Heart- cardiomyocytes, 4-8n
Vascular system-smooth muscle cells
Bone marrow- (megekaryocytes, platelet producing cells), 64 n
Skin- keratinocytes, 12 n
143
Agricultural advantages and generation of monoploids
Monoploid organisms can be used in research for:
Screening genotypes for useful mutations
144
Non-disjunction
improper chromosome segregation during meiosis
145
Non-disjunction during meiosis
Rare, often lethal
Gametes have an incorrect number of chromosomes
146
Non-disjunction of the X chromosomes produces (in flies)
Rare white-eyed female (XXY) in F1: normally only males (XY) carry the white mutation.
Rare viable red eye males (X0) that lack the Y chromosome.
It is the dosage of X chromosomes in the genotype, not the presence of the Y, that determines sex
147
Loss of genetic material
Typically affects one gene
Large-scale chromosome changes= chromosome mutations: affect many genes. ;eads to structural differences that can be detected using a microscope
148
Aneuploid
Abnormal number of one or more chromosomes, but not all
Caused by non-disjunction
Homologs don't seperate in meiosis.
Monosomic
Trisomic
149
Monosomic
One missing chromosome
2n-1
X0 sex determination- bees, some bats, most spiders, some snails...
All are lethal in humans except X0 (turner syndrome)
150
Trisomic
One extra chromosome compared wild type- 2n+1
Most are lethal in humans except: Klinefelter syndrome (XXY), Jacob syndrome (XYY)-few symtoms, Trisomy X (XXX)-few symptoms, Down syndrome (trisomy 21)
151
Karyotype of Down syndrome
Trisomy 21
152
Down Syndrome (trisomy 21)
Genotype: 45 XX or XY
Stereotypical facial structure and neurological issues
Frequency from 1:2,000 un young mothers to 1:50 in "older" mothers
Most cases are sporadic, not heritable: caused by chromosome nondisjunction
153
Trisomies increase in frequency with increased maternal age
All oocytes are present at mother's birth: oocytes arrest in Meiosis I, ageging in the ovaries.
Meiosis I continues upon fertilization.
During ageging the association between homologs weakens: increased chance of non-disjunction with age
154
Trisomies for other chromosomes are often incompatible with development, lead to miscarriages
Actual numbers of aneuploidy is unknown from miscarriages before the pregnancy is known.
Only least serious forms survive to birth: other chromosomes have too many critical genes
155
Gene dosage
The amount of a gene product is dependent on how many copies of the gene there are.
The products of genes typically do not work in isolation
Gene imbalance is not enough to be lethal
156
The effect of chromosome size
Large chromosomes generally have more genes- greater chance of imbalance that is essential for embryonic development
157
Relocation of genetic material
Bigger chromosomal changes: affect more genes
more likely to impact phenotype
More likely to impxct meiosis
158
Non-viable rearrangements
Too damaged
Lacks correct number of centromeres: chromosomes can't seperate properly in mitosis
Lack correct arrangement of telomeres: terminal genes will be eroded in further rounds of mitosis
Loss of required genes or severe impairment of gene function
159
Unbalanced rearrangements:
Changes gene number. Deletions and duplication inolving at least one gene
160
Balanced Rearrangements
No change in gene and centromere number.
Inversions and reciprocal translocations that change gene order
161
Chromosomal translocations
often silent.
are the most common chromosomal rearrangements in humans, 0.4% of individuals contain at least one translocation
162
Williams Syndrome
Phenotype: developmental delays and learning disabilities. Distinct facial features, highly soical personlaities, musical talent and strong language skills.
Genotype: deletion of 27 genes on chromosome 7. Deleted section flanked by 2 repeats, increasing the likelihood of mismatch during cross-over.
163
HEMIZYGOUS
A gene or set of genes that do not have a matching allele on a homologous chromosome
1. Most common for sex chromosomes
2. Can also occur with large scale chromosome arrangements
164
Intraallelic effects
Between alleles of the same gene,
165
Haplosufficient
genes that only require 1 copy to affect the phenotype= Dominant
Example- Albinism: only 1 copy of functional tyrosinase needed to make melanin. Heterozygotes still get melanin= not albino
166
Haploinsufficient
Genes that requires more than 1 copy to affect the phenotype= recessive
Example: Achondroplasia dwarfism
Mutations are often dominant
167
Null allele
non functional
168
Allelic series
describes the dominance hierarchy of multiple alleles
169
Hypomorphic allele
has partial function
170
Incomplete dominance
Appearance of a third phenotype that 'blends' two parental ones.
No clear dominance in the heterozygote.
New phenotype not present in parents.
171
Codominance
More than one allele is dominant.
Heterozygote displays both parental phenotypes: variegation
172
Incomplete dominance in humans
often quantitative traits respoding to a single genetic locus have incomplete dominant alleles.
Skin pigmentation (melanin production levels)
173
Codominance in humans:
ABO blood type
174
Pleiotropy
one allele, affecting two or more phenotypes
175
Chimeric gene
produced from the fusion of 2 genes
176
Retinoblastoma:
only 75% of the poeple who carry the dominant mutant allele develop cancer
177
Polydactyly:
50-80% of the people who carry the dominant mutant allele develop malformations
178
Variable (incomplete) penetrance
Individuals with the same genotype may or may or may not express the phenotype.
Same genotype, different (normal x mutant) phenotypes
179
What determines penetrance of the phenotype?
Modifier genes
Environmental factors
Allelic variation-similar but not identical alleles
Complex interactions of the above: genotype x environment
180
Variable expressivity
The degree or intensity with which a genotype is expressed (100% of individuals show the consequences of the mutation at the phenotypic level, but there are many possible degrees of 'severity') same genotype, variable (non-uniform) mutant phenotypes
181
Marfan syndrome
Autosomal dominant disorder
Tsll individuals, long limbs, fingers
Mild cases: nearsightedness
Severe cases: aorta, skeletal malformations
Mutation in the FBN1 gene that codes to FiBRILLIN-1 an important proteins for connective tissue integrity
182
Neurofibromatosis
Variable penetrance+ Variable expressivity
50% to 80% of the individuals carrying the dominant mutant allele develop variable forms of the diseases
183
Polygenic Inheritance
many genes (and alleles), affecting
Opposite: monogenic- a trait is only affected by 1 gene
Height, skin color, weight, behavioral phenotypes are often polygenic.
Incremental effect on phenotype
184
Polygenic trait
Many genes contribute to a single effect
1 phenotype, multiple genes
185
Pleiotropy
A gene has multiple effects
1 gene many phenotypes
186
How alleles at different loci interact with one another
Additive gene action
Complementary gene action
Duplicate gene action
Dominant epistasis
Recessive epistasis
187
Gene interactions
Metabolic pathways controlling specific phenotypes
188
No gene interaction
2 loci, 2 traits, 4 phenotypes
189
Additive gene action
2 loci, 1 trait, 4 phenotypes
Phenotypic ratio 9:3:3:1
190
Complementary gene action
2 loci, 1 trait, 2 phenotypes
191
Redundancy (duplicate gene action)
2 loci, 1 trait, 2 phenotypes
192
Redundancy
Modified phenotypic ratio 15:1
The dominant alleles of both genes produce the same phenotypic effect
193
Epistasis
The phenotype of a gene masked by alleles of another gene.
An allele of one gene overrides the phenotypic effect of an allele of another gene.
194
In Drosophila
The white mutation produces white eyes.
The cinnabar mutation in a different gene produces bright red eyes.
When both mutations are present in the same fly the eyes are white.
The white mutation is epistatic to the cinnabar mutation.
The cinnabar mutation is hypostatic to the white mutation.
195
Recessive Epistasis
When the recessive allele of one gene masks the effect of the second gene (dominant or recessive)
2 loci, 1 trait, 3 phenotypes
Ex: labrador coat color
Modified phenotypic ratio 9:3:4
196
Dominant epistasis
When the dominant allele of one gene masks the effect of the second gene (dominant or recessive)
2 loci, 1 trait, 3 phenotypes
Ex: squash colour
Modified phenotypic ratio 12:3:1
197
Functional interactions between gene products (porteins)
1. Enzymes in a metabolic/synthesis pathway
2. Proteins in a signaling pathway
3. Proteins in a gene regulation pathway
198
Reject null hypothesis
There is a significant difference between the observed phenotypic ratio and the expected ratio.
These data cannot be explained by a Mendelian dihybrid cross/
199
Crossing over
The cellular mechanism that generates recombination
200
Chiasmata
Sites of crossing over
201
Recombination
a new combination of alleles.
Usually refers to a new combination of alleles of linked genes on the same chromosome
202
Frequency of Recombination
% chance of crossing over during meiosis between specific loci
203
No cross over
no genetic recombination
outcome 100% parental gametes
# of chromatids involved: 0/4
204
Cross over before DNA duplication
of chromatids involved: 4/4
Outcome 100% recombinant gametes
205
Cross over after Meiosis I
of chromatids involved: 4/4
Outcome: 100% parental gametes
206
Cross over happens in Meiosis I
Half of the gametes are recombinant.
Balances variation with stability
# of chromatids involved: 2/4
Outcome: 50% parental gametes, 50% recombinant gametes
207
How often does crossover happen?
Crossing over at a specific locus is RARE
but common on average during meiosis across all chromosomes
208
Still used today
Selective breeding
Human genetic disease mapping
209
Recombination Frequency (RF)
The distance between two markers on a chromosome is directly related to how often crossover events occur between them.
=The percentage of offspring that are recombinant
RF=number of offspring with recombinant genotypes/total number of offspring
210
Synteny
genes on the same chromosome= syntenic genes
