Lecture 15 Mendelian genetics Flashcards
(181 cards)
1
Q
What two assumptions were plant breeders operating under during the early 19th century?
A
- Each parent contributes equally to offspring (correct)
- Hereditary determinants blend in offspring (incorrect)
2
Q
In the 1770’s, the German botanist Josef Gottlieb Kolteuter studied offspring using what method?
A
Reciprocal crosses
3
Q
What are reciprocal crosses?
A
Plants are crossed in opposite directions
4
Q
What did Josef Gottlieb Kolreuter’s studies using reciprocal crosses show?
A
Reciprocal crosses always gave identical results, showing both parents contribute equally to offspring
5
Q
What does it mean that hereditary determinants are blended in offspring?
A
Hereditary determinants in egg and sperm come together and blend together
6
Q
According to the blending theory, what happens to once heritable elements once blended?
A
They could not be separated again once combined
7
Q
Who confirmed that each parent contributes equally to offspring but disproved the blending theory?
A
Gregor Mendel, in the 1860’s
8
Q
What methods did gregor mendel use that led to his successful deductions?
A
Quantitative experiments- due to his studies in physics and maths
9
Q
How many crosses and resulting characteristics did Gregor Mendel observe?
A
24,034 plants
10
Q
When did Mendel’s discoveries burst into prominence and why?
A
1900, meiosis observed and described
11
Q
Why did Mendel chose to study the common garden pea?
A
Ease of cultivation,
feasibility of controlled pollination
Availability of varieties with contrasting traits
12
Q
How did Mendel control pollination and thus fertilization in the pea plants?
A
By manually moving pollen from one plant to another
13
Q
What happens to pea plants if left untouched?
A
They self-fertilize
14
Q
What is a character?
A
An observable physical feature such as flower color
15
Q
What is a trait?
A
A particular form of a character such as purple flowers or white flowers
16
Q
What is a heritable trait?
A
One that is passed from parent to offspring
17
Q
What characters did Mendel look for?
A
Characters with well-defined contrasting alternative traits, such as purple flowers versus white flowers
True breeding traits
18
Q
What are true breeding traits?
A
Observed trait was the only one present for many generations
19
Q
How did Mendel isolate true-breeding strains?
A
By repeated inbreeding (by crossing sibling plants or by allowing self-fertilization)
20
Q
What characters did Mendel focus on in his experiments?
A
Seed shape seed color Flower color Inflated/constricted pod Pod color Flower type Stem length
21
Q
What are the two traits of flower type Mendel focused on? Which was dominant?
A
Axial flowers (dominant) Terminal flowers
22
Q
What did Mendel do before performing any experimental cross?
A
He made sure each potential parent was from a true-breeding strain
23
Q
What was the first step in Mendel’s crosses?
A
Collect pollen from one parental strain and place it on the stigma of another flower whose anthers were removed.
24
Q
Why did Mendel remove anthers of some of the plants?
A
To stop the plant self-fertilizing
25
What were the plants providing and receiving pollen called in Mendel's experiments?
The parental generation, P.
26
What happened after Mendel fertilized the parental generation, P?
Seeds formed and were planted
27
What is the name of the generation made by the parental generation?
The first filial generation, F1
28
What did Mendel and his assistants do with the F1 generation?
Examine and recorded the number of F1 plants expressing each trait
29
What happened after the F1 generation was examined>
The plants self-pollinated to produce the F2 generation, which was characterized and counted.
30
What is a hybrid?
The offspring of crosses between organisms differing in one or more traits
31
What did Mendel do in his first experiment?
Mendel crossed two true breeding parental lineages, differing in just one trait
32
Mendel crossed two true breeding parental lineages, differing in just one trait. What was the F1 generation called?
Monohybrids
33
What happened to the F1 generation in Mendel's first experiment?
They were allowed to self pollinate to produce the F2 generation
34
What is it called when the monohybrids are allowed to self-pollinate?
Monohybrid cross
35
On what traits did Mendel perform his first experiment?
All 7
36
What else did Mendel also do during his first experiment?
Perform a complementary cross
37
What did Mendel observe regarding traits during his first experiment?
Some traits were never expressed in the F1 generation but they reaapear in the F2 generation
38
What was Mendel's conclusion when some traits were not expressed in the first generation but reappear in the second?
Some traits are dominant to the recessive traits
39
What did Mendel observe to do with ratio's during his first experiment?
Ratio of the two traits in the F2 generation is always the same, 3:1
40
Why did Mendel perform reciprocal crosses during his first experiment?
To prove that it does not matter which parent contributes the pollen
41
What did Mendel's monohybrid cross experiment show could not be the case?
The blending theory
42
Why was the blending theory rejected?
- F1 seeds should have intermediate appearance
| - Recessive traits should not reaapear in the F2 generation
43
What did Mendel instead propose, after rejecting blending theory?
Particulate theory- units responsible for inheritance are discrete particles that appear in pairs and separate during gamete formation
44
What is Mendel's unit of inheritance now called?
A gene
45
What are different forms of a gene called?
Alleles
46
What must individuals who produce recessive traits be?
Homozygous for the recessive allele
47
What is the physical appearance of an organism called?
Phenotype
48
What did Mendel correctly suppose about phenotypes?
That they are the result of the genotype
49
How many different genotypes are there for seed shape?
Three: Ss, SS,ss
50
What is Mendel's first law?
The law of segregation
51
What is the law of segregation?
Separation of alleles, or homologous chromosomes, during meiosis so each haploid daughter nuclei contains one pair found in diploid cell, not both
52
How can allele combinations resulting from a cross be predicted?
Using a punnet square
53
What is a gene?
A sequence of DNA that resides as a particular site on a chromosome, called a locus, and encodes a particular gene
54
How are most genes expressed in the phenotype?
Mostly as proteins with particular functions, such as enzymes
55
What can a dominant gene be thought of, given that genes are expressed as phenotypes?
A region of DNA that expresses a functional enzyme
56
What is a recessive gene, given that a dominant gene is a region of DNA that expresses a functional enzyme?
A gene that expresses a non-functional enzyme
57
How did Mendel verify his hypothesis that there are two possible allele combinations for spherical seeds (the dominant trait) in the F1 generation?
By performing a test cross
58
What is a test cross for?
It is a way of showing whether an individual is homo or heterozygous
59
How is a test cross carried out?
The individual in question is crossed with an individual known to be homozygous for the recessive trait
60
What are the two possible results for a test cross?
If tested individual is SS, offspring will all show dominant trait
If tested individual is Ss, offspring will be half dominant and half recessive.
61
What is Mendel's second law?
The law of independent assortment
62
What experiments did Mendel carry out to come up with the law of independent assortment?
Dihybrid crosses
63
What is a dihybrid cross?
A cross between individuals that are identical double heterozygous
64
What happened when Mendel crossed a true breeding plant dominant for two characteristics (SSYY) with a true breeding recessive ssyy?
All offspring in the F1 generation were SsYy (spherical, yellow)
65
What happened to the F1 generation SsYy in Mendel's second experiment?
He conducted a dihybrid cross by allowing them to self pollinate
66
What were the two possibilities for the F2 generation produced by a dihybrid cross in Mendel's second experiment?
The alleles maintain their associations from the parental generation (they are linked)
Or
The segregation of S from s could be independent from the segregation of Y from y
67
What would happen if segregation of S from s was independent from the segregation of Y from y in Mendel's second experiment? (What are the expected results?)
SS, Ss or ss combines with YY, Yy, yy to produce 9 phenotypes occurring in the ratio 9:3:3:1
68
What is it called when parental traits occur in new combinations?
Recombinant phenotypes
69
What is the law of independent assortment?
Alleles of different genes assort independently of one another during gamete formation
70
When is the law of independent assortment?
When genes are on different chromosomes
71
How do geneticists apply Mendel's laws to humans?
Using pedigrees
72
What are pedigrees?
Family trees that show occurrence of phenotypes in several generations of related humans
73
Why are human pedigrees not as clear as Mendel's work on pea plants?
Because humans do not have as many offspring
74
What assumptions are made when making a human pedigree?
-An allele causing abnormal phenotypes is rare, so members outside of the family tree are unlikely to carry it
75
What are key features of a dominant allele pedigree?
- Every affected person has an affected parent
- 1/2 of offspring with affected parent is affected
- Phenotype occurs equally in both sexes
76
What are key features of recessive allele pedigrees?
- Affected people usually have unaffected parents
- In affected families, 1/4 of children are affected
- Occurs equally in both sexes
77
Why do different alleles of a gene exist?
Because genes are subject to mutations
78
What is the name given to an allele of a particular gene that is present in most individuals that gives rise to the expected phenotype?
The wild type
79
What name is given to alleles that are not the wild type?
Mutant alleles
80
What is a genetic locus with a wild type present less than 99% of the time known as?
Polymorphic
81
How is coat colour in rabbits inherited?
Multiple alleles
82
What are the alleles for coat color in Netherland dwarf rabbits?
C
c^ch
c^h
cc
83
What are the possible phenotypes for Netherland dwarf rabbits?
```
Dark gray
Chinchilla
Light gray
Point restricted
Albino
```
84
What happens when genes have alleles that are not dominant or recessive to each other?
Intermediate phenotype is shown
85
Give an example of intermediate phenotypes.
Cross of truebreeding red snapdragon with truebreeding white snapdragon gives F1 generation which is all pink
86
How can it be demonstrated that pink snapdragons are the result of Mendelian inheritance and not blending?
Further cross: F2 generation gives rise to 1 red to 2 pink to 1 white- hereditary particles are sorted not blended
87
What type of dominance gives rise to intermediate phenotypes?
Incomplete dominance
88
What is the name given when two alleles at a locus produce two different phenotypes that both appear in heterozygotes?
Codominance
89
Where can codominance be seen?
ABO blood types in humans
90
Why did Karl Landsteiner find that mixing blood and serum from different individuals sometimes form clumps?
Anitbodies in the serum react with foreign cells because of the antigens on the surface of the foreign blood
91
What three alleles determine blood compatibility?
I^A, I^B, i^O
92
What is it called when a single allele has more than one distinguishable phenotypic effect?
The allele is pleitropic
93
Where can pleitropy be seen?
Allele for coloration pattern of siamese cats, which is also responsible for cross eyedness
94
When does Epistasis occur?
When phenotypic expression of a gene is affected by another gene
95
What two genes determine coat color in labrador retrievers?
- Allele B (black) dominant to b (brown)
| - Allele E (pigment deposition in hair) dominant to e (no pigment deposition in hair)
96
What is the result of having allele e in labrador retrievers?
Yellow hair
97
Why do problems arise as the result of inbreeding?
Close relatives tend to have the same recessive alleles- some of which may be harmful
98
What did G.H Shull experiment on?
2 existing corn varieties
99
What happened when Shull crossed the two corn varieties?
They went from producing 20 bushels of corn per acre to 80
100
What is the name given to the increase in corn yield seen by Shull when he crossed two different varieties?
Hybrid vigor,
| Heterosis
101
What is the mechanism by which heterosis works?
It is not known
102
What is a hypothesis made to explain how heterosis works?
Overdominance- heterozygous condition in certain important genes is superior to the homozygote
103
What is another hypothesis made to explain how heterosis works?
Homozygotes have alleles that inhibit growth, which is less active or absent in heterozygotes
104
What else effects the phenotype of an organism, other than its genotype?
The environment
105
What types of variables can affect gene expression?
Light
Temperature
Nutrition
106
Give an example of a phenotype caused by interaction with the environment as well as genes.
Point restriction in Siamese cats and certain rabbit breeds
107
How is point restriction in Siamese cats and certain rabbit breeds controlled?
Genotype should result in dark fur over entire body
An enzyme that produces dark fur has a mutation, rendering it inactive at temperatures above 25 degrees C
Extremities are cooler
108
How can an experiment demonstrate that dark fur in Siamese cats/ certain rabbits is temperature dependent?
Remove a patch of white fur on point-restricted rabbits back
Apply ice pack to patch
Fur grows back dark
109
What two parameters describe the effects of genes and environment on the phenotype?
- Penetrance
| - Expressivity
110
What is penetrance?
The proportion of individuals in a group with a given genotype that actually show the expected genotype
111
What is expressivity?
The degree to which a genotype is expressed in an individual
112
Complex characters such as height that shows continuous variation is known as what?
Quantitative variation
113
What is quantitative variation usually the result of?
both multiple genes and the environment
114
What name is given to all of the genes that together determine complex characters (such as height)?
Quantitative trait loci
115
Why is it important to determine quantitative trait loci?
The amount of rice grain produced is the result interacting genetic factors, deciphering these leads to higher-yield rice strains
116
How was the pattern of inheritance of genes that don't follow the law of independent assortment worked out?
Studies done on Drosophilia melanogaster
117
Why is Drosophilia melanogaster used in experiments?
Small size
Ease of breeding
Short generation time
118
Who and when did experiments on Drosophilia meanogaster occur to discover the relationship between genes and chromosomes?
Thomas Hunt Morgan
| 1909
119
What was the hypothesis that genes assort independently disproven by Thomas Hunt Morgan?
Crossing Drosophilia of two known genotypes,BbVgvg and bbvgvg
120
What is BbVgvg?
B- wild type gray body
b-mutant black body
Vg- wild type wing
vg- vestigial wing
121
What is the expected phenotype of a cross between BbVgvg and bbvgvg?
Ratio of phenotypes 1:1:1:1
122
What was the actual result of the cross performed between BbVgvg and bbvgvg?
The genes were not assorted independently, they were mostly inherited together
123
How did Morgan explain his results?
The genes were linked because they were on the same chromosome
124
What is a linkage group?
A full set of loci on a given chromosome
125
Why did some flies have some recombinant phenotypes?
Linkage is not absolute, genes can be exchanged between chromatids
126
How do genes recombine?
Two homologous chromosomes physically exchange corresponding segments during prophase I of meiosis by crossing over.
(reciprocal)
127
What name is given to the proportions in which recombinant offspring occur?
Recombinant frequencies
128
How are recombinant frequencies calculated?
By dividing the number of recombinant progeny by the total number of progeny
129
When are recombinant frequencies greater?
When loci are further apart along the chromosome because exchange events are more likely to occur
130
Who discovered that recombinant frequencies could be used to make genetic maps?
Alfred Sturtevant, in 1911
131
What is the distance between genes measured in?
Map units
132
How many recombination frequencies does 1 Map unit correspond to?
0.01, also called a centimorgan
133
What human disease does the parental origin of a chromosome matter?
Hemophilia is inherited from the mother, not father
134
What name is given to plants in which one individual produces both male and female gametes?
Monoecious
135
Give an example of a monoecious plant.
Corn
136
What name is given to plants in which some individuals produce only male gametes and others female gametes?
Dioecious
137
Give an example of dioecious plants.
Date palms
| Oak trees
138
How is sex determined in dioecious organisms?
Chromosomes
139
What type of chromosome determines sex in many animals, including humans?
Sex chromosomes
140
What name is given to chromosomes that do not determine sex?
Autosomes
141
How does abnormal sex chromosome constitutions occur?
Nondisjunction during meiosis
142
What are XO individuals?
Individuals with just an X chromosome (aneuploid)
143
What are the characteristics of XO individuals?
Females with physical abnormalities , usually sterile
144
What is the name given to the condition of having XO chromosomes?
Turner syndrome
145
How is Turner syndrome unusual?
It is the only known case where a person can survive with only one member of a chromosome pair, although most XO conceptions are spontaneously terminated
146
What name is given to the condition resultant from having XXY chromosomes?
Klinefelter syndrome
147
What is the result of Klinefelter syndrome?
Overlong limbs
| Sterile
148
What does observations regarding Turner syndrome and Klinefelter syndrome suggest?
The gene that determines maleness is located on the Y chromosome
149
What two other chromosomal abnormalities helped researchers pinpoint the location of the gene for maleness?
Some individuals are XY but phenotypically female but lack a small portion of Y chromosome
Some men are genetically XX but have a small piece of Y chromosome
150
What did the fragments that were missing/present from
| the sex chromosomes contain?
Sex determining region on the Y chromosome (SRY)
151
What does the SRY gene encode for?
The protein involved in primary sex determination
152
What happens in the presence of functional SRY protein?
Embryo developes sperm producing testes
153
What happens in the absence of the functional SRY protein?
Embryo develops into egg-producing ovaries
| Gene on X chromosome (DAX1) produces anti-testis factor
154
What is the role of the SRY protein in males?
Inhibit the maleness inhibitor encoded by DAX1
155
What is primary sex determination not the same as?
Secondary sex determination
156
What is the result of secondary sex determination?
Outward manifestations of maleness and femaleness
157
What are outward characteristics determined by?
Genes scattered on the autosomes and X chromosome that control hormones
158
In Drosophilia, what are XO individuals?
Sterile males, rather than females seen in mammals
159
What are XXY individuals in Drosophilia?
Fertile females
160
How is Drosophilia sex determined?
Ratio of X chromosomes to autosome sets.
161
Do genes on sex chromosomes show Mendelian pattern of inheritance?
No
162
On which sex chromosome are most genes that affect characters carried?
X
163
What name is given males where there are 2 X genes present in females and only one in males?
Hemizygous for genes on the X chromosome
164
What is the consequence of males only having one gene on the X chromosome?
They only have one copy, so it will be expressed.
165
Name a sex linked character in Drosophilia flies.
Eye color
166
What is the wild type eye color in Drosophilia?
red
167
What happens when a homozygous wild type red eye female is crossed with a mutant, hemizygous white eyed male?
All the sons and daughters have red eyes because red is dominant and all progeny inherited their X chromosome from their mother
168
What was the result of the reciprocal cross (homozygous white eyed female with hemizygous red eye male?
All daughters are red eyed
| All sons are white eyed
169
How many genes are carried on the human X chromosome?
2000
170
What are some sex-linked characters on human X chromosomes?
Red-green colour blindness
171
What are characteristics of a sex-linked human pedigree?
- Phenotype appears more often in males (only one copy of rare allele needed)
- Male can only pass it to his daughter
- Daughters are carriers
- Mutant phenotype can skip a generation
172
Why are X-linked dominant phenotypes rarer than X-linked recessive phenotypes?
People carrying harmful mutations often fail to survive and reproduce
173
Where else is genetic material found?
Mitochondria and plastids
174
How many genes are there in the human nuclear genome?
24,000 genes
175
How many genes are there in the human mitochondrial genome?
37
176
How big are plastid genomes?
5x larger than those of mitochondria
177
Why are organelle genes important?
For organelle assembly and function
178
Why is inheritance of organelle genes different to that of nuclear genes?
Mitochondria/plastids are usually inherited from the mother. (eggs have lots of organelles)
There are hundreds of mitochondria/plastids in a cell, so cell is highly polyploidy
Organelle genes mutate faster than nuclear genes
179
Give an example of plastid gene mutations in some plants and some photosynthetic protists.
Some mutations affect proteins that assemble chlorophyll molecules into photosystems, results in white phenotype instead of green
180
Give an example of mitochondrial gene mutations
Affects one of the complexes in the electron transport chain to result in less ATP production
181
Where does the mutation that affects one of the complexes in the electron transport chain to result in less ATP production have noticeable affects?
Tissues with high energy requirements (nervous system, muscles, kidneys)
