Flashcards in Mendelian Genetics Deck (104):
1
Law of Segregation (Mendel's 1st Law)
Each trait is controlled by particulate factors that occur in pairs.
These factors (alleles) seperate from one another during gamete formation (in meiosis) : each gamete receives only 1 of each pair.
The double number is restored upon fertilization.
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gene
name for one of Mendel's factors
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allele
alternate forms of a gene
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gamete
sex cell; sperm & egg or pollen & ovum
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zygote
formed by fusion of 2 gametes.
2 types: homozygotes (identical alleles), heterozygotes (different alleles)
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genotype
genetics makeup-description of alleles carried by individual
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phenotype
character determined by genotype (oversimplication for first part of course)
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"Law" of Independent Assortment (Mendel's 2nd Law)
Segregation of each gene pair is independent of all other gene pairs.
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Summarize extensions of Mendel's Laws
1) Dominance not general (not always seen)
2) Multiple alleles are possible
3) Several genes can affect the same character
4) One gene can affect multiple phenotypes
5) Some alleles are lethal when homozygous
6) Alleles may not have constant effects on phenotype
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epistasis
masking of the effects of one gene by an allele of another
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Gene whose allele is epistatic must function ______ in the biochemical pathway?
upstream
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What does it mean for a gene to be "essential"?
It is necessary to complete normal development, and when mutated can result in a lethal phenotype
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penetrance
% of carriers of an allele that are affected by it
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expressivity
degree to which the phenotype is altered in the carriers that are affected
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How much do the sperm and egg contribute to inheritance?
Although they differ greatly in size, each contributes about the same to inheritance.
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What does the sperm contribute to the zygote?
Its nucleus, and little else
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What type of cells give rise to gametes and what are the cells that don't called?
germ cells give rise to gametes; somatic cells make up the rest of the cells in the body.
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Although a gene may have multiple alleles in a given population of individuals.....
a single diploid individual can have only a maximum of two of these alleles, one on each of the two homologous chromosomes caring the gene locus.
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The number of possible genotypes in a MULTIPLE allelic series depends on the number of alleles involved. What is the general formula for n alleles?
n(n+1)2 possible genetopes; n are homozygotes and n(n-1)/2 are heterozygotes.
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What happens in the case of complete recessiveness? Complete dominance?
The recessive allele is phenotypically expressed only when it is homozygous. Complete dominance is the phenomenon in which one allele is dominant to another, so that the phenotype of the heterozygote is the same as that of the homozygous dominant.
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What does incomplete (partial) dominance mean?
With incomplete dominance, the phenotype of
the heterozygote lies in the range between the phenotypes of individuals that are homozygous for
either allele involved. The phenotype of the heterozygote is typically referred to as an INTERMEDIATE phenotype, even though it may not be exactly in the middle between the phenotypes of the two homozygotes.
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What is codominance and how is it different from incomplete dominance?
Another modification of the dominance relationship is codominance. In codominance, the heterozygote exhibits the phenotypes of BOTH homozygotes. By contrast, in incomplete dominance, the heterozygote exhibits a phenotype intermediate between the two homozygotes.
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If n is the number of independently assorting, heterozygous gene pairs, how many phenotypic classes are in F2 assuming that a true dominant-recessive relationship holds for each of the gene pairs?
2^n
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If n is the number of independently assorting, heterozygous gene pairs, how many genotypic classes are in F2 assuming that a true dominant-recessive relationship holds for each of the gene pairs?
3^n
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What are some general characteristics of recessive inheritance for a rare trait?
1. Most affected individuals have two normal parents, both of whom are heterozygous. The trait appears in the F1 because a quarter of the progeny are expected to be homozygous for the recessive allele. If the trait is rare, an individual expressing the trait is likely to mate with a homozygous normal individual. The next generation from such a mating would be heterozygotes who do not express the trait. In other words, recessive traits often skip generations.
2. Matings between two normal heterozygotes should produce an approximately 3:1 ratio of normal progeny to progeny exhibiting the recessive trait. However, in the analysis of human populations (families), it is difficult to obtain a large enough sample to make the data statistically significant.
3. When both parents are affected, they are homozygous for the recessive trait, and all their progeny usually exhibit the trait.
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What are some general characteristics of dominant inheritance for a rare trait?
1. Every affected person in the pedigree must have at least one affected parent.
2. The trait usually does not skip generations.
3. On average, an affected heterozygous individual will transmit the mutant gene to half of his or her progeny. If the dominant mutant allele is designated A and its wild-type allele is a, then most crosses will be Aa X aa. From basic Mendelian principles, half the progeny will be aa (wild type), and the other half will be Aa (and show the trait).
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Leptotene stage
Early prophase 1; the extended chromosomes begin to condense and become visible as long thin threads; commits the cell to continuing the meiotic process
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Zygotene stage
Early to middle prophase 1; chromosomes continue to condense. Homologous pairs of chromosomes actively find each other and align roughly along their lengths. Each pair of homologs then undergoes synapsis
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What is synapsis (in a prophase-context)?
the formation along the length of the chromatids of a zipperlike structure called the synaptonemal complex, which aligns the chromosomes precisely, bp for bp.
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What are two things that should be noted about interphase?
During interphase, the nucleus stains brightly with basic dyes. Chromatin is seen (threads) in the nucleus.
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List the terms describing centromere location in relative order (middle to extremities)
metacentric, submetacentric, acrocentric, telocentric
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What are two important discoveries about chromosomes?
1) chromosomes occur in pairs in the cells of higher organisms
2) chromosome types are constant within a species and in different cells from 1 individual
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The centromere is also known as...?
The primary constriction (1° constriction)
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In metaphase, where do the chromosomes line up?
At the metaphase plate (disc shaped region)
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What is the karyotype?
chromosome sets as seen at the end of prophase
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How does one count chromosomes (by definition)?
Count centromeres--each chromosome by definition has 1 centromere
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What is the Harlequin Chromosome Technique?
An experimental method used to demonstrate ‘sister chromatid exchange’ in chromosomes (also convinced people that 1 DNA strand makes up a chromatid); one of the ‘sisters’ is chemically altered and made to fluoresce more brightly (using base analog 5-bromouracil [5BU] replacing thymine) than its sister (through TWO full cycles of interphase+mitosis); there is no gain or loss of genes, only axial rotation of alleles
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What are homologs?
members of a pair of chromosomes (one from maternal, one from paternal). Homologs have the same genes in the same loci where they provide points along each chromosome which enable a pair of chromosomes to align correctly with each other before separating during meiosis
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What do cells that are said to be diploid or haploid have?
2 sets of homologs for diploid, 1 set for haploid
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What is Meiosis I also known as?
It is the "more complicated and important division," known as reductional division
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What is a bivalent?
A physical association between 2 homologs
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What is the biggest difference meiosis I has from meiosis 2 and mitosis?
The fact that homologs physically associate (form bivalents)
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What is meiosis II in essence?
Basically a mitotic division, except there is no DNA replication occurring before meiosis II
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What are the "products" of meiosis I and II
for I: two haploid cells, each chromosome still has 2 chromatids
for II: (two doubles to become) four haploid cells, each chromosome now has 1 chromatid
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In the grasshopper, how do males and females differ (in terms of chromosomes)?
Males have 1 chromosome that has no homolog (initially called the accessory chromosome, later known as the X). Females have two X (XX vs males XO).
Therefore males have 1 less chromosome than females, and are said to be heterogametic, while females are homogametic
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What is the chromosome theory? What did the proof of the theory depend on?
Genes are on chromosomes; the proof depended on the discovery of sex chromosomes
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In humans (and mice), what chromosome determines if one is male or female?
The presence of the Y chromosome (have it and you're a man)
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Compare Turner's syndrome to Klinefelter's
Turners: XO females, underdeveloped sex character, sterile (1/5000)
Klinefelters: XXY males, feminized, sterile, long limbs
(1/1000)
Both have the same "mistake", but occur at different frequencies since Turner's syndrome is usually lethal (80%)
Sidenote: XXX are "normal" females (may be taller, and may be sterile) (1/700)
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What does the Y chromosome do?
The Sry gene on the Y determines the sex of the gonad (i.e. it causes the gonads to become testis), which secretes androgens that cause the rest of embryo to develop as male. Translocation of Sry to XX produces male-like female; deletion on XY leads to female-like male.
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Differentiate 1°, 2°, 3° sex determination
1°: sex of gonad
2°: sex of genitalia
3°: events at puberty
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What happens in testicular feminization (TFM)?
null mutant gene causes missing androgen receptor; therefore, androgen has no effect on cells and only 1° sex determination occurs normally (2° and 3° fail and fall into female mode)---ultimately produces a female (possibly normal looking) that's actually a male
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What is the proband (a.k.a. propositus)?
the person that drew attention to the pedigree in the first place
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What two useful assumptions to make when analyzing pedigrees?
1) assume complete penetrance unless forced to conclude otherwise
2) if you are told the condition is rare, assume unrelated individuals marrying into the family carry ONLY the normal allele (homozygous).
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X-linked inheritance
The term used for the pattern of hereditary transmission of X-linked genes. When the results of reciprocal crosses are not the same, and different ratios are seen for the two sexes of the same offspring, a x-linked trait may well be involved
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autosomes
non-sex chromosomes
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nondisjunction
when homologous chromosomes (meiosis I) or daughter chromosomes (meiosis II) fail to move to opposite poles at anaphase
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What is the ZW sex-determination system, and how is it different from XY (which occurs in most mammals) and X0 (which occurs in grasshoppers and similar)?
In contrast to the XY sex-determination system and the X0 sex-determination system, where the sperm determines the sex, in the ZW system, the ovum determines the sex of the offspring. Males are the homogametic sex (ZZ), while females are the heterogametic sex (ZW). The Z chromosome is larger and has more genes, like the X chromosome in the XY system. Females are hemizygous.
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What happens in the primary disjunction during meiosis in a white eyed female Drosophila (and what are the results when crossed with a normal red-eyed male?)
Nondisjunction in an individual (female) during MI or MII with a normal set of chromosomes (primary nondisjunction) leads to an XX and O egg being produced. Each X carries 1 w allele, and thus XX provides 2 mutant alleles while O egg carries no X at all.
Normal red eye male is w⁺/Y, and produces sperm w⁺ and Y.
Possible outcomes of crossing are XXX, XXY, XO, YO.
XXX and YO die early in development. XXY (females) and XO (males, sterile) have white eyes.
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What happens in secondary disjunction (in MI) for an exceptional white-eyed female XXY
Normal segregation would be:
P w / w / y
F1 w w / y
...with secondary disjunction:
P w / w / y
F1 w / w y
Note that crossing the latter with a normal red-eyed male produces XXX and YY progeny that die. Doing the same with the former produces the same phenotypes of a w / w (normal white-eyed) female and w⁺ / y (normal red-eyed) male (but NOT the same GENOTYPES)
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What is different about XXX and XYY in humans?
They are "mostly normal" female and males (there are abnormalities, but not severe as X0, XXY, XXXY, XXYY)
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X-chromosome-autosome balance system in Drosophila melanogaster
sex determined by ratio of number of X chromosomes to the number of sets of autosomes. Ratio >= 1 female, between 0.5 and 1 is intersex, and 0.5 and below is male
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What is special about the sex determination of baker's yeast (Saccharomyces cerevisiae)?
The species relies on a genic system for sex determination--that is, the sexes are species by simple allelic differences at one or a small # of gene loci.
For baker's yeast, the two MATING TYPES ("sexes") are a and α (alpha). The two types have the same morphologies, but crosses can only occur between individuals of opposite type
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What are the characteristics of X-linked recessive inheritance?
1) Females must usually be homozygous for the recessive in order to express mutant trait.
2) Trait is expressed in males who possess only 1 copy of the mutant allele on the X-chromosome
3) Affected males usually transmit the mutant gene to all their daughters but to none of their sons. The instance of a father-to-son inheritance of a rare trait in a pedigree tends to rule out X-linked recessive inheritance
4) Many more males should exhibit the trait than females.
5) Sons of homozygous affected mother should show trait
6) Sons of heterozygous mothers should show an approx. 1:1 ratio of normal to mutant
7) carrier female crossed with normal male--all will be normal phenotypically, but half will be carriers; therefore half the sons of these carriers will exhibit the trait.
8) Male expressing trait crossed with homozygous normal female will produce all normal children---but all female progeny will be carriers
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X-linked dominant inheritance
Follow the same sort of rules as do X-linked recessives, however...
1) heterozygous females express the trait
2) in general, X-linked dominant traits tend to be milder in females than in males
3) More frequent in females than in males
4) If trait is rare, females with the trait are likely to be heterozygous
4a) pass on the trait to 0.5 male progeny and 0.5 female progeny
5) Males with X-linked dominant trait pass on the trait to all their daughters and none of their sons
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Symbolizing dominant and recessive alleles (this form is used frequently for sex-linked traits)
1) Allele that is considered "normal" is designated "+" (often written as superscript)
2) Gene name used based on phenotype of variant; symbol chosen for gene depends on whether variant is dominant or recessive to the wild type: if recessive, 1st letter of symbol is lowercase, if dominant, 1st letter is uppercase.
3) when describing crosses, a slash '/' is used to seperate the 2 alleles carried by the 2 homologs
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Progeny that violate criss-cross inheritance are called..
matroclinous daughters and patroclinous sons
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linkage
association of genes on the same chromosome (genes that do not appear to assort independently because they are located on the same chromosome exhibit linkage)
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recombination
process of creating new gametic types
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recombination frequency
frequency of non-parental types; i.e. nonparental gametes divided by total gametes expressed as a percentage.
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When alleles are together on the same homolog, they are said to be...
in cis to one another, or in coupling
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When alleles are on different homologs, they are said to be....
in trans to one another, or in repulsion
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Range of recombination frequency
Always between 0 and 50%
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Can meiosis occur in a haploid species? Haploid individual?
Yes for species; neurospora crassa and baker's yeast are examples. If sexual mating system exists, haploid cells can fuse and produce a diploid cell that can undergo meiosis.
No for individual. Meiosis only occurs in diploids
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genetic marker
another name for a mutation or variant that gives a distinguishable phenotype (i.e. it is an allele that marks a chromosome or a gene)
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The best cross to use to test for linkage is the
testcross--a cross of an individual with another homozygous recessive for all genes involved
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The recombination frequency for two linked genes is the same, regardless of what?
regardless of whether the alleles of the two genes involved are in coupling or in repulsion.
Although the actual phenotypes of the recombinant classes are different for the two arrangements, the percentage of recombinants among the total progeny will be the same in each case (within experimental error).
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What are map units (mu) used for?
It is used in measurements of genetic distance between genes, where 1 map unit is defined as the interval in which 1% crossing over takes place
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The farther apart two genes are, the greater is the...
crossover frequency
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What does it means when recombination frequency is high? Low?
recombination frequencies are used as working estimates of map distances between genes; therefore higher means farther from the other genes and lower indicates that the genes are closely linked
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How and what can recombination frequencies be used to predict?
Frequencies observed between genes may be used to predict the outcome of genetic crosses. e.g. if rf%=20% between genes indicates that for a doubly heterozygous genotype (e.g. a+ b+ / a b), 20% of gametes produces on ave will be recombinants (a+ b and a b+, with 10% of each expected)
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Why is there a limit on the recombination frequency in the progeny for any testcross?
Because if the genes assort independently, an equal number of recombinants and parentals are expected in the progeny, so the recombination frequency is 50%. If we get a recombination frequency of 50% from a cross, then we state that the two genes are unlinked. Genes may be unlinked (that is, show 50% recombination) either when the genes are on different chromosomes (a case we discussed before) or when the genes are far apart on the same chromosome.
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What is interference? How does one express the extent of interference?
the presence of one crossover interferes with the formation of another crossover nearby
The extent of interference
is expressed as a coefficient of coincidence (observed divided by expected double crossover frequency); interference = 1 - coefficient of coincidence
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What does a coefficient of coincidence of 1 mean? 0?
A coefficient of coincidence of 1 means that, in a given region, all double crossovers occurred that were expected on the basis of two independent events; there is no interference, so the interference value is zero. If the coefficient of coincidence is zero, none of the expected double crossovers occurred. If one crossover completely prevents a second crossover in the region under examination, the interference value is 1.
84
When is there a direct linear relationship between the genetic map distance and the observed recombination frequency? Why is this the case?
In gene mapping, if no more than a single crossover occurs between linked genes, there is a direct linear relationship between the genetic map distance and the observed recombination frequency, because the recombination frequency then equals the crossover frequency. HOWEVER, in practice we see this relationship only when genetic map distances are small (less than 10 mu).
Thus, map distances based on recombination frequencies of less than 10% are highly accurate. As the distance between genes increases beyond this point, the chance of multiple crossovers increases, and there is no longer an exact linear relationship between map distance and rf%, because some crossovers go uncounted. As a result, it is difficult to obtain an accurate measure of map distance when multiple crossovers are involved.
*Remember also that double crossovers in adjacent intervals of less than 10 map units. Since no double crossover would go undetected in less than 10 map units, there will be no underestimation in distances between them.
85
The map distance between two markers (gene or DNA) depends on the ___________ that occurs between them in meioses.
frequency of crossing-over
86
What are the substages of prophase I, in order?
leptotene, zygotene, pachytene, diplotene
87
What occurs in leptotene?
"thin thread"
Chromatin condensation begins
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What occurs in zygotene?
"paired thread"
-condensation continues; homologs pair, aligning according to their lengths with the help of telemeres. Paired homologs undergo synapsis. Synapsis finishes in this stage, each synapsed set of homologs consists of 4 chromatids and is called a bivalent.
89
What occurs in pachytene?
"thick thread"
condensation continues; crossing over occurs (forms special structure between pair of homologs); synaptonemal complex breaks down, and elongation of chromosomes starts
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What occurs in diplotene?
"two thread"
chromosomes begin to move apart; becomes clear that each homolog is composed of two chromatids; homologs appear to repel each other but cannot seperate
91
What is the most common source of nondisjunction?
Crossing-over not occurring properly in meiosis I, leading to no chiasma in diplotene (i.e. bivalents don't form)
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Why can the homologs not seperate in diplotene?
The bivalent is held together by where crossing over occurs (resulting in chiasmata) and by a "glue" called cohesin between chromatids. Separase is the enzyme used to remove cohesin
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What is a locus?
position of a gene on chromosome (pl. loci)
94
What are two important properties of double cross-overs?
1) They are rarer than expected based on the assumption that crossing over is independent in adjacent intervals
2) if undetected, they lead to underestimations of the distance between genes that are far apart
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How many chromatids does crossing-over require?
Crossing over takes place when 4 chromatids are present. Each crossover involved only 2 of these chromatids.
96
What is the "four products of single meiosis" called?
tetrad
97
What can be studied in yeast that most other eukaryotic organisms can't be?
All 4 products can be studied from a single meiosis (referring to generation of the egg, since in other organisms the 3 polar bodies are lost after MII).
98
What is an auxotrophic mutation? What mutant gene that is critical in yeast genetics is an auxotrophic mutation? Why is the gene important?
a mutation that requires nutritional supplementation not required for the wild type (which is called the prototroph)
mutant adenine-2 gene (ade-2); such mutants produce red colonies when supplemented with adenine.
99
What are the possible tetrads possible with the crossing over of ade2 a / ade2 α?
1) all 4 parental -- parental ditype (PD)
2) 2 parental, 2 nonparental -- tetrotype (T)
3) all 4 nonparental, nonparental ditype (NPD)
100
How can one tell if two genes are linked just from looking at the number of tetrad types (ratio)?
Use the fact that if NPD << PD ==> very likely to be linked
101
What is the simplest way you can get the 3 tetrad types?
No crossover => PD
Single crossover => T
Two crossovers => NPD
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What does one know about the tetrad types if the two genes are on nonhomologous chromosomes?
PD = NPD
103
What's important to remember about chromatid interference?
There is no chromatid interference; chromatids involved in 1 crossover in no way influences chromatids involved in a second crossover.
You should also note that chromatid interference doesn't exist--it should be distinguished from the "real" interference: "chiasma" interference = 1 - coeff. of coincid.
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