Topic 2 Flashcards
First Law
Alleles of a single gene segregates independently from each other
Second Law
Alleles of different genes segregate independently from each other
Chromatid
one leg of a homolog
Homolog
The 2 legs of a chromosome
Products of segregation of a pair of homologs after meiosis
4 chromatids, all different
What does segregation generate?
variability
What does the random orientation of a bivalent on the metaphase plate produce?
2 daughter cells with a different set of chromosomes
Bivalent
1 pair of chromosomes in a tetrad (a pair of homologous chromosomes physically held by at least 1 DNA crossover)
Phenotypic ratio of alternative orientations of bivalents
Same as Mendelian F2 phenotypic ratios
What does 2n=46 mean?
a diploid cell containing 2 copies of each chromosome (contains 46 chromosomes)
of possible random segregation combinations
8,388,608
Meiotic shuffling of chromosomes
cellular mechanism that dictates inheritance patterns
Drosophila
canonical genetic model organism
Thomas Hunt Morgan
Basically gave us the understanding of all genetic inheritance with his fruit flies
What first happened with the flies that started the first experiements?
A fly mutated to have a white eye
What did they discover about the segregation of the white eye mutation?
Sex linked
Morgan’s conclusions
Eye colour co-segregates with sex which doesn’t match 2nd law
Some traits must be stored on the stored on the same unit of segregation (can’t segregate indepenently)
What are chromosomes in regards to segregation?
carriers of genes and the actual unit of segregation
The chromosome theory of inheritance
The pattern of gene transmission reflects the chromosome’s behavior during meiosis
Genes are not inherited in isolation but as a part of a larger structural units shared with other genes with which they segregate
SRY gene
is the determinant factor for the male productive system (its presence makes the embryo develop testes)
What gender would an XX embryo that has a piece of Y the includes SYR?
Male
What gender is an individual that has XY but has a SYR deletion? What is this coondition called?
Female. Sawyer syndrome
How is sex determined n drosophila?
Determined by the ratio of sex (X) to autosomal chromosomes
What are the ratios determining sex in drosophila?
Female - 1
Male - 0.5
Metamale - less than 0.5
Metafemale - more than 1
Intersex - inbetween 0.5 to 1
Non-disjunction events
mechanistc defects in cell division that leads to an abnormal # of chromosomes(ex: failure to correctly separate during anaphase resulting in an abnormal # of chromosomes)
What does non-disjunction lead to in flies?
Aneuploid flies (some are viable but most are lethal), and aberrant inheritance of sex linked traits
X linked recessive in humans
Nearly all affected are males
Carrier female typically is phenotypically normal
All sons of an affected female are affected
Affected male never transmits the trait to male son
X linked dominant in humans
Affected males transfers trait to all daughters but none of their sons
Because females can be heterozygous or homozygous, more females are affected than males
Cytogenetics
Geneticists use stains to identify specific chromosomes and to analyze their structures
Karyotype
A collection of all chromosomes of a species/individual in a condensed state
Monoploidy
The haploid set of chromosomes of a species (n)
Euploidy
A chromosome number that is exactly the multiple of the monoploid set of chromosomes
Polyploidy
A change in the number of copies of an entire set of chromosomes
Aneuploidy
An uneven change of ploidy, when 1 or more chromosomes are missing or in excess
Euploids
Monoploid n
Diploid 2n
Triploid 3n
Tetraploid 4n
Aneuploids
Monosomic 2n-1
Trisomic 2n+1
Natural occurring changes in ploidy
Polyploid - normal development of certain tissues and organisms
Monoploid - part of normal development in some insects (males are 1n, females are 2n)
Monosomy - required for sex determination in some organisms
Plant with higher ploidy level
Look the same as normal ploidy plant, but bigger in size
Autopolyploid
All the chromosomes originated from the same organism
Allopolyploid
1 set of chromosomes originated from one organism and the other from another. The parent organisms need to be related to produce viable progeny
Alloploidy agricultural advantages
Can create new phenotype and eventually new species (ex:brassica)
Autopolyploidy agricultural advantages
Generates new traits valued by customer (ex: bigger, sterile/no seeds)
How are autopolyploids generated?
Mistakes in meiosis that lead to failure in one of meiotic division producing a gamete with twice as many chromosomes as normal
Or
Gametes paused in labs in the metephase stage
How are allopolyploid generated?
Formed by hybridization: crosses between 2 related species
Polyploid tissue in organs (liver, heart, skin, etc.)
Replicate DNA in S phase but do not divide, leading to formation of polyploid tissue. Can help increase cell volume or cell metabolism
Aneuploidy in humans
Defects can typically trace back to defects in meiosis 1. Don’t often carry genetic predisposition or predictable inheritance
Monosomies
1 missing chromosome (autosomic monosomies die, but works only in turner syndrome - missing X)
Trisomies
1 extra chromosome (autonomic trisomies for large chromosomes are lethal)
Why are trisomies and monosomies so deleterious?
too much transcription (trisomies) as well as too little (monosomies) are problematic as they disturb the relative amount of protiens needed to correctly perform a given function
Effect of chromosome size on chromosome imbalances
Larger chromosomes = more genes, likelihood of disrupting several pathways is greater
Chromosome imbalances in chromosome 1
trisomies or monosomies of chromosome 1 are always embryonic lethal
Why cells are able to deal with abnormal # of Xs more effeciently than of autosomes?
Imbalance in Y will restrict fertility because it encodes mostly for sex determinationand function
Imbalance in X will have minor effect because it has naturally evolved to be dosage compensated between 2 sexes
Embryonic cells in female embryos _______ silence a single X
Randomly
Male cells _____ silence their X chromosome
Never
Correct disjunction of homologs produce ….
haploid gametes
Nondisjunction of homologs in the 1st meiotic division produces….
aneuploid gametes
Nondisjunction of homologs in the 2nd meiotic division produces…
aneuploid and haploid gametes
Why are non-disjunction events mostly associated with maternal gametes?
Aging eggs are arrested in meiosis 1 as soon as they are made. Progress is triggered by fertilization. Since it can take a while before this happens, association between homologs weakens over time, making them disjoin more frequently when fertilization does happen
Unbalanced chromosomal arrangements
changes result in loss or gain of genetic material that leads to gene dosage problems or chromosome instability
Balanced chromosomal rearrangements
Changes do not result in gain or loss of genetic material such that gene dosage is not normally affected, but can cause chromosome instability issues
Normal chromosome
A single centromere region
Telomeres in each end (stability)
Dicentric chromosome
More than 1 centromere (chromosome break)
Acentric chromosome
No centromere (spindle fails to capture)
Haploinsufficiency
genes that need to be expressed in both homologs to provide enough protein
pseudodominance
recessive alleles uncovered by the deletion will appear as if they are dominant
Chromosomal loops
association of opposite ends between the inverted region and normal homolog that produces a loop that disrupts homolog segregation in meiosis 1
______ are the most common chromosomal rearrangement in humans
translocations
Robertsonian translocations
produces a large chromosome that is a fusion of both original chromosomes (viable) and a small fragment of remaining part of chromosome (not viable)
