Chapter 15 - Chromosomal Basis of Inheritance Flashcards
Chromosomal Theory of Inheritance
genes, segemnts of DNA coding for a product
are located at specific positions on chromosomes - gene locus
the behavior of chromosomes during meiosis accounts for inheritance patterns
Why did Morgan choose Fruit Flies for Experiment
- short regeneration time allowing mass breeding
- easier to watch over multiple generations
- Only has 4 pairs of chromosomes
Morgans Terms to describe Phenotypic expression
Wild Type: the phenotype for a trait most commonly observed in natural populations
Mutant phenotypes: any other trait alternative to the wild type as they are alleles assumed to have originated as changes or mutations in the wild type allele
Morgans Experimental Conclusions from fruit flies
fly eye color was linked to its sex as mainly it was only white eyed traits showed up in males
- Provided support for the chromosome theory of inheritance
- His work also indicated that genes located on a sex chromosome exhibit unique inheritance patterns
Chromosomal Basis of Sex
species sex are determined by the inheritance of sex chromosomes
- Biochemical, physiological and anatomical features associated with males and females are more complicated with many genes involved in their development
- Y chromosome is much smaller than the X chromosome and - only has short segments at either end which are homologous with regions on the X allowing them to pair in males
Rules of Inheritance of X Linked Genes
- Fathers pass all X-linked alleles to their daughters and none to their sons
- Mothers pass x linked alleles to all sons and possibly to daughters
- If an X linked trait is recessive, a female will only express if she is homozygous for the allele however males will express if the mother carries it
Hemizygous: describes males having 1 locus for x linked genes
Examples of X Linked Disorders
Duchenne Muscular Dystrophy: an X linked disorder which is a progressive weakening of the muscle and loss of coordination
Hemophilia: an X linked recessive disorder defined as the absence of one or more of the proteins required for blood clotting
X Inactivation
- females have two x linked chromosomes, but they do not produce twice as much proteins as 1 x gene is inactivated during early embryonic development
- randomly and independently an X chromosome condenses into a barr body which is a compact object lying along the inside of the nuclear envelope - all mitotic descendents of that cell have the inactive X - They are reactivated in the ovaries which give rise to eggs allowing every female gamete to have an active X after meiosis
Process of X Inactivation
- Inactivation of an x chromosome involved modification of DNA and histones, including the attachment of methyl groups to DNA nucleotides
- Two regions, one on each X chromosome, associates briefly with each other early in embryonic development, and only one chromosomes XIST(X inactive specific transcript) becomes active, resulting in it forming the Barr Body
- Multiple copies of the RNA product of this gene attach to the chromosome, covering it and assisting in X inactiviation
- Two regions, one on each X chromosome, associates briefly with each other early in embryonic development, and only one chromosomes XIST(X inactive specific transcript) becomes active, resulting in it forming the Barr Body
Linked Genes
Two or more genes located near each other on the same chromosome tend to be inherited together in genetic crosses
- Deviate from Mendels Law of Independent Assortment as they are inherited together
Genetic Recombination
the production of offspring with combinations of traits that differ from those found in either P generation parent
Recombination of unlinked genes
- With unlinked genes, some offspring have the chance of having a combination of traits that don’t match those of either parent; nonparental types known as recombinant types
- If 50% of offspring are recombinants, there is a 50% chance of recombination
○ This value and lower shows genes cannot be linked
- If 50% of offspring are recombinants, there is a 50% chance of recombination
- recombination is due to the random orientation of homologous chromosomes at metaphase 1, leading to independent assortment of the unlinked genes
Recombination of Linked Genes; crossing over
- Recombination frequency of less than 50% indicates genes are on the same chromosome(<3% = linked)
- Crossing over accounts for the recombination of linked genes by having two portions of non sister chromatids trade places
- Two genes in close proximity on the same chromosome are more likely to be linked together
- Chi-Square test analyzes the phenotypes of F1 testcross in order to see whether two genes are linked or unlinked.
Process of Chi Square
- If genes are unlinked, phenotypic ratio of offspring is expected to be 1:1:1:1 however if they are linked, it will be1:1:0:0
The observed data is compared to a set of expected data to predict whether genes are unlinked
How are new variations in phenotypes available for natural selection
Physical behavior of chromosomes during meiosis
Differs through:
- Recombination
- Crossing over, independent assortment and random fertilization
Distance between Genes
Map units: the distance between genes equivalent to 1% recombination frequency
- The observed recombination frequency in crosses involving two genes can have a max value of 50%
Any genes on the same chromosome but physically further apart are considered genetically unlinked due to the distance being so great, linkage cannot be observed in genetic crosses
Nondisjunction
where the members of a pair of homologous chromosomes do not move apart properly during meiosis 1 or where sister chromatids fail to separate during meiosis 2.
-error in meiosis 1 results in all offspring having errors, error in meiosis 2 results in half having errors
Anueploidy
zygote will have an abnormal number of a particular chromosome
types:
- Monosomic(2n-1)
- Trisomnic(2n+1)
POlyploidy:
organisms possess more than two complete chromosome sets
- actually common in the plant kingdom as it assists in playing a role in plant evolution
Types of Polyploidy:
- Triploidy(3n):
- Tetraploidy(4n):
Ways Polyploidy Occurs
- Tri: By fertilization of an abnormal diploid egg produced by nondisjunction of all its chromosomes
- Tetra: by the failure of a 2n zygote to divide after replicating its chromosomes leaving mitotic division to produce a 4n embryo
Alterations of Chromosome Structure:
- Errors in meiosis or damaging agents can cause the breakage of chromosomes leading to 4 main structural changes:
- Deletion: when a chromosomal fragment is lost and genes are missing
- Duplication: Broken fragments can become reattached as an extra segment to a sister or nonsister chromatid
- Inversion: Chromosomal fragments can also reattach to the original chromosome in the reverse orientation
- Translocation: crossing over between non-homologous chromosomes
- Deletion: when a chromosomal fragment is lost and genes are missing
- often requires a reciprocal translocation
Syndrome
set of traits characteristic of the type of aneuploidy
Example of Trisomnic disease
Down Syndrome (Trisomy 21): the result of an extra chromosome 21 and characterized by developmental delays, heart and other defects which are treatable and non life-threatening.
Disorders from Aneuploidy on Sex Chromosomes
Klinefelter Syndrome: when males develop and extra X chromosome leading to them having male sex organs that are smaller and produce little to no sperm
Trisomy X: when a female develops a third X chromosome however besides being at risk for learning disabilities, they are generally healthy and have no unusual physical features besides being slightly taller than average Turner Syndrome: monosomy X where females lack the ability to reproduce because while they have sex organs, they do not mature
Genomic Imprinting
he expression of an allele in offspring dependent on whether the allele is inherited from the male or female parent.
- Most are on autosomes
- Occurs during gamete formation and results in the silencing of a particular allele depending on the gene due to genes being imprinted differently in sperm and eggs
Inheritance of Organelle Genes
Extranuclear/cytoplasmic Genes: genes located in mitochondira, chloroplasts plastids and in the cytoplasm
These organelles reproduce themselves and transmit their genes to daughter organelles however not via the process of mendelian inheritance
- passed on through maternal inheritance
