Chromosomal Genetics Flashcards
What is the chromosomal theory of inheritance
Genes have specific locations on chromosomes
The Laws of Independent assortment and segregation occur during meiosis
*this theory was developed before DNA was even determined to carry genes
Why was the fruit fly chosen as the organism for Morgan’s experiment of chromosomal inheritance?
It has a short life span to hatching to mating only takes a couple weeks
Has a simple diploid karyotype - 3 pairs of autosomes and 1 pair of sex chromosomes
XX is female and XY is male
What were the conclusions from Morgan’s fruit fly experiment?
All F1 offspring had red eyes, so the mutant white-eyes must’ve been recessive
Since white-eyes was only expressed in F2 males, the eye colour gene is located on the X chromosome and there is no corresponding locus on the Y chromosome -> eye colour is a sex-linked trait
Explain the X-Y system and X-0 system of sex determination
X-Y (mammals): the sex of the offspring depends on whether the sperm cells contains an X or a Y chromosome
X-0 (grasshoppers and other insects): there is only 1 sex chromosome, X. Females are XX, males are X0. Sex of the offspring is determined by whether the sperm cell has an X chromosome or no sex chromosome
Explain the Z-W system and diploid-haploid system for sex determination
Z-W system (birds, some fishes, and some insects) - the sex chromosomes present in the eggs (not the sperm) determines the sex of the offspring - females are ZW, males ZZ
diploid-haploid (bees and ants) - there are no sex chromosomes. Females develop from fertilized eggs and are thus diploid. Males develop from unfertilized eggs and are haploid (and fatherless)
Describe the nature of the Y chromosome
It only has a few genes that encode for “maleness” (testosterone, development of testes, etc.)
It’s much smaller and shorter than the X chromosome but it has areas at either end that are homologous with the X chromosomes which allows them to act as homologous chromosomes during meiosis
Males are hemizygous for the 1100 genes carried on the X chromosome that they don’t have
If a trait is X-linked, _____ are more likely to display that trait
If a trait is Y-linked, only _______ can display that trait
men
men
Explain what Barr body’s are
What’s an example of this?
female mammals inherit two X chromosomes, but in their early development, one X chromosome is inactivated by DNA methylation into a Barr body and lies along the inside of the nuclear envelope
- barr bodies are reactivated in ovarian cells (for meiosis to produce eggs)
- so females only have 1 functional X chromosome per cell
*it is random which X chromosome gets deactvated - results in genetic mosaics
EX. tortoiseshell colouring in cats
Describe what happens when genes are perfectly linked
What would be the gamete ratios for AB and ab
*linked genes tend to deviate from Mendel’s laws of independent assortment
- if two genes are right next to each other, the alleles will segregate into gametes together because there’s almost zero chance that there can be a cross over between them
Gamete ratios:
50% AB
50% ab
What are recombinant chromosomes - how do we determine how many map units apart the genes are in terms of linked chromosomes?
Ab and aB are recombinant - they have crossed over
To determine the genes’ map units apart you add the total % of recombinant chromosomes
How do you determine the recombination frequency
The # of recombinants / the total # of offspring (x100)
What is aneuploidy and what are the different ways it can occur
Aneuploidy = the incorrect number of chromosomes in an individual (or cell)
Occurs by nondisjunction during anaphase in either:
- anaphase I: a homologous chromosome doesn’t separate
- anaphase II : sister chromatids don’t separate
After fertilization:
the diploid cell can have an extra copy of a chromosome -> trisomy
OR only one copy of a chromosome = monosomy
Explain what Down syndrome is and some of the altered phenotypes it includes
An individual has 47 chromosomes in their cells (3 of chromosome 21)
- nondisjunction in meiosis I
shorter stature, heart defects, developmental delays, shorter than average life span, etc.
What is Klinefelter syndrome ?
males with extra X chromosome - XXY
- low testosterone, can be sterile
- sometimes female-like characters
- barr bodies in a male
- less likely to be affected by X-linked syndromes
What is Jacobs syndrome? and what is XXYY syndrome?
Jacobs:
XYY
- taller, acne, but otherwise healthy
XXYY:
- learning impairment
- sterility
- quite rare
What is triple X syndrome and what is turner syndrome?
XXX:
- taller, but healthy and mostly indistinguishable
- two barr bodies per cell
Turner:
X0 or XX(partial)
- many potential phenotypes ranging from nothing to severe
- cardivascular problems, sterility, shorter stature, neck ‘webing’
What are the alterations of chromosome structure that most likely occur during meiosis I?
Deletion
- missing genes
Duplication: repeats a segment (BCBC)
- has extra of the same genes
Which alterations of chromosome structure can lead to different genes being expressed than original?
Inversion:
- reverses a segment within a chromosome
Translocation:
- moves a segment from one chromosome to a nonhomologous chromosome.
What causes chronic myelogenous leukemia (CML)
caused by reciprocal. translocation during mitosis
switches segments between chromosome 22 and 9 producing Philadelphia chromosome
This causes a fused gene that when expressed, leads to uncontrolled cell cycle division
What is genomic imprinting?
When the phenotypic variation depends on which parent passed on those alleles (100 imprinted genes in humans)
Occurs during gamete formation and results in silencing of particular alleles on certain genes
- offspring will only express one allele of the imprinted gene - the one inherited from the specific parent
DNA in the in the mitochondria and chloroplasts are considered to have _________ genes
whats the other important thing about them?
Extranuclear/cytoplasmic
- do not follow Mendelian inheritance
Mitochondrial and chloroplast genes are ______ inherited
Maternally
*the zygote receives all of its organelles from the egg’s cytoplasm, not the sperm’s
*if a sperm mitochondrion is successful in being transferred, it is destroyed via autophagy
