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
