|Lecture Notes:: 31 October
© R. Paselk 2001
Genetics, Chromosomes, & DNA
Many genes are linked, that is they don't assort independently
as Mendel found.
- Due to chromosome structure - genes on the same chormosome
shoudn't sort independently.
- Rather, expect genes on same chromosome to "travel"
- This simple picture also fails - many genes sort semi-independently:
- Explain with crossing-over
- If genes are far enough apart on a chromosome will get independent
sorting because there will essentially always be a crossover
between genes during meiosis. (Example of Mendel's seven characters
on pea: pea has seven chromosomes, but seed and flower color,
though sorting independently are now known to be on the same
- Can use the frequency of recombination to "map"
genes on chromosomes.
- Can also map chromosomes with stains and fluorescent probes
under the microscope, and now of course have DNA Sequence for
Sex Chromosomes and Sex-linked Genes
Sex and Chromosomes: Different organisms determine sex
in different ways:
- In mammals and fruit flys females are XX while males are
- Note ovum has X while sperm has X or Y
- This means females have two complete sex chromosomes, while
males have one full sex chromosome and one partial chromosome.
- In some insects females are XX, but males are X0, that is
they have only one sex chromosomes - ovum has X, sperm has
X or no sex chromosome.
- In birds, some fish and some insects have ZW system. Similar
to XY system, but males are ZZ and females are ZW.
- Note that ovum is Z or W, while sperm has Z.
- In most species of bees and ants there are no sex chromosomes
- females are diploid, males develop from unfertilized eggs and
Sex linked genes have a different pattern of inheritance.
- Since females have two chormosomes recessive genes will be
masked as they are on somatic chromososmes, but males will express
- as a consequence get a variety of sex-linked traits,
most famoous of which are color blindness, pattern baldness,
and sex-linked diseases.
- X-inactivation in female mammals - calico cat example (black
and orange due to gene on X chromosome, so claicos generally
Deviations in Chromosomal Inheritance
Genetic Disorders due to Alterations in Chromosome Number
- Nondisjunction occurs when chromosomes don't properly separate
- Results in gametes with aneuploidy, an excess or difficiency
- If fertilized egg has extra (2n+1) then trisomic for
that chromosome. (Down's syndrome is trisomic in Chromosome 21.
There are a variety of sex aneuploidys as noted in text, p 273)
- If fertilized egg difficient (n) then monosomic.
- Can also get Polyploidy where there is more than one
complete set of chromosomes. Common in plants and has been important
i the production of new plant species.
Imprinting: Some genes vary in expression depending
on whether they came form egg or sperm. Imprinting is erased for
each new generation of gametes which are reimprinted. Seems to
be due to methylation of DNA interferring with its expression.
Mitochondrial Inheritance: due to mitochondrial DNA,
so maternal inheritance only.
- Last modified 31 October 2001
- © R Paselk