Download Chromosomes and Genetics

Chromosomes
and Genetics
Early Discoveries
There has always been a curiosity about the
human body and how it works
During the middle ages, some individuals
would sneak into caves to dissect corpses
Many would draw their observations
Early Discoveries
Early geneticists had to wait for the
emergence of the light microscope before
investigations could progress
The discovery of the nucleus in 1831 was a
big step towards the discoveries we have
today
Chromosomal Theory
In 1902, Walter S. Sutton and Theodor
Boveri each observed that chromosomes
came in pairs that segregated (separated)
during meiosis
These separated chromosomes then formed
new pairs when the egg and sperm united
Chromosomal Theory
The union of two different alleles in offspring and the formation of
new alleles could now be explained and supported by scientific
evidence
Sutton and Boveri knew that expression of traits like eye colour was
not tied to just the male or female sex cell
They deduced that Mendel’s factors (alleles) must be found on the
chromosomes
Chromosomal Theory
Humans have 44 autosomes (not involved in sex
determination) and 2 sex chromosomes
Sutton hypothesized that each chromosome carries
genes
Genes on the same chromosome are said to be linked
genes
Chromosomal Theory
Chromosomal theory states:
- chromosomes carry genes, the units of heredity
- paired chromosomes segregate (separate) during
meiosis. Each sex cell has half the number of
chromosomes which is why gametes have only one
of each of the paired alleles
Each chromosome contains many different alleles and
each gene has a specific locus on a particular
chromosome
Sex Linked Traits
Thomas Morgan Hunt was among the first to use
Drosophilia melanogaster to study principles of
inheritance
Fruit flies are ideal to study because:
- Reproduce rapidly
- Early mating age
- Small size
- Easy to tell male from female
Sex Linked Traits
When looking at flies, Morgan was interested in tracing
the inheritance of the allele coding for white eyes
He mated a white eye male with a red eye female and all
offspring had red eyes
He then mated to hybrids from the F1 generation
This created ¾ red eyes and ¼ white eyes
Sex Linked Traits
Further examination showed that all the females had red
eyes and only the males had white eyes
Why?!
Morgan turned to cytology (study of cell formation,
structure, and function)
Sex Linked Traits
Previous researchers had found that females have 4
homologous pairs and males only have 3 → the fourth
pair is only partially homologous
Males had one X chromosome paired with a small hook
shaped Y chromosome
Females have two complete X chromosomes
Sex Linked Traits
Since X and Y are not completely homologous, it was
found that they contain different genes
Sex Linked Traits
Morgan concluded that since the Y
chromosome was missing genes, it did not
carry the gene to determine eye colour
Traits determined by genes located on sex
chromosomes like eye colour are called sexlinked traits
Sex Linked Traits
Let’s look at the flies again:
Red eye female → XRXR
White eye male → XrY
XR is allele for red eye dominant
Xr is allele for white eye recessive
No symbol on Y because it doesn’t have an allele for the
trait
Sex Linked Traits
Sex Linked Traits
Although Morgan did not find any white eyed females in
his experiments, they do exist in nature
This is rare because the female will need the recessive
allele on both X chromosomes
Human Sex Linked Traits
In humans, it is estimated that the X chromosome
carries between 100-200 genes while the Y
chromosome has less than 100 genes
Examples of sex linked traits in humans is red-green
colour blindness, hemophilia, near-sightedness, night
blindness
Human Sex Linked Traits
Recessive lethal X-linked disorders in humans, like
infantile spinal muscular atrophy, occur more frequently
in males
Recessive lethal traits result in death or severe
malformation of the offspring
Barr Bodies
A Barr body is a small, dark spot of chromatin located in
the nucleus of a female mammalian cell
These Barr bodies show that not all female cells are
identical; some cells have one X chromosome inactive,
while some have the other
This means that some cells may express a certain trait
while others express its alternate form
Barr Bodies
Example:
If a female is heterozygous for anhidrotic ectodermal dysplasia, she
will have patches of skin that contain sweat glands and patches that
do not
In normal skin, the X chromosome with the recessive allele is
inactivated and sweat glands are produced
In afflicted skin, the X chromosome with the recessive allele is
activated so no sweat glands are produced
Lab Exercise
Page 632
Read through and analyze the pedigree chart
Answer the Analysis questions
This activity is homework and is due on Tuesday.