Download 19.1 - St. Thomas More school Science Student Site

CHAPTER
19
SECTION 19.1
CHROMOSOMES AND GENETICS
CHROMOSOMAL THEORY
• It was not until after Mendel, that scientists began to
have a true understanding of genetics and heredity.
• One of the major discoveries was the chromosome.
• It was eventually determined that all organisms had
these chromosomes and each chromosome carried
some type of genetic information.
• Humans have 46 chromosomes, 44 autosomes (non
sex determining) and 2 sex chromosomes.
• It was determined that each chromosomes carries
thousands of different genes.
• Genes on the same chromosome are said to be
linked genes.
• Over the years the following chromosomal theory of
inheritance was devised:
• Chromosomes carry genes, the units of heredity.
• Paired chromosomes segregate during meiosis. Each sex
cell or gamete has half the number of chromosomes found
in the somatic (body) cells. This explains why each gamete
has only one allele for each gene.
MORGAN’S EXPERIMENTS
AND SEX LINKED TRAITS
• The American, Thomas Hunt Morgan, was among the
first of many geneticists who used the tiny fruit fly,
Drosophila melanogaster, to study the principles of
inheritance.
• There are several reasons why the fruit fly is an ideal
subject for study.
1. REPRODUCE RAPIDLY
• Female Drosophila can reproduce for the first time when
they are only 10 to 15 days old, so it is possible to study
many generations in a short period of time.
• Since genetics is based on probability, the large number of
offspring is ideal.
2. SMALL IN SIZE
• Many individuals can be housed in a single culture tube
without taking up much space.
3. MALE VS. FEMALES
• Males can easily be distinguished from females.
• Males are smaller and have a rounded abdomen with a darkcoloured posterior (back) segment.
• Females are larger and have a pointed abdomen with a
pattern of dark bands.
•
Morgan’s experiments began when he began to notice
white-eyed males amidst many red-eye females.
• He first crossed a true breeding red-eyed female with a
true breeding white-eyed male. His result = all F1
offspring had red eyes.
• He continued his experiment by crossing two F1 flys.
• The F2 generation produced ¾ red eyes and ¼ white
eyes.
• This all seems pretty normal according to Mendelian
genetics; however, Morgan happened to notice that
only males ever developed white eyes.
• This got him thinking that maybe the patterns of
inheritance can differ between males and females.
• Many years later it was proven that in males because
the X and Y chromosomes are not entirely
homologous, a difference in inheritance can result.
• Traits that are affected by the sex of the organism
are known as sex-linked traits.
• Sex-Linked Trait – a trait that is determined by genes
that are located on the sex chromosomes.
• What this meant for Morgan, was that he was correct.
Scientists found that the Y chromosome of
Drosophila does not carry an allele for the eye colour
gene so it can affect inheritance.
• He was wrong however, about only males being
white-eyed.
• Some females can have white-eyes it is just so
uncommon that he didn’t see it in all his crosses.
SEX LINKED CROSSES
• To write a genotype for a sex-linked trait, the letters
X and Y are used.
• For example, lets take a look at Morgan’s original
cross of a homozygous red-eyed female with a
white-eyed male.
XRXR x XrY
XR
Xr
XR
Xr
Y
XR Y
XR
XR
Xr
XR Y
Female
(red-eyed)
Male
(red-eyed)
MORGAN’S F1 AND F2
GENERATIONS
NUMBER OF GENES ON SEX
CHROMOSOMES
• X chromosomes have
between 100-200 genes
depending on the species.
• Y chromosomes have less
than 100 genes.
HUMAN SEX-LINKED GENES
• COLOUR BLINDNESS
• Red green colour blindness is recessive allele found only
on the X chromosome.
• More males have colour blindness because females have
to have two X chromosomes with the recessive allele,
while males only require the one recessive allele.
• HEMOPHILIA
• Extremely slow blood clotting.
• NIGHT BLINDNESS
RECESSIVE LETHAL
DISORDERS
• Recessive lethal disorders – a trait that, when both
recessive alleles are present, it results in death or
severe malformation of the offspring.
• Recessive lethal disorders occur more commonly in
males, because they require only one of the
recessive alleles due to their single X chromosome.