Transcript chromosomes

CHAPTER 15
1.
2.
Genes have specific loci (positions) on
chromosomes.
It is the chromosomes that undergo
segregation and independent assortment
1. Behavior of homologous chromosomes during
meiosis accounts for Mendel’s law of
segregation.
2. Behavior of nonhomologous chromosomes
during meiosis accounts for Mendel’s law of
independent assortment
①
Organism studied: Drosophilia melanogaster (fruitfly)
①
②
③
④
⑤
⑥
⑦
4 pairs of chromosomes (3 pairs of autosomes & 1 pair of sex
chromosomes)
Morgan crossed a white-eyed male with a red-eyed.
female & got all red-eyed F1 offspring.
He then crossed the F1 flies & got a phenotypic ratio of 3
red-eyed : 1 white-eyed.
Upon closer examination, he found that the white-eyed
trait showed up only in males.
Conclusion: white-eyed trait is linked to sex
This provided support for the chromosome theory of
inheritance because it showed that a specific gene was
carried on a specific chromosome
It also showed that genes located on sex chromosomes
exhibit a unique pattern of inheritance
1.
2.
Located on the same chromosome.
Tend to be inherited together.
3.
Evidence:
Morgan showed that the genes for body color &
wing size in flies are linked by test-crossing an F1
female heterozygous for both traits with a male
homozygous recessive for both traits
2. in the F2 progeny, he saw that the parental allele
combinations showed up more than would be
expected if the two genes had assorted
independently
1.
1.
If the genes for body color & wing size are
linked, why didn’t 100% of the F2 progeny
show the parental phenotype?
Crossing over accounts
for the recombination
of linked genes.
② Genetic recombination
creates recombinants
(offspring with
combinations of traits
different from those
found in either parent)
①
1.
Function of the recombination frequency –
percentage of recombinant offspring
1. The higher the r.f., the greater the distance
between the genes because the more likely a
crossover will occur (Sturtevant)
2.
The distance between linked genes is
expressed in map units
1. 1 map unit = 1% recombination frequency
①
a specific type of genetic map based
on recombination frequencies.
map units
1.
2.
Locates genes by looking at chromosomal
features (such as stained bands) under the
microscope.
Comparisons between linkage maps,
Cytogenetic maps, & other types of physical
maps of the same chromosome have shown
that, although the order of genes is the
same, the spacing between the genes in not
1.
2.
A gene located on either sex chromosome is
called sex-linked
In humans, the terms has traditionally been
used to describe genes linked to the X
chromosome
1. in this case, mothers can pass sex-linked alleles
to both sons & daughters BUT fathers can only
pass sex-linked alleles to their daughters
1.
2.
3.
4.
Will only show in females if they inherit 2
recessive alleles.
Will show in males with the inheritance of 1
recessive allele since they have only one X
chromosome
Therefore, males are much more likely to
have sex-linked recessive disorders than
females.
Examples: color blindness, hemophilia,
Duchenne muscular dystrophy.
1.
2.
3.
4.
One X chromosome in each cell becomes
almost completely inactivated during
embryonic development.
The inactivated X chromosome is called a Barr
body.
Which X chromosome becomes inactivated is
random in each cell; therefore, females contain
some cells with an active X derived from their
father & some cells with an active X derived
from their mother.
This means that if a female is heterozygous for
a sex-linked trait, she will show mosaicism
Caused by nondisjunction – when members of a pair
of homologous chromosomes or sister chromatids fail
to separate during meiosis.
2. Nondisjunction results in aneuploidy – the abnormal
# of a particular chromosome
3. A zygote that has a chromosome number of:
1.
4.
1.
2.
2n + 1 is said to be trisomic
2n – 1 is said to be monosomic
1.
2.
3n = triploid
4n = tetraploid
An organism that has more than 2 sets of
chromosomes is polyploid (common in plant
kingdom)
1.
2.
3.
4.
5.
Down syndrome (trisomy 21) – results from
extra chromosome #21
Klinefelter syndrome – extra X
chromosome in a male (XXY)
Turner syndrome – missing X chromosome
in a female (XO)
cri du chat – deletion in chromosome #5
chronic myelogenous leukemia (CML) –
due to chromosomal translocations
Variation in phenotype
depending on whether
an allele is inherited
from the male or
female parent
2. Occurs during the
formation of gametes
and results in the
silencing of one allele
of certain genes
1.
1.
2.
Mitochondria, chloroplasts, & other plant
plastids contain small circular DNA molecules.
This DNA carries genes that code for proteins,
tRNA & rRNA.
1.
3.
(ex) some of the genes in mitochondria code for
enzymes involved in oxidative phosphorylation.
mtDNA is only inherited along the maternal
line because the mtDNA in a sperm cell does
not enter an egg during fertilization.