Download Chapter 15: The Chromosomal Basis of Inheritance

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Transcript
Chapter 15: The Chromosomal Basis of Inheritance
Law of Independent Assortment
Alleles on nonhomologous chromosomes separate
independently during gamete formation.
• Thomas Hunt Morgan
– In early 1900’s used fruit flies and figured out the concept
of linked genes
• Symbols
+ = wildtype (normal)
– For example, red eyes in fruit flies are the wildtype and
white eyes are “mutants”.
– Used w+ for red and w for white
• Linked genes – genes that are on the same chromosome
• Punnett squares from prior chapter assume that genes are
NOT linked!
• Linked genes change the possible offspring because linked
genes change the gametes. Independent assortment will not
affect linked genes.
• Crossovers can affect linked genes!
Examples
• For fruit flies
b+ = gray body (wildtype)
b = black body
vg+ = normal wings (wildtype)
vg = vestigial wings (small)
• If the wing gene and body color genes are not linked
then what results are expected from the cross below?
Cross:
b+bvg+vg x bbvgvg
• If you actually perform the cross on the prior
slide and you do not get the expected 1:1:1:1
ratio in the offspring, this indicates gene
linkage and possibly crossovers!
So if genes are linked….
• Mommy makes fewer sex cells (only 2 instead of 4) because
the b+ and vg+ stay together in her eggs. The Punnett square
and possible offspring would look like this:
Of the genes are linked and crossovers occur…
• A crossover occurred between mommy’s homologous
chromosomes, which switched the vg and vg+ alleles.
• The Punnett square would have to include 4 different
possible sex cells for mommy (the non-crossovers and
the crossover scenarios). The bold genotypes are the
result of crossovers.
bvg
b+vg+
b+vg
b+bvg+vg b+bvgvg
bvg+
bbvg+vg
bvg
bbvgvg
• This Punnett square looks like the one for no linkage
EXCEPT that the ratio would not be 1:1:1:1.
• The bolded genotypes from the crossover gametes
would have a different frequency than 25%.
• The farther apart two genes are on a chromosome, the
higher the probability of a crossover occurring.
Mapping Chromosomes
• Where on the chromosome is the gene located?
• Give the following information:
– Crossover frequencies (recombination frequencies)
• b-vg = 17%
• b-cn = 9%
• cn-vg = 9.5%
Sex-linked genes
• Sex-linked genes are genes on the sex
chromosomes
• Sex chromosomes determine the gender in some
species
• In humans, XX is female and XY is male.
• The Y chromosome is much smaller and does not
contain all of the genes that the X does.
• Males determine the sex of a child.
• Sex-linked recessive traits are more common in
males than in females. Why?
• In females, one of the X’s must be inactivated and is
called the Barr body. The one that is inactivated is
random. A female will have a mosaic of two types of
cells: those with the active X derived from the father
and those with the active X derived from the mother.
Sex-linked problems
1. Cross a woman who is heterozygous for hemophilia
with a man who is normal. Hemophilia is sex-linked
and recessive. What are the possible offspring?
Include gender in your answer.
2. In fruit flies, white eyes are recessive and sex-linked
while red eyes are dominant. Cross a red-eyed female
(whose father had white eyes) with a white-eyed male.
What are the possible offspring? Include gender in your
answer.
Sex-linked Pedigrees
Chromosome disorders
• Nondisjunction – when a pair of chromosomes does not
separate correctly in meiosis causing gamete to have too few
or too many chromosomes
• XXY – Klinefelter syndrome
– Male born sterile, possible slight mental retardation,
overdeveloped breasts with higher risk of breast cancer
• XYY – Taller male
• XXX – Normal female
• X – Turner syndrome
– Female born sterile, short, possible slight mental
retardation, delayed puberty
• Down syndrome – extra 21st chromosome
Breakage of Chromosome Structure
• Duplication – fragment attached to a sister chromosome
• Translocation – a fragment of a chromosome breaks off and
joins another chromosome (not crossover)
– Example – Chronic myelogenous leukemia results from
exchange of large part of chromosome 22 with small piece
of chromosome 9. This leads to a distinctly different
(short) chromosome 22 called Philadelphia chromosome.
• Inversion – fragment of a chromosome reattaches but in
reverse orientation
• Deletion – chromosomal fragment lacking a centromere is
lost
– Example - Cri du chat results from the deletion of piece of
chromosome 5
• Causes mental retardation, small head, and cries like a
cat
• X-inactivation and calico cats
– Gene for fur color is sex-linked with one allele yielding
black and one orange. A female can end up with cells that
have both active X with orange alleles or active X with
black alleles. Males typically cannot be calico because
they only inherit one X chromosome.
• Genomic imprinting - certain genes can be imprinted
depending on whether the gene resides in a male or female.
This means that the same gene may have different effects
depending on gender. In gamete forming cells, genomic
imprint is erased and re-imprinted according to gender of the
individual.
– Example – in mice, paternal gene for Igf2 is expressed and
the maternal allele is not. Igf2 is insulin-like growth factor.
• Heterozygous mice with mutant allele from father =
dwarf
• Heterozygous mice with normal allele from father =
normal size