Download Genes on Chromosomes - Capital High School

Genes on Chromosomes (Linkage & Sex)
Recombination
• The reassortment of genes or
characteristics into different combinations
from those of the parents.
• happens during crossing over in prophase
of meiosis.
• Offspring with a new combination of
characteristics/genes are called
recombinants.
Gene Linkage
• Genes often move together in crossing
over because the are close together on the
same chromosome and therefore called a
linkage group.
• Discovered because typical expected
ratios of 1:1:1:1 were coming out 1:1 or
7:1:1:7
– More offspring had parental characteristics
and fewer recombinants because genes were
linked
• How can geneticists make use of these
unusual ratios?
– find which gene a chromosome is located on
– identify all the genes in a linkage group
– estimate how far apart the loci of genes are
on a chromosome
Inheritance and Gender
Pedigree analysis/chart
• shows the members of a family and how
they are related to each other
• can be used:
– to follow a trait through many generations
– see if the trait is dominant or recessive
– see if the trait is sex-linked or not
• square is male, circle female
• affected individuals are darkened in
Colorblindness: p = recessive colorblind allele.
See Fig. 11.22 &
11.23 in text
Given the following blood types, deduce the
genotypes of each individual.
(A and B are codominant and both are dominant
over O)
A_
O_
B_
A_
A_
B_
A_
O_
A_
AB
AB
A_
B_
A_
_
Given the following individuals affected by the “pruple
condition”, deduce all phenotypes.
A = normal allele (good)
a = purple allele (bad)
“A” dominant over “a”.
Sex Linkage or X-linked
• sex linkage - the association of a characteristic
with gender because the gene controlling the
char. is located on a sex chromosome
• male 23rd chromosomes are XY and female
XX
• X and Y do not carry the same genes
• XX, XY, X, XXY, XYY, XXX, and even more
are all possible.
Males (23rd – XY)
• with only one X chromosome, all alleles
on the X will be expressed – dominant or
recessive
Scientists may have saved
the reputation of the ‘Y’,
or male-determining
human chromosome,
whose functions were
poorly understood.
Researchers from the
Whitehead Institute for
Biomedical Research in
Cambridge (USA) and
Washington School of
Medicine in St Louis
(USA) have identified
some 78 genes on the
chromosome, instead of
the 40 or so it was
thought to contain.
Females (23rd – XX)
• females inactivate one of the two X
chromosomes in each cell during
embryonic development. This is called a
Barr body – an inactive and dense
stainable structure found in the nucleus.
• X carries hundreds of genes but few, if
any, of these have anything to do directly
with sex.
Sex-linked conditions
• all are recessive alleles on the X
chromosome
• examples
– hemophiliacs - blood does not clot very well.
– colorblindness – difficulty distinguishing
between shade of red and green.
– muscular dystrophy - defective proteins
causing defective muscles.
Hemophilia