Download Chromosomal Basis of Inheritance

Chromosomal Basis of
Inheritance
Chapter 15
Objectives
• Understand the concept of “Linked Genes”
• Understand how recombination of genes
affect genetic variability
• Understand how frequency of
recombination of linked genes is related to
their loci distance from one another
• Be familiar with patterns of inheritance for
genes on sex chromosomes
• Be familiar with errors that may occur in
chromosomal inheritance due to problems
associated with meiosis
Karyotype
• Using a microscope, it is
possible to count and
characterize the individual
chromosomes during the time
they are coiled and
condensed. A photograph of
the entire set of chromosomes
can be made. Then the
images of the individual
chromosome can be cut out
and arranged by shape and
size in an orderly
arrangement called a
karyotype
http://www.dost-dongnai.gov.vn/portals/0/tinkhcn/200811/20081111/hoi%20chung%20down.gif
http://www.contexo.info/DNA_Basics/chromosomes.htm
Linked Genes
• Linked genes are those that reside on the
same chromosome and tend to be inherited
together
– Autosomal genes reside on the autosomal
chromosomes (pairs 1-22)
– Sex-linked genes are found on the sex
chromosomes (pair 23, usually on the X)
Recombination of Genes
• Genetic recombination is
said to have occurred when
offspring exhibit new
combinations of traits not
seen in the parent
• Independent assortment
may recombine genes that
are unlinked
• Linked genes can become
unlinked through
recombination events like
crossover (during meiosis)
Gene Mapping
• Maps of genes on chromosomes can be
constructed from recombination data
• Recombination data for linked genes
reflects the “distance” of the 2 loci
from one another
• The farther apart 2 loci are from one
another the more frequent the observed
recombination due to crossover
Linkage map: genetic map
based on recombination
frequencies
Sex Linked Inheritance
• Males and females
differ in their sex
chromosome
combination (females
XX; males XY)
• Barr bodies
• Because the X contains
genes and the Y “does
not”, inheritance
patterns of sex-linked
genes vary between the
sexes
– recessive traits more
prevalent in males
Errors in Inheritance
• Genetic disorders can
occur due to:
– changes in
chromosome number
• nondisjunction
– changes in gene
sequence
• deletion
• duplication
• inversion
– changes in gene
location
• translocation
Nondisjunction
• Failure of a pair of
chromosomes to separate
during gamete formation
• Genetic disorders like:
–
–
–
–
–
Down’s syndrome (#21, 3n)
Turner syndrome (XO)
Metafemale (XXX)
Kleinfelter Male (XXY)
Jacob’s Male (XYY)
Changes in Gene Sequence
• Deletion: loss of a chromosomal segment
• Duplication: addition of a chromosomal segment
• Inversion: flipping of information on a
chromosome
Changes in Gene Location
• Translocation: the movement of information
from one member of a homologous pair to a
chromosome of a different (non homologous) pair