Chapter 12 Chromosomal Patterns of Inheritance
Autosomes
Sex Chromosomes
X-linked alleles
Drosphila melanogaster
Total chromosomes = 8
Autosomes = 6
Sex chrom = 2
Eye color in fruit flies – sex-linked trait
F1 generation from white-eyed male and redeyed female (XrY x XRXR)
XRY
x
XRXr
XR Y
XR
Xr
Do the Punnett square with sex chromosome bearing
the allele
2 FEMALES: Red eyes (1 is RR and 1 is Rr)
2 MALES: 1 with white eyes (r_), 1 with red eyes (R_)
Human X-Linked Disorders
Color Blindness: grandfather to grandson via
carrier daughter
Muscular Dystrophy – test for duchenne; inject
immature muscle cells
Hemophilia
Queen Victoria and Prince Albert
Fragile X Syndrome
Base triplet repeats on X chromosome
Gene Linkage: several genes on 1 chromosome
Crossing over: genes that are close together should
cross together
With Crossing Over: 2 parent chromatids (no crossing
over) and 2 recombinant chromatids (crossing over
occurred)
Linkage Map
Chromosome map
Linkage Data
Distance = No. map units
Assume that 1% crossing over = 1 map unit
Direct relationship between frequency of
crossing over and distance between alleles
Perform crosses to point to map distance between
pairs of alleles - > e.g.:
Distance between black-body and purple-eye alleles =
6 map units;
Distance between purple eye and vestigal wing alleles
= 12.5 map units;
Distance between black-body and vestigial wing
alleles =18.5 map units;
Order of genes?
Black body – 6 units – purple eyes – 12.5 units –vestigial wings
Changes in Chromosome Number
Polyploidy – more than 2 sets of chromosomes
Plants exhibit
Aneuploidy: Nondisjunction during meiosis
Monosomy = 2n -1
Trisomy = 2n +1
Trisomy 21 = Down’s syndrome
Amniocentesis – check fetal cells in amniotic fluid
Chorionic Villi Sampling – check fetal cells in
chorionic membrane / may risk natural abortion of
child if perform CVS
Karyotypes
Changes in Chromosome Structure
Accidents – Non-disjunction
Non-disjunction in Meiosis I
n + 1 n+ 1
n–1 n-1
Non-disjunction in Meiosis II
n + 1 n+ 1
n n
Fertilization
Egg: n+1
Sperm: n
Zygote: 2n + 1
Abnormal Sex Chromosomes
XY normal male
XX normal female
XXY
Klinefelter syndrome 1 out 2000 (1/800)
Female genetalia
XYY
meiosis in sperm formation
Jacobs Syndrome
XXX
normal female
1 out of 1500
XO
Turner syndrome
1 out of 2500
1 out 1000
XXYY
XXXY
XXXXY
Alterations of Chromosome Structure
Deletion: part of chromosomes is deleted
Lost fragment of chromosome
Chromosome becomes shorter
Duplication: part of chromosomes is replicated
Fragment is replicated
Chromosome becomes longer
Inversion: two (adjacent) fragments of chromosome
exchange places
2 fragments exchanging places
Chromosome length remains the same
Translocation: fragment from 1 chromosome breaks
off and “travels” to another chromosome and binds
with it
Fragments break off and bind with another
chromosome
Chromosome length can be longer or shorter