Transcript Chapter 15 Chromosomal Basis of Inheritance

Chapter 15 Chromosomal
Basis of Inheritance
Chromosome Theory
• Chromosome theory of
inheritance – states that genes
have specific loci on the
chromosome and they are what
go through segregation and
independent assortment
• Thomas Hunt Morgan is
credited with providing support
to theory with work on fruit
flies (Drosophila Melanogaster)
- showed early framework
for sex linked genes
Sex-linked Genes
• Chromosomal basis of sex determination is
simply X or Y.
- one allele from each parent
- XX is female
-XY is male
- ovaries produce only X chromosomes ,
whereas sperm produce X and Y that
segregate during meiosis
Figure 15.6
44 +
XY
44 +
XX
Parents
22 +
X
22 +
X
+
or 22
Y
Sperm
44 +
XX
Egg
or
44 +
XY
Zygotes (offspring)
(a) The X-Y system
22 +
XX
(b) The X-0 system
22 +
X
76 +
ZW
76 +
ZZ
(c) The Z-W system
32
(Diploid)
16
(Haploid)
(d) The haplo-diploid system
Sex-linked Genes
• Sex linked gene- gene located on the sex chromosome
- historically referred to gene on X chromosome
- fathers may pass sex linked allele to daughters but not
sons
- mothers could pass alleles to both
- males have only one locus meaning if they receive
recessive allele from the mother, they express the trait
*leads to male more prominently expressing
sex-linked recessive disorders
Ex: Muscular dystrophy – lack of muscle
protein dystrophin located on the X
chromosome
Hemophilia- absence of one or more
proteins for clotting blood
X Inactivation in Females
• Barr body – inactive X chromosome in a cell of a
female
-genes will not be expressed
-occurs randomly and independently in
each embryonic cell
-creates 2 cell types: active X from father and
active X from mother
-once inactivated all mitotic descendants
have inactive X
• possible cause comes from multiple copies of
RNA transcript gene on Barr body called Xinactive specific transcript (XIST)
Gene Linkage
• Linked genes – genes located on the same
chromosome that are inherited together in genetic
crosses
• Genetic recombination – production of offspring with
combinations of traits different from either parent
*For unlinked genes recombination is caused by
random orientation of homologous
chromosomes – gives 50% frequency
*Crossing over of homologous chromosomes
accounts for the break in connection between
genes on the same chromosome and allows for the
recombination of linked genes
Gene Maps
• Genetic map – ordered list of genetic loci along a
chromosome
• Linkage map – genetic map based on recombination
frequencies
map unit – distance between genes;
1 unit = 1% recombination frequency
• Alfred Sturtevant contributed the idea that the farther
apart two genes are, the higher the probability of
crossover and thus higher frequency of recombination
-possible for genes on the same chromosome to be
physically linked but not genetically linked
Abnormal Chromosome
Number
• Sex linked traits and mutations are not the only
ways of changing phenotypes
• Physical and chemical disturbances, errors in
meiosis, or damage to chromosome can alter
genes and lead to disorders
• Nondisjunction – condition in which members of
a pair of homologous chromosomes do not move
apart in meiosis I, or sister chromatids fail to
separate in meiosis II
-leads to abnormal chromosome
numbers
Abnormal Chromosome
Number
• Aneuploidy – condition in which offspring have
an abnormal number of a particular chromosome
-trisomic – 3 chromosomes present or 1
extra
-monosomic – 1 chromosome present or 1
missing
• If nondisjunction occurs during mitosis and early
in embryonic development then the condition can
be passed to a large a number of cells and have a
greater effect on organism
Abnormal Sets
• Polyploidy- term for alteration of chromosome
sets or multiple chromosome sets
- refers to somatic cells after mitosis
-triploidy – 3 sets of chromosomes
-tetraploidy – 4 sets
-condition more common in plants
Ex: wheat – hexaploid
strawberries - octoploid
Alterations of Structure
Human Disorders
• Aneuploid gametes can result
from nondisjunction in meiosis
creating aneuploid zygote
-some conditions can cause
embryos to spontaneously
abort
-some survive birth and
beyond but have set of
symptoms labeled a
syndrome
Ex: Down syndrome or
trisomy 21
Aneuploidy of Sex
Chromosomes
• Klinefelter syndrome- males born with extra X
chromosome (XXY)
-occurs 1 in 2,000 births (2004)
- occurs 1 in 500 – 1,000 male births (2014)
-presence of sex organs but sterile individual
-some breast enlargement or female
characterstics may occur
-normal intelligence
-Males with extra Y (XYY) exhibit taller
stature than average but not known
syndrome ( 1 in 1,000)
Aneuploidy of Sex
Chromosomes
• Females with extra X chromosome (XXX) are
normal and healthy
• Turner’s syndrome – (X0) only known
monosomy in humans
- 1 in 2,500 female births
-individuals appear female but sex
organs do not mature and become
sterile
- normal intelligence
Altered Chromosome Disorders
• Cri du chat – deletion in chromosome 5 that
results in mental retardation, small head, facial
abnormalities – cat sound cries
-most die as infants or early childhood
• Chronic myelogenous leukemia – results from
translocation of chromosomes
- results from long portion of chromosome
22 and tip of chromosome 9 exchange(forms
Philadelphia chromosome)
- gene activated causes uncontrolled cell cycle
progression
Inheritance patterns
• Genomic imprinting - patterns of methylation
passes from parent to offspring keeping record of
what occurred during development
-permanently regulates expression of
either maternal or paternal allele of
certain genes
• Extranuclear genes – found in organelles such as
mitochondria and chloroplasts contain small
circular DNA that can code for proteins and RNA