Download The Chromosomal Basis of Inheritance

The Chromosomal
Basis of Inheritance
Chapter 15
REVIEW OF MENDEL’S
DISCOVERIES:
The arrangement of chromosomes at
metaphase 1 of meiosis and their
movement during anaphase 1 account
for the segregation and independent
assortment of the alleles for seed color
and shape.
Let’s look at figure 15.1 on text page
270 together!
REMEMBER:
Principle of Segregation: during the
formation of gametes, the two traits
(alleles) carried by each parent
separate.
Principle of Assortment: States that
each allele pairs of different genes
segregates independently during
gamete formation – applies when genes
for two characteristics are located on
different pairs of homologous
chromosomes.
Beyond Mendel: Thomas Morgan


Thomas Morgan was the first scientist to associate a
specific gene with a specific chromosome, early in the 20th
century.
Morgan chose to work with fruit flies (Drosophila
melanogaster), because:





They are prolific breeders – a single mating will produce hundreds of
offspring
A new generation can be bred every two weeks
They have only four chromosomes – easily distinguishable with a
light microscope
They have three pairs of autosomes and one pair of sex
chromosomes
These characteristics make the fruit fly a convenient
organism for genetic studies!
Normal v/s Mutant Traits
The normal phenotype for a character
(most common in natural populations) is
referred to as WILD TYPE.
 Traits that are alternatives to the wild type
are called MUTANTS – because they are
due to alleles assumed to have originated
as changes in the wild-type allele.

Morgan – Tracing a Gene to a Specific
Chromosome in Drosophila
Figure 15.3

Wild-type
Drosophila flies
have red eyes
(bottom).

Morgan discovered
a mutant male with
white eyes (top).

This variation made
it possible for
Morgan to trace a
gene for eye color to
a specific
chromosome.
Sex-Linked Inheritance
When Morgan bred his
mutant male to a wild-type
female, all F1 offspring had
red eyes. The F2 generation
showed a typical Mendelian
3:1 ratio of traits, but the
recessive trait – white eyes –
was liked to sex.
All females had red eyes,
but half the males had white
eyes. Morgan hypothesized
that the gene responsible
was located on the X
chromosome and that there
was no corresponding locus
on the Y chromosome.
Linked Genes



Number of genes in a cell is FAR greater than
the number of chromosomes.
Linked Genes are genes that are located on the
same chromosome and that tend to be inherited
together .
Inheritance patterns with linked genes tend to
deviate from expected Mendelian ratios:
 Morgan
was the first to trace a gene to a specific
chromosome.

Sex-linked genes are genes located on a sex
chromosome (X or Y chromosome).
Figure 15.9
EVIDENCE FOR LINKED
GENES IN DROSOPHILA
This example explains
that body color AND wing
size must be linked –
look at the expected v/s
observed ratios in the
offspring!
Pages 293
• Morgan determined that
– Genes that are close together on the same
chromosome are linked and do not assort
independently
– Unlinked genes are either on separate
chromosomes of are far apart on the same
chromosome and assort independently
b+ vg+
Parents
in testcross
Most
offspring
X
b vg
b vg
b vg
b+ vg+
b vg
or
b vg
b vg
Genetic Recombination

Genetic Recombination is the general term for the
production of offspring with new combinations of
traits inherited from two parents

Organisms that have these are called
“recombinants”

“parental types” would be offspring that
phenotypically match either parent
In crossing over during prophase of meiosis I, chromatids of
paired homologous chromosomes break, and homologous
chromatid fragments switch places…crossing over. This
creates recombinant chromosomes.
NOTICE the parental types and the recombinants created
during Meiosis I in the diagram above.
Mapping Genetic Loci

Genetics can use recombination data to map a chromosome’s
genetic loci!


Called a genetic map: an ordered list of the genetic loci
along a particular chromosome.


A linkage map is a genetic map based on recombination frequencies.
Assuming that cross-over possibility is approximately equal at
all points on a chromosome, the further apart two genes
are, the HIGHER the probability that a cross-over will
occur between those two genes – the higher the
recombination frequency


Method discovered by Alfred Sturtevant
REASONING: the greater the distance between two genes, the more
points there are between them where crossing over can occur.
The distances between genes are called Map units, or
centimorgans in honor of Thomas Hunt Morgan.

Equivalent to a 1% recombination frequency.
b = body color
The probability of a crossover between two genetic loci
is proportional to the distance separating the loci.
cn = cinnabar eyes
(brighter red)
vg = wing size
This simplified map
shows just a few of the
genes that have been
mapped on Drosophila
chromosomes II.
Notice that more than one
gene can affect a given
phenotypic characteristic,
such as eye color.
Constructing a Linkage Map
• Determine the sequence of genes along a
chromosome based on the following
recombination frequencies:





A-B = 8 map units
A-C = 28 map units
A-D = 25 map units
B-C = 20 map units
B-D = 33 map units
The Sex Chromosomes

In humans and other animals, there are two
varieties of sex chromosomes: X and Y.
 XX
= girl
 XY = boy

Anatomical signs of sex begin to emerge in
humans when the embryo is about 2 months old.
then, the rudiments of gonads are generic –
they can develop into either ovaries or testes,
depending on hormonal conditions within the embryo.
 Y chromosome must be present to produce testes.
 Before
Sex-Linked (x-linked) Genes

Show up more often in the sex that has only one copy of
the X- chromosome!



Fathers pass sex-linked alleles to all of their daughters but none
of their sons.
Mothers can pass sex-linked alleles to both sons and daughters.
If a sex-linked trait is due to a recessive allele, a female
will express the phenotype only if she is homozygous.
BUT, a male receiving the recessive allele from his
mother will ALWAYS express the trait – because his
other allele is the Y chromosome!

Human examples of sex-linked disorders:



Duchenne muscular dystrophy
Hemophilia
Baldness

Sex-linked genes follow specific patterns of
inheritance
XAXA
XaY
(a)
A father with the disorder will transmit the
mutant allele to all daughters but to no sons.
When the mother is a dominant homozygote,
the daughters will have the normal phenotype
but will be carriers of the mutation.
Ova
Xa
Y
Sperm
XA XAXa XAY
A a
A
XA X Y X Y
XAXa
XAY
(b)
If a carrier mates with a male of normal
phenotype, there is a 50% chance that
each daughter will be a carrier like her
mother, and a 50% chance that each son
will have the disorder.
XA
Sperm
Y
Ova XA XAXA XAY
Xa XaYA XaY
(c)
If a carrier mates with a male who has
the disorder, there is a 50% chance that
each child born to them will have the
disorder, regardless of sex. Daughters
who do not have the disorder will be
carriers, where as males without the
disorder will be completely free of the
recessive allele.
Figure 15.10a–c
XAXa
XaY
Sperm
Xa
Y
Ova XA XAXa XAY
Xa
XaYa XaY
X-Inactivation in Female Mammals

Only one X chromosome stays active in females –
the other becomes inactivated during embryonic
development.
 The
inactive X in each cell of a female condenses into a
compact object – a Barr Body

Barr body chromosomes are reactivated in the
ovary cells that give rise to ova.
 See

page 291-292 and discussion of tortoiseshell cat.
XIST is a gene that is active ONLY on the Barrbody chromosome.
Mary Lyon and the inheritance of inactivation: page 292
Mutations

Mutations are changes in DNA that also change
the protein produced.
 Can
happen spontaneously (no reason they
occurred).
-ORmutagens – substances or agents that can cause
changes in DNA
 By

Ex. Chemicals, radiation, x-rays, viruses
“Unseen” Changes

Changes can occur without the organism showing an
effect!
 NOTE:
changes DO occur that are not true “mutations” –
protein production not affected

These occur in the INTRON areas of chromosomes,
and as long as the length of the code is not affected,
they will not be seen in the organism…
Mutations in Reproductive Cells

Changes in reproductive cells:
 code
is changed in sperm or egg
 won’t effect the parent, but CAN effect the
offspring produced from those altered sex
cells
 can be POSITIVE – new, adaptive traits
Mutations in Body Cells

Changes in body cells:
 Are
not passed on to offspring, but can cause
MAJOR complications for the individual
effected

Ex. Solar radiation – skin cancer
 Again
the process is thought to be cumulative
effect of exposure to mutagens over time.
Point Mutation v/s Chromosomal Mutation

Point Mutation: change in a single base
pair of DNA.

Chromosome Mutation: change in a
large portion of an entire chromosome –
effects MANY genes.
Point Mutations

Point mutation: change in single base pair of
DNA
 if
the mutation does not effect the length of the code,
it will just change the amino acid in that position

Ex: SUBSTITUTION
 If
the mutation does change the length of the
code, is called a frameshift mutation, and are
two types:

Ex: INSERTIONS AND
DELETIONS
Gene Mutations – Effect Only ONE Base in the Code

Examples: Substitutions, Insertions, Deletions
Gene Mutations only affect ONE point of the code -- often called Point Mutations
Chromosomal Mutations

Can effect chromosome number AND
chromosome shape:
 Mistakes
in numbers of chromosomes
such as nondisjunction (where members
of a pair of homologous chromosomes do
not move apart properly resulting in
offspring that have):

Aneuploidy – abnormal chromosome number –
can be (Trisomy or Monosomy or Polyploidy)
Either type of meiotic error will produce gametes with an
abnormal chromosome number.
Trisomy 21 – Down Syndrome
Karyotype showing trisomy 21 – individual has three #21
chromosomes. Child will exhibit facial characteristics of Down
syndrome.
Trisomy 16 – Major Cause of Miscarriage in 1st
Trimester Pregnancy
Monosomy X – Turner Syndrome
XO individuals are phenotypically female, but their sex organs do not
mature at adolescence, and they are sterile. Most have normal
intelligence.
XXY – Klinefelter Syndrome
XXY – Klinefelter Syndrome – have male sex organs, but testes are
abnormally small and the man is sterile. Often includes breast enlargement
and other feminine body characteristics.
Males XYY – not characterized by a particular syndrome, but
usually are somewhat taller than average.
Fragile X Syndrome

Named for the physical appearance of an
abnormal X chromosome, the tip of which
hangs on to the rest of the chromosome
by a thin thread of DNA.
retardation – more common in males
and usually inherited by Mother.
 Mental
Autosomal Dominant Genes – body cells, not passed on to
offspring
 Autosomal Recessive Genes – body cells, not passed on to
offspring
 X-linked recessive Genes – sex cells, passed on to offspring
 Y-linked – only in males
 Chromosomal Abnormalities – if affects sex chromosomes,
passed on to offspring
 Multifactorial – genetic component (gene or chromosome)
plus a significant environmental influence


Mitochondrial DNA comes from Mom – maternal
inheritance, because mitochondria passed on by zygote
all come from the cytoplasm of the ovum