Download Genes and Chromosomes

Genes and
Chromosomes
•The Chromosome
Theory of Heredity
•Mutations
•Regulation of Gene
Expression
Objectives
• State the chromosome theory of heredity
• Explain how gene linkage affects inherited
traits
• Describe the process of crossing-over and
explain how it increases genetic variety
• Describe gene mapping
• Describe the process of sex determination and
the patterns of inheritance for sex-linked traits
Chromosome Theory of Heredity
• Mendel’s work was incomplete because he
never asked an important question: Where in
the cell are the factors that control heredity?
Where are the genes?
What do you see in this picture?
What do you recall about chromosomes?
Chromosome Theory and Heredity
Key terms
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Chromosome theory of heredity
Linked gene
Linkage group
Recombinant
Sex chromosome
Autosome
X chromosome
Y chromosome
Sex-linked
Genes and Chromosomes
• Nucleuscontains
chromsomes
– chromosome=threadlike
structure in a cell that
contains the genetic
information that is
passed on from one
generation of cells to the
next
Genes and Chromosomes
• Walter Sutton
– 1902, discovered gene location
– Chromosome theory of heredity genes are
located on the chromosomes and each gene
occupies a specific place on a chromosome
• Each gene may exist in several forms or alleles
• Each chromosome contains just one of the alleles for
each of its genes
– Sutton’s development of the chromosome theory is an
example of how the work of one scientist builds on the work
of another scientist
Gene Linkage
• Genes on a chromosome are linked together
– They are inherited together
• Linked genes do not undergo independent assortment
• Linked genes=genes that are inherited in a group
Gene Linkage
– Thomas Hunt Morgan studied drosphilia
• Effects of gene linkage
– Morgan crossed purebred gray bodies and normal wings with
purebred black bodies and small wings
» Gray (G) black (g) Normal wings (W) small wings (w)
• F1 should have been gray with normal wings (GgWw)
• When F1 crossed with black small-winged drosphilia
(ggww) Morgan did not observe the expected results
• Most gray-bodied drosphilia had normal wings and
most black-bodied flies had small wings
• Gene for body color and gene for wing size were
somehow connected, or linked
• They could not assort independently
Gene Linkage
• Linkage groups
– Morgan studied more and more genes
• Discovered genes fell into distinct linkage groups of genes
that always tended to be inherited together
• The linkage groups (chromosomes) assorted independently,
but all genes on one group were inherited together
• Because homologous chromosomes contain the same genes,
there is one linkage group for every homologous pair of
chromosomes(drosphilia has four linkage groups, four pairs
of chromsomes)
– A cobra has 38 chromosomes. How many linkage groups would
this make?
Tomorrow!
• Crossing over in linked genes
• Gene mapping
• Sex linkage
Crossing-over
• During prophase I of meiosis, homologous
chromosomes may exchange sections of their
chromatids in a process called crossing-over
– Increases genetic variety
Crossing-Over
• Linkage groups explains some of the results of
the drosphilia crosses but does not provide a
complete explanation
– 83% have gene combinations like their parents
– 17% have new gene combinations
• Recombinants=individuals with new combinations of
genes
Crossing-Over
• If the genes for body color and wing size are
linked, why aren’t they linked all the time?
– Morgan proposed that linkages could be broken
some of the time
• If two homologous chromosomes were positioned side
by side, sections of the two chromosomes might cross,
break, and reattach.
– This process would rearrange the genes on the chromosome
and produce new linkage groups
Gene Mapping
• Further reasoned that crossing-over occurs at
random along the linkage groups, and the
distance between two genes determines how
often crossing-over occurs between them
– Close together crossing-over is rare
– Far apart crossing-over more common
Gene Mapping
• Knowing the frequency with which crossingover between two genes occurs makes it
possible to map the positions of genes on a
chromosome
– Today we have detailed maps of Drosphilia that
pinpoint the locations of more than 1500 different
genes
Sex Linkage
• 1905 American biologist Nettie Stevens
discovered that not every chromosome has a
corresponding homologous chromosome
– Discovered female mealworm contain 20 large
chromsomes and male contain19 large and one
small
• One of male chromosome pairs is not homologous
– The pair has very different shapes
» Same thing was found in drosphilia
Sex Linkage
• These “mismatched” chromosmes are the sex
chromosomes
– female sex chromosomes=two matching sex
chromosomes (XX)
– Male sex chromosomes=two dissimilar sex
chromosomes(XY)
• Y chromosome=small and hook shaped
• The other chromosomes, which are the same
in both males and females, are called
autosomes
Sex Linkage
• Sex determination
– The sex chromosomes in the male’s gametes
determine the sex of the offspring
X
X
X
XX
XX
Y
XY
XY
Sex Linkage
• Sex determination
– When female gametes are produced, meiosis
separates one of the X chromosomes into each
egg cell
– In the male, meiosis separates the X and Y
chromosomes so that 50% carry X chromosome
and 50% carry Y chromosome
• When an X sperm fertilizes an egg a female is formed
• When a Y sperm fertilizes an egg a male is formed
Sex Linkage
• Genes on sex chromosomes
– Sex chromosomes also carry genes that affect
other traits
• Sex-Linked a gene located on one of the sex
chromosomes
Non-Disjunction
• Occurs during meiosis
• Some gametes contain extra chromosomes
• Some gametes are missing chromosomes
Klinefelter’s Syndrome
• Male XXY sex chromosomes
– Sterile
– Show female characteristics
– Underdeveloped testes
– Breast development
– Poor beard growth
Turner’s Syndrome
• FemaleX_
– Mental retardation
– Sterile
– Short in stature
– Underdeveloped ovaries
– Increased chance of thyroid problems
Tomorrow!
• Gene Mapping Lab!
Board work 21
1. How are genes related to chromosomes?
2. How does crossing-over make genetic
mapping possible?
3. What are sex chromosomes? Autosomes?
4. Why are the effects of recessive sex-linked
alleles seen more often in males than in
females?
Mutations
Mutations
• Mistakes in duplicating genetic info and
transmitting it to the next generation are rare,
but they do occur
– Mutations=change in the genetic material of the
cell
• Not all are harmful
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No effect
Slight effect
Harmless
beneficial
Mutations
• Mutations in reproductive cells (germ cells)
– Germ mutations
• inheritable
• Mutations that affect other cells of the body
– Somatic mutations
• Cancer
• 2 levels
– Chromosomal mutations
– Gene mutations
Chromosomal mutations
• Involve
– Segments of chromosomes
– Whole chromosomes
– Entire sets of chromosomes
• Results in change in number or structure
• 4 types
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deletions
Duplications
Inversions
translocations
Chromosomal mutations
• Deletions
– The loss of part of a chromosome
Chromosomal mutations
• Duplications
– Opposite of deletion, segment of chromosome is
repeated
Chromosomal mutations
• Inversions
– Part of a chromosome becomes reversed
Chromosomal mutations
• Translocations
– Part of one chromosome breaks off and attaches
to another, nonhomologous chromosome
Chromosomal mutations
• Nondisjunctions
– Involve whole chromosomes or complete sets of
chromosomes
• Failure of homologous chromosomes to separate normally
during meiosis
– Not coming apart
– 1 chromosome involveextra copy in one cell and
loss from another
– More than 1dramatic increase in number,
producing triploid (3N) or tetraploid (4N) organisms
• Extra sets of chromosomespolyploidy
– Almost always fatal in animals
– Plants are often larger and hardier
Gene mutations
• Involve
– Individual genes
• Cause
– Chemical change that affects DNA
Gene mutations
• Point mutationsaffect no more than a single nucleotide
Gene mutations
• Insertion or deletion of nucleotide
– Frameshift mutationscompletely change the
polypeptide product produced by a gene
Board Work 22
1. Compare a chromosomal mutation and a
gene mutation.
2. What is a somatic mutation? How does it
differ from a germ mutation?
3. How does nondisjunction result in
chromosomal mutations?
Regulation of Gene Expression
• Gene interactions
– Incomplete dominance
– Codominance
– Polygenic inheritance
• Gene expression in
prokaryotes
– Operon
– Repressor
– Gene activation
• Gene expression in
eukaryotes
Regulation of Gene Expression
• As biologists have intensified their studies of
gene activity, it has become clear that
interactions between different genes and
between genes and their environment are
critically important
Gene Interactions
• Dominance
– How genes interact with each other
• Remember….
– A gene is a section of DNAcodes for a
polypeptide
– Dominant allele codes codes for a specific
polypeptide that works, recessive for one that
does not work
Gene Interactions
• Incomplete dominance
– Inheritance in which an active allele does not
entirely compensate for an inactive allele
White
carnation
r r
R Rr Rr
R Rr Rr
R=red
r=white
Red carnation
F1 generation
All pink
Pink carnation
R r
R RR Rr
r Rr rr
F2 generation Pink carnation
1 red
2 pink
1 white
Gene Interactions
• Codominance
– Condition in which both alleles of a gene are
expressed
• Written as capital letters with subscripts or superscripts
– Ex: B1 and B2 or R and R’
• Seen in many organisms
– cattle=red hair is codominant with white hair (HRHW)
» Look roan or pinkish white
– Chickens=black feather are codominant with white feathers
(FBFW)
» Erminant chickens (speckled black and white)
Gene Interactions
• Polygenic Inheritance
– A trait that is controlled by two or more genes
– Many traits are produced by the interaction of
many genes…polygenic
• Shape of your nose
• Color and markings on an animal’s coat
Gene Expression in Prokaryotes
• The genes of a single organism cannot be
activated ate the same time
– Make many molecules it did not need
• Waste energy
– Must be able to produce the product of a gene
quickly and in adequate amounts
Gene Expression in Prokaryotes
• When the product of a gene (a specific
protein) is being actively produced by a cell,
we say that the gene is being expressed
– Within a single organism, some gene are rarely
expressed, some are constantly expressed, and
some are expressed for a time and then turned off
• How does a cell “know” when to make a protein and
when not to
– Turn off and turn on?
Gene Expression in Prokaryotes
• The Operon
– Genes and regions of DNA that operate together;
consists of a gene cluster and regions involved in
the regulation and expression of that cluster
– Consists of
• Operator=region of chromosome near the cluster of
genes in an operon to which the repressor binds when
the operon is “turned off”
• Promoter=region of chromosome next to the operator
in an operon to which RNA polymerase binds at the
beginning of transcription
Gene Expression in Prokaryotes
• The Operon
– The gene cluster in the operon studied by Jacob
and Monod produces enzymes that break down
lactose
• Bacteria does not produce enzymes in large amounts
unless lactose is present
– Lactose induces production of enzymes to break down lactose
for use as food
– This operon systeminducer because it induces the
production of enzymes
» Enzymes not produced in the absence of lactose
Gene Expression in Prokaryotes
• The Repressor
– When repressor nears the operator it attaches
itself to the operator so that it sits between the
promoter and the genes
• Position blocks the access of RNA polymerase
Gene Expression in Eukaryotes
• Inducers induce the activation of genes
– Bind directly to DNA and either start or increase
transcription of particular genes
– mRNA produced during transcription may be altered
before it is used to make protein during translation
• The presence of DNA sequences that do not code for protein
– Exons=sequences that are complementary code for protein
“expressed”
– Introns=segments that are not complementary and do not code
for protein “intervening”
Board work 23
1. How do gene interactions affect gene
expression?
2. Compare incomplete dominance and
codominance.
3. What is a polygenic trait?
4. “Mutations in introns are less likely to affect
phenotype than mutations in exons.? Defend
or refute this statement.