Biology 1406 notes Exam 4 - Genetics and Gene expression
Ch. 9, 11
Some complicating details of inheritance (genes do not follow Mendel’s laws of inheritance)
Gene linkage – genes on same chromosome
9.17
- inheritance does not follow Mendel’s Law of Independent Assortment Why?
- linked genes separated by crossing over in meiosis
Which phase?
- sex cells produced by crossing over cannot be included in Punnett square
9.18
Why?
- the farther apart the genes are on the chromosomes, the more frequently they are separated
(recombination frequency)
9.19
- recombination frequency data used to make chromosome maps – where genes are located
on the chromosome
- through evolutionary time alleles of genes that work well together came to be close together
on chromosome
Sex-link genes – if sex is determine by sex chromosomes
Sex determination
9.20
- genetically determined in many organisms
- sex chromosomes (all other chromosomes called autosomal)
X - Y sex chromosomes
XX ♀
XY
♂
X - 0 sex chromosomes
XX ♀
X
♂
Z - W sex chromosomes
ZW ♀
number of chromosomes
2N ♀
ZZ ♂
1N ♂
(fertilized egg = 2N)
- Environmentally determined in some organisms
- temperature of nest
- chemicals produced by other individuals can change from one sex to another
- Some organisms are both male and female (hermaphroditic)
sex-limited genes – control sexual traits (sex organs, sexual characteristics)
– expressed under right conditions
– mostly not on sex chromosomes – on autosomal chromosomes
- inheritance typical "Mendelian"
Inheritance of sex chromosomes and Sex – linked genes
9.21-9.23
Sex chromosomes are not exactly homologous
(in humans ≈ 1100 on X only, ≈ 80 on Y only, 15 on both)
SRY
X
sex-linked genes on X or Y
but not both
Y
Recessive alleles must be
homozygous to be expressed
in female; just one recessive
will be expressed in male
XX
♀
XY
♂
9.22
Example: hemophilia
red-green color blindness
Duchenne’s muscular dystrophy
Female offspring get X from mother and X from father
Male offspring get X from mother and Y from father
Pattern of inheritance of sex-linked genes
9.21
How does this affect phenotype ratios in males and females differently?
examples:
Use of pedigrees in studying inheritance of human traits
Why is this necessary?
Determine the inheritance pattern in some sample problems.
9.9, 9.22
Mutations
- changes in DNA copied in replication
- in humans 175 new mutation in 7 billion letters each generation
- inheritable mutations
- only in gametes
- passed to next generation
- produce alleles of a gene or a new gene (genetic variation)
- nonfunctional alleles often recessive
- lethal alleles 9.9
- most lethal alleles are recessive. Why?
- we all have lethal recessive alleles
Inbreeding increases the probability of these recessive lethal alleles being expressed. Why?
Examples:
1. autosomal recessive “loss – of – function”
cystic fibrosis
phenylketonuria (PKU)
PP
Pp
pp
2.
9.9-9.10
- normal
- “
- PKU
sex-linked recessive
hemophilia
9.22
Duchenne muscular dystrophy - most frequently occurring fatal genetic disease among
children
females : XD XD – normal
XD Xd - “
Xd Xd - Dmd
males : XD Y - normal
Xd Y - Dmd
1/3,500 in US males
1/3,500
* 1/3,500 = 1/12,250,000 in US females
Why is this different for males and females?
Gene codes for protein called dystrophin which connects cytoskeleton to connective tissue (3685
amino acids)
3. autosomal dominant
9.9
Huntington’s disease - causes deterioration of nerve cells and is always lethal
- usually expressed later in life after reproductive age
AH AH
AH A
A A
- Huntington’s
“
- normal
normal gene produces huntingtin - protein that regulates about 50 other genes including growth factor
for some types of nerve cells
abnormal protein has 3 - 215 repeats of GTC (glutamin) which causes clumps and no growth factor
4. autosomal dominant (incompletely dominant allele - lethal and beneficial)
- Sickle-cell allele
- has beneficial effects and lethal effects
9.13
H - normal allele describing hemoglobin
HS - sickling allele – result of point mutation which changed one codon and one amino acid
in hemoglobin
Sickling hemoglobin folds together in low oxygen environment; red blood cells then
fold or “sickle”
Sickled cells are destroyed, block capillaries and reduce blood volume
Genotypes
Phenotypes
HH
normal hemoglobin only
H HS
some normal hemoglobin, some sickling hemoglobin – sickle cell trait
HS HS
sickling hemoglobin only – sickle cell disease
Sickle cell disease is lethal; normal hemoglobin gives no protection from malaria; sickle cell trait
(heterozygous individuals) are resistant to malaria and survive best
Control of gene expression
Ch 11
When, where and how much of protein determines structure and function of organism
- only about 1.5 % of human DNA is translated ( 25,000 genes)
- some used to make different types of RNA (mRNA, tRNA, rRNA, sRNA, miRNA)
- about 3 % of DNA used as “switches” (regulatory) to control gene expression
-only some genes are expressed, others not (“turned off”)
Ex.
cell specialization, development,
cell function under different conditions,
sex determination
11.2
How can genes be turned on or off?
prokaryotic - ex. lac operon in E. coli
11.1
- series of genes and promoter and operator promoter - RNA polymerase attaches
operator - repressor attaches (“on/off switch”)
regulatory gene – translated to repressor protein
eukaryotes - more complex
11.2 - 11.7
Epigenetics is inheritance of traits transmitted by mechanisms other than nucleotide sequences
DNA packing into chromosomes
- packed genes not expressed
- methylation prevents transcription
- --CH3 to cytosine nucleotides
11.2
Inactivation of X chromosome - one X permanently packed
Why does this occur only in females?
Describe an example of this.
transcription factors
11.3
- transcription factors – series of proteins that attach to enhancer area, allow RNA
polymerase attachment to DNA for transcription
- enhancers – area of DNA for attachment of transcription factors
- promoter – area of DNA where RNA polymerase attaches
RNA splicing - modified mRNA in nucleus
11.4
- introns (noncoding sections) removed
- exons linked in several alternate patterns
- caps and tails added
- smallRNA (sRNA) and micro RNA (miRNA) used to cut mRNA
Proteins modified (after translation) into active form or broken down
11.6
Genomic imprinting - either paternal or maternal allele "silenced"
unknown inheritance patterns
-segregation distorter gene ex. tt in mice is lethal, but if Tt then t gets into 90% of sperm instead of 50% as
expected
Cytoplasmic inheritance - genes in mitochondria
- mitochondria only from mother (in egg)
Homeotic genes
11.8 – 11.11
homeotic genes – “master control genes” called “Hox” genes
- turn on sets of genes to produce body regions
- homeoboxes - part of gene that binds to protein to allow gene transcription
- identical in many organisms worms to humans
- very ancient, preserved through evolutionary time
cell signaling genes - important in body formation – laying out body plan
- embryonic development depends on cell signaling
Ex. thalidomide and cyclopamine
11.10 - 11.11
*How is this connected to the cell cycle of cell division?
The genetic tool kit – maybe 200 – 300 genes that control body development
- very important in understanding evolutionary change
(15.11)
Biology 1406 Exam 4 Review Genetics and Control of Genes Ch. 9, 11
Describe some variations that complicate simple “Mendelian” genetics and change expected genotype ratios.
What are linked genes? Why do they not follows Mendel’s law of independent assortment?
What event in meiosis causes linked genes to somewhat follow Mendel’s law of independent assortment?
What are recombinant phenotypes (recombinants)?
Why are the frequencies of recombinant phenotypes different for different pairs of genes?
How are gene maps drawn from recombinant frequencies?
What is a possible advantage of gene linkage?
Describe how sex is determined in mammals. How is this different from environmentally determined sex?
What are sex-limited genes and how do they differ from sex-linked genes?
Explain how the inheritance of sex-linked genes differs from sex-limited (and other autosomal) genes.
What are autosomal genes?
Why are the X and Y chromosomes not homologous, even though they pair in meiosis? Do they cross over?
Why are sex-linked diseases more common in males than in females?
Sex-linked traits frequently have a very distinct phenotype ratio pattern. What is this and why does it happen?
What are pedigrees and why are they necessary in the study of human traits?
Complete pedigree analyses assigned and be able to distinguish between autosomal dominant, autosomal recessive, sexlinked dominant and sex-linked recessive traits.
Define the term inheritable mutations. Where do these mutations first occur?
What are lethal alleles and why are most lethal alleles that we see recessive and not dominant?
What is inbreeding? How, and why, does inbreeding affect the expression of recessive lethal alleles?
Describe the inheritance of the following genetic diseases in humans: phenylketonuria (PKU - a recessive lethal), sicklecell anemia (an incomplete dominant lethal), Duchenne muscular dystrophy (recessive sex linked) and Huntington’s
disease (a dominant lethal).
How can a dominant lethal allele survive in a species? How can a lethal allele be both harmful and beneficial?
What percentage of human DNA is translated (codes for protein or RNA)?
What is meant by the term “gene expression”?
If all of the cells in an individual’s body have the same DNA why are the cells different?
Describe the method that prokaryotes use to regulate genes - the operon. Use the lac operon in E. coli as an example.
Describe some of the ways that eukaryotes regulate gene expression.
What is DNA packing and how does this affect gene expression?
What is X chromosome inactivation and why does it occur only in female mammals? Describe an example.
What are transcription factors and how do they bring about gene expression?
What are enhancer and promoter areas?
Describe mRNA splicing. What are introns and exons?
How can several proteins be produced by one mRNA molecule?
What is cytoplasmic inheritance? Where are these genes located? What is the inheritance pattern?
What is genomic imprinting?
How are segregation distorter genes different from other genes?
What are homeotic genes? How are they important in embryonic development?
What is the evolutionary significance of the homeobox region of homeotic genes?
Why would a simple mutation to a homeotic gene cause a very different body?
How do cell signaling genes work and why are they important in development of the body?