Genetics For Pathophysiology

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allele

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an alternative form of a gene (one member of a pair) that is located at a specific position (Locus) on a specific chromosome.
Example: Hgb A vs. Hgb S

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Organisms have _____ alleles for each trait.

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two

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Locus

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Position of a gene along a chromosome

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Polymorphism

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Locus that has two or more alleles that occur with appreciable frequency

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Homozygous

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Loci on a pair of chromosomes have identical genes
Example
O blood type (OO)‏

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heterozygous

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Loci on a pair of chromosomes have different genes
Example
AB blood type (A and B genes on pair of loci)‏

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Phenotype

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An organism's expressed traits (green or yellow). In Mendel's experiment, the F2 generation had a 3:1 phenotypic ratio of plants with green pods to plants with yellow pods

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Genotype

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An organism's genetic makeup (GG, Gg or gg). The genotypic ratio of the F2 generation was 1:2:1 (1GG:2Gg:1gg).

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Because some alleles are dominant over others, the phenotype of an organism does not always reflect its genotype.
True or False

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True

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recessive phenotype

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(yellow) is only expressed with the organism is homozygous recessive (gg).

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A pea plant with green pods may be either ....

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homozygous dominant (GG) or heterozygous (Gg).

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Testcross

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The breeding of an organism of unknown genotype with a homozygous recessive.

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If all the progeny of the testcross have green pods...

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then the green pod parent was probably homozygous dominant since a GG x gg cross produces Gg progeny.

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If the progeny of the testcross contains both green and yellow phenotypes...

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then the green pod parent was heterozygous since a Gg x gg cross produces Gg and gg progeny in a 1:1 ratio.

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Hemophilia

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is a sex-linked trait where XH gives normal blood clotting and is dominant to the hemophilia allele Xh.

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Give the genotypes of:
1) a woman with normal blood clotting whose father had hemophilia

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1) the woman has normal clotting so she has one XH but she got Xh from her father

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Give the genotypes of:
2) a normal man whose father had hemophilia.

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the man is XHY since he got the Y from his father and he is normal so must be XH

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XH Xh * XH Y

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each child has a 1/2 chance of being male and males have a 1/2 chance of being affected; so 1/4 chance of a child with hemophilia
If this couple has a daughter, what is the probability that the daughter will be a carrier of the hemophilia trait?
1/2 chance of being a carrier
What is the probability a daughter would have hemophilia?
0 chance that a daughter would have hemophilia
If this couple has a son, what is the probability he will have hemophilia?
1/2 chance

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Who decides what proteins must be made?
Who determines the sequence of Amino Acids?

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Ribosome
T-RNA
M-RNA

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Pyrimidines nitrogen bases

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cytosine and thymine

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Purines nitrogen bases

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adenine and guanine

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Transcription

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DNA → RNA

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Translation

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RNA → protein

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Transcription

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RNA is synthesized from the DNA template

Results in the formation of messenger RNA (mRNA)‏

mRNA moves out of the nucleus and into the cytoplasm

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RNA Polymerase

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An enzyme binds to the complex of transcription factors.
The RNA polymerase proceeds down one strand moving in the 3′ → 5′ direction.

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Transcription

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Some 50 different protein transcription factors bind to promoter sites, usually on the 5′ side of the gene to be transcribed.

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Working together, they open the DNA double helix...

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RNA polymerase & protien transcription factor

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There is no ____ on RNA, instead you have _____

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T, U pyrimidine

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Purpose of RNA Polymerase

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travels along the DNA strand, it assembles ribonucleotides into a strand of RNA. Each ribonucleotide is inserted into the growing RNA strand following the rules of base pairings.

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What needs to be removed before the RNA polymerase does its job?

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nucleosomes

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Synthesis of the RNA proceeds in the

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5---> 3 direction

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Translation

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Process by which RNA directs the synthesis of a polypeptide...Site of protein synthesis is the ribosome

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The ribosome moves along the _____sequence to translate the ______________ sequence

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mRNA...amino acid

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tRNA contains a sequence of nucleotides ________ complementary to the triad of nucleotides on the ______ strand _______.‏

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anticodon
mRNA
codon

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Translation Story

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The ribosome binds to mRNA at a specific area.
The ribosome starts matching tRNA anticodon sequences to the mRNA codon sequence.
Each time a new tRNA comes into the ribosome, the amino acid that it was carrying gets added to the elongating polypeptide chain.
The ribosome continues until it hits a stop sequence, then it releases the polypeptide and the mRNA.
The polypeptide forms into its native shape and starts acting as a functional protein in the cell.

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DNA Replication

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Untwisting and unzipping of the DNA strand
Single strand acts as a template
Complementary base pairing by DNA polymerase
Adenine-thymine; cytosine-guanine

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The first major step for the DNA Replication

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to take place is the breaking of hydrogen bonds between bases of the two antiparallel strands. The unwounding of the two strands is the starting point. The splitting happens in places of the chains which are rich in A-T. That is because there are only two bonds between Adenine and Thymine (there are three hydrogen bonds between Cytosine and Guanine).

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Helicase

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is the enzyme that splits the two strands.

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origin of replication for DNA replication

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The initiation point where the splitting starts is called "origin of replication".The structure that is created is known as "Replication Fork".

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One of the most important steps of DNA replication is...

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is the binding of RNA Primase in the initiation point of the 3'-5' parent chain.

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RNA Primase

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can attract RNA nucleotides which bind to the DNA nucleotides of the 3'-5' strand due to the hydrogen bonds between the bases.
RNA nucleotides are the primers (starters) for the binding of DNA nucleotides.

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elongation process of DNA replication

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5'-3' Template: The 3'-5' proceeding daughter strand -that uses a 5'-3' template- is called leading strand because:
DNA Polymerase can "read" the template and continuously adds nucleotides (complementary to the nucleotides of the template, for example Adenine opposite to Thymine etc).

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the lagging strand

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the DNA Pol I -exonuclease- reads the fragments and removes the RNA Primers.
The gaps are closed with the action of DNA Polymerase (adds complementary nucleotides to the gaps) and DNA Ligase (adds phosphate in the remaining gaps of the phosphate - sugar backbone).

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semiconservative replication

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Each new double helix is consisted of one old and one new chain.

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Leading Strand contains

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RNA Primase
DNA Polymerase III
DNA Polymerase I

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Lagging Stands

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RNA Primase
DNA Polymerase III
DNA Polymerase I
Ligase

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Termination

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Last step of DNA replication

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Termination happens when...

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DNA Polymerase reaches to an end of the strands.
These ends of linear (chromosomal) DNA consists of noncoding DNA that contains repeat sequences and are called telomeres.

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semiconservative replication

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Each new double helix is consisted of one old and one new chain.

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Leading Strand contains

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RNA Primase
DNA Polymerase III
DNA Polymerase I

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Lagging Stands

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RNA Primase
DNA Polymerase III
DNA Polymerase I
Ligase

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Termination

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Last step of DNA replication

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Termination happens when...

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DNA Polymerase reaches to an end of the strands.
These ends of linear (chromosomal) DNA consists of noncoding DNA that contains repeat sequences and are called telomeres.

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Somatic cells

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Contain 46 chromosomes (23 pairs)‏
Diploid cells

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Gametes

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Contain 23 chromosomes
Haploid cells
One member of each chromosome pair

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Meiosis

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Formation of haploid cells from diploid cells

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Autosomes

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The first 22 of the 23 pairs of chromosomes in males and females
The two members are virtually identical and thus said to be homologous

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Sex chromosomes

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Remaining pair of chromosomes
In females, it is a homologous pair (XX)‏
In males, it is a nonhomologous pair (XY)‏

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Euploid cells

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Cells that have a multiple of the normal number of chromosomes
Haploid and diploid cells are euploid forms

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polyploid cell

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When a euploid cell has more than the diploid number
Triploidy: a zygote having three copies of each chromosome (69)‏
Tetraploidy: four copies of each (92 total)‏
Both triploid and tetraploid fetuses don’t survive

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tetraploidy

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four copies of each (92 total)

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Triploidy

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a zygote having three copies of each chromosome (69)‏

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Aneuploidy

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A somatic cell that does not contain a multiple of 23 chromosomes

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trisomy

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A cell containing three copies of one chromosome is trisomic

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Monosomy

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is the presence of only one copy of any chromosome.Monosomy is often lethal, but infants can survive with trisomy of certain chromosomes
“It is better to have extra than less”

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disjunction versus nondisjunction

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disjunction is normal versus
Usually the cause of aneuploidy
Failure of homologous chromosomes or sister chromatids to separate normally during meiosis or mitosis

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Partial trisomy

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Only an extra portion of a chromosome is present in each cell

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Chromosome mosaics

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Trisomies occurring only in some cells of the body

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Down syndrome

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Best-known example of aneuploidy
Trisomy 21
1:800 live births
Mentally retarded, low nasal bridge, epicanthal folds, protruding tongue, poor muscle tone
Risk increases with maternal age

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trisomy X.

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This is a female that has three X chromosomes.
Termed “metafemales”
Symptoms are variable: sterility, menstrual irregularity, and/or mental retardation
Symptoms worsen with each additional X

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Turner syndrome

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Females with only one X chromosome
Characteristics
Absence of ovaries (sterile)‏
Short stature (~ 4'7")‏
Webbing of the neck
Edema
Underdeveloped breasts; wide nipples
High number of aborted fetuses
X is usually inherited from mother

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Klinefelter syndrome

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Individuals with at least two Xs and one Y chromosome
Characteristics
Male appearance
Develop female-like breasts
Small testes
Sparse body hair
Long limbs
Some individuals can be XXXY and XXXXY. The abnormalities will increase with each X.

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Chromosome breakage

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If a chromosome break does occur, physiological mechanisms will usually repair the break, but the breaks often heal in a way that alters the structure of the chromosome

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Agents of chromosome breakage

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Ionizing radiation, chemicals, and viruses

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Types of Mutations

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UV light
Spontaneous Mutations: Transversions
Inversions
Deletion
Duplication
Silent Mutations
Nonsense Mutations
Frameshift Mutation
Translocation
Insertion

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Cri du chat syndrome

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Cry of the cat”
Deletion of short arm of chromosome 5
Low birth weight, metal retardation, and microcephaly

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Duplication

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Presence of a repeated gene or gene sequence
Rare occurrence
Less serious consequences because better to have more genetic material than less (deletion)‏
Duplication in the same region as cri du chat causes mental retardation but no physical abnormalities

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Inversions

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Two breaks on a chromosome
Reversal of the gene order
Usually occurs from a breakage that gets reversed during reattachment
ABCDEFG may become ABEDCFG

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Translocations

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The interchanging of material between nonhomologous chromosomes
Translocation occurs when two chromosomes break and the segments are rejoined in an abnormal arrangement

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Fragile sites

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Fragile sites are areas on chromosomes that develop distinctive breaks or gaps when cells are cultured
No apparent relationship to disease

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Fragile X syndrome

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Site on the long arm of the X chromosome
Associated with mental retardation; second in occurrence to Down syndrome
Higher incidence in males because they have only one X chromosome

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Single-Gene Disorders

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Recurrence risk
The probability that parents of a child with a genetic disease will have yet another child with the same disease
Recurrence risk of an autosomal dominant trait
When one parent is affected by an autosomal dominant disease and the other is normal, the occurrence and recurrence risks for each child are one half

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Single-Gene Disorders
Autosomal dominant disorder

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Abnormal allele is dominant, normal allele is recessive, and the genes exist on a pair of autosomes

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Penetrance

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The percentage of individuals with a specific genotype who also express the expected phenotype

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imcomplete penetrance

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Individual who has the gene for a disease but does not express the disease
Retinoblastoma (eye tumor in children) demonstrates incomplete penetrance (90%)‏

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Expressivity

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the variation in a phenotype associated with a particular genotype
This can be caused by modifier genes

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Examples of Expressivity

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von Recklinghausen disease
Autosomal dominant
Long arm of chromosome #17
Disease varies from dark spots on the skin to malignant neurofibromas, scoliosis, gliomas, neuromas, etc.

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Single gene disorder
Autosomal recessive disorder

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Abnormal allele is recessive and a person must be homozygous for the abnormal trait to express the disease
The trait usually appears in the children, not the parents, and it affects the genders equally because it is present on a pair of autosomes

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Autosomal recessive disorder recurrence risk
Single gene disorder

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Recurrence risk of an autosomal dominant trait
When two parents are carriers of an autosomal recessive disease, the occurrence and recurrence risks for each child are 25%

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Consanguinity

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Mating of two related individuals
Dramatically increases the recurrence risk of recessive disorders

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Sex-Linked Disorders

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The Y chromosome contains only a few dozen genes, so most sex-linked traits are located on the X chromosome and are said to be X-linked

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Sex-Linked Disorders

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Sex-linked (X-linked) disorders are usually expressed by males because females have another X chromosome to mask the abnormal gene

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X-linked recessive

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Most X-linked disorders are recessive
Affected males cannot transmit the genes to sons, but they can to all daughters
Sons of female carriers have a 50% risk of being affected