Patterns of inheritance Flashcards
Gene
Length of DNA which determines the sequence of amino acids in a ppc
Locus
Position of a gene on a chromosome
Allele
different form of a gene occupying same position on homologous chromosomes
Genotype
There combination of alleles present in an organism
Phenotype
The physical observable characteristics that are expressed in an organism
Heterozygous
A genotype consisting of two different alleles
Homozygous
A genotype consisting of two identical alleles
Dominant
An allele which is expressed in the phenotype irrespective of the other allele present for the gene
Recessive
An Allele which is masked by a dominant allele and is only expressed when there are 2 recessive alleles present for that specific gene
Multiple alleles
When there are 3 or more alleles present for one specific gene e.g. blood group I’A, I’B, I’O
Co-dominant alleles
Both alleles are equally dominant and have an impact on the phenotype in a heterozygote, usually causing an intermediate characteristic.
Pure breeding
Genotype is homozygous for characteristic
Homogametic
Both sex chromosomes are identical e.g females
Heterogametic
Both sex chromosomes are different e.g. males
Monohybrid inheritance
Inheritance of a single gene
Test cross
The organism of unknown phenotype is crossed with homozygous genotype to determine if an organism is heterozygous or homozygous for that characteristic
Proportion of polymorphic gene loci
n of polymorphic gene loci/ total number of loci
Inversion mutation
Segment of chromosomes breaks off and is inserted bit eh same place but in the opposite direction telative to the rest of the chromosomes
Insertion mutation
-Extra nucleotide is added to DNA sequence
-Frameshift of codons in mRNA
-Leads to altered sequence of amino acids in translation
-Results in a non-functioning ppc
Deletion mutation
-Nucleotide is deleted
-Causes frameshift in the codons in mRNA
-Leads to altered sequence of amino acids in translation
-Results in non-functioning ppc
Substitution mutation
-Single nucleotide is inserted into the DNA sequence in place of the correct nucleotide
-Different codon
-Could be silent, missense or nonsense
Translocation
Movement of gene fragment from one chromosome location to another
Nonsense mutation
Triplet for amino acid is replaced for stop codon- ppc is shortened and does not function
Missense mutation
Triplet for amino acid is replaced for a different one. Ppc will probs have a different 1 structure, which will alter 2,3,4.
Silent mutation
Triplet for amino acid is replaced with triplet for the same amino acid. ppc remains unchanged
Frameshift mutation
All triplets are affected- ppc is completely altered
Beneficial mutation
improved ppc function, natural selection will select individuals with improved protein function, increases the frequency of beneficial allele within the population
Cystic Fibrosis causes
Recessive allele on chromosome 7 which codes for transporter protein called CFTR
CFTR sits in ccm of cells in alveoli- allows cl ions to pass out of cells
CF symptoms
Abnormally thick mucus in the lungs and pancreas
Prone to bacterial infections
CF treatment
-Daily physiotherapy to aid the clearance of mucus as excess mucus traps bacteria and leads to infection
-Administer pancreatic enzymes as mucus blocks pancreatic duct- these aid digestion of lipids and proteins
-Antibiotics to treat bacterial infections
-Males may need fertility treatment as mucus can block sperm ducts
-Maintain childhood vaccines e.g. MMR
CFTR normal
Transport proteins transport cl ions out of cell
High conc of ions outside cell
Lowers water potential compared to the cell cytoplasm
Draws water out of the cells which dilutes mucus
Makes it thin enough to be moved by ciliated epithelium
CFTR with CF
Transport proteins do not pump cl ions out of the cell
No water is drawn from the cells
Mucus remains thick and viscous
Ciliated epithelium unable to clear it
Mucus accumulates (goblet cells produce it)
Bacteria get trapped- more likely bacterial infections
Phenylketonuria (PKU)
Abnormal base sequence of DNA coding for enzyme. Affects single locus on chromosome 12
Sufferers lack essential enzyme phenylalanine hydroxylse
enzyme: phenylalanine -> tyrosine-> melanin
PKU symtonms
-Lack of PH, lower levels of phenylalanine so fairer skin & hair
-Phenylalanine accumulation in blood- severe brain damage in children
-Mental retardation
-Routine testing at birth- if affected, strict diet without phenylalanine
Huntington’s disease
-Insertion mutation on chromosome 4
-Autosomal dominant allele
-40+ repeats of CAG: gene stutter
-Caused by accumulation of protein fragments in neurones in brain-> neurone death
Huntingtons symptoms
Reduced ability to send brain impulses
So, reduced speech, movement, thought processing
Progressive disorder
Sickle Cell anaemia
-Codominant
-Caused by base substitution mutation at a single locus on chromosome 11
-Valine is substituted in place of glutamic acid
-Affects B globin
-makes little difference when Hb is bound to O2
-RBC become sickle shaped
-Distorted- dec O2 carrying ability, stick in capillaries, prevent normal cells passing
SCA symptoms
-Severe anaemia, tired easily, breathless easily, can be fatal
-Pain, cramps
-Common in Africa & India- provides resistance to malaria as plasmodium can’t live & reproduce within affected RBC
Duchenne Muscular Dystrophy (DMD)
Dominant allele carried on the X chromosome
DMD gene codes for a protein called dystrophin required for muscle contraction
Mutations of gene result in non-dystrophin
Codominance and blood groups
A and B are codominant, O is recessive
A: I’A I’A or I’A I’O
B: I’B I’B or I’B I’O
AB: I’A I’B
O: I’O I’O
HLA
All cells have antigens on ccm
HLAs are very important in organ transplants
The equivalent of major histocompatibility complex
HLA antigens are coded for by 6 gene loci on chromosome 6
Each HLA gene exhibits codominance. Inherited from parents expressed equally.
Genes for 6 HLAs inherited together so example of genetic linkage
Autosomal linkage
Non sex chromosomes
Two or more genes that are located on the same chromosome
these alleles are normally inherited together.
Autosomes
non sex chromosomes
Nail Patella Syndrome & blood group
(example of autosomal linkage)
non-sex linked- affects males and females equally
-underdeveloped knee caps and nails, inability to stretch elbows
-Dominant
-allele for NPS found on chromosome 9 very close to ABO group gene. So, NPS and blood group gene usually inherited together due to crossing over during meiosis
Sex linkage
When gene that it codes for is carried on the sex chromosome
Haemophilia A
-Recessive allele carried on the X chromosome
-Individuals fail to produce clotting factor VII
-Recessive allele expresses altered protein that does not function
-Increases clotting time, internal bleeding etc
-Males only have 1 X chromosome, while females have two copies, this condition is almost always found in males
Model organisms- Drosophila
-Short life cycle-> large n individuals can be produced in short period of time
-Short regeneration time so many generations can be studied in a short time
-Minimal culturing requirements
-Only 4 pairs of chromosomes- genetic simplicity
-No ethical or moral issues
bad:
-anatomy of the brain and major organs are v different from humans
-lack of in-depth cognitive abilities
Dihybrid crossing
4x4
The pattern of inheritance of two characteristics which are determined by 2 genes at different loci
Dihybrid crossing genotypes vs alleles in gametes
Parent genotype: GgRr
Alleles present in gametes: GR, Gr, gR, gr
what are chromosome mutations
Change in the n of chromosomes, change in structure of chromosomes
Translocation
Piece of chromosomes breaks off and transferred to another chromosome
e.g. Down’s syndrome. Long arm of chro 21-> chro 14
Non-disjucntion
when homologous chromosomes fail to separate in meiosis I or sister chromatids fail to separate in meiosis II
Changes diploid n. chromosomes. Gamete has +1 or -1
e.g. DS- 3 copies of chro. 21
Turner’s- single X chro
Klinefelter’s- only affects males. XXY.
Parental carrier testing
couples risk of having genetic condition assessed by testing for the presence of recessive allele
Pre-natal testing
embryos tested for chromosome mutations and other inheritable conditions
New-born screening
babies tested shortly after birth to identify genetic conditions that require immediate treatment
Ethical concerns about eugenic screening
-may lead to an abortion
-risk of possible damage to embryo
-risk of false positives or negatives
-embryo cannot consent
Pedigree analysis
Genetic counsellors use family trees to construct pedigree trees to calculate the probability of having a child with the inherited disease
Genetic counselling consist of
Evaluating family history and medical records
Ordering genetic tests
Evaluating results
Helping parents understand and reach decisions about future
Provide emotional support
Ethical dilemmas of genetic counselling
-Difficult decisions- abort?
-genetic counsellors may learn sensitive information about family- e.g. father is not the biological father
what is the function of HLAs?
to help the immune system distinguish the body’s own proteins from proteins made by pathogens
state the location of factor viii gene in a cell
X chromosome in nucleus
