Introduction To Genetics Flashcards
What makes up the human genome?
Complete (haploid) set of genetic material in an organism that contains 23 pairs of chromosomes:
- 22 pairs of autosomes
- 1 pair of sex chromosomes (XX = female, XY = male)
What makes up our genotype?
Genes, which are segments of DNA, within the genome encode RNA and usually protein
What is the central dogma of molecular biology?
DNA encodes mRNA which encodes protein
What is phenotype?
Anatomy, physiology and psychology which is influenced by our genotype and the environment so it can change over time
What is a monogenetic disease? How do you investigate it?
Single gene abnormality e.g. cystic fibrosis - investigate with molecular genetic tests e.g. PCR
What is a multifactorial/polygenetic disease? How do you investigate it?
Multiple genes and environmental influences are involved e.g. heart disease - assess RISK with molecular genetic tests e.g. PCR
What ii a chromosomal disease? How do you investigate it?
Abnormality of chromosome structure or number e.g. down syndrome - investigate by cytogenetics e.g. karyotype
Are all genetic diseases inherited?
No as new mutations can occur
What is a genetic polymorphism?
If different versions of a gene exist in a population, the gene is polymorphic - different versions are called ALLELES with the most common one in the population (usually not ass. with disease) is the wild-type
What is the difference between homozygous and heterozygous chromosomes?
Homo: 2 alleles over both the chromosome pairs are identical
Hetero: 2 alleles over both the chromosomes pairs are different or unmatched
What is the difference between dominant and recessive alleles?
Dominant alleles determine phenotype in heterozygous individuals
Recessive alleles only determine phenotype if they are present in as a homozygous pair - if an individual is heterozygous, they are a ‘carrier’
What is a punnet square?
Simple grid used to draw the expected outcomes of matings which can be used to predict the probably that a new individual will have a particular genotype
How are punnet squares drawn?
- Genotypes of parents are written along top and down the left side of the square
- Dominant alleles are represented by capital letters
- Recessive alleles are represented by the same letter as dominant allele but in lower case
- 4 possible genotypes resulting are added to the grid and % chances of genotypes can be worked out
What is Mendel’s law of uniformity?
If homozygous dominant and homozygous recessive individuals are mated, the offspring are identical to one another i.e. there is no blending of dominant and recessive
What is Mendel’s law of segregation?
During formation of gametes, the members of each gene pair separate so that each gamete carries only one copy of the gene
What is Mendel’s law of independent assortment?
Each gene pair segregates independently of other gene pairs (unless they are EXTREMELY close together)
What is autosomal dominant inheritance/vertical pattern of inheritance?
Dominant alleles are expressed and determine phenotype when they are present as a single or duplicate copy so an affected person must have an affected parent (i.e. it does not skip generations) and usually involve a GAIN of function in protein encoded - low frequency if they are ass. with disease e.g. familial hypercholesterolaemia, HD and achondroplasia
What is achondroplasia?
Premature differentiation of chrondrocytes at growth plates due to Fibroblast Growth Factor Receptor 3 (FGFR3) causing bone growth abnormality and short stature
What is autosomal recessive inheritance?
Recessive alleles are only expressed and determine phenotype when 2 copies are present and usually involve a LOSS of function in protein - heterozygous individuals or ‘carriers’ are healthy and can be common in disease-causing alleles esp. in consanguinity (4% in compared to 2% normally) because there is a higher risk both parents are carriers of the same recessive allele increasing risk of having affected children
What is consanguinity?
Sharing of blood and refers to matings where the partners have at least one common ancestor considered back to great-grandparent level i.e. second cousins
What is genetic load?
The hidden detrimental component of our genome i.e. the lethal pre-natal mutations and harmful recessive disorders for which we are carriers
What is the exception to Mendel’s law of inheritance?
Co-dominance where genes do not have dominant or recessive alleles, but heterozygous individuals have a blend of 2 the phenotypes e.g. A and B blood group encoding A and B surface antigens (O is recessive because it is the absence of either)
What is overdominance?
Homozygous dominant individuals have a more severe effect than heterozygotes; often homozygosity is incompatible with life
What is incomplete penetrance?
Not all individuals with a disease-causing genotype exhibit the disease clinically
What is genomic imprinting?
Alleles have different effects depending on the parent of origin; they may have no effect if inherited from one or the other parent
What are sex-linked effects?
Where the gene is on the sex chromosome (XX/XY) so if a gene is on both the X and Y chromosomes it will be inherited identically to autosomal genes - most on X chromosome are recessive whereas Y-linked genes and X-linked dominant are rare with males being more affected by X-linked disease receiving it from carrier mother (50% chance)
What are examples of sex-linked recessive diseases?
Most commonly affect males: Red/green colour blindness Duchenne muscular dystrophy Fragile X syndrome Haemophilia A
What is mitochondrial inheritance?
Mitochondria contain DNA which is inherited entirely from the mother
What is genetic linkage?
Combinations of genes tend to be inherited together because they are located closely together on one chromosome pair
What is dynamic mutation?
Genetic diseases occur with increasing severity in consecutive generations due to expansion of a repetitive DNA sequence
What is mosaicism?
When different cells within the same individual have different genotypes usually as a result of an error in DNA replication and chromosome separation in the very early embryo (2-8 cell stage) - if 1 cell line is abnormal (e.g. in Trisomy 21), but the other cell line is normal the abnormal effects may be ameliorated
Is mosaicism common?
It is difficult to predict the occurrence or its effects but it may not be uncommon - affected individuals may not even be aware of it until as offspring is born who is clearly affected by disease e.g. if all gametes are affected
How do you record a genetic history?
- When taking a genetic disease history, draw a family pedigree showing who is affected to help determine the mode and risk of inheritance
- For each individual record name, DOB, date/cause of death and medical conditions
- Consider consanguinity, miscarriages, terminations of pregnancy, still births and infant deaths
When is genetic counselling offered? Why?
In the process of family planning to access the risk to offspring or of late onset disease in adults
What symbols are used in pedigrees?
Male = square on L
Female = circle on R
Person of unknown sex/unborn child = diamond
Unaffected = hollow Affected = shaded Carrier = half-shaded Deceased = slash Person of unknown genetic status = question mark
Marriages/matings = horizontal lines Children = vertical line and then on a horizontal line in BIRTH ORDER from L to R
