Introduction to Genetics Flashcards
How many pairs of chromosomes do genetically normal individuals have?
23:
- 22 pairs of autosomes
- 1 pair of sex chromosomes
What are genes?
Segments of DNA withint he genome that encode messenger RNA and usually protein which make up out genotype.
Define phenotype
Observed characteristics of the genotype (physiological, anatomical, psychological)
Genotype + environment
What are the 3 classifications of genetic disease?
What tests are used to detect them/establish risk?
Give examples for each one
Monogenetic:
- Single gene abnormality
- Detected by molecular genetic tests (PCR)
- E.g. cystic fibrosis
Multifactorial:
- Multiple genes and environmental factors involved
- Molecular genetic tests detect risk
- E.g. heart disease
Chromosomal:
- Abnormality in chromosome structure or number
- Investigated by cytogenetic tests
- E.g. Down’s syndrome
What is genetic polymorphism?
When different versions of a particular gene (alleles) exist within a population
Define alleles
Different versions of a particular gene that exist within a population
Define wild type
The most common allele of a particular gene to exist within a population.
(usually not associated with disease)
Define homozygous
2 identical alleles within a chromosome pair
Define heterozygous
2 different genes within a chromosome pair
What is the effect of dominant alleles?
They are expressed: determine the phenotype in heterozygous individuals
What are Mendel’s laws of inheritance?
Law of uniformity: if homozygous dominant and homozygous recessive are mates, the offspring are identical to eachother, there is no blending of dominant and recessive.
Law of segregation: during gamete formation, members of each gene pair separate so that each gamete only contains one copy of the gene.
Law of independent assortment: each gene pair segregates independently of other gene pairs.
What is the pattern of inheritance for autosomal dominant genes?
An affected person must also have an affected parent.
What is the distribution of inheritence of autosomal recessive and dominant genes amongst males and females?
Inherited equally and both exhibit dominant phenotype
What would be the risk of disease in the offspring of a dominany heterozygote and a healthy homozygote?
50%
When do recessive alleles determine the phenotype?
If 2 copies are inherited (homozygous recessive individuals)
What occurs with the function of proteins encoded by dominant and recessive genes?
Dominant = gain of function
Recessive = loss of function
What is the risk of an offspring being affected by a gene carried by 2 unaffected parents?
25%- the parents must be heterozygotes as they are unaffected, the chance of the offspring inheriting both disease causing recessive alleles is therefore 25%
What is consanguinity? How does it affect risk of inheriting recessive alleles?
‘Mixing of blood’: mating of 2 individuals with at least one common ancestor as far back as great-grandparents.
Increases the risk of expression of recessive disease causing allele as the likelihood of offspring inheriting 2 recessive alleles for the same disease is increased.
What is genetic load?
Recessive alleles for harmful disorders of which are carriers.
What are exceptions to Mendel’s Laws of Inheritance?
- Co-dominance
- Over-dominance
- Incomplete penetrance
- Genomic imprinting
- Sex-linked effects
- Mitochondrial inheritance
- Genetic linkage
- Dynamic mutation
- Mosaicism
What is co-dominance?
Alleles for a particular gene are neither dominant or recessive, phenotype will be a blend of both alleles.
E.g. Blood types (A & B= dominant, O= recessive)
AA + BB = AB blood type
What is over-dominance?
Homozygote dominant individuals have a more seve effect that heterozygote, often incompatible with life (early embryo may not be viable)
What is incomplete penetrance?
Not all individuals with a disease causing genotype clinically exhibit the disease
What is genomic imprinting?
Alleles have different effects depending on the parent of origin - may have no effect if inherited from one parent but effect if inherited from the other.
What are sex-linked effects?
What will be the pattern of inheritance if it exists on both X and Y?
Are males or females more affected by X-linked genes? Why? Which parent are they inherited from?
Give an example of an x-linked genetic disease
When the genes exist on the sex chromosomes.
If the gene exists on both X and Y chromosomes it wil be inherited in the same way as autosomal genes.
Males more affected by X-linked genes as they do not have a nother X chromosome with a different copy of the disease to dominate. (x-linked diseases often recessive). Inherited from the mother.
- E.g. red-green colourblindness, Duchenne’s muscular dystrophy, haemophilia A, fragile X syndrome.
What risk does X-linked inheritance give to females and males?
50% risk of carrier in females
50% of affected in males
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 as they are located close together on the chromosome.
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 cells of the same person contain different genotypes.
Usually caused by error in DNA replication.
