Component 2 - Inheritance Flashcards
What is genetics?
The study of the mechanism by which an organism inherits characteristics from its parents
What is monohybrid inheritance?
The inheritance of a SINGLE characteristic, controlled by one gene (each parent having two alleles for that gene)
e.g. inheritance of height in pea plants
What is Mendel’s first law of segregation?
The characteristics of an organism determined by factors (alleles) which occur in pairs. Only one of a pair of factors can be present in a single gamete
How do we tell if a dominant phenotype is heterozygous or homozygous?
May be crossed with the homozygous recessive
What is pure breeding?
Refers to a variety bred for several generations with itself, so that all individuals are homozygous
All offspring will have the same phenotype as their parents when selfed
What is co-dominance?
Both alleles express themselves equally in the heterozygote phenotype
The heterozygotes show a phenotype intermediate between those of the two homozygotes
What are multiple alleles?
When there are more than two alleles for one particular gene
What are sex linked genes?
Have their loci on the X chromosome
e.g. males in humans (XY), have only one copy of these genes, whereas females (XX) have two
What are the 2 well known human sex-linked traits?
Red-green colour blindness
Haemophilia
What is Duchenne’s muscular Dystrophy?
Sex linked recessive disorder, 2/3 cases are inherited from parents, 1/3 are due to a new mutation
Affects about 1/5000 males at birth
Mutation on the X chromosome affects production of DYSTROPHIN
What is dihybrid inheritance?
The inheritance of TWO genes located on two different chromosomes
Each gene will have two alleles
What is Mendel’s second Law?
Law of independent assortment
Either one of a pair of contrasted characters may combine with either of another pair
What is autosomal linkage?
This is a dihybrid cross where BOTH the genes are located close together on the SAME chromosome
What is the outcome of dihybrid cross?
Assumes that all possible allele combinations are equally probable due to random assortment of homologous chromosomes
What if 2 genes are close together on the same chromosome in autosomal linkage?
Likely to be more common in the parental combinations and are said to be linked
Give some example of where characteristics are due to an interaction with the environment
Himalayan rabbit ears
Hydrangea flower colour
Name some human characteristics that have both genetic and environmental influences
Heart disease
Hair colour
Proportions of twitch and tonic muscle
What is epigenetics?
Involve heritable changes in gene function, without changes to the base sequence of DNA
When does epigenetic control of gene expression take place
After DNA replication
What is epigenetics caused by
Variation in the environment that alters gene expression
What are the two ways in which transcription is inhibited?
Increased methylation of the DNA
Decreased acetylation of associated histones
Where are histones and what are they
Found in the eukaryotic cells
They package the DNA and help it to condense into chromosomes
What are histones covered in and what does it do?
Covered in chemicals referred to as tags (accumulate throughout life)
Tags are the epigenome - reason why identical twins do not stay identical
Determine the shape of the histone - DNA complex
What is epigenetic silencing?
Where inactive genes can be wrapped tightly and become inaccessible - ensuring they are not transcribed
How are active genes different to inactive genes?
They are looser and more easily read and transcribed
What does more acetylation of histones result in
DNA less tightly wound around them
More transcription
What is DNA methylation?
When a methyl groups is added to a cytosine nucleotide in DNA
This is caused by enzymes where they can add or remove them
What is gene silencing?
Over-methylation
The more methyl groups causes DNA to become more tightly wound around histones which inhibits transcription
What is under-methylation and what is it associated with?
Fewer methyl groups, less tightly wound, easier transcription
Gene activation
How can epigenetics affect gene expression?
Different epigenetic changes can occur in cells of the same tissue and in different tissues
This results in different expression of the same gene in different parts of the same organism
What is a mutation?
A change in DNA or amount of DNA, the structure or the arrangement of the bases
Happens in somatic cells during mitosis
What must happen for a mutation to be inherited?
Must effect the DNA of the gametes
Are mutation rates high or low
Normally low
Therefore less impact on evolution that other sources of variation
What do organisms with short lifecycles have in terms of mutation?
Higher rates of mutation
Due to more frequent meiosis
What are mutagens?
Chemicals or radiation which increase the rate of mutation
Ionisation radiation examples:
Gamma-radiation
UV
X-rays
Chemicals which increase rate of mutation:
Polycyclic hydrocarbons
What is a mutagen that causes cancer?
Carcinogen
What are the two types of mutation?
Chromosome mutation
Gene mutation
What is chromosome mutation?
Large changes in chromosome structure
Affecting many genes and visible under light microscope in karyotype
What is the result of chromosome mutation?
Non-disjunction in meiosis
e.g. Downs syndrome
What is gene mutation?
Small changes in the base sequence, only affecting a single gene
No change visible in karyotype
e.g. Sickle Cell Anaemia - a substitution of 1 base during interphase
This DNA change will change the structure of the protein produces and the phenotype
When might there be a “silent” mutation?
When the code is degenerate
Describe what occurs to the DNA base sequence in Sickle cell anaemia
A single base substitution in the gene for Beta Haemoglobin with CTT replaced by CAT
Causes the 6th amino acid Glutamic acid to be replace by Valine
This change in the primary structure causes the secondary, tertiary and quaternary structure of the haemoglobin to change
When might there be a “neutral” mutation
If the amino acid altered is NOT essential to the tertiary/quaternary structure of the protein
What happens to the haemoglobin molecule after its protein structure has been changed?
Becomes insoluble at low oxygen tensions
Tends to stick to other Hb molecules to form long fibres
Fibres distort the blood cells, making them sickle shaped
Mutated Hb is poor at binding to oxygen
Why are deletion and addition mutations much more severe?
They cause “frame shift” mutations
All subsequent codons are altered
Result in non-functional protein
What is cancer?
The uncontrolled division of cells and the production of a tumour
In malignant tumours, cells maybe break off and be carried in the blood to form secondary tumours
What do tumour suppressor genes do?
Mutate so there is a lack of proteins which would halt cell division
What do proto-oncogenes do?
Mutate produces an oncogene which releases an excess of proteins to stimulate cell division or excess receptors foe these proteins
How are mutations important?
Increase variation in a population, but most mutations are harmful, rare that beneficial mutations arisw
Describe Down syndrome in terms of chromosomes
Non-disjunction of chromosome 21 during anaphase results in oocytes with an extra chromosome
Why are mutations so rare?
Most are recessive to the ‘normal allele’ and therefore must await replication in the gene pool
Most gene mutations occur in the “junk DNA”, which does not code for proteins
Describe Polyploidy in terms of chromosomes
One gamete receives both sets of chromosomes
This gamete is then fertilised by a second gamete and the zygote produced has 3 sets of chromosomes
