3.2 Flashcards
Homologous chromosomes
carry the same sequence of genes but not necessarily the same alleles of those genes
maternal and paternal chromosome pairs are called?
homologous chromosomes
Homologous chromosomes are chromosomes that share:
The same structural features (e.g. same size, same banding patterns, same centromere positions)
The same genes at the same loci positions (while the genes are the same, alleles may be different)
Why must homologous chromosomes be separated in the gametes
Homologous chromosomes must be separated in gametes (via meiosis) prior to reproduction, in order to prevent chromosome numbers continually doubling with each generation
How many different types of chromosomes are there?
23 types of chromosomes in humans
- each carry different groups of genes
What are karyotypes?
the number and types of chromosomes in a eukaryotic cell
How are gametes formed?
by the process of meiosis
Genotype
The gene composition (i.e. allele combination) for a specific trait
- genotype of a particular gene will typically be either homozygous or heterozygous
Phenotype
The observable characteristics of a specific trait (i.e. the physical expression)
- phenotype is determined by both the genotype and environmental influences
What will the dominant allele do to the recessive allele?
The dominant allele will mask the recessive allele when in a heterozygous state
When will the recessive allele be expressed?
The recessive allele will only be expressed in the phenotype when in a homozygous state
When does co-dominance occur?
when pairs of alleles are both expressed equally in the phenotype of a heterozygous individual
What are ABO blood groups joined by?
a single gene with multiple alleles (A, B, O)
- The A, B and O alleles all produce a basic antigen on the surface of red blood cells
Which red blood cell alleles are co-dominant?
A & B
- each modify the structure of the antigen to produce different variants
Which red blood cell is recessive?
The O allele
- it does not modify the basic antigenic structure
Why are blood transfusions are not compatible between certain blood groups?
humans produce antibodies against foreign antigens
What type of blood can AB blood groups receive?
AB blood groups can receive blood from any other type (as they already possess both antigenic variants on their cells)
What type of blood can A blood groups receive?
cannot receive B blood or AB blood (as the isoantigen produced by the B allele is foreign)
What type of blood can B blood groups receive?
cannot receive A blood or AB blood (as the isoantigen produced by the A allele is foreign)
What type of blood can O blood groups receive?
O blood groups can only receive transfusions from other O blood donor (both antigenic variants are foreign)
What does a monohybrid cross determine?
A monohybrid cross determines the allele combinations for potential offspring for one gene only
How are genetic diseases caused
caused when mutations to a gene (or genes) abrogate normal cellular function, leading to the development of a disease phenotype
- Many genetic diseases in humans are due to recessive alleles of autosomal genes, although some genetic diseases are due to dominant or co-dominant alleles
When will an autosomal recessive genetic disease occur?
will only occur if both alleles are faulty
- Heterozygous individuals will possess one copy of the faulty allele but not develop disease symptoms (they are carriers)
Example of an autosomal recessive genetic disease
cystic fibrosis
When will an autosomal dominant genetic disease occur?
autosomal dominant genetic disease only requires one copy of a faulty allele to cause the disorder
Homozygous dominant and heterozygous individuals will both develop the full range of disease symptoms
Example of an autosomal dominant genetic disease
Huntington’s disease
If a genetic disease is caused by co-dominant alleles, it will require what?
If a genetic disease is caused by co-dominant alleles it will also only require one copy of the faulty allele to occur
-heterozygous individuals will have milder symptoms due to the moderating influence of a normal allele
Example of a co-dominant genetic disease
sickle cell anaemia
Cystic fibrosis
Cystic fibrosis is an autosomal recessive disorder caused by a mutation to the CFTR gene on chromosome 7
Individuals w cystic fibrosis produce mucus which is unusually thick and sticky
This mucus clogs the airways & secretory ducts of the digestive system, leading to respiratory failure and pancreatic cysts
Heterozygous carriers who possess 1 normal allele won’t develop disease symptoms
Huntington’s disease
Huntington’s disease an autosomal dominant disorder caused by a mutation to the Huntingtin (HTT) gene on chromosome 4
The HTT gene possesses a repeating trinucleotide sequence (CAG) that is usually present in low amounts (10 – 25 repeats)
More than 28 CAG repeats is unstable 7 causes the sequence to amplify (produce even more repeats)
When the number of repeats exceeds ~40, the huntingtin protein will misfold and cause neurodegeneration
This usually occurs in late adulthood and so symptoms usually develop noticeably in a person’s middle age (~40 years)
Symptoms of Huntington’s disease include uncontrollable, spasmodic movements (chorea) and dementia
Which are more common, out of recessive conditions or dominant conditions?
Recessive conditions
bc the faulty allele can be present in carriers without causing disease
Dominant conditions may often have a late onset, as this does not prevent reproduction and the transfer of the faulty allele
What does sex-linkage refer to?
when a gene controlling a characteristic is located on a sex chromosome (X or Y)
- sex-linked conditions are usually X-linked - as very few genes exist on the shorter Y chromosome
What is a gene mutation?
a change to the base sequence of a gene that can affect the structure and function of the protein it encodes
Mutations can be spontaneous (caused by copying errors during DNA replication) or induced by exposure to external elements
What is a pedigree
a chart of the genetic history of a family over several generations
How many chromosomes in a diploid nucleus
46
Organisation of eukaryotic chromosomes can be summarised as follows:
Organisation of eukaryotic chromosomes can be summarised as follows:
- complexed with 8 histone proteins to form anucleosome
- Nucleosomes are linked by H1 histone to form a string ofchromatosomes
- These then coil to form asolenoid structure, condensed to form a30 nm fibre
- fibres then form loops, which are compressed and folded around a scaffold to formchromatin
- Chromatin will then supercoil during cell division to formchromosomes
What are diploid nuclei?
Nuclei possessing pairs of homologous chromosomes
These nuclei will possess two gene copies (alleles) for each trait
All somatic (body) cells in the organism will be diploid, with new diploid cells created via mitosis
Diploid cells are present in most animals and many plants
What are haploid nuclei?
Nuclei possessing only one set of chromosomes are haploid
These nuclei will possess a single gene copy (allele) for each trait
All sex cells (gametes) in the organism will be haploid, and are derived from diploid cells via meiosis
Haploid cells are also present in bacteria (asexual) and fungi (except when reproducing)
What are heterosomes?
sex chromosomes
When are the sex chromosomes homologous?
- they are homologous in females
What did Cairn use to measure the length of DNA?
autoradiography
What is the process of autoradiography?
Cells are grown in a solution containing radioactive thymidine (tritiated thymidine – 3H-T)
The tritiated thymidine is incorporated into the chromosomal DNA of the cell (3H-T is used as thymidine is not present in RNA)
The chromosomes are isolated by gently lysing the cells and fixing the chromosomes to a photographic surface
The surface is then immersed in a radioactively-sensitive emulsion containing silver bromide (AgBr)
The radiation released from the tritiated thymidine converts the Ag+ ions in silver bromide into insoluble metal grains
Following a period of exposure, excess silver bromide is washed away, leaving the silver grains to appear as small black dots
When the photographic film is developed, the chromosomal DNA can be visualised with an electron microscope
How did Cairn use autoradiography?
- Cairns used autoradiography to visualise the chromosomes whilst uncoiled - more accurate indications of length
- used ecoli and measured length
- also provided evidence for semi-conservative replication
How many diploid chromosomes do roundworms have?
4
How many diploid chromosomes does rice have?
24
How many diploid chromosomes do humans have?
46
How many diploid chromosomes do chimpanzees have?
48
How many diploid chromosomes do dogs have?
78
What are the rules for genome sizes?
Viruses and bacteria tend to have very small genomes
Prokaryotes typically have smaller genomes than eukaryotes
Sizes of plant genomes can vary dramatically due to the capacity for plant species to self-fertilise and become polyploid
