Inheritance 2 Flashcards
Define homologous chromosomes
A pair of chromosomes in diploid cells. One inherited from mother, one from father. Same size, same genes in the same position but may have different alleles.
What are the sex chromosomes
X and Y
What are autosomes
All chromosomes except sex chromosomes
What is a gene and what are the different types?
A gene is a specific DNA sequence that codes for a polypeptide
*Sex linked genes=genes on sex chromosomes
*Autosomal-linked genes=genes on autosomes
*Linked genes=all genes on the same chromosome. They are in a linkage group. Likely to be inherited together. (Not the same with sex/autosomal linked genes)
What is an allele
An allele is a form of gene➡️ result of mutation
*Dominant allele represented by capital letter and always placed in front (e.g. B)
*Recessive allele represented by small letter and always placed behind (e.g. b)
What is a locus (plural loci)
Locus (plural: loci)=position of gene on chromosome
Describe diploid cells
Diploid cells have two sets of chromosomes (2n) =2 copies of each gene = 2 alleles
*1 maternal and 1 paternal
* Somatic cells=all cells except for gametes
*In humans: 2n=46
➡️ Result of mitosis
*Homozygous= have two identical alleles of a gene
*Can be homozygous dominant(e.g. BB) or homozygous recessive (e.g. bb)
*Heterozygous=have two different alleles of a gene (e.g. Bb)
Describe haploid cells
Haploid cells have only one set of chromosomes (n) = Only one copy of each gene = 1 allele
*Gametes/germ cells except
*In humans: n=23
➡️ Result of meiosis
When does meiosis occur and what is its importance?
Meiosis also known as reduction division occurs before fertilisation in sexual reproduction
Importance:
*To produce gametes/sex cells with half the no. Of chromosomes
*Maintain the diploid no. Of chromosomes in each generation
To produce genetic variation in offspring
Describe what happens during prophase I of meiosis
- Condensation of chromatin/chromosome
*Centrosomes move to opposite poles - Nucleur envelope breaks down
*Nucleolus disappears - Spindle fibres form
➡️ Start attaching to centromere and pull
*synapsis occurs
➡️Process where homologous chromosomes pair up
➡️ Each pair is called a bivalent/tetrad
*Crossing over takes place between non-sister chromatids
➡️At chiasmata (singular: chiasma)
What does crossing over mean
During synapsis; homologous chromosomes coil around each other intimately
*Remain in contact at chiasmata
*Site of crossing over: chiasma
*Part of chromatids break
*Reconnect to another non-sister chromatid
Result in:
➡️ Exchange of genetic material [NOT genes]
➡️ Linkage groups broken/linked genes on same chromosome are separated
➡️ Genetic variation
Describe what happens during metaphase I of meiosis
*Centrosomes reach opposite poles
* Spindle fibres are fully formed
➡️ Attached to chromosomes at centromeres
*Bivalent/tetrads line up across equator/metaphase plate
➡️Chromosomes line up in pairs
*Independent assortment of homologous chromosome pairs occurs
➡️Each pair lines up independently of others on equator
➡️Results in gametes that are genetically unique
Describe what happens during Anaphase I duringmeiosis
- spindle microtubules shorten
- homologous chromosomes separate
➡️ with centromeres leading towards poles
*Centromeres DO NOT divide
Describe what happens during telophase I and cytokinesis of meiosis
*Chromosomes reach the poles
*Chromosomes partially decondenses
*Nucleolus reforms
*Nucleur envelope reassembles
*Spindle fibres breaks down
*Cytokinesis occurs
*Number of chromosomes in each daughter cell are now halved
➡️2 haploid cells formed
Describe cytokinesis in animal cells
In animals cells:
*Cell membrane drawn together
➡️By contractile ring of micro filaments
➡️Forms a cleavage furrow
➡️ Creating a drawstring effect
*Cell membrane fuses
*To divide cell into two
*Organelles are shared out
Describe cytokinesis in plants
*Vesicles transported to equator
➡️ To form cell plate at equator
*Cell wall laid out
*Did cytoplasm divided into two
Describe the brief interphase between meiosis I and meiosis II
*Called interphase II/interkinesis
* Growth
*Synthesis of protein and other substances
*No DNA replication
Plant cells go straight into meiosis II
*No reformation of nucleolus and Nucleur envelope
Describe what happens during prophase II
*Condensation of chromatin
*Spindle fibres form
➡️ Start attaching to centromere
*Centrosomes move to opposite poles
*Nuclear envelope breaks down
*Nucleolus breaks down
Describe what happens during metaphase II during meiosis
*Centrosomes reach opposite poles
* Spindle fibres are fully formed
*Chromosomes line up at the metaphase plate/ equator
*At right angles to first equator (right angle to first metaphase)
*Chromosomes attached to spindle fibres at centromere/Kinetochore
*Independent assortment of sister chromatids occurs
Describe anaphase II
*Centromere of each chromosome divides
*Sister chromatids split at the centromere
*Spindle microtubules shorten
*Chromatids pulled to opposite poles
➡️With centromeres leading towards poles
Describe telophase II and cytokinesis of meiosis
*Chromatids reach the poles
*Chromosomes decondense
➡️ Become long and thin
*Nucleolus reforms
*Nucleur envelope reassembles
*Spindle fibres breaks down
*Cytokinesis occurs
➡️ 4 haploid cells formed
How does meiosis cause genetic variation?
- Crossing over @ Prophase I
- Random/Independent assortment of homologous chromosomes @ Metaphase I
- Independent assortment of chromatids @ Metaphase II
- Possible chromosome mutation
Number 1 and 2 most important
How does crossing over @ Prophase I during meiosis cause genetic variation
- Crossing over @ Prophase I
*Between non-sister chromatids of homologous chromosomes
*At chiasma
➡️Exchange of genetic material [NOT genes]
➡️ Linkage groups broken/linked genes on same chromosome are separated
➡️ New combination of alleles within each each chromosome
➡️ Genetic variation
How does random/independent assortment of homologous chromosomes @ metaphase I
*Each pair lines up independently of other on equator
*Possible combinations =2^n
➡️Results in gametes that are genetically unique
Apart from meiosis what can also cause genetic variation?
Besides meiosis, fertilisation can also cause genetic variation!
This is due to:
- Random mating
- Random fusion of gametes
Define genotype
Genotype: the alleles possessed by an organism
* Represented by same alphabet (e.g. B and b=same gene, different allele)
*gene short form name is usually in italics, whereas gene full name and protein is not italicized (e.g. HBB=haemoglobin beta gene➡️codes for beta globin protein)
Define phenotype
Phenotype: the observable characteristics of an organism
➡️ can be influenced by genotype, environment
Define dominant allele
*Expressed in the phenotype even when only one copy of the allele is present
*Expressed in heterozygous and homozygous for the allele (e.g. Bb and BB)
Define recessive allele
*Expressed in the phenotype only when two copies of the allele are present
*Expressed only in a homozygous form the allele (e.g. bb)
What are the 5 things we need to have in a genetic diagram?
1) parental phenotypes (don’t forget the X in between them)
2) Parental genotypes
3) Gametes (should be circled)
4) Offspring (F1) genotype and phenotype (you must draw this in a punnet square)
4) Probability/ratio depends on what the questions wants
What is a test cross
Test cross=method to determine exact genotype of a dominant phenotype organism
➡️Is it homozygous dominant or heterozygous for the gene?
➡️Cross with individual homozygous recessive for the gene
For example : how to determine if genotype is TT or Tt?
Cross with tt
Define purebred
A cross of same purebred organisms always produce offspring with the same genotype. Homozygous individuals.
What is parental generation?
Parental (P) generation=initial generation
What is F1 generation
Result of crossing 2 different purebreds (i.e. 2 homozygous of different phenotype.) F1 offspring are all heterozygotes.
Stands for first filial generation/first set of offspring
What is F2 generation
Offspring results of self pollination of F1 plants OR cross of 2 heterozygotes.
What is dihybrid inheritance?
*Involves the crosses of two genes at once
➡️ 2 genes are on different chromosomes
➡️ 2 alleles each
➡️ Code for different traits
For example gene for tall/short plants and gene for purple/white flowers
9:3:3:1 ratio
What 3 laws did Mendelian genetics prove?
Law of segregation: during gamete formation, the alleles for each gene segregate from each other (during anaphase I) so that each gamete carries only one allele for each gene.
Law of independent assortment: Each pair lines up independently of others in equator (during metaphase I). Genes for different traits can segregate independently during the formation of gametes.
Law of dominance: some alleles are dominant while others are recessive; an organism with at least one dominant allele will display the effect of the dominant allele.
What is sex linkage
*Sex-linkage=allele/gene carried on the sex chromosomes
*Usually X chromosome
*Y chromosome is shorter than X
➡️Carries fewer genes than X
➡️Do not have the same genes in the same loci
➡️Some different genes from X
➡️Some genes exist as a single copy only either on X or Y chromosome= hemizgous
Types of sex-linked inheritance patterns:
*X-linked recessive
*X-linked dominant
Describe X-linked recessive
*Phenotype determined by recessive allele on X
*Phenotype will be expressed if there are no dominant alleles of the same gene present
*I.e. in females with both recessive alleles on X, or males with one recessive allele on X
*To show genotypes for sex-linked genes:
a) Use symbols for gender (XX-female, XY-male)
b) Write alleles as subscripts
Describe X-linked dominant
*Phenotype determined by dominant allele on X
*Phenotype is shown in homozygous dominant and heterozygous females, and males with the dominant allele
*Affected males will not pass the dominant allele on X to sons (since sons must inherit Y)
*But will pass it on to daughters (Daughters must inherit one X from mum and one X from dad)
What do the symbols in a pedigree diagram represent
In a pedigree diagram:
⚪️ is female
⬜️ is male
⚫️ and ⬛️ is affected
Describe codominance
*Codominance=when both alleles are fully expressed in the heterozygote
*More than 2 phenotypes are possible
*Phenotype of heterozygote different from either homozygote
*When 2 heterozygote (F1) are crossed, the resulting phenotype is 1:2:1 ratio
What is multiple alleles?
Genes with more than 2 alleles
E.g. ABO gene for human blood groups
*I^A=A allele ➡️Codominant
*I^B=B allele ➡️ Codominant
*i = O allele ➡️ recessive to I^A and I^B
Describe gene interactions
AKA epistasis
*When different genes at different loci
*Interact to affect ONE phenotype
Describe autosomal linkage
Linked genes: 2 genes are in the same chromosome
➡️ They are in an autosomal linkage group (not to be confused with sex/autosomal-linked genes!)
➡️Likely to be inherited together
➡️No independent assortment
➡️ No separation during anaphase
The closer the loci are the lower the chance of crossing over.
We will use brackets in genotypes for example: (EA)(EA) to show they are on the same chromosome. EEAA without brackets indicates they are on the different chromosomes.
How can we determine whether the 2 genes are linked or just normal dihybrid?
Cross an individual heterozygous for both genes (EA)(ea) with a homozygous recessive individual (ea)(ea).
In dihybrid the ratio will be 1:1:1:1
In autosomal linkage the expected ratio is 1:1
But getting a 1:1 ratio is rare
1:1 ratio = total linkage
Linked genes can be separated by rare cross over events during prophase I.
The closer the loci are the lower the chance of crossing over
What is the cross over value?
Cross over value= measure of distance between 2 gene loci on chromosomes
This is the percentage that belongs to the recombinant class
The smaller the value, the closer the loci are together
The closer the loci are together the lower the chance of crossing over.
What is sickle cell anemia?
Inherited blood disorder
Affects the structure of haemoglobin
Cause: base substitution in gene coding for beta globin
Base substitution (T is replaced by A) in gene coding for beta globin results in
➡️different mRNA codon
➡️ different tRNA brings a different amino acid to ribosome
➡️Leads to a change of 6th amino acid in polypeptide chain
➡️ altered primary structure
➡️ glutamic acid is polar whereas valine is non-polar
➡️ changed secondary, tertiary and quaternary structure
In mutated
Describe the genotypes of sickle cell anaemia
*Hb^A=Normal B-globin allele
*Hb^s=Mutant B-globin allele
*Hb^AHb^A=2 normal B polypeptide alleles
*Hb^sHb^s=2 mutant B polypeptide alleles (sickle cell anaemia)
*Hb^AHb^s= 1 normal and 1 mutant
Has the sickle cell trait/”carriers”
Half of the haemoglobin is normal
Codominant
What is albinism and what are the symptoms?
*Classic form caused by autosomal recessive mutation
*Melanin is missing from eyes, skin and hair
Symptoms:
*Pale/white skin and hair
*Pink eyes
*Increased susceptibility to sunburn/skin cancer
*Rapid, jerky eye movements
*Tend to have poor vision
*Tend to avoid bright light
How is albinism caused?
*Classic form caused by autosomal recessive mutation
*In TYR gene coding for enzyme tyrosinase
*Involved in conversion of tyrosine to DOPA and DOPA into to dopaquinone, which is needed for melanin production in melanocytes (cells that produce melanin).
What is tyrosinase?
Tyrosinase is an oxidase enzyme
Has two copper atoms, binds 1 O2 molecule
Transmembrane protein found at membrane of melanosomes
(large, specialised organelles in melanocytes)
Tyrosine→DOPA→dopaquinone→melanin
Tyrosinase is needed to convert tyrosine to DOPA and DOPA to dopaquinone.
Mutation results in absence/inactive tyrosinase
Describe haemophilia and its symptoms?
*Sex-linked
*Gene for blood clotting protein (factor VIII) is carried on X chromosome
*Caused by X-linked recessive mutation
*X^H=normal allele
*X^h=haemophillia allele
→Factor VIII (8) is not produced
→This reduces clotting of blood
Symptoms:
*Excessive bleeding
*Bleeding into joints
*Large bruises
*Internal bleeding
describe Huntington’s disease (HD) and its symptoms
*Neurological disorder (i.e. neurones die)
*Usual onset in middle age btwn 28-65yo
*Caused by an autosomal dominant mutated allele
*Mutation at the Huntingtin (HTT) gene on chromosome 4
Symptoms:
*Involuntary movements/uninhibited motor control
*Mood changes
*Progressive mental deterioration
*Brain cells lost
How is Huntington’s disease (HD) caused?
*Normal recessive allele has 10-35 repeats of CAG
*Mutant dominant allele has equal to or greater than 36 repeats of CAG
→CAG codes for glutamine/polyglutamine
→Huntingtin protein misfolded
*More repeats=earlier onsets
→E.g. onset even in babies if many repeats
What is cystic fibrosis and what are the symptoms?
*Inherited genetic disease
*Faulty, autosomal recessive allele of the CFTR gene
Symptoms:
*Thick and sticky mucus produced at lungs
*Mucus not moved effectively by cilia→mucus accumulates
*Mucus traps bacteria →more infection
*Reduced gaseous exchange→due to longer diffusion pathway
*Diffuculting in breathing, wheezing
*coughing→ cause lungs to be scarred
*blocked pancreatic duct→reduced digestion, damage of pancreatic tissues causes diabetes
*Blocked sperm ducts/oviducts
→reduced fertility
How is cystic fibrosis caused?
Faulty, autosomal recessive allele of the CFTR gene
Normal CFTR protein:
*Transmemebrane protein at cell surface membrane
*Acts as chloride channel
*Has binding site for ATP
*Cl- moves out of cell via active transport
*Water is drawn out from cell
→Normal/less viscous mucus formed
Faulty CFTR allele:
a) Base deletion→faulty CFTR (most common)
b) Base substitution → STOP codon →Incomplete CFTR
Faulty CFTR protein:
*No functional channels for Cl- ions.
*Cl- ions do not move out
*less water leaves cell
→Formation of thick, sticky mucus on cell surface membrane
Define gene expression
Gene expression is the process where gene is being transcribed to mRNA and then translated to protein.
Define gene regulation/ control
Gene regulation/control is the process that enables gene expression in specific cells and specific time.
➡️to ensure gene is expressed in correct cell at the correct time in the correct context.
➡️Some genes are transcribed all the time to produce constitutive proteins
➡️Others are only “switched on” when their protein products are required
➡️why?
⚪️No wastage of energy and recourses
⚪️ No expression of protein that may interfere with cell function
What is a structural gene?
⚪️ Gene that code for proteins needed for cell structure or function
⚪️ E.g. proteins that form part of cellular structure, enzymes rRNA etc
What is a regulatory gene?
⚪️Genes that code for regulatory proteins that controls gene expression/ transcription
⚪️E.g. transcription factors, including repressions and activators
Transcription factors can bind to a specific DNA sequence and control gene transcription
⚪️Activators=increase gene expression
⚪️Repressor=reduce/inhibit gene expression (bind to operator)
What is a repressible enzyme?
A repressible enzyme is one whose synthesis can be prevented when a repressor protein binds to an operator, a specific sequence of DNA.
Repressor stops binding of RNA polymerase.
What is an inducible enzyme?
An enzyme whose synthesis only occurs when a substrate is present.
Inducer that binds to a regulatory protein .
➡️ gene is switched on.
E.g. lac enzymes
What is an operon?
Gene control in prokaryotes: operons
⚪️ Found in prokaryotic DNA
⚪️Makes up a unit of gene expression in bacterium
⚪️Regulatory gene is close to the operon
Operon consist of:
⚪️One or more structural genes, under control of a single promoter
⚪️Control region of DNA, which are specific sequence recognised by regulatory proteins and enzymes
⚪️Promoter=where RNA polymerase binds
⚪️Operator=where repressor binds
What is lac operon?
⚪️Found in Escherichia coli bacteria
⚪️ The bacteria lives on sugar, I.e. glucose and lactose
Lac operon=3 genes next to each other
Code for 3 repressible, inducible enzymes
LacZ=codes for Beta-galactosidase (breaks down lactose into galactose and glucose)
LacY=codes for permease (increases lactose uptake)
LacA=codes for transacetylase
What is meant by parental type and recombinant offspring?
Parental type are offspring that show the same combinations of characteristics as their parents.
Recombinant are offspring that show different combinations of characteristics from their parents. The more recombinance you get in the offspring the more crossing over has taken place and the further apart the jeans are in the chromosome.
What is the regulatory gene for lac operon?
*Regulatory gene, LacI that codes for repressor is close by
*Repressor is a constitutive protein: produced all the time
*Repressor protein is an allosteric protein= has 2 binding sites
1. DNA-binding site (to bind to operator region)
2. Lactose-binding site
*When lactose binds to its site, the shape of the protein changes
*DNA binding site is closed
*Repressor is inactivated
What happens to lac operon when there is no lactose?
When there is no lactose:
*Regulatory gene codes for a protein called a repressor
*Repressor not bound to lactose
*Repressor binds to the operator region close to the gene that codes for B-galactosidase
*Because the repressor is bound to the operator, RNA polymerase cannot bind to DNA at the promoter region.
*No transcription of the 3 structural genes
*Genes are switched off
What happens to lac operon when lactose is present?
When lactose is present:
* lactose is taken up by bacterium
* Lactose acts as an inducer and binds to repressor protein
* Repressor changes shape, and cannot bind to the operator region on DNA
*RNA polymerase cannot move bind to promoter and move to operator and genes
*Transcription occurs
*genes are switched on
What are the benefits of the lac operon?
Benefits of this mechanism:
*Allow bacteria to produce 3 enzymes only when lactose is available
*When both glucose and lactose are available, bacteria prefers glucose-can repress the lac operon using another transcription factors
*3 enzymes produced in equal amounts
*Avoids waste of energy and materials
Describe gene control in eukaryotes
*Genome is larger than prokaryotes
*have more more different transcription factors and mechanisms to regulate gene expression
*In humans, 10% genes code for transcription factors
What is the function of transcription factors?
*General TFs=form part of the protein complex that binds to the promoter region with RNA polymerase cannot move
➡️ transcription
* activate appropriate gene in sequence
➡️ allow correct pattern of development of body regions
*determination of sex in mammals
*allow responses to environmental stimuli e.g. temp
*Products of proto-oncogenes and tumour suppressor genes and other Tags, regulate the cell cycle, growth and apoptosis.
*TFs carry out responses to hormones
In the gene control of plants, what is the role of gibberellin in stem elongation?
Without GA:
*Transcription factors (i.e. PIF) attached to DELLA protein
*TF cannot bind to DNA
When GA binds to receptor:
*Causes DELLA protein destruction
*TF binds to DNA
*TF recruits RNA polymerase to bind to DNA
*Growth genes switched on/transcribed ➡️production of amylase
Explain the relationship between gene protein and phenotype.
A gene codes for a protein which determines the phenotype
