1.7: Genetics Flashcards
How may mutation affect the phenotype?
- All organisms show variation
- Members of the same species have common characteristics but they are not identical
- they show phenotypic variation
- They have the same genes but different alleles
What are the four causes of variation in gentoype?
- Crossing over of non-sister chromatids during prophase I of meiosis
- Independent assortment during metaphase I of meiosis
- Random fusion of gametes during sexual reproduction (Punnett Square)
- DNA mutations 2 types gene and chromosome
what is mutation
Any change in the composition of the DNA in an organism
What is chromosome mutation?
Change in the chromosome structure number of chromosome
What is gene mutation?
change in the base sequence of a gene
What are gene mutation also known as?
Point mutations
What can point mutations lead to?
A new allele
- if the allele confers a selective advantage it can be passed onto offspring.
What are the three types of gene mutations (clue: SAD)
- substitution
- addition
- deletion
What is substitution in gene mutation?
A nucleotide in a gene is replaced by a different one (e.g. sickle cell anaemia)
What is addition in gene mutation?
Extra nucleotides are added to the gene (e.g. Huntingtons Chorea)
What is deletion in gene mutation?
Nucleotides are lost from the gene (e.g. cystic fibrosis)
What are the chromosome mutations called?
- translocation
- Polysomy
- Polyploidy
What is translocation (chromosome mutation)?
This is when a section of a chromosome is lost or moved to other chromosomes.
5% of people with Downs syndrome have this type of mutation
What is polysomy (chromosome mutation)?
This is when whole chromosomes are duplicated (chromosomes fail to separate during anaphase 1)
Most people with Downs syndrome have an extra chromosome 21 called trisomy
21
What is polyploidy (chromosome mutation)?
This is when sets of chromosomes are duplication
This is common in many crop plants
What do addition and deletion mutations do to the triplet code?
alter the way the DNA triplet code is read (causes frame shifts) and
will therefore alter the sequence of amino acids produced and change the protein structure.
What is different about substitution mutations?
Substitution mutations may not always change the protein structure.
What is a nonsense mutation caused by?
caused if the substituted base makes a stop codon and causes the protein produced to be smaller
What causes a missense mutation?
caused when the substituted base makes a protein with a different amino acid sequence.
What causes a silent mutation?
caused when the substituted base still codes for the same amino acid
What mutagens caused by environmental factors are there?
- radiation (X-rays, UV rays and gamma rays)
- chemicals (benzopyrene in tobacco smoke)
What is a carcinogen?
A mutagen, which causes a mutation in the genes which control mitosis. This can lead to uncontrolled cell division and cancer.
Recap the structure of haemoglobin?
- Globular protein (soluble) with 4o structure
- 4 polypeptide chains per molecule
- 2 α chains and 2 β chains
- Each chain contains a haem group
- Each haem group contains an Fe2+ ion
Recap the function of haemoglobin?
- O2 molecule binds to each haem group
- 4O2 molecules transported per haemoglobin molecule
- reversible binding of O2
- S-shaped binding curve
What causes sickle cell anaemia?
A base substitution mutation in the gene which codes for the β polypeptide.
How does base substitution cause sickle cell?
DNA has a base substitution. The 6th triplet changes from
CTT to CAT
mRNA has a base substitution. The 6th codon changes from GAA to GUA
The beta chain has a different A.A. A Glu is substituted by a Val.
Glu is polar and soluble. Val is not. Therefore the beta chain becomes less soluble and fibrous
This causes the RBC to become sickle shaped
What does the sickle shaped RBC do to the body?
- This reduces efficiency of oxygen transport = tiredness and shortness of breath
- Or can block blood vessels (sickle cell crises) which is very painful and can damage organs
- which can lead to death
What does PKU stand for?
- Phenylketonuria
- (autosomal recessive metabolic genetic disorder)
What enzyme is affected by PKU?
- Phenylalanine hydroxylase hepatic enzyme
- Non-functional
What reaction is catalysed by this enzyme in PKU?
Conversion of amino acid phenylalanine to the amino acid tyrosine
- which increase phenylalanine conc. increases
Explain why individuals with PKU are often very pale?
- Low levels of tyrosine.
- Tyrosine needed for melanin production
Why are all babies in the UK tested for PKU at birth?
High levels of Phe – prevent “normal” brain development in babies and young children
Why should babies with PKU have little phenylalanine in their diet?
High levels of Phe – to prevent “normal” brain development in babies and young children
What is a gene?
A section of DNA which codes for a protein
What is an allele?
Different forms of the same gene
What is a locus?
The position of a gene on a chromosome
What is a phenotype?
The observable characteristics of an organism
What is a genotype?
The alleles an organism has
What is a dominant allele?
The allele which exerts an affect on the phenotype of the heterozygote
What is a recessive allele?
The allele which has no affect on the phenotype of the heterozygote
What is a co-dominant allele?
When both alleles exert an affect on the phenotype of the heterozygote
What is a gamete?
A sex cell e.g. sperm or ovum
What is a zygote?
The fertilized female gamete
What is pure breeding?
Indicates that the organisms genotype is homozygous
What is the homozygous?
A genotype in which the 2 alleles of a gene are the same
What is a heterozygous?
A genotype in which the 2 alleles of a gene are different
What do genetic diagrams allow us to do?
to predict and explain the characteristics of the offspring produced by 2 parents.
What is monohybrid inheritance?
Genetic diagrams involving the inheritance of a single gene
what can monohybrid inheritance genetic diagrams show?
- They can show dominant & recessive inheritance e.g. Tall pea plants crossed (bred) with dwarf pea plants produce all tall pea plants. The tall allele is dominant to the dwarf allele
-Or they can show co-dominance inheritance.
Red salvia plants crossed (bred) with white salvia plants produce all pink salvia plants. The red allele and the white allele are co-dominant
What are the rules for predicting the genotypes and phenotypes expected?
- Identify the interaction between alleles
- Give alleles symbols
Always pick letters that look different when written as capitals compared to lower case - State parents phenotype
- State parents genotype
- Show the possible gametes from each parent– always draw in circles
6 Draw a Punnet square to show the random fertilisation of the gametes - Add offsprings genotype to Punnet square.
- Add offsprings Phenotype to Punnet square
- State the Phenotypic ratio
What does Mendel’s first law state?
“in diploid organisms, characteristics are determined by
alleles that occur in pairs. Only one of each pair of alleles can be present in a single gamete”.
This is the law of segregation
What does a gene code for?
A polypeptide
What does a gene not and does code for?
A gene does not code for a genetic disease BUT an altered allele may code for a different structural protein or enzyme that may lead to a genetic disease.
What is the phenotypic ratio of cystic fibrosis?
- 2 unaffected: 2 cystic fibrosis
1:2 chance of cystic fibrosis - Probability of 0.5 of cystic fibrosis so 50% chance of cystic fibrosis.
What happens in co-dominance?
both alleles exert an effect on the phenotype of the heterozygote.
What are multiple alleles?
when a gene has more than two alleles. Most genes have more than 2 alleles coding for a characteristic
Eg. The gene for human blood groups has 3 alleles
What are the 3 alleles and are they codominant/recessive?
IA
IB
Io
IA and IB are codominant
Io is recessive to both
what are the possible genotypes and phenotypes of human blood groups?
What is sex linkage?
- Refers to genes carried on the sex chromosomes
- Since the X chromosome is usually bigger than the Y the X carries more genes
so usually…
A sex –linked gene is one found on the X chromosome but absent on the Y
What are family trees (a family pedigree)?
Family trees can be drawn to show the occurrence of particular conditions.
What is sex determination? (X and Y chromosomes)
In humans males are heterogametic (XY) and females are homogametic (XX)
What are the two types of dihybrid inheritance?
unlinked
and
linked
Unlinked dihybrid inheritance
Inheritance of 2 separate genes at different loci on different chromosome i.e. unlinked
Linked dihybrid inheritance
Inheritance of 2 separate genes at different loci on the same chromosome ie linked (called linkage)
