Lecture 16: HOW ARE WE ALL DIFFERENT? Flashcards
What is Mendel’s first law?
Law of Segregation
What is the Law of Segregation?
When gametes form, alleles are separated so that each gamete carries only one allele for each gene
What is Mendel’s second law?
Law of Independent Assortment
What is the Law of Independent Assortment?
The segregation of alleles for one gene occurs independently to that of any other gene
What is Mendel’s third law?
Law of Dominance
What is the 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 an allele?
An alternative form of a gene (one member of a pair) that is located at the same place on a chromosome. An allele inherited for a gene may be different from mum and dad
What is recombination?
Shuffling of DNA in chromosomes during meiosis
What happens first before recombination?
Homologous chromosomes (same genes, different DNA sequences) undergo replication to form sister chromatids (same DNA sequence)
What happens after homologous chromosomes replicate?
The homologous pairs of sister chromatids pair up and undergo recombination
What is the result of recombination?
Each gamete gets one copy of the chromosome, each with a unique combination of alleles
What is the exception to Mendel’s second law?
When two genes are close together on a chromosome - genetic linkage
How are genetically linked alleles inherited?
Often together, rarely independently
What is a recessive allele?
A version of the gene that does not encode a functional protein
What happens when there is a recessive mutation (Aa)?
When there is a mutation in either the gene from mum or dad there will be no problem as the normal copy from the other parent can mask the mutated one
What happens when there is a homozygous recessive mutation (aa)?
When both parents have the mutation for an enzyme there will be an accumulation of the precursor and no product formed (an observable effect)
What is the dominant allele for pea pigment?
Functional CHS enzyme is produced
What is the recessive allele for pea pigment?
Mutation so no CHS mRNA (therefore no CHS protein)
When is there no product?
If there is too recessive alleles present
What do we inherit?
A unique combination of alleles from our parents
How identical is the DNA of any two people?
99.4% so it differs at about 20 million places (0.6% of the total 3.2 billion base pairs)
How many of the different base pairs in individuals might affect how our genes function?
5% (1 million)
What determines how some genetic differences affect us?
Environment
What causes alkaptonuria?
A lack of the enzyme needed to complete the final step of breaking down phenylalanine
What is alkaptonuria?
Black urine caused by a genetic recessive defect meaning there is an accumulation of protein 4
What happens if there is a defect in the first enzyme?
It leads to an accumulation of phenylalanine and to phenylketonuria (PKU)
How many people does phenylketonuria affect?
1 in 100,000
What happens if there is a defect in the second enzyme?
It leads to tyrosinemia Type 2
How may people does tyrosinemia type 2 affect?
1 in 250,000
What happens if there is a defect in enzyme 3?
It leads to tyrosinemea type 3
How many people does tyrosinemai type 3 affect?
Very rare
What happens if there is a defect in enzyme 4?
It leads to alkaptonuria
How many people does alkaptonuria affect?
1 in 250,000
What can PKU lead to?
Intellectual disability, seizures, behavioural problems and mental disorders because biochemistry is complex networks
What does increased phenylalanine cause?
An increase/build up of phenylpyruvate and phenylethylamine which could be toxic to the brain
What can lack of tyrosine cause?
A decrease in neurotransmitters (dopamine, adrenaline)
What can phenotype be affected by?
Environment and genetics
How can environment affect PKU?
A low phenylalanine diet prevents intellectual disability and associated problems in PKU individuals
Where is phenylalanine transported?
Across the blood-brain barrier
What does genetic variation mean (PKU)?
The efficiency of the transporter differs between individuals
What do individuals with less efficient transporters have?
Lower levels of phenylalanine in the brain and less damage (higher IQ)
