16.1. Inheritance Flashcards
Darwinian inheritance…
Suggests some individuals in the population have advantageous alleles causing them to have select and reproductive advantages.
Mendelian inheritance…
Mendel crossed hundreds of pea plants and recorded the traits of 24,034 progeny.
He applied mathematics to propose the laws of Mendelian inheritance:
- Law of segregation.
- Law of independent assortment.
Law of segregation…
Phenotypic traits do not blend in the offspring and remain as two different phenotypes.
Genes can exist as variants (alleles). A gene with more than one allele is polymorphic.
Alleles are either dominant or recessive.
During production of gametes, the two copies of each gene segregate so that each gamete receives only one copy of the gene.
The zygote receives two copies, one from each parent.
Law of independent assortment…
The alleles of different genes are divided across the gametes independently of one another.
Parental traits appear in new combinations in some F2 progeny.
The law generally applies for genes that are on separate chromosomes but not genes on the same chromosome.
Codominance of blood groups…
The ABO gene encodes a glycosyltransferase:
- Enzymes encoded by IA allele add an N-acetylgalactosamine to the H antigen.
- Enzymes encoded by IB allele add a galactose to H antigen (4 amino acid difference).
- Enzymes encoded by I allele are inactive (due to frameshift mutation).
The presence or absence of the AB antigens on the red blood cell membrane is controlled by the FUT 1 gene with alleles (H and h).
FUT 1 gene encodes a fucosyltransferase:
- Enzymes coded by H alleles add a fucose to a precursor.
- Enzymes coded by h alleles are inactive.
Bombay phenotype…
People homozygous for the FUT 1 h allele cannot produce the H antigen regardless of ABO genotype.
Their blood does not express any of the ABH antigens on the cell surface.
These individuals have antibodies against A, B and H antigens, they can only receive blood from other people who have the Bombay phenotype.
Genetic variation…
All species show genetic variation and is essential for evolution.
Mutation: a heritable change in genetic information of a cell or organism.
Recombination: the rearrangement of genetic material. This happens during sexual reproduction.
These are the two processes that introduce variation.
Types of mutations…
Spontaneous mutations, caused by:
- Chemical reactions that alter nucleotides.
- Errors in DNA replication.
- Errors during meiosis.
Induced mutations, caused by:
- Damaging chemicals.
- Ionising / UV radiation.
- Other mutagens.
Point mutations…
A change in 1 or 2 nucleotides in DNA.
The effect of the mutation on the amino acid sequence of the protein determines the type of point mutation.
Synonymous (silent) point mutation:
- Where one of the bases is modified.
- This does not change the amino acid sequence, as DNA has a level of redundancy.
Missense point mutation:
- Where the base that is modified leads to a change in the amino acid sequence, changing the protein that is produced.
- For example D5V.
Nonsense point mutation:
- Mutation that results in a premature termination of translation due to a codon being changed into 1 or 3 stop codons.
- This results in a truncated protein.
Frameshift point mutation:
- Either an insertion or a deletion of one or two bases.
- This causes a frameshift as it alters the reading frame of the mRNA base sequence.
Chromosomal mutations…
Involve larger segments of DNA and often have more severe consequences.
They can cause multiple diseases simultaneously.
Can be caused by deletion, duplication, inversion, insertion and translocation.
Silent mutations…
The mutation does not alter the amino acid sequence of a protein.
Or, the mutation does cause a change in the amino acid sequence but has no effect on the protein function.
Loss-of-function mutations…
Results in a change in the amino acid sequence of a protein.
Frequently shows recessive inheritance, because the wild-type allele encoding for the normal protein is often sufficient to maintain a normal phenotype.
Gain-of-function mutations…
Results in a change in the amino acid that results in an abnormally functioning protein.
In most cases, this is detrimental.
Frequently shows dominant inheritance, because the wild-type encoding for the normal protein is often insufficient to prevent the abnormal activity of the mutant protein:
- RAS is a small GTPase that cycles between the active and inactive states.
- A mutation that affects its GTPase activity results in constitutive activation.
- Constitutively active RAS promotes survival and proliferation of tumour cells.
- This is commonly observed in cancer cases.
Harmful or beneficial mutations…
A monogenic disease with recessive inheritance, caused by a mutation of the haemoglobin gene.
The mutated form of haemoglobin distorts the red blood cells into a crescent shape at low oxygen.
Individuals carrying both copies of the mutated allele suffer from severe anemia.
However, heterozygotes (carriers) of this mutant gene are resistant to malaria - so it can have some benefits too.
