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Genes: Genes are sequences of DNA that code for a particular protein. These are exons (expressed).

Features of the genetic code:

-Degenerate: More than one triplet codon can code for a given amino acid. This means that, for example, a substitution mutation could have no effect on the amino acid sequence.

-Non-overlapping: The code is read in triplets which are treated as separate entities.

-Universal: The fundamental rules of the genetic code are conserved across nature.

Evolution by natural selection: Natural selection describes how the individuals best suited to survive in a given environment will survive and reproduce, passing on those advantageous genes (or alleles- better) to their offspring. As a result of this the allelic frequency for advantageous traits increases, and certain characteristics become more common. Over time, this leads to larger scale changes and evolution.

Lac Operon: Understanding the vocabulary

Promoter: Where RNA pol binds to begin transcribing a certain gene.

Operator: Where regulatory elements/transcription factors bind to encourage or discourage transcription.

Transcription factor: A protein which binds to DNA upstream of the promoter. It helps bind RNA pol to begin transcription.

Repressor: A protein which binds to an operator sequence on DNA in order to discourage transcription.

Homeobox gene: A gene that codes for a transcription factor protein. They have key developmental roles.

Regulatory gene: A gene which codes for a repressor protein.

Monohybrid + dihybrid inheritance:

See GCSE inheritance for monohybrid inheritance

Dihybrid inheritance: When two different characteristics coded for by genes on different chromosomes are studied , and the relative ratio of the offspring's genotypes are estimated.

RG

Rg

rG

rg

RG

RRGG

RRGg

RrGG

RrGg

Rg

RRGg

RrGg

RRgg

Rrgg

rG

RrGG

RrGg

rrGG

rrGg

rg

RrGg

Rrgg

rrGg

rrgg

Phenotype ratio: 9:3:3:1

Parents heterozygous

CoDominance: This phenomenon describes when two phenotypes are expressed at the same time. This is common in feather colouring/fur colour and is when two different colours are expressed. White flowers and pink flowers will breed to give a codominant pink flower (one allele for red, one allele for white.

Epistasis: The interaction between genes which cause suppression of certain genes. Recessive epistasis describes how if an individual inherits 2 copies of a recessive allele, this genotype will cause the silencing of another gene at another locus. 

Dominant epistasis ratio: 12:3:1

Recessive epistasis ratio: 9:3:4

Learn these ratios for inheritance based questions!

Questions are often asked on mice fur colouring due to epistatic alleles causing gene silencing of primary fur colour.

Definitions:

-Allele: An alternative form of a gene.

-Dominant: One copy of the allele is required in order for the phenotype to be expressed.

-Recessive: Two copies (homozygous) of the allele are required for the phenotype to be expressed.

-Homozygous: Has two copies of the same allele.

-Heterozygous: Has two different alleles.

-Genotype: The combination of alleles e.g Gg

-Phenotype: The characteristic/impact of the genotype.

Lac Operon: A mechanism developed by E-Coli to allow them from using glucose as their primary respiratory substrate to using lactose as their primary substrate, when in a lactose (and glucose-free) environment.

Repressor

RNA Pol

X

L

P

O

Z

Y

A

Transcription

L

P

O

Z

Y

A

Lactose

Lac Operon: The lac operon uses repressors, and the inhibition of those same repressors in order to switch genes on/off. When lactose is present, it binds to the repressor and stops the repressor binding to the operator. Therefore RNA pol is not discouraged from binding to the promoter, and transcription occurs. The genes coded for include lactose hydrolysing enzymes as well as proteins which increase the permeability of the bacterium to lactose.

Meiosis and variation:

Crossing over: Chromosomes line up and pressure dynamics in the nucleus cause equivalent portions of each chromosome to break off and re anneal on the other chromosome. This means the offspring will have a combination of alleles.

Independent segregation of chromosomes: In meiosis I, the homologous chromosomes line up in pairs. Since only one of the homologous chromosomes will go into the daughter cell, i.e the division of genetic material is random

Random fertilisation of gametes: Usually only one egg is fertilised by only one sperm. However, many millions of sperm compete for one egg, and that egg is genetically different each month (in the case of humans)

Sex-Linkage: Since the 23rd pair of chromosomes dictate the sex of an individual, a human can either be XX (female) or XY (male). The Y chromosome is shortened and serves no real purpose. This means that if a male inherits a recessive copy of an allele, it is almost 100% likely that the phenotype will be displayed despite only one copy of the allele being present. 

X

B

Y

X

B

X

X

B

B

X

B

Y

X

b

X

B

X

b

X

b

Y

Sex linked diseases date back to the days of Rasputin, who once claimed he healed Alexei Nikolaevich back to health after suffering from the sex-linked disease.

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