Component 2 - Inheritance Flashcards by Alex Ik

What is genetics?

The study of the mechanism by which an organism inherits characteristics from its parents

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What is monohybrid inheritance?

The inheritance of a SINGLE characteristic, controlled by one gene (each parent having two alleles for that gene)
e.g. inheritance of height in pea plants

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What is Mendel’s first law of segregation?

The characteristics of an organism determined by factors (alleles) which occur in pairs. Only one of a pair of factors can be present in a single gamete

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How do we tell if a dominant phenotype is heterozygous or homozygous?

May be crossed with the homozygous recessive

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What is pure breeding?

Refers to a variety bred for several generations with itself, so that all individuals are homozygous
All offspring will have the same phenotype as their parents when selfed

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What is co-dominance?

Both alleles express themselves equally in the heterozygote phenotype
The heterozygotes show a phenotype intermediate between those of the two homozygotes

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What are multiple alleles?

When there are more than two alleles for one particular gene

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What are sex linked genes?

Have their loci on the X chromosome
e.g. males in humans (XY), have only one copy of these genes, whereas females (XX) have two

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What are the 2 well known human sex-linked traits?

Red-green colour blindness
Haemophilia

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What is Duchenne’s muscular Dystrophy?

Sex linked recessive disorder
Causes progressive muscle weakness and degeneration
Affects about 1/5000 males at birth
Mutation on the X chromosome affects production of DYSTROPHIN

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What is dihybrid inheritance?

The inheritance of TWO genes located on two different chromosomes
Each gene will have two alleles

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What is Mendel’s second Law?

Law of independent assortment
Either one of a pair of contrasted characters may combine with either of another pair

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What is autosomal linkage?

This is a dihybrid cross where BOTH the genes are located close together on the SAME chromosome

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What is the outcome of dihybrid cross?

Assumes that all possible allele combinations are equally probable due to random assortment of homologous chromosomes

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What if 2 genes are close together on the same chromosome in autosomal linkage?

Likely to be more common in the parental combinations and are said to be linked

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Give some example of where characteristics are due to an interaction with the environment

Himalayan rabbit ears
Hydrangea flower colour

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Name some human characteristics that have both genetic and environmental influences

Heart disease
Hair colour
Proportions of twitch and tonic muscle

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What is epigenetics?

Involve heritable changes in gene function, without changes to the base sequence of DNA

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When does epigenetic control of gene expression take place

After DNA replication

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What is epigenetics caused by

Variation in the environment that alters gene expression

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What are the two ways in which transcription is inhibited?

Increased methylation of the DNA
Decreased acetylation of associated histones

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Where are histones and what are they

Found in the eukaryotic cells
They package the DNA and help it to condense into chromosomes

What are histones covered in and what does it do?

Covered in chemicals referred to as tags (accumulate throughout life)
Tags are the epigenome - reason why identical twins do not stay identical
Determine the shape of the histone - DNA complex

What is epigenetic silencing?

Where inactive genes can be wrapped tightly and become inaccessible - ensuring they are not transcribed

How are active genes different to inactive genes?

They are looser and more easily read and transcribed

What does more acetylation of histones result in

DNA less tightly wound around them More transcription

What is DNA methylation?

When a methyl groups is added to a cytosine nucleotide in DNA This is caused by enzymes where they can add or remove them

What is gene silencing and what processes are involved?

Over-methylation and de-acetylation The more methyl groups causes DNA to become more tightly wound around histones which inhibits transcription

What is under-methylation and what is it associated with?

Fewer methyl groups, less tightly wound, easier transcription Gene activation

How can epigenetics affect gene expression?

Different epigenetic changes can occur in cells of the same tissue and in different tissues This results in different expression of the same gene in different parts of the same organism

What is a mutation?

A change in DNA or amount of DNA, the structure or the arrangement of the bases Happens in somatic cells during mitosis

What must happen for a mutation to be inherited?

Must effect the DNA of the gametes

Are mutation rates high or low

Normally low Therefore less impact on evolution that other sources of variation

What do organisms with short lifecycles have in terms of mutation?

Higher rates of mutation Due to more frequent meiosis

What are mutagens?

Chemicals or radiation which increase the rate of mutation

Ionisation radiation examples:

Gamma-radiation UV X-rays

Chemicals which increase rate of mutation:

Polycyclic hydrocarbons

What is a mutagen that causes cancer?

Carcinogen

What are the two types of mutation?

Chromosome mutation Gene mutation

What is chromosome mutation?

Large changes in chromosome structure Affecting many genes and visible under light microscope in karyotype

What is the result of chromosome mutation?

Non-disjunction in meiosis e.g. Downs syndrome

What is gene mutation?

Small changes in the base sequence, only affecting a single gene No change visible in karyotype e.g. Sickle Cell Anaemia - a substitution of 1 base during interphase This DNA change will change the structure of the protein produces and the phenotype

When might there be a "silent" mutation?

When the code is degenerate

Describe what occurs to the DNA base sequence in Sickle cell anaemia

A single base substitution in the gene for Beta Haemoglobin with CTT replaced by CAT Causes the 6th amino acid Glutamic acid to be replace by Valine This change in the primary structure causes the secondary, tertiary and quaternary structure of the haemoglobin to change

When might there be a "neutral" mutation

If the amino acid altered is NOT essential to the tertiary/quaternary structure of the protein

What happens to the haemoglobin molecule after its protein structure has been changed?

Becomes insoluble at low oxygen tensions Tends to stick to other Hb molecules to form long fibres Fibres distort the blood cells, making them sickle shaped Mutated Hb is poor at binding to oxygen

Why are deletion and addition mutations much more severe?

They cause "frame shift" mutations All subsequent codons are altered Result in non-functional protein

What is cancer?

The uncontrolled division of cells and the production of a tumour In malignant tumours, cells maybe break off and be carried in the blood to form secondary tumours

What do tumour suppressor genes do?

Mutate so there is a lack of proteins which would halt cell division

What do proto-oncogenes do?

Mutate produces an oncogene which releases an excess of proteins to stimulate cell division or excess receptors foe these proteins

How are mutations important?

Increase variation in a population, but most mutations are harmful, rare that beneficial mutations arisw

Describe Down syndrome in terms of chromosomes

Non-disjunction of chromosome 21 during anaphase results in oocytes with an extra chromosome

Why are mutations so rare?

Most are recessive to the 'normal allele' and therefore must await replication in the gene pool Most gene mutations occur in the "junk DNA", which does not code for proteins

Describe Polyploidy in terms of chromosomes

One gamete receives both sets of chromosomes This gamete is then fertilised by a second gamete and the zygote produced has 3 sets of chromosomes