Inheritance

Define gene

Define gene

a sequence of DNA that codes for a polypeptide and which occupies a specific locus on a chromosome

Define allele

a variant nucleotide sequence for a particular gene at a given locus on a chromosome

Define dominant allele

allele that is expressed in homozygote or heterozygote → always expressed if present

Define recessive allele

allele that is only expressed in the homozygote

Define locus

the specific site on a chromosome occupied by a gene

Define phenotype

physical appearance or characteristic

Define genotype

the alleles contained in an individual

Define monohybrid

the inheritance of a single gene

Define homozygous

genotype with 2 alleles the same for a particular gene

Define heterozygous

individual with 2 different alleles for a particular gene

Define codominance

when both alleles contribute to the phenotype

Define F1

First filial generation → offspring from 1st cross in question

Define F2

Second filial generation → offspring from 2nd cross in question

Define autosome

Pairs of chromosomes that carry identical genes but can have different alleles

Define sex chromosome

pair of chromosomes of different lengths that carry different genes and control gender

Define linkage

Description of genes that are on the same chromosome and therefore do not separate independently at meiosis

Define carrier

a phenotypically normal female with one normal, dominant allele and one mutant recessive allele

Define sex linkage

a gene carries by a sex chromosome so that a characteristic it encodes is seen predominantly in one sex

Define mutation

a change in the amount, arrangement or structure of DNA (or RNA)

Define gene (point) mutation

a change in the nucleotide sequence in DNA

Define chromosome mutation

a change in the chromosome structure or number

Define mutagen

an environmental factor that increases the mutation rate

Define somatic cells

body cells, non-reproductive cells (not gametes)

Define S phase

Stage of cell cycle when DNA is replicated → therefore mutation may occur here

Define carcinogen

an agent/factor that causes cancer

Define euploidy

cells with complete sets of chromosomes

Define aneuploidy

cells with too few or too many chromosomes

Define trisomy

the presence of an additional copy of one chromosome

Define polyploidy

Having more than 2 complete sets of chromosomes

Define silent mutation

a point mutation that alters the bases of mRNA code but does not alter the sequence of amino acids in a polypeptide

Define disjunction

the separation/segregation of homologous chromosomes during Anaphase I in meiosis

Define non-disjunction

a faulty cell division in meiosis following which one of the daughter cells receives 2 copies of a chromosome and the other receives none → causes trisomy or triploidy

Define proto-oncogene

a DNA sequence that codes for a protein that contributes to cell division

Define oncogene

a mutated proto-oncogene that leads to uncontrolled mitosis and results in cancer

Define tumour suppressor gene

a DNA sequence that regulates the rate of mitosis → prevents rapid cell division

What was Gregor Mendel’s experiment investigating and what did he use to show this? Why did he use this?

• investigated monohybrid inheritance in garden peas

• used peas because they were:

  • easy to grow

  • flowers self-fertilised and cross-fertilise

  • make flowers and fruit in the same year

  • makes large number of seeds from each cross → when phenotypes of next generation counted, numbers make them statistically meaningful

What is a test cross, back cross and why is it used?

• cross between an individual with the phenotype of the dominant characteristic, but unknown genotype, with an individual that is homozygous recessive for the gene in question

• used to determine whether an individual with a dominant phenotype is heterozygous or homozygous dominant

  • if homozygous dominant → all offspring will display dominant phenotype

  • if heterozygous → 50% will display dominant phenotype and 50% will display recessive phenotype

What is incomplete dominance? Give an example

• when neither of the alleles are completely dominant so an intermediate between the 2 alleles is expressed in the heterozygous phenotype

• e.g. red carnations + white carnations = red, white and pink carnations

What was Mendel’s first law of inheritance?

the characteristics of an organism are determined by factors (alleles) which occur in pairs. Only one pair is present in each gamete

What is dihybrid inheritance?

• the simultaneous inheritance of 2 unlinked genes (i.e. genes on different chromosomes)

• e.g. RrYy

What is Mendel’s 2nd law of inheritance and what is it based on?

• each member of a pair of alleles may combine randomly with either of another pair of alleles on a different chromosome

• based on dihybrid ratio

What is the dihybrid phenotype ratio?

9:3:3:1

How is dihybrid inheritance tested for?

cross the genotype being tested with an individual that’s homozygous recessive for both genes

What are recombinant and parental genotypes? Use parents with DE and de chromosomes to explain

• recombinant: when crossing over of chromosomes causes genes that were previously linked to be separated (e.g. De and dE)

• parental: when the genotypes produced are the same as the parental genotypes (DE and de)

Why are recombinant genotypes less likely to be seen than parental?

crossing over is a rare event → recombinant types only seen when crossing over occurs

What causes an increase in recombinant genotypes and why?

the distance between the 2 genes on a chromosome → the further apart 2 genes are, the more opportunity for crossing over to occur

What would be assumed if ratios of genotypes in offspring are not Mendelian?

genes are linked

What is the monohybrid phenotype ratio?

3:1

How many chromosomes do humans have?

46

Which gamete is homogametic sex cell and what does this mean?

• female gamete

• gametes are identical with respect to the sex chromosomes

Which gamete is heterogametic sex cell and what does this mean?

• male gamete

• gametes are of different types with respect to the sex chromosomes → sperm could have contain an X or a Y chromsome

Why are carriers of sex linked diseases always females?

• females have 2 X chromosomes and therefore can be heterozygous with the recessive gene not being expressed

• males only have 1 X chromosome so whichever gene is present will be expressed whether recessive or dominant

What are the characteristics of mutations?

• spontaneous → may happen without apparent cause

• random → happen with equal probability anywhere in the genome of diploid organisms

Which factors can increase mutation rates?

• ionising radiation → gamma rays, X-rays and UV light

  • radiation joins adjacent pyramidine bases in a DNA strand → during replication, DNA polymerase may insert incorrect nucleotide

• mutagenic chemicals → polycyclic hydrocarbons (cigarette smoke), methanal (formaldehyde) and mustard gas

  • some chemicals have flat molecules → slide between base pairs in double helix

  • prevent DNA polymerase inserting correct nucleotide

What are the different kinds of point mutations?

• addition → base added. if occurs in 3 places, extra amino acid added

• duplication → same base incorporated twice

• subtraction → base is deleted. if occurs in 3 places, 1 less amino acid when translated

• inversion → adjacent bases on same DNA exchange

What are the ways in which a point mutation might affect the polypeptide produced?

• new codon may code for same amino acid → silent mutation

• if amino acid has similar chemical nature as one substituted, effect may be small

• if mutation is at significant site, may make significant difference to activity of protein

  • if protein were an enzyme → active site could be destroyed

What causes sickle cell anaemia?

• substitution point mutation in gene that produces beta polypeptide of haemoglobin

  • CTC (codes for glutamate) becomes CAC (codes for valine)

  • glutamate side chain is large and hydrophilic whereas valine side chain is small and hydrophobic → when oxygen tension low, affected haemoglobin in red blood cell aggregate

  • cell membrane collapses → red blood cell becomes sickle shaped

What are the symbols used to represent normal haemoglobin and sickle cell haemoglobin?

• HbA (allele: Hbᴬ) → normal

• HbS (allele: Hbˢ) → sickle cell

What type of inheritance is used in sickle cell anaemia? What does this mean?

• co-dominance

• when both Hbᴬ and Hbˢ present, both HbA and HbS haemoglobin produced

What causes down’s syndrome?

• individual with 3 copies of chromosome 21 → trisomy 21

• 47 chromosomes in total

What is translocation down’s?

• when a fragment of chromosome 21 attaches to chromosome 14 in a gamete

• when fertilised, 2 normal copies of chromosome 21 with an additional one attached to chromosome 14 → have normal number of chromosomes (46)

Why might polyploidy occur?

• defect in spindle at meiosis → all chromosomes at anaphase I or chromatids at anaphase II move to same pole of cell

  • results in diploid gamete

  • when fertilised by normal gamete, a triploid zygote is formed → may survive but won’t be able to reproduce as can’t make homologous pairs at meiosis

• if 2 diploid gametes fuse → tetraploid (4n)

• endomitosis (replication not followed by cytokinesis) happens in early embryo → 4 sets of chromosomes produced and continue to produce tetraploid cells in mitosis

  • infertile triploids undergo endomitosis and make fertile hexaploids (fertile bc can make homologous pairs)

Why is polyploid more common in plants than in animals?

• can reproduce asexually

• hermaphrodite and don’t use chromosomes to determine sex

What are the possible reasons for mutation of a proto-oncogene to an oncogene?

• mutation causes chromosomes to rearrange and places proto-oncogene next to DNA sequence that permanently activates it

• there is an extra copy of proto-oncogene → results in too much of its product being made = excessive mitosis

Define epigenetics

the control of gene expression by modifying DNA or histones, but not by affecting the DNA nucleotide sequence

What is DNA methylation?

• addition of methyl or hydroxymethyl group to cytosine

• methylated cytosine can still pair with guanine at transcription

  • but if heavily methylated → less likely to be transcribed

What is histone modification?

• occurs after translation → acetyl group attaches to lysine (amino acid); methyl group attaches to lysine; arginine or phosphate group attaches to serine and threonine

• changes alter histone-DNA interaction

  • unmodified histone packs tightly → less available = reduced transcription

  • modified histone, relaxed coiling → RNA polymerase has more access to DNA = increased transcription

Identify the structures in this diagram and explain what has happened

• histone (blue cylinder) and nucleosome (coiled structure)

• histone modification → coiling more relaxed so transcribed genes previously unavailable

What are the consequences of epigenetic changes?

• genomic imprinting → gene permanently switched off due to methylation in parent may be passed on to next generation

• X inactivation → epigenetic changes can switch off whole chromosomes. female mammals only use one X chromosome

  • tortoiseshell cats show random inactivation of either X chromosome → alternative X chromosomes activated in adjacent groups of cells

• implicated in autoimmune conditions, mental illness, diabetes and cancers