Pearson Baccalaureate Biology Standard Level 2nd Edition Print and Ebook Bundle for the IB Diploma
Complete summaries and study guides on the chapters of Pearson Baccalaureate's Biology Standard Level IB book. Detailed summaries of up to 10 pages with illustrations. These documents helped me get a good 6 in the IB exams of May 2019 :)
--> This document is on Chapter 3: Genetics
Terminology
Genotype: symbolic representation of the pair of alleles, Ex: Aa, GG
Phenotype: characteristics/traits of an organism; dependent on genotype
Dominant allele: always expressed in the phenotype even when paired with a different allele, capital letter
Recessive allele: only expressed when in a homozygous state (aa), usually masked by dominant allele
Homozygous: two identical alleles of a gene
Heterozygous: two different alleles of a gene → paternal gene different than maternal one
Co-dominant alleles: pair of alleles that are both expressed in phenotype (ex. ABO blood system; Ia & Ib)
Karyogram: an image showing the homologous pairs of increasing length (karyotype)
Autosomal: first 22 chromosome pairs on the Karyogram; do NOT define the gender
Sex chromosomes: last pair of chromosomes on the Karyogram: define the gender
Karyotype: number and appearance of the chromosomes in one’s cells
Genome size: total length of DNA in an organism’s nucleus
Genome: whole genetic information of an organism (30’600 in humans)
Sister chromatids: two identical DNA molecules formed during DNA replication. Only after splitting of
the centromeres are they renamed chromosomes.
Homologous chromosomes: 22 matching pairs of chromosomes on karyotype, which pair up with each
other during meiosis. Parental and maternal. Same genes on the same locus with different allele.
Gene: heritable factor consisting of a length of DNA. It is a chemical message (“make this protein”) and
influences a specific characteristic. ~21′ 000 𝑔𝑒𝑛𝑒𝑠 in human genome
Alleles: Different versions of gene (A/a; B/b); differ from each other by one or a few bases.
Mutation: random, rare change in genetic material. Often involves a change in the sequence of bases in
DNA. → altering of mRNA during transcription = different protein
Gametes: sex cells (sperm/egg)
Somatic cells: ordinary body cells
Haploid cells: cells with half of the genetic material (n=23); egg cells and sperms, produced during
meiosis; only one allele
Diploid cells: produced during mitosis; 23 PAIRS of chromosomes (2n=46) maternal + paternal one
Monohybrid inheritance: inheritance of ONE trait (e.g. height)→ looking at one factor at a time Dihybrid
inheritance: simultaneous inheritance of two or more factors → looking at more than one factor
Gene locus: location of gene (allele) on chromosome
Carrier: Lifeform carrying a recessive allele in their genotype which isn’t expressed in the phenotype
Pure breeding: Both parents are “breed true”; parents have homozygous alleles → offspring’s phenotype
same as parent
Co-dominance: Pair of dominant alleles. Both alleles are equally expressed in the phenotype
Sex-linked disease: faulty allele is found on the 23th pair of chromosome
Autosomal disease: faulty allele(s) is found on one of the first 22
homologous pairs of chromosomes
Chorionic villus Sampling: prenatal diagnosis, sampling placental tissue and
testing it for any chromosomal abnormalities of the Karyotype.
Amniocentesis: prenatal diagnosis, sampling of amniotic fluid containing
fetal tissue and testing it for any chromosomal abnormalities of the
Karyotype
Genes
A heritable factor consisting of a length of DNA. It is a chemical message
(“make this protein”) and influences a specific characteristic.
~𝟐𝟏′ 𝟎𝟎𝟎 𝒈𝒆𝒏𝒆𝒔 in human genome. Genes occupy their
corresponding placed, called locus, on a chromosome. Transducin gene:
controlling protein called transducing which enables colour vision. Mutation of the gene stops a person
from being able to make the protein properly and thus the information about colour is not transmitted
from eye to the brain. = achromatopsia → mutation of gene on chromosome 1
, - We possess two copies of each gene in our bodies. One from our mother and one from our
father. They are located at the same locus but simply on the other copy of your chromosome pair.
→ these two genes do not necessarily have to be identical as genes come in many forms.
Alleles:
Variations of genes differing from each other by one or a few bases. A single base pair difference in gene
= mutated allele which can have a drastic impact.
→ e.g. Colour vision (allele to produce transducin); skin pigmentation (allele to make pigments); blood
type (allele for type A, B etc.)
Base substitution mutation:
Change/alteration in the base sequence (genetic code) of a gene = mutated allele = mRNA is altered
during DNA transcription = different amino acid is produces = changes composition/structure of
resulting protein; beneficial mutations (immunity to HIV), detrimental mutations and neutral
mutations (change in base sequence still codes for same AA), which do not affect the organism
- HIV immunity:
LRP5 is a gene that helps the immune system make a specific type of protein acting as a receptor on their
surfaces. This receptor is used by the human immunodeficiency virus to infect the cells. Mutated LRP5
gene = no receptor = no HIV infection.
→ people with mutated allele of LRP5 are naturally immune to HIV.
- Sickle cell disease:
Base substitution mutation in the section of the haemoglobin gene’s DNA = sixth codon in sequence
GAG → GTG ; mRNA is altered during DNA transcription and as a result the amino acid valine is added
instead of glutamic acid. → different shape to the haemoglobin molecule → mutated red blood cells with
a sickle shape = oxygen cannot be carried as efficiently within the red blood cells + blood flow can be
slowed or blocked. (higher resistance to malaria)
- Cystic fibrosis:
Gene called CFTR found on chromosome 7 plays a key role in the production of mucus, a fluid essential
for our health. The standard allele allows mucus-producing cells to function properly. A mutation of the
gene = faulty allele = causes cystic fibrosis = over-production of mucus = difficulty with respiratory and
digestive system. → autosomal recessive disease
Revolution of genes:
Genes/production of genes can adapt over time. Originally, humans only drank milk when they were
infants and when they got older their bodies stopped digesting milk (lactose). Nowadays, we (our gene
coding for lactase enzyme) have adapted to our agricultural-bases lifestyle. → more people show the
genetic code allowing humans to digest lactose.
Gene therapy:
Process of taking a beneficial gene from a person possessing it and putting it into a person who does not
have it and needs it to be healthy. → tricky procedure (using virus = risky)
Different types of genetic conditions:
Condition Example
Chromosomal anomaly Caused by an extra chromosome e.g. Down
Syndrome
Autosomal genetic disease Caused by a gene found on one of the 22 non-sex
chromosomes e.g. cystic fibrosis (1st
chromosome)
Sex-linked genetic disease Caused by a gene found on the X or Y
chromosome of the 23rd pair of chromosomes e.g.
colour blindness on X chromosome
Sequencing DNA: e.g. Sager technique
In order to find out what gene does what, a list must be made showing the order of all nucleotides in the
DNA code. = genome. Sequence consists of codons (three nucleotides).
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