IB Biology HL - D3.2

alleles

different versions of the same gene

genotype

combination of alleles

phenotype

physical traits

expression may occur due to environmental factors or genetics

phenotype plasticity

changing of traits to adapt to organism’s environment by varying gene expression

eg. tanning (increase of melanin production)

recessive allele

only expressed in homozygous genotypes

result of DNA mutation

dominant alleles

expressed in phenotype regardless if homozygous or heterozygous

monohybrid cross

breeding experiment that observes phenotypes of one trait

dihybrid cross

breeding experiment that observes phenotype of two traits

carrier

heterozygous organism with recessive allele (unaffected)

PKU

phenylketonuria

recessive genetic disease for phenylalanine hydroxylase

phenylalanine cannot convert into tyrosine

low tyrosine = low melanin

incomplete dominance

expression of two different alleles to produce intermediate phenotype

eg. pink 4 oclock flowers

codominance

expression of two different alleles (same gene) resulting in two distinguishable phenotypes

eg. ABO blood type

AB type blood

both alleles for type a and b blood are (co)dominant

i allele (O type blood) is recessive

their blood have both a and b antigens

gene pool

all genes of individuals in population that use sexual reproduction

single nucleotide polymorphism

SNP

single base substitutions that often have no effect on phenotype

segregation

separation of two alleles of the same gene into different gametes

occurs in anaphase I

recombinant (F2)

offspring that have a different combination of traits then the parental generation

independent assortment

alleles for different genes segregate into gametes independently

inheriting one trait does not influence the inheritance of another

autosomal chromosomes (autosomes)

contains genes that do not determine biological sex

sex chromosomes

contains genes that determine biological sex

human genome details

around 20000 genes

22 autosomes (180000 genes), 2 sex chromosomes (1600 genes)

gene linkage

genes on same chromosome do not assort independently

occurs most often on autosomes

recombinants (linked genes)

only occurs from crossing over of non-sister chromatids during prophase I

recombinants (unlinked genes)

results from random orientation of bivalents that line up in metaphase I

variation

defining feature of life
two types: discrete and continuous

continuous variable

no distinct categories

trait influenced by many genes

environmental factors may influence trait

ex. body mass, skin colour

discrete variable

separate categories with no intermediates between

trait influenced by one or few genes

environmental factors generally don’t influence trait

ex. ABO blood groups, numbers of eggs laid by bird