8.1+ 8.2- genetic information and gene pools Flashcards
independent (random) assortment
-chromosome pairs from mother and father are distributed into gametes randomly
-0-23 chromosomes could coma from maternal or paternal
-results in many new combinations of alleles and introduces considerable genetic variation
crossing over (recombination)
- large multi enzyme complexes cut and join bits of the maternal and paternal chromatids together at chiasmata
-also a potential source of mutation which also introduces new combinations
random fertilisation as a source of genetic variation
-male and female gametes from two unrelated individuals fuse to form new genetic material
-introduces considerable genetic variation
-this is completely random
-several ova mature in ovary each month, random which one reaches ovulation first
-combination of all sources of genetic variation ensure genetic variation
genotype
-genetic make up of an organism with respect to a particular feature
phenotype
-the physical traits expressed as a result of the interactions of the
genotype with the environment
homozygote
-individual where both of the alleles coding for a particular characteristic are identical
heterozygote
-where the two alleles coding for a particular characteristic are different
dominance
-phenotype expressed whether the individual is homozygous for the characteristic or not
recessive
-only expressed when both alleles code for the recessive feature aka. homozygous recessive
codominance
-both alleles are expressed and the proteins they code for act together without mixing to produce a given phenotype
monogenic (monohybrid) crosses
-when one gene is considered at a time in a genetic cross
-use a punnet square with one parent on top and one along side
e.g. 2 heterozygous parents plants- Rr and Rr (round seed phenotypes) produce 3 round and one wrinkles
-show genotype and phenotype
polygenic
-most traits are polygenic, determined by several interacting genes
mendels two laws of inheritance
- law of segregation- one allele is inherited from each parent
- law of independent assortment- different traits are inherited independently from each other
multiple alleles
-Although individuals can only possess two alleles for a
given gene, some genes have more than two alleles
(multiple alleles) in a population.
-inherited in a normal way but there are more than 2 possible phenotypes
blood groups as an example of codominance
-expressed as iO iB and iA
-iO is recessive so someone with iOiB or iOiA will have blood group A or B, must have iOiO to be group O
-however, iA and iB are codominant
-this means someone with iAiB will be group AB, they have both antigens on the surface of their erythrocytes
-both alleles are expressed and produce their proteins without mixing
what is autosomal linkage
-when 2 or more genes that are being inherited are located on the same autosome (non sex chromosome)
-giveaway in exams is when it says ‘genes are on the same chromosomes’ or that they are linked
-key is to identify allele pairs in parent gametes
-There is rarely crossing over with genes on the same
chromosome, so when a parent is heterozygous assume the dominant alleles are inherited together and the recessive alleles are inherited together.
-use the foil method to find all possible combinations of alleles that could be inherited
fruit flies (Drosophila melanogaster) and autosomal linkage
-genes for body colour and wing shape are on same chromosome
-grey body is dominant over black
-normal wing shape is dominant over vestigial (small) shape
-parents heterozygous for each so both are (GN,gn)
what is a dihybrid cross?
-a cross where the inheritance of two genes is considered at the same time
-links to mendel and his peas, 2 characteristics were round or wrinkled and green or yellow
dihybrid question- tips
-e.g. crossing 2 parent peas
-must label ;
-parental genotype
-parental phenotype
-possible gametes
-offspring genotype
-offspring phenotype
-proportion of each phenotype
dihybrid inheritance example
e.g. crossing round yellow and green wrinkled
-parent g= RRYY + rryy
-possible gametes= RY + ry
-offspring g= RrYy
-offspring p= all round + yellow
-if parents are heterozygous, use foil, should have 4 possible gametes for each
-use these potential gametes to create punnet square and determine offspring
-say ratio of each at end
dihybrid ratio if parents are heterozygous for both genes
-will always be 9:3:3:1
-unless there is autosomal linkage
what is gene linkage
- when genes for 2 characteristics are found on the same chromosome and are close together so they are linked and inherited as a single unit
what is sex linkage
-genes carried on the X chromosome are said to be sex linked
-can linked to sex linked diseases
-can cause red green colour blindness
haemohpilia- sex linked disease
- haemophilia (protein needed for clotting of blood is missing) genes involved in blood clotting cascade are often on X chromosome, so it is sex linked
-male only needs one haemophilic allele to have haemophilia, as only has one X chromosome, female with one is only a carrier
why do we use hardy weinberg?
-to calculate expected frequencies of genotypes
hardy weinbergs 5 assumptions
- large population
- mating is random
- no mutations occur
- no immigration or emigration (isolation)
- no natural selection taking place
hardy weinberg equations
p= frequency of dominant allele
q= frequency of recessive allele
p2= proportion of individuals that
are homozygous dominant (AA) 2pq= proportion of individuals that are heterozygous (Aa)
q2= proportion of individuals that are homozygous recessive (aa)
p+q=1 and p2+ 2pq+ q2=1
disruptive selection
-opposite extreme traits are favoured
-average trait is eliminated
-e.g. in environment with black and white rocks, black and white bunnies can camoflauge from predators but grey die
stabilising selection
-most common type of selection
-average survives
e.g. size of baby, too heavy or light have higher mortality rate
directional selection
-one extreme trait is favoured
-e.g. giraffes with the longest necks
two types of genetic drift
-population bottlenecks
-founder effect
what is a population bottleneck
-when a populations size becomes very small very quickly
-usually due to catastrophic environmental event, hunting or habitat destruction
-makes the surviving population have much less genetic diversity in most cases
-gene pool shrinks and allele frequency also changes dramatically
-remaining population is vulnerable to complete loss of some alleles, may even become so genetically different that it becomes a new species
what is the founder effect
-loss of genetic variation when a small number of individuals leave the main population and set up a seperate new population, producing a voluntary population bottleneck
-unusual genes may become amplified
what is genetic drift
- random changes in the gene pool of a population that occur by chance, not because they confer any advantage or disadvantage to offspring
-as a result, gene frequencies will sometimes increase or decrease over time
-in small populations this can have a major effect as uncommon alleles can be lost, decreasing variation
