8.1+ 8.2- genetic information and gene pools

Question 1

independent (random) assortment

Answer 1

-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

Question 2

crossing over (recombination)

Answer 2

• 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

Question 3

random fertilisation as a source of genetic variation

Answer 3

-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

Question 4

genotype

Answer 4

-genetic make up of an organism with respect to a particular feature

Question 5

phenotype

Answer 5

-the physical traits expressed as a result of the interactions of the
genotype with the environment

Question 6

homozygote

Answer 6

-individual where both of the alleles coding for a particular characteristic are identical

Question 7

heterozygote

Answer 7

-where the two alleles coding for a particular characteristic are different

Question 8

dominance

Answer 8

-phenotype expressed whether the individual is homozygous for the characteristic or not

Question 9

recessive

Answer 9

-only expressed when both alleles code for the recessive feature aka. homozygous recessive

Question 10

codominance

Answer 10

-both alleles are expressed and the proteins they code for act together without mixing to produce a given phenotype

Question 11

monogenic (monohybrid) crosses

Answer 11

-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

Question 12

polygenic

Answer 12

-most traits are polygenic, determined by several interacting genes

Question 13

mendels two laws of inheritance

Answer 13

1. law of segregation- one allele is inherited from each parent
2. law of independent assortment- different traits are inherited independently from each other

Question 14

multiple alleles

Answer 14

-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

Question 15

blood groups as an example of codominance

Answer 15

-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

Question 16

what is autosomal linkage

Answer 16

-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

Question 17

fruit flies (Drosophila melanogaster) and autosomal linkage

Answer 17

-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)

Question 18

what is a dihybrid cross?

Answer 18

-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

Question 19

dihybrid question- tips

Answer 19

-e.g. crossing 2 parent peas
-must label ;
-parental genotype
-parental phenotype
-possible gametes
-offspring genotype
-offspring phenotype
-proportion of each phenotype

Question 20

dihybrid inheritance example

Answer 20

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

Question 21

dihybrid ratio if parents are heterozygous for both genes

Answer 21

-will always be 9:3:3:1
-unless there is autosomal linkage

Question 22

what is gene linkage

Answer 22

• 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

Question 23

what is sex linkage

Answer 23

-genes carried on the X chromosome are said to be sex linked
-can linked to sex linked diseases
-can cause red green colour blindness

Question 24

haemohpilia- sex linked disease

Answer 24

• 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

Question 25

why do we use hardy weinberg?

Answer 25

-to calculate expected frequencies of genotypes

Question 26

hardy weinbergs 5 assumptions

Answer 26

1. large population
2. mating is random
3. no mutations occur
4. no immigration or emigration (isolation)
5. no natural selection taking place

Question 27

hardy weinberg equations

Answer 27

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

Question 28

disruptive selection

Answer 28

-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

Question 29

stabilising selection

Answer 29

-most common type of selection
-average survives
e.g. size of baby, too heavy or light have higher mortality rate

Question 30

directional selection

Answer 30

-one extreme trait is favoured
-e.g. giraffes with the longest necks

Question 31

two types of genetic drift

Answer 31

-population bottlenecks
-founder effect

Question 32

what is a population bottleneck

Answer 32

-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

Question 33

what is the founder effect

Answer 33

-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

Question 34

what is genetic drift

Answer 34

• 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