inheritance and evolution

Question 1

what is a test cross used for

Answer 1

to determine the genotype of an organism with a dominant phenotype

basically using a punnet square to find ratio of a certain phenotype

Question 2

non sex and sex chromosomes

Answer 2

22 pairs of non sex chromosomes - known as autosomes

1 pair sex chromosomes either XX or XY

females= XX
males= XY

Question 3

key notes on sex linkage

Answer 3

• man CANT pass sex linked trait onto son but can pass it onto daughter
• usually X linked
• only mothers can pass on X chromosome to sons

Question 4

why are recessive sex linked traits more likely to occur in males

Answer 4

• no other allele on Y chromosome
  -so the recessive allele is always expressed
  -only require 1 recessive allele to be expressed whereas females require 2
  (homozygous recessive)

Question 5

pedigree charts tips ( the chart with squares and circles)

Answer 5

• give numbers
• refer to phenotype/trait

Question 6

dihybrid inheritance

Answer 6

inheritance of 2 diff characteristics

each characteristic controlled by diff gene

genes are on separate/diff pairs of homologous chromosomes ( so alleles or diff characteristics can combine e.g seed colour and seed shape to produce a gamete )

use FOIL method to find gametes as there are 4 alleles in genotype

Question 7

why are diff types of gametes produced

Answer 7

independent segregation of homologous chromosomes during meiosis

2 genes on diff chromosomes

Question 8

what is the expected ratio if there are 4x4 diff gametes in a punnet square

Answer 8

9:3:3:1

Question 9

why might the observed and expected ratios not be similar

Answer 9

• small sample size- low n.o offspring, sampling error greater
• random fusion of gametes at fertilisation
• epistasis

Question 10

statistical tests

Answer 10

• chi squared
• correlation coefficient
• T test

Question 11

what is the chi squared test used for

Answer 11

• to compare the difference between the expected and observed ratios from results of a genetic cross

-for categoric data e.g hair/eye colour

Question 12

chi squared equation

Answer 12

x²= ∑(o-e)² divided by e

o = observed frequencies

e= expected

Question 13

what is correlation coefficient used for

Answer 13

if theres a correlation between 2 variables

Question 14

what is T test used for

Answer 14

comparing means

Question 15

null hypothesis in genetics

Answer 15

there is no significant difference between the phenotype ratio expected of….. and the one observed

any change from expected ratio is due to chance

Question 16

what is the unit to look for in the charts for chi squared

Answer 16

0.05

p-values (probability) less than 0.05 = significant = the probability that its due to chance is less than 0.05- reject null hypothesis

if value greater than 0.05 column: not significant + accept null hypothesis - higher probability that its due to chance

in some tables given:
1. find degrees of freedom
2. look in 0.05 column
3. compare value given to value in table
4. if value is higher, what is its

Question 17

autosomal linked genes

Answer 17

22 pairs of non sex chromosomes

2+ genes present on same chromosome at diff loci

usually inherited together- so fewer genetic combos of alleles in gametes as no independent segregation

leading to reduced variety of gametes and offspring

Question 18

autosomal linked genes example for ur understanding

Answer 18

e.g AaBb - if the 2 genes were linked, only 2 gamete would be produced : AB and ab

due to 1 chromosome having both gene A and gene B on them

and another having both gene a and gene b on them

if not linked then 4 diff types of gamete would be produced:
- one homologous pair would have A and a in it
- the other would have B and b in it
- so 4 diff types of gametes produced

Question 19

expected ratio for linked and unlinked genes

Answer 19

unlinked = 9:3:3:1
linked : 3:1

expected ratio assumes no crossing over/recombination occurred

Question 20

epistasis

Answer 20

2+ genes interact to contribute to a phenotype

non-linked genes where one masks / inhibits the expression of the other

Question 21

species

Answer 21

exist as one or more populations
can breed tg to produce fertile offspring

Question 22

population

Answer 22

group of organisms of same species occupying a particular space at a particular time that can potentially interbreed

Question 23

gene pool

Answer 23

all the alleles of all the genes of all individuals in a population

Question 24

allelic frequency

Answer 24

number of times an allele of a particular gene occurs within the gene pool

Question 25

why is Hardy weinberg principle useful

Answer 25

- can determine frequencies of carriers - can identify evolution

Question 26

factors causing allele frequencies to not change

Answer 26

- population large and isolated ( no flow of alleles in or out pop) -mating in populations random -no mutations of gene -no selection

Question 27

Hardy weinberg principle

Answer 27

remember to always multiply by population size or divide by population size FREQUENCY OF ALLELES: ( have allele but not genotype) p+q=1 p= frequency of DOM allele q= frequency of RECESSIVE allele FREQUENCY OF GENOTYPES: p² + 2pq + q² = 1.0 p² = frequency of homozygous dominant genotype e.g AA q² = frequency of homozygous recessive genotype e.g aa 2pq = frequency of heterozygous genotype e.g Aa

Question 28

genetic factors causing genetic variation/diversity

Answer 28

- gene mutations -crossing over - independent segregation -random fertilisation of gametes

Question 29

characteristics mainly influenced by genetic factors

Answer 29

- controlled by 1-2 genes - expressed as distinct phenotypes w no intermediates e.g tongue roller -represented as distinct groups

Question 30

characteristics mainly influenced by environment

Answer 30

- controlled by many genes -no separate categories but have range of intermediates -produce normal distribution curve on graph

Question 31

stabilising selection

Answer 31

environment not changing select for mean within population organisms w extremes of the range are less likely to survive e.g fur thickness

Question 32

directional selection

Answer 32

when environment changing selects for alleles for phenotype towards the extreme of a range more likely to survive and reproduce change in range of phenotypes

Question 33

disruptive selection

Answer 33

opposite of stabilising dont want mean average same frequency of phenotypes selects phenotypes at the 2 extremes at the expense of the intermediate phenotypes e.g favours extremes of ranges at random points

Question 34

speciation

Answer 34

evolution of new species from exisiting ones

Question 35

general speciation mechanism

Answer 35

- evolution occurs as a result of change in allele frequencies in population -members of same species are reproductively isolated from other species -new species arise when genetic differences due to gene pools, lead to inability of members of population to interbreed and produce fertile offspring - new species arise from existing species

Question 36

types of speciation

Answer 36

allopatric sympatric

Question 37

allopatric speciation

Answer 37

new species form from DIFF populations in diff areas 1) variation in population already present due to mutations 2) geographical isolation causes split of groups e.g formation of an island, preventing breeding between 2 pops causing reproductive isolation (cant breed to produce fertile offspring) 3) no gene flow between gene pools (of separated populations) 4) diff selection pressures (e.g predators/food) will occur in diff environments causing a particular different phenotypes to be selected for 5) organisms with selected / beneficial phenotype/allele are more likely to survive and reproduce 6) passing on advantageous allele 7) frequency of selected phenotypes and alleles in pop increases 8) allelic frequency in 2 separate gene pools will change over time due to mutation - diff species develop

Question 38

sympatric speciation

Answer 38

new species form from population living in SAME area 1) random mutations cause reproductive isolation as no reproduction between organisms of same species in same habitat 2) involves disruptive selection 3) no gene flow between gene pools 4) allelic frequencies change due to mutations ( diff alleles passed onto offspring) 5) allelic frequencies in separated groups become so different 6) new species arise

Question 39

genetic drift and speciation

Answer 39

1) GD leads to speciation 2) occurs by chance, not due to selection 3) allele of particular gene passed onto offspring more often than other alleles of same gene -by chance 4) frequency of this allele increases over generations 5) much greater effect in smaller pops, with small variety in gene pool 6) can lead to rapid speciation

Question 40

How many alleles of a gene can be found in diploid organisms?

Answer 40

2 as diploid organisms have 2 sets of chromosomes (chromosomes are found in homologous pairs) ○ But there may be many (more than 2) alleles of a single gene in a population

Question 41

Explain the evidence from a pedigree diagram which would show that the allele for [named phenotype] is dominant

Answer 41

Give numbers ● parents have [Named phenotype] but offspring WITHOUT [named phenotype] ● So both parents must be heterozygous / carriers of recessive allele ○ If it were recessive, all offspring would have [named phenotype]

Question 42

Explain the evidence from a pedigree diagram which would show that the allele for [named phenotype] is recessive

Answer 42

Give numbers ● Parents WITHOUT [named phenotype] have offspring WITH [named phenotype] ● So both parents [n & n] must be heterozygous / carriers of recessive allele

Question 43

Explain the evidence from a pedigree diagram which would show that the allele for [named phenotype] on the X-chromosome is recessive *sex linked*

Answer 43

● Mother [n] WITHOUT [named phenotype] has child [n] WITH [named phenotype] ● So mother [n] must be heterozygous / carrier of recessive allele

Question 44

Explain the evidence from a pedigree diagram which would suggest that [named recessive phenotype] is caused by a gene on the X chromosome

Answer 44

Only males tend to have [named recessive phenotype]

Question 45

Explain the evidence from a pedigree diagram which would show that the gene for [named phenotype] is not on the X chromosome/ sex-linked

Answer 45

● father with trait has daughter WITHOUT trait ● Father would pass on allele for trait on X chromosome so daughter would have [named phenotype] OR ● mother with trait has son WITHOUT [named phenotype] ● Mother would pass on allele for [named phenotype] on X chromosome so son would have [named phenotype] this assumes males are XY and females are XX, as in humans. There has been a question previously about birds where males are XX and females are XY. In this case, swap father for mother and son for daughter.

Question 46

to find degrees of freedom

Answer 46

number of categories - 1