inheritance and evolution Flashcards
what is a test cross used for
to determine the genotype of an organism with a dominant phenotype
basically using a punnet square to find ratio of a certain phenotype
non sex and sex chromosomes
22 pairs of non sex chromosomes - known as autosomes
1 pair sex chromosomes either XX or XY
females= XX
males= XY
key notes on sex linkage
- 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
why are recessive sex linked traits more likely to occur in males
- 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)
pedigree charts tips ( the chart with squares and circles)
- give numbers
- refer to phenotype/trait
dihybrid inheritance
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
why are diff types of gametes produced
independent segregation of homologous chromosomes during meiosis
2 genes on diff chromosomes
what is the expected ratio if there are 4x4 diff gametes in a punnet square
9:3:3:1
why might the observed and expected ratios not be similar
- small sample size- low n.o offspring, sampling error greater
- random fusion of gametes at fertilisation
- epistasis
statistical tests
- chi squared
- correlation coefficient
- T test
what is the chi squared test used for
- to compare the difference between the expected and observed ratios from results of a genetic cross
-for categoric data e.g hair/eye colour
chi squared equation
x²= ∑(o-e)² divided by e
o = observed frequencies
e= expected
what is correlation coefficient used for
if theres a correlation between 2 variables
what is T test used for
comparing means
null hypothesis in genetics
there is no significant difference between the phenotype ratio expected of….. and the one observed
any change from expected ratio is due to chance
what is the unit to look for in the charts for chi squared
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
autosomal linked genes
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
autosomal linked genes example for ur understanding
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
expected ratio for linked and unlinked genes
unlinked = 9:3:3:1
linked : 3:1
expected ratio assumes no crossing over/recombination occurred
epistasis
2+ genes interact to contribute to a phenotype
non-linked genes where one masks / inhibits the expression of the other
species
exist as one or more populations
can breed tg to produce fertile offspring
population
group of organisms of same species occupying a particular space at a particular time that can potentially interbreed
gene pool
all the alleles of all the genes of all individuals in a population
allelic frequency
number of times an allele of a particular gene occurs within the gene pool
why is Hardy weinberg principle useful
- can determine frequencies of carriers
- can identify evolution
factors causing allele frequencies to not change
- population large and isolated ( no flow of alleles in or out pop)
-mating in populations random
-no mutations of gene
-no selection
Hardy weinberg principle
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
genetic factors causing genetic variation/diversity
- gene mutations
-crossing over - independent segregation
-random fertilisation of gametes
characteristics mainly influenced by genetic factors
- controlled by 1-2 genes
- expressed as distinct phenotypes w no intermediates e.g tongue roller
-represented as distinct groups
characteristics mainly influenced by environment
- controlled by many genes
-no separate categories but have range of intermediates
-produce normal distribution curve on graph
stabilising selection
environment not changing
select for mean within population
organisms w extremes of the range are less likely to survive
e.g fur thickness
directional selection
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
disruptive selection
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
speciation
evolution of new species from exisiting ones
general speciation mechanism
- 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
types of speciation
allopatric
sympatric
allopatric speciation
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
sympatric speciation
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
genetic drift and speciation
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
How many alleles of a gene can be found in diploid organisms?
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
Explain the evidence from a pedigree diagram which would show that the allele for [named phenotype] is dominant
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]
Explain the evidence from a pedigree diagram which would show that the allele for [named phenotype] is recessive
Give numbers
● Parents WITHOUT [named phenotype] have offspring WITH [named phenotype]
● So both parents [n & n] must be heterozygous / carriers of recessive allele
Explain the evidence from a pedigree diagram which would show that the allele for [named phenotype] on the X-chromosome is recessive
sex linked
● Mother [n] WITHOUT [named phenotype] has child [n] WITH [named phenotype]
● So mother [n] must be heterozygous / carrier of recessive allele
Explain the evidence from a pedigree diagram which would suggest that [named recessive phenotype] is caused by a gene on the X chromosome
Only males tend to have [named recessive phenotype]
Explain the evidence from a pedigree diagram which would show that the gene for [named phenotype] is not on the X chromosome/ sex-linked
● 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.
to find degrees of freedom
number of categories - 1
