Genetics and populations YEAR 2 Flashcards
genotype
The genetic constitution of an organisms. Describes all the alleles that an organism has
phenotype
observable or biochemical characteristics of an organism. Result of the interaction between the expression of the genotype and the environment. The environment can alter an organisms phenotype
gene
a length of DNA, that is a sequence of nucleotide bases coding for a particular polypeptide.
allele
when genes exist in 2 or more forms. An allele is one of the different forms of a gene
Only one allele of a gene can occur at the locus of any one chromosome
locus
position of agene on a particular DNA molecule
homologous chromosomes
in diploid organisms chromosomes occur in pairs. There are therefore 2 loci that each carry one allele of a gene.
If the allele on each of the chromosomes if the same then the organisms is said to be homozygous for the character.
If the 2 alleles are different then the organisms is said to be heterozygous
homozygous dominant
when two dominant alleles are displayed.
Homozygous recessive is when 2 recessive alleles are displayed
codominant
when 2 alleles contribute to the phenotype. Both represented or blend together in the phenotype. Both alleles are equally dominant
Write alleles as subscript above the characteristic they are coding for
- BOTH ALLELES EXPRESSED IN THE PHENOTYPE
multiple alleles
more than two alleles for each gene
however only two may be present at the loci of an individuals homologous chromosomes
genetic crosses
Come up with parents genotypes and then cross them to show gamete combinations produces. Then cross gametes using a punnet square
monohybrid inheritance
inheritance of a single gene
law of segregation
in diploid organisms, characteristics are determined by alleles that occur in pairs. Only one of each pair of alleles can be present in a single gamete
pure-breeding
can be bred for almost any character. When generations are repeatedly bred with each other to give rise to plants that have consistently the same characteristics
why do exact ratios not always occur in reality
Small sample size
fertilisation is random
* linked genes
* epistasis
* lethal genotypes
very common exam question asking for these two reasons
sample size and fertilisation appears on all markschemes so answer with these ones
dihybrid inheritance
when two characters, determined by two different genes located on different chromosomes are inherited
Mendel’s ratio for dihybrid inheritance
9:3:3:1
law of independent assortment
each member of a pair of alleles may combine randomly with either of another pair. Which allele a gamete receives for gene A has no baring on the allele it receives for gene B
sex- linkage
any gene that is carried on the X or Y chromosome
X chromosome is much longer than the Y chromosome so for most of the length of the X chromosome , there is no equivalent portion of the Y chromosome. Characteristics controlled by recessive alleles on non-homologous portion of the X chromosome will appear more frequently in males as there is no homologous portion on the Y that might have the dominant allele
examples of sex linked disorders
- X-linked genetic disorder is a disorder caused by a defective gene on the X chromosome
eg haemophilia. When blood clots slowly. This is caused by a recessive allele with an altered DNA sequence that codes for a faulty protein that doesnt function
eg colour blindness
linkage group
all the genes on the same chromosome
autosomal linkage
where two or more genes are carried on the same autosome
-produces 2 possible combinations of the alleles in the gametes, whereas normal chromosomes produce 4
autosome
any chromosome that is not a sex chromosome
they are homologous and match in length, containing the same genes at the same loci
assumptions in autosomal linkage
- autosomal linkage predicts solely parental type offspring (offspring that have the same combination of characteristics as their parents)
- in reality recombinant offspring (offspring that have different combination of characteristics to their parents) are often produced. This is due to crossing over during meiosis
crossing over breaks the linkage between the genes and recombines the characteristics of the parents
epistasis
when the allele of one gene affects or masks the expression of another in the phenotype
recessive epistasis
recessive genotype on one gene inhibits the expression of other gene
- 9:3:4
expressed (dominant): expressed (recessive): epistasis
dominant epistasis
dominant genotype on one gene inhibits the expression of other gene
12:3:1
epistasis (inhibited): expressed (dominant): expressed (recessive)
complementary epistasis
dominant genotype required on both genes to achieve final product
chi squared test
- used to test the null hypothesis
- means of testing whether any deviation between observed and expected numbers in an investigation is significant or not
chi squared=
sum of (observed numbers (O)- expected numbers (E)) ^2
——————————————————————————–
expected numbers (E)
χ2 = ∑(O – E)^2
—————-
E
value obtained is then read off on a chi- squared distribution table to determine whether any deviation from the expected results is significant or not. Critical value p= 0.05
If probability is equal to or greater than 0.05 the deviation is said to be not significant and the null hypothesis would be accepted
If the deviation is less than 0.05 the deviation is said to be significant and the null hypothesis is rejected
null hypothesis
- used to examine the results of scientific investigations and is based on the assumption that there will be no statistically significant difference between sets of observations, any difference being due to chance alone
criteria for chi squared test
- sample must be relatively large, at least over 20
- data must fall into discrete categories
- only raw counts and not percentages, rates etc can be used
- used to compare experimental results with theoretical ones
degrees of freedom
number of classes (categories) minus one
if asked why you would use chi squared test
categorical data
hardy- weinberg principle
mathematical equation used to calculate the frequencies of the alleles of a particular gene in a population
let probability of allele A= p
and probability of allele a = q
p+ q= 1
as o ly four possible arrangements of the two alleles, it follows that the probability of all four added together must equal1.
AA+Aa+ aA+ aa =1
p^2 + 2pq +q^2=1
hardy weinberg assumptions
proportion of dominant and recessive alleles of an gene in a population remains the same from one generation to the next. This can be the case provided that the following conditions are met:
* no mutations arise
* the population is isolated, so there is no flow of alleles into or out of the population
* no selection, so all alleles are equally likely to be passed to the next generation
* population is large
* mating within the population is random
