Genetics and populations YEAR 2

Question 1

genotype

Answer 1

The genetic constitution of an organisms. Describes all the alleles that an organism has

Question 2

phenotype

Answer 2

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

Question 3

gene

Answer 3

a length of DNA, that is a sequence of nucleotide bases coding for a particular polypeptide.

Question 4

allele

Answer 4

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

Question 5

locus

Answer 5

position of agene on a particular DNA molecule

Question 6

homologous chromosomes

Answer 6

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

Question 7

homozygous dominant

Answer 7

when two dominant alleles are displayed.
Homozygous recessive is when 2 recessive alleles are displayed

Question 8

codominant

Answer 8

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

Question 9

multiple alleles

Answer 9

more than two alleles for each gene
however only two may be present at the loci of an individuals homologous chromosomes

Question 10

genetic crosses

Answer 10

Come up with parents genotypes and then cross them to show gamete combinations produces. Then cross gametes using a punnet square

Question 11

monohybrid inheritance

Answer 11

inheritance of a single gene

Question 12

law of segregation

Answer 12

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

Question 13

pure-breeding

Answer 13

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

Question 14

why do exact ratios not always occur in reality

Answer 14

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

Question 15

dihybrid inheritance

Answer 15

when two characters, determined by two different genes located on different chromosomes are inherited

Question 16

Mendel’s ratio for dihybrid inheritance

Answer 16

9:3:3:1

Question 17

law of independent assortment

Answer 17

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

Question 18

sex- linkage

Answer 18

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

Question 19

examples of sex linked disorders

Answer 19

• 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

Question 20

linkage group

Answer 20

all the genes on the same chromosome

Question 21

autosomal linkage

Answer 21

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

Question 22

autosome

Answer 22

any chromosome that is not a sex chromosome
they are homologous and match in length, containing the same genes at the same loci

Question 23

assumptions in autosomal linkage

Answer 23

• 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

Question 24

epistasis

Answer 24

when the allele of one gene affects or masks the expression of another in the phenotype

Question 25

recessive epistasis

Answer 25

recessive genotype on one gene inhibits the expression of other gene
- 9:3:4
expressed (dominant): expressed (recessive): epistasis

Question 26

dominant epistasis

Answer 26

dominant genotype on one gene inhibits the expression of other gene
12:3:1
epistasis (inhibited): expressed (dominant): expressed (recessive)

Question 27

complementary epistasis

Answer 27

dominant genotype required on both genes to achieve final product

Question 28

chi squared test

Answer 28

• 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

Question 29

null hypothesis

Answer 29

• 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

Question 30

criteria for chi squared test

Answer 30

• 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

Question 31

degrees of freedom

Answer 31

number of classes (categories) minus one

Question 32

if asked why you would use chi squared test

Answer 32

categorical data

Question 33

hardy- weinberg principle

Answer 33

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

Question 34

hardy weinberg assumptions

Answer 34

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