genetics and meiosis Flashcards
meiosis
nuclear division that produces 4 haploid cells from a diploid cell. The gametes are used in sexual reproduction.
4 things in prophase I
1 Chromosomes condense and become visible
2 crossing over occurs
3 centrioles migrate to opposite poles forming spindle fibre
4 nuclear envelope breaks down and nucleolus disappears
chromosomes of bivalents are joined at the
chiasmata
2 things in metaphase I
- homologous pairs chromosomes are arranged randomly on equator
2 homologous chromosomes are still attached at chiasmata
1 thing in anaphase I
spindle fibres attached at centromeres contract and pull homologous chromosomes apart- one pair to each pole
telophase I
1 spindle fibres break down
2 chromsones uncoil
3 in animals nuclear envelope and nucleolus reform
what is the division after anaphase I called
reduction division
cytokinesis is animals
organelles distributed
cell surface membrane pinches inwards creating cleavage furrow which contracts
cytokinesis in plants
organelles distributed
vesicles form Golgi apparatus gather along the equator of spindle and merge to create new cell membrane
prophase II
nucleolus and nuclear envelop disappear, centriole move to opposite poles, chromosomes condense
metaphase II
chromosomes arrange themselves on equator. Spindle fibres attach to centromere of each chromosome
crossing over
the exchange of alleles between non sister chromatids of homologous chromosomes
anaphase II
centromeres divide and are pulled by spindle fibre to opposite poles carrying chromatids with them. Chromatids are separated
locus
position of gene on chromosome
gene 3 marks
a specific sequence of DNA occupying a position on a chromosome that codes for a specific protein, Each gene consists. of 2 or more alleles
phenotype
observable features of an organisms determined by its genes and environment
genotype
alleles an organism has
homozygous
having 2 identical alleles of a gene BB/bb
dominant allele
has the same effect on a phenotype whether or not another allele is present
co dominance
co dominant alleles each effect phenotype when both are present
multiple allele
the existence of 3 or more alleles of a gene eg blood groups
sex linkage
genes or alleles present in sex chromosomes are inherited together
sources of genetic mutation meiosis
crossing over-new combination of alleles on the 2 chromosomes
independent assortment- different combination of alleles in daughter cells due to random alignment of homologous Paris along the equator of the spindle
fusion of gametes- at fertilisation there is a random fusion of gametes creating variation between zygotes
crossing over is more likely to occur
further down the chromosomes away from the centromere
work out the number of possible chromosomes combinations
2^n where n is the number of chromosomes in a haploid cell 23 in humans
chi squared is used to
find the difference between expected and observed results
if chi squared value is bigger than critical value then
differences are due to chance otherwise differences are significant and caused by something else
haploid
containing one complete set of chromosomes
diploid
containing two complete set of chromosomes
need for reductive division
reduces number of chromosomes to half original number diploid to haploid so when gametes fuse cell with have correct number of chromosomes
homologous pair of chromosomes
2 chromosomes with similar gene loci but different alleles- one inherited form each parent
allele
alternative form of a gene
F1
first generation of offspring from a genetic cross
F2
the offspring produced when F1 individuals reproduce with one another
autosomal linkage
the presence of 2 genes on the same autosome, tend to b’e inherited together and do not assort independently
why is haemophilia only found in men
factor vII is a gene found on t a non-homologous region of the x chromosome, Males (xy) only have one copy of the allele to be diseasedwhereas females (xx) have 2.
epistasis
interaction of 2 different genes at different loci- one gene may effect expression of another
continuous variation
variation within a range eg- mass and height
what is variation
differences amongst individuals of the same species
what is variation caused by
genes and environment
what is genetic variation caused by (5)
1 independent assortment of chromosomes during mitosis
2 crossing over between non sister chromatids of homologous chromosomes during meiosis
3 random mating between organisms within species
4 random fertilisation of gametes
5 mutation
discontinuous variation
can only take particular values eg gender and shoe size
monogenic
characteristics influenced by only one gene
natural selection
process in which fitter individuals who are better adapted to the environment survive and pass on the advantageous genes to future generations
evolution
process by which the frequency of alleles in the gene pool change over time
genetic drift
small change in alleles frequency occurs as a result of the fact not all the individuals in a population reproduce. effect amplified in very small groups isolated forms eh rest of the population
genetic bottleneck effect
rapid reduction of population size which has an effect on genetic variation in future generations
founder effect
decrease in genetic diversity which occurs when the population descends from a small number of ancestors
selection
process by which organisms that are better adapted to the environment survive and breed while those less well adapted do not
speciation
process by which new species arise after a population becomes seperated and cannot interbreed
antibiotic resistance
1 random mutation occurs in bacteria
2 mutation bacteria can survive and reproduce
3 they pass mutant allele over many generations
3 ways natural selection can act on a population
directional selection against an extremes
stabilising selection against both extremes
disruptive selection against the mean
species
populations of similar organisms with similar morphological and physical features occupying the same niche and can reproduce to make fertile offspring. Reproductively isolated from other populations
allele frequency
relative proportions of alleles of a gene present in a population
3 causes of allele frequency change in a population
mutation, migration, natural selection
hardy Wienberg principle in words
in a large randomly mating population there is a fixed relationship between gene and genotype frequency. Frequencies remain constant unless there is mutation, migration or natural selection
