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Terms in this set (50)
a special type of cell division that is used to produce male and female gametes
sex cell (sperm - males, ova - females)
how any chromosomes does a human body cell contain?
46 chromosomes (diploid)
how any chromosomes does a gamete contain?
23 chromosomes (haploid)
how does meiosis lead to genetic variation within a population?
genes get shuffled, and that is why you look similar, but not identical to your parents
stage 1 of meiosis
when the cell is not dividing, the chromosomes are not visible. they are unwound as single strands of DNA that are being used to make proteins (protein synthesis)
stage 2 of meiosis
chromosomes replicate due to DNA replication. nuclear membrane disappears
stage 3 of meiosis
homologous pairs of chromosomes line up randomly "independent assortment" at the cell equator. they exchange sections of DNA, and therefore alleles, by the process "crossing over"
stage 4 of meiosis
chromosomes get pulled apart and the parent cell begins to divide
stage 5 of meiosis
parent cell splits into 2
stage 6 of meiosis
chromosomes line up in single file at the cell equator and get pulled apart
stage 7 of meiosis
four daughter cells (or gametes) are produced, each with half of the original number of chromosomes
independent assortment as a cause of variation
when homologous pairs of chromosomes line up during meiosis, they do so randomly. this means that it is completely random which combination of alleles end up in a particular gamete.
independent assortment in humans
independent assortment of our 23 pairs of chromosomes means there are over 8 million possible combinations.
crossing over as a cause of variation
during meiosis, homologous chromosomes can swap parts of chromosomes, breaking up gene combinations that were inherited together. this increases the variation between gametes.
- creates new combinations of alleles in the gametes that are not found in either parent
- increases genetic diversity of the population
- creates new combinations of linked genes
genes found on the same chromosome that tend to be inherited together. this linkage reduces genetic variation in the offspring (crossing over will not affect them)
segregation as a cause of variation
during meiosis, pairs of alleles are segregated (separated) when the homologous chromosomes split so that each gamete only receives one allele for each pair. segregation results in all daughter cells being unique
- a permanent change in the base sequence of DNA
- random events that occur as a result of chance
- can result in changes to large sections of chromosomes or to specific genes
when can mutations be passed to offspring?
only when the mutation occurs in the reproductive cells (gametes). that gamete then needs to be involved in the fertilisation process, the resulting offspring will have that allele (mutation) in every cell in its body.
have no observable effect on the organism
detrimental and may affect the survival of the organism
give the organism a survival advantage over other members in the population, and it may live on in conditions where others die. a beneficial mutation is more likely to establish itself in the gene pool as the individuals carrying this mutation will be able to survive and reproduce, passing on the mutation (new allele).
is mutation our only source of new alleles?
yes, but it's not the only source of variation. already existing alleles can contribute to variation through meiosis (crossing over, independent assortment and segregation).
- often caused by problems that occur during meiosis or exposure to mutagens
- can result in changes in the number of chromosomes in a cell or changes in the structure of a chromosome
- 4 main types of block mutations
part of the chromosome breaks off, reducing its length and removing genes
rearrangement of genetic information on the chromosome, not affecting the overall length
movement of the genetic material from one chromosome to another, will lengthen the chromosome
some genetic material on the chromosome is copied, resulting in duplication of genes and an increase in length
gene (point) mutations
occur on a smaller scale than chromosomal mutations as they involve changes to the base sequence within a gene, as opposed to a block of genes or an entire chromosome. bases (e.g. A, T, C & G) can be inserted, deleted or substituted.
- some alleles are incompletely dominant, which means that when the individual is heterozygous, the phenotype is a combination of both the recessive and dominant alleles.
- one allele is still considered dominant but it can not completely overshadow the protein production of the recessive allele
incomplete dominance example
in some flowers, the gene to make the protein for red pigmented flowers is incompletely dominant over the gene that results in the protein for blue pigmented flowers. a heterozygous individual is producing both proteins to contribute to the phenotype, in this case purple flowers.
- occurs when a single gene has more than one dominant allele
- an individual who is heterozygous for two co-dominant alleles will express the phenotypes associated with both alleles
a white cow (WW) crossed with a red cow (RR) has offspring with red and white spots (RW). all offspring have the genotype RW and therefore show the red and white spot phenotype.
alleles that produce a phenotypic effect that causes the organism to die
why do lethal alleles usually arise?
due to mutation in an essential gene. this mutation means that the gene cannot produce a functioning version of an essential protein. it can be completely dominant, incompletely dominant or recessive
genes that have more than two different alleles
multiple alleles example
genes responsible for the human blood groups have multiple alleles. some of these alleles show complete dominance while others show co-dominance. human blood groups (known as the ABO groups) are controlled by 3 alleles.
- occurs when the genes for a certain characteristic are carried on the sex chromosomes
- they do not determine the sex of the organism, they just happen to occur on the sex chromosomes
total number of different alleles that exist within a population
how often a certain allele occurs in a gene pool
calculated by: occurance of that allele/total number of alleles
factors that cause changes to allele frequency in a gene pool
- natural selection
- sexual selection
- founder effect
- bottleneck effect
- genetic drift
change in frequency of alleles in the gene pool of a population
can changes in allele frequency be seen as evidence of evolution within that population?
natural selection - stabilising selection
when selective pressure selects against the 2 extremes of a trait, the middle phenotypes are retained and become more common
natural selection - directional selection
when selective pressure selects against one extreme of the trait, the population's trait distribution shifts toward the other extreme
are only advantageous in particular environments, a change in environment could mean they're no longer advantageous
natural selection - disruptive selection
e.g. the beak size of finches in the Galapagos. more specialised beaks were able to utilise different food sources giving them a survival advantage.
a form of natural selection, certain individuals with selected traits are selected to reproduce, as a result, those traits are passed onto the next generation e.g. peacocks
gene flow (migration)
occurs when individuals move in or out of a population. when the migrating individuals interbreed with the population they contribute to the gene pool of the local population.
How many types of tRNA exist?
What kind of protein synthesis involves enzymes that are synthesized depending on presence or absence of a particular substrate?
Why did the theory of Beadle and tatum's experiment have to be modified later on in the years?
Where is the location of DNA for a eukaryote?
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