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40 terms

Genetic Variation

Bio - alleles and glossary
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Mendel's first law
Law of segregation
Mendel's second law
Law of independent assortment
Law of independent assortment
each member of a pair of homologous chromosomes separates independently of the members of other pairs so the results are random. inherited independently. alleles maintain identity and are not changed by their association with other alleles.
Law of segregation
members of a pair of homologous chromosomes separate during the formation of gametes and are distributed to different gametes so that every gamete receives only one member of the pair metaphase (really during synapsis)
Alleles
different forms of a gene for a single trait
Dominant
observed trait of an organism that masks the recessive form of a trait
Recessive
describes a trait or an allele that is expressed only when two recessive alleles for the same characteristic are inherited
Heterozygous
having dissimilar alleles at corresponding chromosomal loci (genotype made of two different alleles)
Homozygous
having identical alleles at corresponding chromosomal loci (genotype made of same alleles)
Test cross back cross
the crossing of an individual of unknown genotype (hetero or homo) with a homozygous recessive individual to determine the unknown genotype
Incomplete Dominance
one allele is not completely dominant over the other allele intermediate inheritance - hetero = combinded phenotype eg pink
Co-Dominance
situation in which both alleles of a gene contribute to the phenotype of the organism. some cells show 1 trait some the other eg roan.
Lethal alleles
Where the homozygous condition (usually homozygous recessive) is lethal. The ratio will be a multiple of three eg. 2:1. when an essentail gene is mutated = lethal phenotype.
Multiple alleles
three or more forms of a gene that code for a single trait (such as blood types)
Polygenes
When multiple independent pairs of genes have similar and additive effects on the same characteristic. e.g: inheritance of skin color in humans.work together have continous variation.
Sex-linked genes
gene located on the X or Y chromosome. harmful and recessive genes which would not be expressed in the homo form would show up more frequently.
Dihybrid cross
predicts results of combining each of the gametes inmating parent looking at two traits. mating of two organisms that differ in two characters. ratio of 2xhetero mating is 9:3:3:1 (16).
Genotype
the particular alleles at specified loci present in an organism. genetic constitution of an organism with respect to a trait.
Autosome
any chromosome that is not a sex chromosome
Phenotype
The physical traits that appear in an individual as a result of its gentic make up.
Linkage
(genetics) traits that tend to be inherited together as a consequence of an association between their genes
Evolution
change in a kind of organism over time; process by which modern organisms have descended from ancient organisms
Affects of Evolution
Species become better adapted to their environment if not they will become extinct
Why adapt
to deal with changes i.e increased competition, changes in environment, new diseases, new predators.
Darwins observations
most living things produce more offspring than # which survive to adulthood, despite this population sizes remain roughly constant,variation exists between some species, characteristics can be passed on from one generation to the next.
why Natural selection
1. each species shows variation. 2.there is competition within each species for food, better living space, water, mates etc 3. better adapted species more likely to survive. 4. survivors will pass on their better genes to their offspring and so on.
Mutation
A change in the nucleotide sequence of DNA. Only source of new alleles
Migration
new alleles entering into a population by individuals entering.
genetic drift
The gradual changes in gene frequencies in a population due to random events
Natural selection
process by which individuals that are better suited to their environment survive and reproduce most successfully; also called survival of the fittest
sexual recombination
independent assortment and crossing over during meiosis, and fertilization
Crossing over
exchange of genetic material between adjacent chromatids of homologous chromosomes during prophase I of meiosis. Site of exchange on chromosome is chiasma. recombination shuffles the allele content bteween sister chromatids making gametes unique.
Pure Breeding
Individuals that are homozygous that will always produce the same offspring when crossed together.However may still not be the desired pure breed individuals further crossing needed- uncertainity remains.
Meiosis
produces 4 haploid cells which develop into gametes. occurs in sex cells. maintaining chromosomal number from generation to g2 while promoting genetic diversity and variability w/i pop.
Stabilising Selection
Maintains the population in a stable form because it favours the average phenotype compared to extremes in either direction
Directional Selection
Natural selection in which individuals at one end of the phenotypic range survive or reproduce more successfully than do other individuals.
Disruptive Selection
Natural selection in which individuals on both extremes of a phenotypic range survive or reproduce more successfully than do individuals with intermediate phenotypes.
Founder Effect
when a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool isn't reflective of the source population
Bottleneck effect
Genetic drift resulting from the reduction of a population, typically by a natural disaster, such that the surviving population is no longer genetically representative of the original population. Reduces genetic variation.
Gene
sequence of DNA that codes for a protein and thus determines a trait