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Bio - alleles and glossary

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)


different forms of a gene for a single trait


observed trait of an organism that masks the recessive form of a trait


describes a trait or an allele that is expressed only when two recessive alleles for the same characteristic are inherited


having dissimilar alleles at corresponding chromosomal loci (genotype made of two different alleles)


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


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)


When multiple independent pairs of genes have similar and additive effects on the same characteristic. e.g: inheritance of skin color in 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).


the particular alleles at specified loci present in an organism. genetic constitution of an organism with respect to a trait.


any chromosome that is not a sex chromosome


The physical traits that appear in an individual as a result of its gentic make up.


(genetics) traits that tend to be inherited together as a consequence of an association between their genes


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.


A change in the nucleotide sequence of DNA. Only source of new alleles


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.


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.


sequence of DNA that codes for a protein and thus determines a trait

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