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Genetics Review


Human Genetics Terminology


Genes: units of information on chromosome about specific traits that are passed from parents to offspring


Alleles: different molecular forms/traits on the same gene


Homozygous dominant: the trait is composed of a pair of dominant alleles (e.g. AA)


Homozygous recessive: the trait is composed of a pair of recessive alleles (e.g. aa)


Heterozygous: the trait is composed of a pair of non-identical alleles (e.g. Aa)


Genotype: refer to the particular alleles


Phenotype: refer to the particular observable traits (appearance)


P: parental generation


F1: first-generation offspring


F2: second-generation offspring

Hybrid: a genetic cross that gives a pair of non-identical alleles for a trait

True-breeding lineage: a genetic cross that gives a pair of identical alleles for a trait


Testcrosses: an experimental test that is used to identify the unknown genotype given that the phenotype is known.  It is done by crossing the unknown gene to a known homozygous recessive gene.  The test results indicate whether the organism tested is homozygous dominant or heterozygous (i.e. AA or Aa)


Mendel's Theory of Segregation: Diploid (2X) cells have pairs of genes, on pairs of homologous chromosomes.  The two genes of each pair are separated from each other during meiosis, so they end up in different gametes.


Mendel's Theory of Independent Assortment: By the end of meiosis, genes on pairs of homologous chromosomes have been sorted out for distribution into one gamete or another independently of gene pairs of other chromosomes.


Punnett-square: a method that is used to explain and visualize the possibilities of different combination of outcome of a cross










 where A represents the dominant trait, a represents the recessive one


Incomplete Dominance: one allele of a pair is not fully dominant over the other, which results the heterozygous phenotype somewhere in between the two homozygous phenotypes.  For example, red flower is incomplete dominant over white flower.  When they are crossed, the offspring will be pink depending on the degree of dominance.


Codominance: a pair of non-identical alleles specifies two phenotypes, in a way that none of them is dominant over the other.  As a result, they are both expressed in the heterozygous offspring.  For example, a black-dotted dog is crossed with a white-dotted dog.  Assume that the pattern of the dog skin is a case of codominance.  The resulting offspring will be a black- and white-dotted dog. 



Pleiotropy: the phenotypic outcome of a single gene that can have positive or negative effects on two or more traits.  For example, a defected type of hemoglobin (red blood cell) can cause many negative effects such as kidney failure, poor physical development, pneumonia and heart disease.

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