Dat Biology- Heredity

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Law of Segregation

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Refers to the separation of alleles (and their chromosomes) to individual gametes. One member of each chromosome pair migrates to an opposite pole so that each gamete contains only one copy of each chromosome.

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Law of independent assortment

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Refers to the independent assortment of alleles (and their chromosomes). The process is independent because the migration of homologues within one pair of homologous chromosomes to opposite poles does not influence the migration of homologues of other homologous pairs.

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Testcross

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Mating of an individual with a mystery genotype with an individual whose genotype is known. Ex: Crossing P_ x pp will yield all dominant trait if PP and half dominant, half recessive if Pp.

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Incomplete dominance

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Sometimes the alleles for a gene do not exhibit the dominant and recessive behaviors. Instead, their combined expression yields a blending of the two alleles. Ex: snapdragons are red (dominant) and white (recessive) and a cross yields pink.

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Codominance

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In this pattern, both inherited alleles are completely expressed. Ex: human blood types

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Multiple alleles

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Seen in blood types- alleles IA, IB, or i. ( Gives A, B, or O blood and a combination of IA and IB gives AB blood).

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Epistasis

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Occurs when one gene affects the phenotypic expression of another gene. Ex: pigmentation, one gene turns of the production of pigment while a second gene controls either the amount of pigment produced or the color ofthe pigment. If the first gene codes for no pigment, then the expression of the second gene has no effect.

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Pleiotropy

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Occurs when a single gene has more than one phenotypic expression. Ex: round or wrinkled peas also influences other genes involved in starch production. Sickle-cell disease is pleiotropic.

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Linked genes

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Genes that reside on the same chromosome and thus cannot segregate independently because they are physically connected. Usually they are inherited together.

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Sex linked inheritance

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Sex linked genes typically reside on the X chromosome (very few on Y). Males only inherit one X chromosome and thus are susceptible to sex linked diseases. Therefore, whichever X allele is inherited will be expressed, regardless of whether it is dominant or recessive. Ex: hemophilia, color blindness.

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X inactivation

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During embryonic development in females, one of the two X chromosomes in each cell does not uncoil into chromatin and instead becomes a Barr body. This process is random but some cells will have one allele silenced while others will have the opposite one silenced. Ex: calico cats

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Nondisjunction

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Failure of one or more chromosome pairs or chromatids to properly separate during meiosis or mitosis. Produces gametes with extra or missing chromosomes, mosaicism, or polyploidy.

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Point mutations

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Occur when a single nucleotide in the DNA of a gene is incorrect. Occurs when a different nucleotide is substituted for the correct one, if a nucleotide base-pair is omitted, or if an extra base pair is inserted.

Most point mutations have a deleterious effect on gene function.

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Aneuploidy

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A genome with extra or missing chromosomes. Most often caused by nondisjunction and produces sterile zygotes. Some do survive: Down syndrome (trisomy 21) and Turner syndrome (XO females)

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Chromosomal aberrations

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Caused when chromosome segments are changed.

Duplications- occur when a segment is repeated on the same chromosome.
Inversions- occur when chromosome segments are rearranged in reverse orientation.
Translocations- occur when segment is moved to another chromosome.