Lecture 14 Flashcards
complete dominance, incomplete dominance and codominance
Codominance: Inheritance pattern: pattern that occurs when the heterozygote expresses both alleles simultaneously.
Eg: human carrying A and B alleles for the ABO antigens of red blood cells produces both the A and B antigens (has AB blood type)
Molecular basis: Codominant alleles encode proteins that function slightly differently from each other. In a heterozygote, the function of each protein affects the phenotype uniquely.
Incomplete dominant:
Inheritance pattern: occurs when heterozygote has a phenotype intermediate to the phenotypes of dominant and recessive homozygotes, as when a cross between red-flowered and white flowered plants produces pink.
Molecular basis: 50% of the protein encoded by the normal allele is not sufficient to produce the normal trait.
- Explain why multiple alleles at one locus exist in a population, not in a diploid organism.
In the population as a whole, there are many copies of each chromosome, so any gene can have multiple alleles present in the different copies.
Compare pleiotropy and polygenic inheritance.
pleiotropy is when one gene affects multiple characteristics (e.g. Marfan syndrome) and polygenic inheritance is when one trait is controlled by multiple genes (e.g. skin pigmentation).
Polygenic inheritance: Occurs when a trait is controlled by several gene pairs, each of which is additive; usually results in continuous variation.
polygenic inheritance and epistasis.
Epistasis. 2 or more genes involved. In such cases, one of two scenarios occurs:
1) Genes can mask each other’s presence (more common) or 2) combine to produce an entirely new trait
Polygenic: the heredity of complex characters that are determined by a large number of genes, each one usually having a relatively small effect.
recessively inherited disorder & a dominantly-inherited disorder: for both autosomal and X-linked disorders.
Autosomal dominant: One altered copy of the gene in each cell is sufficient for a person to be affected by an autosomal dominant disorder. In some cases, an affected person inherits the condition from an affected parent. In others, the condition may result from a new variant in the gene and occur in people with no history of the disorder in their family.
Autosomal recessive :
In autosomal recessive inheritance, variants occur in both copies of the gene in each cell. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition. Autosomal recessive disorders are typically not seen in every generation of an affected family.
X-linked dominant disorders are caused by variants in genes on the X chromosome. In males (who have only one X chromosome), a variant in the only copy of the gene in each cell causes the disorder. In females (who have two X chromosomes), a variant in one of the two copies of the gene in each cell is sufficient to cause the disorder. Females may experience less severe symptoms of the disorder than males. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons (no male-to-male transmission).
X-linked recessive disorders are also caused by variants in genes on the X chromosome. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a variant would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, males are affected by X-linked recessive disorders much more frequently than females. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons (no male-to-male transmission).
X-inactivation
: process by which one X chromosome (either maternally or paternally derived) is randomly inactivated in early embryonic cells. From this point on, all descendent cells will have the same X-chromosome inactivated as the cell from which they arose, thus a female is a mosaic composed of two types of cells, one which expresses only the paternal X-chromosome, and another which expresses only the maternal X-chromosome.
