Extensions to Mendelian inheritance

describe the different types of dominance

complete - one allele is dominant to another

incomplete - alleles are incompletely dominant relative to each other (mixing)

codominant - alleles are codominant relative to each other

what is some information about codominance

• both traits are visible in the heterozygote

• expression of two different alleles for a locus occurs

• if two heterozygotes have children, genotypic & phenotypic ratios = 1:2:1

what is overdominance

the heterozygote has characteristics that are better for survival/reproductive success in a particular environment then either homozygote

what are de novo mutations

mutations that are present in children but not in the parents of the children - new mutations

what is mosaicism & 2 different types

de novo mutation that is expressed early in development leads to cells differing in genetic make-up

• somatic mosaicism: present in some cells but not gametes

• germ-line mosaicism: restricted to the gamete lineage

what is a lethal alleles

one that has the potential to cause the death of an organism

• sometimes in utero

• variants of essential genes

• 2:1 phenotypic & genotypic ratio (since 1 variants results in death)

what is penetrance

the proportion of carriers that will have any phenotypic expression

• individuals have same genotype as their family but don’t express the phenotype

what is reduced/incomplete penetrance

when the frequency of expression of a phenotype is >100%

• individual with disease genotype doesn’t show the phenotype but can pass it on

what is variable expressivity

the severity of expression of the phenotype among individuals with the same disease-causing genotype

• penetrance: light (on/off?)

• expressivity: how bright?

what is pleiotropy

one variant has multiple phenotypic effects

• most variants are pleiotropic

• genes have different function depending on cell location

what is genetic heterogeneity & 2 types

a single disease phenotype can be caused by variants are different alleles or loci in different families

• allelic heterogeneity: different variants within a gene/locus can be seen in different patients with a certain genetic condition

• locus heterogeneity: same clinical phenotype can result from variants at any one of the several different loci that lead to disease

what is a complementation test

a test conduct to determine if variant alleles in the same gene or different genes

how can you interpret the results of a complementation test

when organisms homozygous for variants that show the same phenotype but are in different genes are crossed, the progeny are wild-type

• variants complement one another

• locus heterogeneity

when organisms homozygous for variants that show the same phenotype and are the same genes are crossed, the progeny are mutant

• variants fail to complement

• allelic heterogeneity

what is anticipation

a genetic trait becomes more strongly expressed or is expressed at an earlier stage as it is passed down

• common in disease caused by dynamic mutations

what are dynamic mutations

a trinucleotide repeat that expands during gametogenesis and interferes with gene expression

• number of repeats affected severity & age of onset

what is a dihybrid ratio

when 2 genes affect different traits, a dihybrid cross gives an F2 9:3:3:1 phenotypic ratio

what is complementary gene action

need a dominant allele for each gene to produce the trait

• e.g. CcPp = purple but ccPp or Ccpp = white

• distinguished by 9:7 F2 dihybrid ratio (56%:44%)

what is epistasis

the phenotype produced by a variant of one gene (epistatic allele) blocks/masks the phenotype of alleles of another gene

• usually occurs as genes act in the same pathway

what is recessive epistasis

distinguishing F2 ratio = 9:3:4

• a recessive epistatic allele blocks/masks phenotypic effects of alleles when it is homozygous

what is the Bombay phenotype

refers to an absence of substance H which binds A and/or B sugars to blood cells

• present as type O

• both parents would be heterozygous for lose of function in FUT1 gene

what is dominant epistasis

dominant allele at one loci masks an allele at second loci regardless of second loci allele

• ratio 12:3:1

what are duplicate genes

distinguished by 15:1 F2 dihybrid ratio

• either dominant alleles is required

what environmental aspects can affect phenotype

• temperature

• chemicals

what are sex-influenced characteristics

inherited characteristics where the same gene can be expressed differently in males & females

• can be hormone influence

• e.g. baldness is autosomal dominant in XY but recessive in XX

what are sex-limited characteristics

characteristics only appears/develops in one sex

• e.g. ovary development & milk yield

summary of ratios

what is genomic imprinting

for some genes, one of the alleles is transcriptionally inactive

• no mRNA is produced

• depends on the parent from whom they received it

• imprinted = transcriptionally silenced genes

• imprinted genes = heavily methylated or modified chromatin of specific histone types (epigenetic changes)

what is cytoplasmic inheritance

inheritance of parental characteristics through non-chromosome DNA

• mitochondrial DNA

• inheritance is matrilineal (XX)

• autosomal as it can affect either sex

what are the 2 types of mutations in mitochondrial DNA

homoplasmy - every mitochondrial genome carries the causative variant

heteroplasmy - contain mixed population of normal & variant genomes in each cell

what are some pedigree characteristics of mitochondrial inheritance

• all children of affected female will be affected

• affected males WON’T pass it on