Chapter 2.3: Mendelian Traits vs. Non-Mendelian Traits

only 2 alleles (one dominant, one recessive) and conforms to Mendel’s 2 laws (segregation and independent assortment)

what makes it a mendelian trait?

lethal alleles

a phenotypic class does not survive to reproduce; Ex. spontaneous abortion

multiple alleles

many variants or degrees of a phenotype occur; ex. cystic fibrosis

incomplete dominance

a heterozygote’s phenotype is intermediate between those of two homozygotes; example familiar hypercholesterolemia

codominance

a heterozygote’s phenotype is distinct from and not intermediate between those of the two homozygotes; Ex. ABO blood types

epistasis

one gene masks or otherwise affects another’s phenotype; ex. bombay phenotype

penetrance

some individuals with a particular genotype do not have the associated phenotype; ex. polydactyly

expressivity

a genotype is associated with a phenotype of varying intensity; ex. polydactyly

pleiotropy

the phenotype includes many symptoms, with different subsets in different individuals; ex. porphyria variegata

phenocopy

an environmentally caused condition has symptoms and a recurrence pattern similar to those of a known inherited trait; ex. infection

genetic heterogeneity

different genotypes are associated with the same phenotype; hearing impairment

lethal alleles

some allele combinations are lethal (not necessarily all)

Lucien Cuénot

lethal genes were discovered by who?

coat color in mice

What was Cuénot studying when discovering lethal genes?

expected phenotype ratio of 3 Y: 1 W but got 2:1; the allele was lethal in homozygous dominant

how did Cuénot discover lethal genes?

early onset, late onset, conditional, semi lethal

what are the 4 categories of lethal alleles?

early onset

lethal allele which results in death of an organism at early stage of life; ex. during embryogenesis

late onset

lethal allele which kills organisms at their final stage of life

conditional

lethal allele which kill organism under certain environmental conditions only; ex. temperature sensitivity alleles kills organisms at high temperature but not low temperature

semi lethal

lethal allele which kill only some individuals of the population but not all

Tay-Sachs disease

example of human lethal alleles; resulting from an inactive gene for the enzyme hexoaminidase; homozygous individuals develop neurological symptoms before 1 year of age (recessive)

hemophilia

example of human lethal alleles; results from an x-linked recessive allele, lethal when untreated

Huntington disease

example of human lethal alleles; dominant lethal allele causes this disease, characterized by progressing central nervous system degeneration

multiple alleles

while an individual can only carry two alleles for a gene, it doesn’t mean more don’t exist in the population

300

some genes may have as many as ____ alleles

multiple alleles

if a gene in the population has more than 2 alleles (3+) it is called this.

True

True or False: any two of these multiple allels can be present in the genome of a diploid organism, but a haploid organism or a gamete can have just one of the in its genome

multiple alleles

Phenylketonuria (PKU) is an example of what?

phenylketonuria (PKU)

disorder where the enzyme that breaks down phenylalanine is deficient so it accumulates.

there are hundreds of possible alleles and the combination of the hundreds of possible alleles will determine the phenotype that presents (severe mental retardation, moderate PKU, milk PKU, asymptomatic PKU)

how is PKU a multiple alleles trait?

incomplete dominance

when an organism is heterozygous for a trait, it will show a third phenotype which is a blend of the other two; neither allele is dominant

intermediate phenotype

what is the phenotype called that is a combo of the two main phenotypes in incomplete dominance? (IDK if that makes any sense sorry hanna lol)

familial hypercholesterolemia

example of incomplete dominance inheritance

codominance

when both alleles are expressed in heterozygotes; ex. AB blood type

epistasis

occurs when two or more genes contribute to the same phenotype; often describes when one gene masks or modifies the genotype of a second gene

H gene

the _____ is epistatic to the ABO gene

the H protein is needed for the A or B antigens to attach to the cell, so if someone has a hh genotype (no H protein) they will have type O blood regardless of their ABO blood group

how does Epistasis work with the H protein and RBCs

locus

a position on a chromosome

Bombay Phenotype

what is it called when the hh genotype causes type O blood?

penetrance

percentage of individuals with a given genotype who exhibit the phenotype

expressivity

extent to which genotype is expressed at the phenotypic level (may be due to allelic variation or environmental factors)

incomplete penetrance

the phenotype is not always observed among individuals carrying the genotype; ex. people with polydactyly DD or Dd only 80% show polydactyly

ok

Example of polydactyly and penetrance/expressivity:

one form of polydactyly is 65% penetrant (only 65% of those with the dominant allele have extra digits). within those who express the gene how many extra digits they have varies as well as those digits functionality. 

pleiotropy

one gene has many symptoms or controls several functions

the symptoms it causes depends on where the lack of enzyme in the production of heme takes place

how is porphyria variegate an example of pleitropy

porphyria variegate

example of pleiotropy