5.4 Non-Mendelian Genetics, 5.5 Environmental Effects on Phenotype

Non-Mendelian Genetics

varying degrees of dominance

many traits are produced through multiple genes acting together

some traits are determined by genes on the sex chromosomes

some genes are adjacent or close to one another on the same chromosome and will segregate as a unit

some traits are the result of non-nuclear inheritance (ex. chloroplasts and mitochondria)

Degrees of Dominance

alleles can show varying degrees of dominance

mendel worked with traits that showed complete dominance

-homozygous dominant and heterozygous individuals are phenotypically(appear) the same in complete dominance

includes incomplete dominance, codominance, and multiple alleles

Incomplete dominance

neither allele is fully dominant

F1 generation has a phenotype that is a mix of those of the parent generation

ex. red flowers crossed with white flowers produce pink offspring

Codominance

2 alleles that affect phenotype are both expressed

ex. human blood

-AB blood: A & B are both expressed

ex. cows can have red (RR) and white (WW) hair(RW)

Multiple Alleles

genes that exist in forms with more than 2 alleles

ex. human blood groups

-alleles are I^A,I^B,i

Multiple Genes

in many cases, 2 or more genes are responsible in determining phenotypes

includes epistasis and polygenic inheritance

Epistasis

the phenotypic expression of a gene at one locus affects a gene at another locus

ex. coat color in labs and some mice

one gene codes for pigment and a second gene determines whether or not that pigment will be expressed in the hair

Polygenic inheritance

the effect of two or more genes acting on a single phenotype

ex. height, human skin color

Pleiotropy

when a single gene controls multiple traits

typically in multiple body systems

since these traits are determined by the same gene, they will NOT segregate independently

ex. Marfan Syndrome

-an autosomal dominant connective tissue disorder caused by a mutation in the FBN1 gene(codes for proteins that make strong fibers that add strength to the body’s connective tissues)

Sex-linked Genes

Y-linked gene- genes specifically found on the Y chromosome

-very few Y-linked genes, so very few disorders

X-linked genes- genes found on the X chromosome

Inheritance of X-linked genes

fathers can pass X-linked alleles to all of their daughters, but none of their sons

mothers can pass X-linked alleles to both daughters and sons

if an X-linked trait is due to a recessive allele

-females will only express the trait if they are homozygous

-males only have 1 X chromosome, so they will express the trait if they inherit it from their mother(they are called hemizygous, since the term heterozygous does not apply)

due to this, males are much more likely to have an X-linked disorder

X-linked disorders

duchenne muscular dystrophy- progressive weakening of muscles

hemophilia- inability to properly clot blood

color blindness- inability to correctly see colors

X-inactivation

females inherit 2 X chromosomes

during development, most of the X chromosomes in each cell becomes inactive

-the inactive X in each cell of a female condenses into a Barr body(helps to regulate gene dosage/hormones in females)

Linked Genes

genes on the same chromosome that tend to be inherited together

crossing over still occurs, but the genes do not assort independently

Genetic Recombination

production of offspring with a new combination of alleles from parents

parental types- offspring with the parental allele combination of alleles from parents

recombinants- offspring with new allele combinations from the parents

due to independent assortment, 50% recombination is the max: indicates that genes are unlinked, or on different chromosomes

Linked genes: crossing over

linked genes show parental types in offspring at higher than 50%

during crossing over homologous chromosomes exchange corresponding segments, but crossovers BETWEEN two genes that are close together are not common

the further apart two genes are on the same chromosome, the higher the probability that crossing over event will occur between them and the higher the combination frequency (50% being the max)

Mapping Distance

experiments performed by Sturtevant allowed scientists to map genes and their locations on chromosomes

linkage map- genetic map that is based on recombination frequencies

the distance between genes are map units

-1 map units is equivalent to 1% recombination frequency (this is an estimate of physical distance)

-express the relative distances along chromosomes

-50% recombination means that the genes are far apart on the same chromosome or on 2 different chromosomes

note: chromosomes may be longer than 50 map units, BUT 50% recombination is still the max we can measure (so to determine the distance between genes further than this we must look at the recombination frequencies of multiple genes to construct a linkage map)

Non-Nuclear DNA

some traits are located on DNA found in the mitochondria or chloroplasts

both chloroplasts and mitochondria are randomly assorted to gametes and daughter cells

in animals, mitochondria are transmitted by the egg, NOT the sperm, which means all mitochondrial DNA is maternally inherited

in plants, mitochondria and chloroplasts are transmitted in the ovule, NOT the pollen, which means both mitochondrial and chloroplast determined traits are maternally inherited

Environmental Effects on Phenotype

various environmental factors can influence gene expression and lead to phenotype plasticity

Phenotype plasticity

individuals with the same genotype exhibit different phenotypes in different environments

ex. temperature can change coat color in rabbits and Siamese cats, like fox white in snow brown in forest

soil pH can affect flower color

UV exposure can increase melanin production in the skin