Chapter 14c: Extensions and Deviations of Mendelian Genetics

sex-linkage

-a gene located on a sex chromosome

-patterns of inheritance in males and females differ

linkage

-two or more genes are on the same chromosome

-genes do not assort independently

codominance

-heterozygotes have phenotypes of both alleles

-more than two phenotypes are seen because heterozygotes have a unique phenotype

incomplete dominance

-heterozygotes have phenotypes of both alleles

-the pattern is common and more than 2 phenotypes are seen

-typically seen in pigments

multiple allelism

-in a pop. there are more than 2 common alleles for the locus

-more than 2 phenotypes are often seen

pleiotropy

-a single gene affect many traits

-Marfan Syndrome

polygenic (quantitative trait)

-a trait that is influenced by many genes and often exhibits continuous variation rather than distinct phenotypes

-each gene adds a small amount of phenotype

gene interaction

-the phenotype associated with an allele depends on which alleles of another gene are present

-leads to one allele being associated with diff phenotypes

epistasis

-a particular form of the gene interaction in which particular alleles of one gene mask the expression of the alleles of another gene

-shows that there are many forms of gene interaction and leads to surprising results in which an expected phenotype is not seen

environmental factors

-phenotype is influenced by the environment experienced by the individuals with the same genotype

-same genotypes can lead to different phenotypes

complete dominance

in the traits Mendel studied, only the phenotype associated with the dominant allele appeared in heterozygotes

Marfan syndrome

-caused by the mutation of a pleiotropic gene

-mutations in gene affect multiple traits related to connective tissue formation such as: near sightedness, scoliosis, and heart valve defects

essential gene

product necessary for life

-mutation can lead to a lethal phenotype

-can be X-linked or autosomal

lethal allele

-an allele whose expression results in the death of the individual organism expressing it

-may be dominant(Huntington's kills you when older) or recessive (Tays-Sachs kills you young)

-if allele is dominant it will kill the homozygote and heterozygote

novel phenotypes

-may happen when 2 non allelic pairs (2 diff genes) affect the same trait then the gene products interact

-maintain 9:3:3:1 ratio

-in chickens the R and P genes interact to create the comb phenotype

epistasis in mice

produces 9 agouti: 3 black: 4 albino

coat color of mice

-controlled by 2 genes, A and C

-A and C produce functional enzymes

-a and c do not produce functional enzymes

recessive epistasis

only the homozygous recessive allele at one locus masks or modifies the phenotype from another locus (A in photo)

epistasis in labrador retrievers

-controlled by two genes , E and B

-E and B produce functional enzymes

-e and b do not produce functional enzyme

dominant epistasis

-the presence of one copy of the dominant allele completely masks the effect from the second locus

-dominant allele epistatic (masks) other locus

ratio for:

F1: A/a B/b * A/a B/b

Gene Interactions: None

A/- B/- : A/- bb : a/a B/- : a/a b/b

9 : 3 : 3 : 1

ratio for:

F1: A/a B/b * A/a B/b

Gene Interactions: a/a epistatic to B or b

A/- B/- : A/- bb : a/a B/- : a/a b/b

9 : 3 : (3+1) 4

ratio for:

F1: A/a B/b * A/a B/b

Gene Interactions: A is epistatic to B or b

A/- B/- : A/- bb : a/a B/- : a/a b/b

(9+3) 12 : 3 : 1

ratio for:

F1: A/a B/b * A/a B/b

Gene Interactions: a/a is epistatic to B or b

OR b/b epistatic to A and a

OR a/a = b/b = same phenotype

A/- B/- : A/- bb : a/a B/- : a/a b/b

9 : (3+3+1). 7

ratio for:

F1: A/a B/b * A/a B/b

Gene Interactions: A epistatic to B or b

OR B epistatic to A or a

OR A and B give the same phenotype

A/- B/- : A/- bb : a/a B/- : a/a b/b

(9+3+3) = 15 : 1

penetrance

-the percentage of individuals with the same genotype, at a specific locus, that actually exhibit the associated phenotype

-depends on both the genotype and the environment

complete pentrance

100% of individuals with the genotype all have the same phenotype

incomplete pentrance

less than 100% of individuals with the same genotype have the same phenotype

expressivity

-degree to which a penetrant gene is phenotypically expressed

-depends on genotype and the environment

-individuals show range of phenotype

constant expressivity

all individuals with a given genotype express the same phenotype

variable expressivity

individuals with the same genotype have related phenotypes that vary in intensity

incomplete penetrance and variable expressivity

identical known genotypes produce a broad range of phenotypes due to varying degrees of gene activation and expression

effect of environment on phenotype

- age of onset

- sex

- temperature

- chemicals

temperature sensitivity to tyrosinase

-determines coat color by converting tyrosine into black pigment

-siamese cats are homozygous recessive for siamese allele (c^s, c^s)

-c^s allele encodes for temp sensitivity so tyrosinase is only active at low temps

extranuclear inheritance

-all comes from mom (uniparental inheritance)

-mitochondrial gene (involved in TCA cycle, oxidative phosphorylation)

-chloroplast genes

Leber's Hereditary Optic Neuropathy (LHON)

-caused by defects in ETC

-causes blindness in adults and due to maternal inheritance

mode of transmission

describes the trait as autosomal or sex-linked and the type of dominance

pedigrees

or family trees, are used to learn the mode of transmission for the given trait

autosomal recessive traits

-Affected individuals must be homozygous for recessive allele

-Affected offspring often have unaffected parents

-Unaffected parents of affected offspring are heterozygous (carriers)

-Trait often skips generations (e.g. sickle cell anemia)

autosomal dominant traits

-Homozygous or heterozygous individuals will display the phenotype

-Affected offspring are heterozygous if only

-if one parent is heterozygous, about ½ of the offspring will be affected

-Autosomal dominant traits do not skip generations

X-linked recessive traits

-Females only exhibit if they are homozygous for recessive allele

-Trait is never passed from father to son

- About ½ of the sons of a carrier mother will be affected

-All daughters of an affected male and an unaffected non-carrier female are carriers

-Trait often skips generations

X-linked dominant traits

-Males and females are equally likely to be affected

- All daughters of an affected father are affected, but no sons

- Affected sons always have affected mothers

- Trait does not skip generations

Y-linked traits

-very easily predicted since t goes straight from father to son

-few genes are on Y chromosome