mendelian genetics - BIO2133

gene

a discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA in some viruses) and located in a specific location on a chromosome; responsible for the physical trait of an organism

locus (loci)

a specific location on a chromosome

alleles

alternative forms of a gene

dominant allele

an allele whose phenotype is expressed even in the presence of the other allele

recessive allele

an allele whose phenotype is expressed only if both alleles are found in the gene

homozygous genotype

a genotype carrying the same alleles of a gene (BB or bb)

heterozygous genotype

a genotype carrying the 2 different alleles of a gene (Bb)

genotype

messages in DNA; the genetic makeup of an individual represented by the alleles carries at a specific locus on a gene (BB, Bb, bb)

phenotype

any characteristic that can be observed and measured in an individual (ex. tall plant, round seed, etc.)

trait

a specific characteristic of an organism that can vary between individuals (ex. height, weight, hair colour, etc.)

Mendelian inheritance (simple inheritance of traits)

inheritance of traits controlled by only one gene

discrete traits

traits that have a set number of different forms (ex. pea plat can be green or yellow)

no blending inheritance

a law of Mendel that states that traits are not mixed and changed forever; in F2, some pea plants looked like the parent (purple) and some exhibited traits different to the parent (white)

hereditary factor

each trait has two alternative copies of these and one copy comes from the father while one copy comes from the mother

law of dominance

when two different alleles for a trait are present, one (dominant) may mask the expression of the other (recessive)

law of segregation

the two copies for each unit factor (alleles) must be equally segregated during gamete formation, providing only one copy to each gamete

pure-breeding line

a group of identical individuals that always produce offspring of the same phenotype when intercrossed

cross-fertilization

group of organisms with purebred parents of two different phenotypes that produce offspring (F1) that are phenotypically the same as one parent

monohybrid cross

a breeding experiment between hybrids for a single trait (ex. flower colours, seed shape, etc.)

dihybrid test cross

crossing two individuals with 2 different alleles at two different loci (ex. pea colour AND shape)

recombinant phenotypes

combination of phenotypes that are not present in the parents; this heritable source of variation occurs due to meiosis

recombinant phenotypes

combination of phenotypes that are not present in the parents; this heritable source of variation occurs due to meiosis

test cross (back cross)

a controlled experiment where an organism with an unknown genotype at one (or more) loci is crossed with a homozygous recessive for the gene(s) in question

thalassemia

a disease caused by a single recessive gene on Chr16/11 that reduces the amounts of hemoglobin in blood

sickle cell anemia

disease caused by a recessive gene on Chr11 that leads to abnormal hemoglobin sickle shaped red cells

cystic fibrosis

disease caused by a single recessive gene on CHR7 that leads to a defective cell membrane protein which causes excessive mucus production in the lungs

Tay-sachs disease

buildup of fatty deposits in the brain due to a recessive gene on CHR15 that prevents the production of an enzyme for it

phenylketonuria (PKU)

inability to break down the amino acid phenylalanine caused by a recessive gene on CHR12 that blocks the expression of an enzyme for it

hypercholesterolemia

a recessive gene on CHR19 that leads to a missing protein that removes cholesterol from the blood

Huntington disease

a recessive gene on Chr4 that leads to progressive mental and neurological damage

forward genetics

using a trait to identify the gene associated

reverse genetics

start with a gene, then test its effects on the trait

incomplete dominance

one gene, 2 alleles, results in the expression an intermediate phenotype that is distinct from either parent (ex. white + red flower = white flower); 1:2:1 ratio (P & G)

codominance

one gene, both alleles fully and simultaneously expressed as phenotypes in a heterozygous genotype (ex. ABO blood groups); 1:2:1 ratio (P & G)

multiple alleles

one gene, more than 2 alleles in the population, resulting in a distinct phenotype for every genotype (ex. ABO blood group system); 1:2:1 ratio (P&G)

pleitropy

one gene determines multiple traits, it expresses itself in many different phenotypes seemingly unrelated (ex. beta-globin gene)

additive interaction

a type of polygenic inheritance where two genes interact to produce new phenotypes; 9:3:3:1 phenotypic ratio

epistasis

a type of polygenic inheritance where one gene masks or modifies the effect of another gene

recessive epistasis

homozygous recessive of one gene masks both alleles of another gene; 9:4:3 phenotypic ratio

reciprocal recessive epistasis

homozygous recessive of each gene masks the dominant allele of the other gene; 9:7 phenotypic ratio

bombay phenotype

an example of recessive episatasis; arises from homozygosity for a mutant recessive allele (hh); the homozygous recessive of the H gene completely overrides the dominance of the ABO blood group gene (i.e., presence of hh but IAIB will not lead to an AB blood type but a O blood type)

dominant epistasis

the dominant allele of one gene hides the effects of both alleles of another gene; 12:3:1 phenotypic ratio

locus heterogeneity

a type of polygenic inheritance where alleles at one or two or more different genes produce the same phenotype

recombination

genes exchange alleles (ex. linked genes in fruit flies); 1:1:1:1 phenotypic ratio

polygenic inheritance

many genes, continuous variation (ex. height)

multifactorial inheritance

genes + environment + threshold (ex. diabetes)

wild-type (A+)

if an allele frequency is overwhelmingly common (>99%)

mutant

if an allele frequency is overwhelmingly uncommon (<1%)

monomorphic gene

if a gene has only one common allele

polymorphic gene

if a gene has more than one common allele

complementation

mutations in different genes; the normal allele of one parent can provide what the mutant allele of the same gene from the other parent cannot

noncomplementation

mutations in the same gene; offspring receive only the mutant allele from each parent (homozygous for this gene)

linkage analysis

a test cross between a dihybrid and a homozygous when traits assort independently; done to determine if genes are linked

penetrance

how many members of a population with a particular genotype show the expected phenotype

expressivity

the degree or intensity with which a particular genotype is expressed in a phenotype

incomplete penetrance

not all carriers will develop the disease, only a small percentage

variable expressivity

all of the organisms exhibit a certain phenotype and are homozygous recessive for a specific locus, but a range exists for the phenotype (ex. all yellow lab are homozygous recessive for the E locus but the shade of their coat ranges from a light cream to a deeper, reddish golden colour)

full penetrance

nearly everyone who inherits the mutated gene will develop the disease

modifier genes

produces subtle, secondary effects on phenotype; can affect penetrance, expressivity, dominance and pleitropy

DNA methylation

adding a methyl group to the DNA sequence to reduce or stop gene expression

epigenetics

alter function (increase or decrease; inhibit or stop gene activity) of DNA and histones

histone modification

how DNA is coiled around histones; if histones squeeze DNA too much, DNA cannot be easily read

genomic imprinting

when the sex of the parent who contributes the particular allele determines whether the allele is expressed