Genetic Inheritance

genotype

the genetic makeup of an organism

phenotype

observable physical traits determined by the genotype

dominant

a trait expressed with just one allele

recessive

a trait that needs two alleles for expression

homozygous

when both alleles for a gene are identical (ex. AA)

heterozygous

when both alleles for a gene are different (ex. Aa)

gene

a unit of heredity transferred from parent to offspring that determines a characteristic of the offspring

chromosome

threadlike structure of nucleic acids and protein that carries genes

diploid

nucleus containing two sets of chromosomes (one from each parent)

haploid

a nucleus containing one set of chromosomes (23)

allele

form of a gene occupying a given position on a chromosome

fertilization

joining of sperm and egg to form an embryo (46 total chromosomes)

main dominance patterns

complete dominance, codominance, incomplete dominance

complete dominance

• homozygous dominant: AA → expresses “A” phenotype

• heterozygous dominant: Aa → expresses “A” phenotype

• homozygous recessive: aa → expresses “a” phenotype

codominance

multiple dominant alleles allow for the expression of two or more phenotypes simultaneously (ex. white and red cow = roan cow - white red spotted)

single allele

one variant of the gene (recessive and dominant)

multiple alleles

when a gene has more than two alleles (ex. blood type)

incomplete dominance

heterozygous genotype results in an intermediate phenotype between the alleles’ coded phenotypes (ex. homozygous red flower and homozygous white flower produce a heterozygous pink flower)

penetrance

percentage of population expressing a phenotype with a specific genotype

expressivity

wide variability of phenotypes emerging from a single genotype

law of dominance

mendelian law stating that in a heterozygote, the dominant allele will take the lead while the recessive one remains hidden

law of segregation

mendelian law stating that during reproductive cell (gamete) formation, alleles separate to ensure genetic variability

law of independent assortment

mendelian law stating that the inheritance of one gene doesn’t impact the inheritance of another (for unbiased mix of genetic traits)

recessive disease inheritance

two recessive alleles needed for disease to express (except for x-linked diseases for men - only one recessive allele is needed)

dominant disease inheritance

only one dominant allele is needed for disease to express