Biology 30 - Chapter 17

selective breeding

process of choosing and breeding specific organisms for particular physical features or behaviours

true breeding

organisms that are homozygous for a particular trait or set of traits and produce offspring that exhibit the same characteristics generation after generation

monohybrid cross

cross of two individuals that differ in one trait

Dominant

describes a trait which always appears (is expressed) in an individual that is either heterozygous (Aa) or homozygous (AA) for that trait

Recessive

refers to a type of trait which does not appear (is not expressed) in an individual that is heterozygous (Aa) for that trait

complete dominance

a condition in which the dominant allele of a gene completely conceals the presence of the recessive allele of a gene; an individual with one recessive and one dominant allele has the same observable physical characteristic as an individual with two dominant alleles

law of segregation

Mendel’s first law of inheritance, stating that all individuals have two copies of each factor (gene); these copies segregate (separate) randomly during gamete formation, and each gamete receives one copy of every factor (gene)

Genotype

the combination of alleles for any given trait; compare phenotype

Phenotype

the visible physical and physiological traits of an organism

Homozygous

describes an individual with two identical alleles for a trait (AA or aa); compare heterozygous

Heterozygous

describes an individual with two different alleles for a trait (Aa)

Punnett square

simple grid used to illustrate all possible combinations of simple genetic crosse

test cross

cross of an individual of unknown genotype with a homozygous recessive individual; used as a method to determine the unknown genotype

dihybrid cross

cross of two individuals that differ in two traits

law of independent assortment

Mendel’s second law of inheritance, stating that the two alleles for one gene segregate (assort) independently of the alleles for other genes during gamete formation

incomplete dominance

a condition in which neither of two alleles for the same gene can completely conceal the presence of the other; compare complete dominance

Co-dominance

describes a situation in which two alleles may be expressed equally; occurs when two different alleles for a trait are both dominant

chromosome theory of inheritance

theory proposed by Walter Sutton that genes are carried on chromosomes

linked genes

genes found on the same chromosome

crossing over

the process by which non-sister chromatids exchange genes during prophase I of meiosis, allowing for the recombination of genes

chromosome mapping

process for determining the relative position of genes on a chromosome

map unit

distance between points on a chromosome where a crossover is likely to occur in 1% of all meiotic events

recombinant type

describes offspring that have a different combination of alleles than the chromosomes of their parents; also known as recombinants; compare parental type

parental type

describes offspring that have chromosomes that are identical to those of their parents; compare recombinant type

recombination frequency

percentage of times that a crossover occurs as gametes are formed

sex-linked trait

trait controlled by genes on either the X or Y chromosome

Barr body

structure formed when the inactive X chromosome condenses tightly

multiple alleles

pattern of inheritance in which a gene has more than two alleles for any given trait

order of dominance

sequence indicating which alleles are dominant to other alleles

continuous trait

a trait for which the phenotypes vary smoothly from one extreme to another

polygenic trait

trait that is controlled by many genes

Pedigree

diagram that uses symbols to illustrate the patterns of relationships and traits among a family over many generations

genetic screening

any of several methods of identifying people who are at risk of developing particular genetic conditions or of passing these conditions on to their children

genetic counsellor

person who uses an understanding of genetics to predict and explain traits in children