Ch 9 - Mendelian Genetics

Chromosome theory of inheritance

the principle that genes and their alleles are carried on the chromosomes

Character

a heritable characteristic

Trait

a particular variation in a genetic or phenotypic character

Allele

alternative forms of a gene; genes governing variation of the same character that occupy corresponding positions (loci) on homozygous chromosomes

Homozygous

a true breeding organism having 2 identical alleles for a given characteristic

Heterozygous

having 2 different alleles for a given characteristic

Hemizygous

a condition in which only one copy of a gene or DNA sequence is present in diploid cells

Dominant allele

an allele that is always expressed when it’s present, regardless of whether it is homozygous or heterozygous

Dominance

the masking effect of one allele over another

Dominant

refers to the allele expressed when more than one allele is present

Recessive allele

an allele that is not expressed in the heterozygous state

Recessive

an allele that is masked by a dominant allele

True-breeding

refers to an individual that passes traits without change from one generation to the next; organisms for which sexual reproduction produces offspring with inherited traits identical to those of parents

Self-pollination

fertilization in which sperm nuclei in pollen produced by anthers fertilize egg cells housed in the carpel of the same flower; can also occur in hermaphroditic animals

Cross-pollination

fertilization of one plant by a different plant

P Generation

the parental individuals used in an initial cross

F1 Generation

the first generation resulting from a cross; all plants had purple flowers

F2 generation

the next generation of plants from self-fertilization of the F1 offspring; plants had some purple and some white flowers, 3:1 ratio

Mendel’s law of segregation

Mendel’s first law; when an individual produces gametes, the alleles of a gene pair separate, so that each gamete receives only one member of the pair of alleles

Mendel’s law of independent assortment

Mendel’s second law; alleles of two (or more) different genes get sorted into gametes independently of one another

Testcross

a cross of an individual of unknown genotype (which may be either heterozygous or homozygous) for a particular characteristic with a homozygous-recessive individual for that same characteristic

Monohybrid

an F1 heterozygote produced from a genetic cross that involves a single character

Monohybrid cross

a genetic cross between two individuals that are each heterozygous for the same pair of alleles

Dihybrid

a zygote produced from a cross that involves two characters

Dihybrid

a zygote produced from a cross that involves two characters

Dihybrid cross

a cross between 2 individuals that are heterozygous for 2 pairs of alleles

Phenotype

the physical appearance of an organism; observable/expressed traits of an individual, including the organism’s appearance, development, biochemical/physiological properties, and behaviour

Genotype (broad)

refers to the genetic makeup of an organism

Genotype (narrowed)

term can be used to refer to the alleles or variants of a gene that are carried by an organism; contributes to phenotype

Along with genotypes, what are the other two factors that determine phenotype in complex traits?

Epigenetic factors and non-inherited environmental factors

Incomplete dominance

a type of inheritance in which one allele for a specific trait is not completely expressed over its paired allele in F1 hybrids; results in a 3rd phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles

Example of incomplete dominance

Red flower + White flower = Pink flower

Codominance

a cross between organisms with 2 different phenotypes produces offspring with a 3rd phenotype in which both of the parental traits appear together

Example of Codominance

Brown cow + White cow = Brown-spotted White cow

Multiple alleles

occurrence of a gene that exists as 3 or more alleles in a population; when a single gene has more than 2 allelic forms in a population

Example of Multiple alleles

Human blood types

Pleiotropy

the determination of more than one character by a single gene; one gene influences multiple traits

Example of pleiotropy

Sickle cell anemia and marfans

Epistasis

occurs when 2 or more different gene loci contribute to the same phenotype, but not additively; often described as when one gene masks or modifies the phenotype of a second gene; 2 or more genes interact to influence a single trait

Example of epistasis

Mice fur, Labrador retriever coats

Polygenic inheritance

the additive effect of two or more gene loci on a single phenotypic characteristic

Example of polygenic inheritance

Skin colour in humans