Genetics

Gregor Mendel

silly little monk in czech republic; worked with pea plants 1956-1866 and published results in 1866

knew there must be a set of principles to govern inheritance and factors that must be passed down generations

didn’t actually know what genes were but knew how they worked (the word “gene” was not used until much later)

Mendel’s Laws

1.) segregation of alleles: 2 copies of each gene (allele); alleles do not blend; meiosis packages into separate gametes; alleles separated during meiosis

2.) independent assortment of alleles: gene pairs assort independently; random lining up of tetrads; if close together on the chromosome, they may stay together/become linked

Gene

unit of heredity; part of a linear sequence of such units occurring in the DNA of chromosomes

traits (proteins)

Allele

pairs of genes for one trait on homologous chromosomes

one of at least two alternative forms of a gene

Dominance

complete: genes that mask or hide recessive genes; if gene is present, it is expressed

incomplete: one allele only partly masks another allele (ex: red and white snap dragon parents produce pink in F1 generation)

codominance: both genes expressed, but not mixed (ex: red and white camellias produce flowers with blotches of both colors but are not blended)

Recessive

genes masked or hidden by dominant genes

for gene to be expressed, 2 of the alleles are needed must be inherited (ex: yy)

Genotype & Phenotype

geno: genes that are physically found on the homologous chromosomes (ex: homozygous dominant YY, homozygous recessive yy, heterozygous Yy)

pheno: how the genotype is expressed; what is observable (ex: pea color; enzymes; height)

Homozygous & Heterozygous

homo: set of alleles are the same (ex: YY or yy)

hetero: sets or alleles are different from one another; may also be rereferred to as hybrid (ex: Yy)

Pea Plants Experiments

chosen for quick life cycle and rapid generations; can crosspollinate and self-pollinate (Mendel removed anthers so it couldn’t pollinate pistil, then used little paintbrush to transfer pollen)

wide variety of characteristic each with 2 variants (ex: white and purple flower color)

Characters & Traits

characters: a heritable feature (ex: flower color; seed shape (smooth or wrinkled)

traits: each variant for a character (ex: purple or white flower)

Generations

F1: offspring from true-breeding parents

F2: offspring of F1 generation; results in 1:2:1 ratio (25% homozygous dom: 50% heterozygous dom: 25% homozygous recessive)

Testcross

cross between an organism with the dominant phenotype and a homozygous recessive organism

if dominant parent is homozygous; phenotype all same

if dominant parent is heterozygous; half the phenotype is the same

Backcross

cross between a hybrid and one of its parents; proves dominance of parents

Dihybrid Cross

looks at two different characteristics (ex: in corn we looked at purple smooth, purple wrinkled, yellow smooth, yellow wrinkled)

cross involves two different pairs of genes and heterozygous parents

Polygenic Inheritance

trait is a result of the sum of several genes [ex: in wheat, 2 gene loci control kernel color; in capsicum annuum (bell pepper)]

Pleiotropy

condition where a single gene has more than one effect on an organism’s phenotype

ex: S gene is tobacco affects the corolla, anther, calyx, leaf, and capsule

Multiple Alleles

one gene can have multiple alleles (more than the 2 or 4 we often look at in punnett squares)

ex: v pattern in clovers; 7 different alleles

Epistasis

type of gene interaction in which a gene at one locus (i.e. gene location) masks or suppresses the effects of a gene at another locus

enzyme produced by dominant W allele catalyzes pathway that synthesizes a compound used by the gene M enzyme

(ex: corn kernels)

Environment Affect on Phenotype

our environment influences gene expression

ex: anthocyanin expression in hydrangeas: acidic soil (pH ≤ 5.5) will result in blue flowers; more alkaline (pH ≥ 6.5 soil results in pink flowers

Cytogenetics

the study of chromosome behavior

Barbara McClintock

botanist/cytogeneticist who introduced the concept of transposition after studying corn

transposable genetic elements: gene or gene fragment can get inside another gene and then inactivate gene or change its expression; movement of a piece of chromosome to another chromosomal location

ex: speckles on corn kernels

Transposable Genetic Elements

“jumping genes”- a DNA sequence (called a transposon) capable of being moved from one chromosomal location to another

the transposon can deactivate or disrupt gene function while present, then restore the function back its normal function before the mutation took place

ex: purple specks in yellow kernels are areas where pigment was not interrupted by the transposon, and the yellow areas are where the transposons did not allow the purple pigment to be expressed

Transposition Example in Grapes

transposition of Gret 1 into VvmybA1 (dark anthocyanin)

inactivates pigment genes and creates green/white grapes

Gret 1 can jump back out and mutates surrounding DNA, resulting in red grapes

Polyploidy

having an extra set of chromosomes

common in plants such as wheat or strawberries; 30-70% of angiosperms are thought to be polyploid

can be beneficial to plants: bigger plant parts such as stems, roots, fruits, or seeds

Polyploidy vs Pleiotropy 

Aneuploidy

abnormal chromosome number caused by a nondisjuction event, which is a failure of separation in either homologous chromosomes (meiosis 1) or sister chromatids (miosis II)

Allopolyploid

polyploid generated from two species

ex: upland cotton: produces fibers that are longer, stronger, and whiter than diploid counterparts; valuable in agriculture

Autopolyploid

polyploid generated from one species; offspring may be viable and self-fertile, but not always 

Chromosomal Mutations

deletion: gene removed

inversion: genes switch places

translocation: crossing over between nonhomologous chromosomes; (ex: rye)

duplication: two copies of gene