8.1 Genes are particulate and are inherited according to Mendel’s laws
Mendel’s Pea Plants Father of genetics Crossed pea plants b/c available for experimentation and easy to control Pea flowers have both female and male gametes; they normally self-fertilize, so Mendel removed the male stamens
Vocabulary Trait: characteristic to be observed (ex. Seed shape, seed color) Alleles: versions of traits (ex. wrinkled or smooth seeds) Parental (P) generation F 1 (first filial) generation: offspring of parents F 2 (second filial) generation: offspring of offspring
Homozygous: same alleles (AA or aa), can be homozygous dominant or homozygous recessive; aka purebred or true-breeding Heterozygous: different alleles (Aa); aka hybrid Genotype: combination of alleles (AA, Aa or aa) Phenotype: physical appearance of trait (internal or external) Monohybrid cross: purebred parents of different alleles are crossed; F1 gen. is allowed to self-fertilize
We can predict the outcome of monohybrid crosses in ratios and percentages We can also use the procedure to determine the genotype if it is unknown
Test Cross to determine genotype Dominant alleles: need just one to show phenotype (uppercase) Recessive alleles: need two to show phenotype (lowercase)
Mendel’s 1 st Law Law of segregation – When an individual produces gametes, the two copies of the gene separate, so that each gamete receives only one copy – Punnett squares help to show the possible gamete combinations from two parents
Mendel’s 2 nd Law Law of independent assortment – Alleles of different genes/traits assort independently of each other in gamete formation – b/c the chromosomes assort independently – Ex: blue eye color genes and blond hair color alleles are not tied together – Ex: gametes combine at random; an AaBb individual will produce gametes that randomly get the A or a, and randomly receive the B or b
Dihybrid cross: crossing parents who are heterozygous for the chosen traits
8.2 Alleles and genes interact to produce phenotypes
Genes and their alleles are not always as simple as dominant versions and recessive versions Environment and other genes can affect how genes are expressed
Mutations provide genetic variety – Changes in the genetic code that produce different versions of alleles – Wild-type allele: allele that is most frequent in individuals (vs. the mutant allele[s]) – Mutations are the source of evolutionary change
Different Types of Dominance Incomplete dominance: alleles are not completely dominant or recessive – Heterozygotes show a BLEND – Ex: RR (red) x WW (white) = RW (pink) Codominance: more than one allele is able to produce a phenotypes – Heterozygotes have a completely different genotype – Ex: RR (red) x WW (white) = RW (red / white spotted)
Gene interactions Epistasis: phenotype expression of one gene affects the expression of another Labrador coat color BB or Bb = black (with EE or Ee) bb = brown (with EE or Ee) ee = yellow, no matter what the B alleles are E determines the expression of B and is epistatic to B
Human examples: red hair, albinism Individuals have alleles for other hair colors or production of skin pigment, but the red hair or albinism genes hide the others
Most complex traits are determined by multiple genes interacting together - “quantitative” traits: must be measured rather than looked at qualitatively -examples: higher grain yield of corn from hybrid varieties - hybrid vigor: interaction of multiple alleles sometimes results in enhancement of qualities
Environmental interactions w/genes Sequencing of human genome was not the “end product” we thought; turns out an organism’s genotype does not determine its entire phenotype Penetrance: how many individuals who have the gene express the gene – BRCA1 breast cancer gene; not everyone who has it develops breast cancer; incompletely pentrant Expressivity: degree to which a gene is expressed – BRCA1 may cause both breast & ovarian cancer in some women but only breast cancer in others; variable expressivity
Example: extra toes on cats - Gene has high penetrance (always causes extra toes) but variable expression (how many toes can be different)
gene: sequence of DNA at a particular locus (location) on a chromosome
Linked Genes If test crosses do not come out with expected ratios, then they are not assorting independently and the genes must be linked. The genes are being inherited together.
- G and R are linked. - In this GgRr individual, the only possible gametes are GR and gr (not Gr or gR). -GgRr x ggrr cross -Gametes: GR, gr x gr -Expected results: -What if this cross produces gray, purple offspring? They come from recombination. G = gray body g = black body R = red eyes r = purple eyes
Suppose out of 100 offspring, you got: 46 gray/ red 46 black/purple 4 gray/purple 4 black/red. Eight percent of the offspring resulted from crossing over. These offspring are recombinant.
Recombination Homologous chromosomes swap pieces while crossing-over during Prophase 1 of meiosis.
If linked genes give unexpected ratios, how do we predict phenotypes? -Recombination frequency: calculated by dividing the number of recombinant offspring by the total number of offspring 8/100 = 0.08 = 8% -Frequency is higher for loci that are farther apart on chromosomes b/c crossing over is more likely between genes that are farther apart.
Recombination frequences are converted to map units on genetic maps. Genetic maps and studying linkage has helped geneticists isolate genes, target them, and identify individuals with particular alleles – Animal breeding – Agriculture – Medically significant human mutations
Sex-Linked Genes Some genes are linked to the X or Y chromosomes X and Y chromosomes contain very different genes Females XX Males XY Males have only one copy of X, so they display X-linked diseases much more often
Dominant or recessive disorder? Males can only pass on mutations in their X chromosome to daughters Daughters can be heterozygous carriers of mutations
Other sources of genetic information Mitochondrial DNA Plastid DNA (ex: chloroplast) Inherited maternally AKA cytoplasmic inheritance – Nucleus in sperm is only surviving part of male gamete – Egg cell contains organelles and cytoplasm – ABRACADABRA, you inherit your mother’s mtDNA
Bacteria have a single chromosome They reproduce asexually (binary fission) and produce clones Yet they evolve, much to our dismay (antibiotic resistance) BUT HOW?
Bacterial DNA can still mutate Mutations create diversity and variety But there’s only one way to pass it on, since they don’t reproduce sexually with each other bacterial conjugation, aka horizontal gene transfer
A sex pilus draws the two cells together Genetic material from the donor will be transferred to the recipient through a conjugation tube Donor DNA can recombine with the recipient cell’s chromosome About half the DNA is transferred to chromosome, the other half lost
Plasmids: additional circular pieces of DNA in bacteria (besides their single circular chromosome) These usually contain metabolic process genes or antibiotic resistance genes These can transfer to recipient cells as well THIS IS REALLY REALLY BAD FOR HUMAN BEINGS