biology - genetic variation

chromosome

long, coiled DNA molecule carrying many genes

DNA

molecule that stores genetic information. polymer made of repeating units called nucleotides

nucleotides have a phosphate group, a deoxyribose sugar and one of four nitrogenous bases

gene

a section of DNA that influences a specific characteristic. order of base sequences determines the gene

allele

an alternative version of a gene. you get one allele from each parent.

genotype

heterozygous (Aa) = dominant trait

homozygous dominant (AA) = dominant trait

homozygous recessive (aa) = recessive trait

gamete

mature haploid cell that unites during reproduction to form a zygote

haploid (n) vs diploid (2n)

cell containing a single set of unpaired chromosomes

cell containing two chromosome sets, one from each parent

meiosis process

interphase - DNA is copied

prophase 1 - chromosomes pair up with matching partners and swapping over happens

metaphase 1 - chromosome pairs line up in the middle of the cell

anaphase 1 - pairs are pulled apart to opposite sides of the cell

telophase 1 and cytokinesis - cell splits into two new daughter cells

prophase 2 - chromosomes get ready to split

metaphase 2 - chromosomes line up in a single line down the cell

anaphase 2 - the sides of each chromosomes (sister chromatids) are pulled apart to opposite ends

telophase 2 and cytokinesis - cells divide again, resulting in four haploid cells

in meiosis 1, sister chromatids stay together. in meiosis 2, they separate

crossing over (FOUR SOURCES OF VARIATION)

prophase 1, non sister chromatids exchange DNA segments, forming recombinant chromatids with new allele combinations

independent assortment (FOUR SOURCES OF VARIATION)

at metaphase 1, each homologous pair aligns independently and randomly. this creates 2ⁿ possible chromosome combinations (n = number of pairs)

random fertilization (FOUR SOURCES OF VARIATION)

any sperm can fuse with any egg, so two genetically different gametes combine to form a unique zygote

mutation (FOUR SOURCES OF VARIATION)

a change in the DNA base sequence can create a new allele. mutations are the original source of new genetic information

kinds of asexual reproduction

binary fission - parent cell copies genetic material, grows lager and splits down middle to form two identical daughter cells

fragmentation - organism splits off into pieces and the different pieces grow into a full individual

budding - new organism/plant shoots out from the parent. the bud gets bigger until it separates to become an entirely different new clone

parthenogenesis - an embryo grows from an unfertilized egg when a female’s egg triggers its own cell division

spore formation - spores carrying genetic material float through the air on wind or rain or other animals or bugs to take root in a different location to the parents

vegetative propagation - new plants grow from the stem or roots of a parent plant NOT from a new seed

segregation

paired alleles separation during meiosis = each gamete gets one (anaphase 2)

each gamete only carries one allele for each trait so that when sperm meets egg, offspring end up with the correct diploid number

prophase and synapsis

homologous pairs line up close in a process called synapsis

non sister chromatids can touch at points called chiasmata then break and reattatch to each other

linked genes

genes close together on the same chromosome are ‘linked’ meaning they tend to be inherited together as they are unlikely to be separated by crossing over → this reduces expected variation in offspring as alleles don’t sort independently

sex linked genes

these genes are on the x or y chromosome. x is larger to some x-linked genes have not matching gene on y.

example = red green colour blindness is x-linked. male needs one affected allele, female usually needs two

x linked traits can be inherited differently from typical genes

test crossing

• crossing a dominant trait expressing individual to see if it is heterozygous or homozygous dominant. cross it with a homozygous recessive to see if original individual has recessive gene present - if recessive individual is born, its heterozygous and if enough offspring are born with no recessive phenotypes being expressed, then you can be pretty certain, the original individual is homozygous dominant

dihybrid cross

cross between individuals involving two pairs of contrasting traits (genetically determined characteristic or condition)

mendel’s laws

law of segregation - alleles for different traits separate into different gametes so that the haploids have one allele

law of independent assortment - inheritance of one trait does not affect the inheritance of another because homologous pairs align randomly (provided genes are on different chromosomes)

solving dihybrid cross

FOIL method

RrYy

first = RR

outer = Ry

inner = rY

last = ry

ratio

9:3:3:1 if genes are on different chromosomes

if you see a ratio that deviates significantly from 9:3:3:1 in a large sample, suspect gene linkage

co-dominance

both alleles in a heterozygous cross are fully and equally expressed

multiple alleles

a gene having more that two possible alleles in a population (e.g. blood types I^A, I^B, i)

incomplete dominance

alleles are blended together so no allele is completely dominant

lethal allele

an allele causing death in an organism (typically when homozygous)

lethal alleles cause death before being born if you have two (WHEN THEY’RE HOMOZYGOUS)

lethal alleles removed from phenotypic ratio

carrier

an individual possessing one recessive allele without trait expression

solving pedigree charts

start with recessive allele bearing person

mutation

permanent change in the base sequence of DNA

mutagens are environmental agents increasing the rate of mutation e.g. harmful rays, chemicals, ect

ONLY WAY TO MAKE NEW ALLELES

gene pool

total set of alleles in a population

ways in which mutations occur

nucleotides can be deleted, inserted or swapped and this can change the protein shape as protein chain may no longer work properly/cannot fold properly

mutations can occur in coding (1%) and non coding (99%) - non coding mutations till impact phenotypes as non-coding impacts expression

kinds of mutations

somatic = body cell, not inheritable, often caused by environmental factors

gametic = sex cell, inheritable, every cell in offspring will carry gene

causes of mutation

spontaneous → occurs during dna replication when polymerase makes a mistake (gametic)

induced by mutagens → physical or chemical mutagens like sun/cigarettes (somatic)

mutation impacts

silent mutations occur when bases that change still code same protein (GTA = protein one and GAA = protein 1)

harmful = protein function is impacted and can cause diseases like cystic fibrosis

beneficial = highly unlikely. causes survival advantage AS THOSE WITH THE MUTATION SURVIVE LONG ENOUGH TO HAVE OFFSPRING

epigenetics

changes in gene expression without DNA sequence shifts

plasticity

a genotype’s capacity to produce varied phenotypes in environment. also results in evolutionary advantage as it allows survival in fluctuating conditions

environmental variation

organisms are rarely at genetic maximum - instead, their environmental factors like nutrition, temperature and light determine variation

norm of reaction

range of phenotypes produced by a singular genotypes across different environments → genes provide potential while environment depends point within range

multifactorial

genes affected by both genotype and environment