AP Biology - Meiosis & Genetics

character

character

A heritable feature that varies among individuals

ex:) flower color

trait

Each variant for a character

ex:) purple or white color for flowers

true-breeding

varieties that, over generations of self-pollination, have produced only the same variety as the parent

ex:) a purple flower plant produces seeds in successive generations from self-pollination that all produce purple flower plants.

Hybridization

the mating/crossing of two true-breeding varieties

P generation

Parental generation; the first two individuals (true bred) that mate in a genetic cross

F1 generation

1st generation; hybrid offspring of the P generation

F2 generation

the 2nd generation; offspring of F1 self-pollination or cross-pollination w/ other F1 hybrids

tracking inheritance into the F2 gen allowed Mendel to

create the law of segregation and the law of independent assortment

Mendel found that some "heritable factors" could be

hidden in the presence of other factors

ex:) When F1 plants (all purple from P gen) self-pollinated, white flowers reappeared in F2 gen (was not completely erased by purple allele).

allele

An alternative version of a gene at similar locations

ex:) gene for flower color -> purple allele, white allele, etc

A gene's DNA/sequence of nucleotides

have a specific locus along the chromosome and can vary slightly

Variations in DNA can

affect encoded protein function and an inherited character

ex:) purple allele = synthesis of purple pigment, white allele = no synthesis

an organism inherits

two alleles (one from each parent) for each character

the dominant allele

determines the organism's appearance

the recessive allele

has no noticeable effect on the organism's appearance

Law of Segregation

Mendel's law that states that the 2 alleles for a heritable character segregate during gamete formation and end up in different gametes bc homologs are separated during meiosis

true-breeding gametes

identical allele in all gametes

hybridization gametes

half dominant, half recessive allele gametes

All allele combinations have

random/equal chances of being made

Punnett square

diagram that can be used to predict the allele composition (genotype) and phenotype combinations of the offspring of a known genetic cross

homozygote

an organism that has a pair of identical alleles for a gene encoding a character (said to be "homozygous" for that gene)

heterozygote

organism that inherits two different alleles for a given gene (said to be "heterozygous" for that gene)

phenotype

An organism's physical appearance, or visible traits.

genotype

genetic makeup of an organism

testcross

Breeding an organism of unknown genotype with a homozygous recessive individual to determine the unknown genotype (The ratio of phenotypes in the offspring reveals the unknown genotype)

monohybrids

offspring that were heterozygous for one particular character being followed in a cross (monohybrid cross) helped derive the law of segregation

the law of independent assortment was derived from

following two characters simultaneously, instead of one

dihybrid

a hybrid that is heterozygous for the 2 characters being followed in a cross (dihybrid cross)

Law of Independent Assortment

Mendel's second law, stating that each pair of alleles segregates, or assorts, independently of each other pair during gamete formation (applies when genes located on different chromosomes (not homologs) or when they are far apart on the same chromosome)

independent event

The outcome of one event does not affect the outcome of the second event

ex:) gene segregation

multiplication rule

rule stating that the probability of two or more independent events occurring together can be determined by multiplying their individual probabilities.

ex:) 1/2 chance of getting either dominant or recessive allele. Getting 2 recessive alleles: 1/2 x 1/2 = 1/4 chance of that outcome

mutually exclusive event

Events that cannot occur at the same time.

addition rule

rule stating that the probability of any one of two or more mutually exclusive events occurring can be determined by adding their individual probabilities.

ex:) an allele can come from the egg or sperm, but not both at the same time/same gamete

multicharacter crosses are equal to

multiple independent monohybrid crosses occurring simultaneously

ex:) create a monohybrid cross for each character, then use the multiplication rule to find the multicharacter genotype.

to find the probability of a certain condition

calculate the probabilities of each outcome with the multiplication rule, then use the addition rule to find the probability of fulfilling the specified condition

complete dominance

The situation in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable.

ex:) offspring look like one of the parents

incomplete dominance

neither allele is completely dominant, and the hybrids have a phenotype somewhere in between the 2 parental varieties

ex:) red x white flowers = pink flowers

codominance

the two alleles each affect the phenotype in separate, distinguishable ways

ex:) heterozygotes for M and N alleles have BOTH M and N molecules on their red blood cells (shows both phenotypes)

Having dominance means

the trait is seen in the phenotype, NOT that it "subdued" the recessive allele. They don't actually interact.

ex:) one dominant allele (heterozygous) codes for enough enzyme to synthesize branched starch so dominant homo/heterozygotes have the same phenotypes (round seed shapes)

observed dominant/recessive relationship of alleles depends on

the level at which we examine the phenotypes

ex:) Tay-Sachs Disease recessive at the organismal level bc 2 alleles are needed incomplete dominance at the biochemical level bc heterozygotes have an intermediate (betw. normal and abnormal) enzyme activity level codominant at the molecular level bc heterozygotes make equal #s of normal and dysfunctional enzymes

being dominant does not always equal to

being more common than the recessive allele in a population

ex:) being born w extra digits = dominant, but low frequency (1/400). The recessive allele (5 digits when homozygous) is more prevalent

multiple alleles

most genes exist in more than two allelic forms

ex:) ABO blood groups blood type is determined by 2 of 3 possible alleles (IA, IB, i), with phenotypes being type A, B, AB, or O blood (A, B, both, or no carbs on your RBCs)

pleiotropy

the property of a gene that causes it to have multiple phenotypic effects

Ex:) pleiotropic alleles causing hereditary diseases as well as their symptoms

epistasis

the phenotypic expression of a gene at one locus alters that of a gene at a second locus

ex:) lab retrievers one gene determines coat color (black - B, brown - b) another gene determines if pigment is deposited (E) (color depends on 1st genotype) or not (e). If 2nd gene is recessive (ee), the coat is yellow, regardless of the 1st gene.

quantitative characters

Characters that vary in the population along a continuum (in gradations), not discretely one or the other

ex:) height, skin color

polygenic inheritance

An additive effect of two or more genes on a single phenotypic character.

ex:) there are at least 180 genes affecting height

the converse of pleiotropy (one gene, multiple characters)

phenotypes are RANGES of possibilities influenced by

environmental factors as well as genotype (nature vs nurture)

multifactorial character

characters where the environment contributes to the quantitative nature of them. Many factors, both genetic and environmental, collectively influence phenotype.

pedigree

a family describing the traits of parents and children across generations by collecting family history info for a certain character

a pedigree can be used to figure out

the probability that a child will have a certain genotype/phenotype, what the genotype of an unknown person is, and if a trait is recessive or dominant

recessive genetic disorder alleles

code for either a malfunctioning protein or no protein at all

carrier

a phenotypically normal heterozygote that may transmit the recessive allele for a disorder to their offspring

disorders are not

evenly distributed amongst different populations bc of differences in genetic histories and isolation

close relatives and consanguineous mating

increases chances of passing on recessive alleles compared to unrelated individuals

cystic fibrosis

common lethal genetic disorder that occurs in people (1/2500 of European descent, less in others) with two copies of the recessive allele plasma membrane chloride transport channels are defective/absent = excessive secretion of mucus and other pleiotropic effects. Fatal if untreated.

sickle-cell disease

common recessive inherited disorder in African descendants (1/400) cause by the substitution of 1 amino acid in hemoglobin in RBC. sickle-cell hemoglobin aggregates in rods that deform RBCs into sickle shapes when oxygen is low = clumps and blocks vessels = pain, organ damage, stroke, paralysis, etc

organismal level: incompletely dominant bc heterozygotes don't have full disease, but still an affected phenotype

molecular level: codominant bc both normal and sickle-cell hemoglobin is made in heterozygotes

being heterozygous/a carrier of sickle-cell disease can be an advantage because

malaria attacks (common in Africa) are less intense. Heterozygotes allow sickle-cell disease to continue/not get erased by evolutionary processes when homozygotes die from it

dominantly inherited disorders might not be common because

the recessive allele is more prevalent in a population (even though it's a dominant allele)

deadly dominant diseases are less common than deadly recessive ones because

recessive alleles are only lethal when homozygous and can be carried by otherwise healthy heterozygotes dominant alleles guarantee the disease and can cause early death, preventing it from being passed on

deadly dominant disorders usually have

symptoms that appear after the reproductive age

ex:) Huntington's disease lethal dominant disorder that has no obvious phenotypic effect until about 35-45 y/o irreversible degenerative disease of the nervous system with a 50% chance of passing it on

more people are at risk of diseases with

a genetic component and an environmental influence (multifactorial)

ex:) heart disease, diabetes, cancer, alcoholism, etc

lifestyle dramatically impacts ____________ no matter what the genotype is for multifactorial characters

phenotypes

ex:) exercise, diet, smoking, stress, etc can reduce or lead to/worsen cancer or heart disease as well as the genotypes for it

genetic counselors can provide

info to parents concerned about family history or a disease, especially if its in the family/they're carriers Mendelian probabilities, laws, and rules can be used to determine the chance a child has a trait (each are independent events), what the parent genotypes are, and more.

heredity

the transmission of traits from one generation to the next

variation

genetic differences in offspring

genetics

The scientific study of heredity and inherited variation

genes

hereditary units of coded info that is passed down by parents. Programs specific traits that emerge as fertilized eggs develop into adults

genes have specific

sequences of nucleotides

genes program cells to

synthesize specific enzymes and other proteins that produce inherited traits all together

gametes

reproductive cells in plants and animals that transmit genes during fertilization, sperm and eggs unite and pass genes from both parents to offspring

somatic cells

all body cells except reproductive cells

locus

a gene's specific location along the length of a chromosome

asexual reproduction

a single individual is the sole parent and passes copies of all its genes to its offspring without the fusion of gametes. Offspring are genetically identical to the parent.

clone

a group of genetically identical individuals from one that reproduces asexually

asexual reproduction can be bad because

there is no genetic variation = one thing (disease, predator, etc) could wipe out the population

mutations cause

genetic variation in asexual reproduction

Sexual Rproduction

2 parents have offspring that have unique combinations of genes from each parent, resulting in genetic variation

life cycle

The generation-to-generation sequence of stages in the reproductive history of an organism. (conception ->production of own offspring)

Humans have ___ chromosomes

46 (23 pairs). Can be distinguished by size, centromere position, and the pattern of colored bands produced by certain chromatin-binding stains

karyotype

a micrograph of chromosomes in a single cell, arranged by length

homologous chromosomes/pair (homologs)

Chromosomes that pair during meiosis and are the same length, have the same centromere position, and staining pattern

homologs carry genes

controlling the same inherited characters ex:) an eye color gene will have a similar version at an equivalent locus on the homologous chromosome

sex chromosomes

Chromosomes that determine the sex of an individual

Female sex chromosomes

XX

Male sex chromosomes

XY

the male sex chromosomes are exceptions to homologs because

most X genes do not have Y counterparts and vice versa

autosomes

Chromosomes that are not directly involved in determining the sex of an individual.

one chromosome of each pair is from each parent, so we actually inherit

two sets of 23 chromosomes (maternal and paternal)

diploid cell

A cell containing two sets of chromosomes (2n), one set inherited from each parent.

ex:) somatic cells

haploid cells

A cell containing only one set of chromosomes (n).

ex:) gametes

fertilization

the union of gametes. Begins the life cycle

zygote

the fertilized egg (now diploid bc it has the 2 haploid sets of chromosomes) mitosis of the zygote generates somatic cells

meiosis

type of cell division that produces gametes in sexually reproducing organisms reduces the number of chromosome sets to counter doubling at fertilization

life cycle varieties

alternation of meiosis and fertilization occurs at different times in each species. 3 main variations: animal, plant, fungi/protists

animal variation

germ cells (made in the gonads - testes and ovaries) undergo meiosis, making gametes (sperm and eggs, the only haploid cells). Fertilization restores the diploid state (zygote). Zygote divides by mitosis, germ cell are made to repeat the cycle

plant variation

alternation of generations haploid multicellular gametophytes make gametes. Fertilization forms a diploid zygote. The zygote will give rise to a diploid multicellular sporophyte that will undergo meiosis and produce haploid spores. The spores will then develop into the gametophytes to repeat the cycle

fungi/protist variation

multicellular haploid hyphae grow towards each other and form a zygosporangium, containing multiple haploid nuclei from the two parents within a single cell. The haploid nuclei fuse to form diploid nuclei (zygotes) in the zygospore. The diploid nuclei undergo meiosis to make haploid nuclei that are released spores that germinate and divide by mitosis to make new, multicellular haploid fungi and repeat the cycle

in meiosis there are ____ divisions, _____

2; resulting in 4 daughter cells with 1 set of chromosomes

sister chromatids vs homologous chromosomes

sister chromatids: 2 copies of 1 chromosome homologous chromosomes: individual chromosomes from different parents w different versions of a gene

stages of meiosis

Meiosis I (prophase I, metaphase I, anaphase I, telophase I, cytokinesis)

Meiosis II (prophase II, metaphase II, anaphase II, telophase II, cytokinesis)