Quiz 4 Flashcards
genetics
scientific study of heredity and variation
heredity
transmission of traits from one generation to the next
variation
differences in appearance of offspring from parents and siblings
how are traits inherited?
through genes, specific sites on chromosomes
- physical characteristics (like big muscles) not inherited
what are genes and how are they passed on
units of heredity; made up of segments of DNA packaged into chromosomes
- each has a specific location on a chromosome (locus)
- passed on to the next generation via reproductive cells called gametes (sperm and egg)
asexual reproduction and its benefits
mitosis - single individual passes genes to offspring without fusion of gametes
*good in times of environmental uncertainty!
clone
group of genetically identical individuals from the same parent (created via mitosis)
sexual reproduction
meiosis - two parents give rise to offspring with unique combinations of genes (from both parents) through the fusion of gametes
exceptions to rules of asexual/sexual reproduction
some protists reproduce “sexually” without sperm/egg gametes
homologous chromosomes and how many do humans have
the 2 chromosomes in each pair; one from mother and one from father
- same length/shape and carry genes controlling the same inherited characteristics
- humans have 23 pairs of chromosomes; females have 23 homologous pairs and males have 22 homologous pairs
karyotype
ordered display of pairs of chromosomes from a cell
sex chromosomes
determine biological sex of an individual
- females are homologous (XX)
- males are XY (Y is shorter, may have broken off ancestral X)
autosomes
remaining 22 pairs of chromosomes in humans
what happens to homologous chromosomes during DNA replication?
each chromosome is replicated and each replicated chromosome makes 2 identical sister chromatids
*4 chromatids for a homologous pair
what are haploid cells and where are they produced in humans
haploid cells: single set of chromosomes
- gametes (sperm or egg) are human haploid cells (haploid number: 23)
- gametes produced by cells in gonads (testes and ovaries) that undergo meiosis
how do organisms maintain chromosome number?
fertilization and meiosis alternate in organisms with sexual life cycles
- 3 main types
life cycle
generation-to-generation sequence of stages in the reproductive history of an organism
- in organisms with sexual life cycles, fertilization and meiosis alternate to maintain chromosome #
- 3 main methods of alternation
life cycles in animals
- gametes are only haploid cells; undergo no further division before fertilization
- gametes fuse during fertilization to become a diploid zygote
- zygote divides by mitosis to form a multicellular organism
life cycles in plants/some algae
includes both diploid and haploid multicellular stage!
- diploid sporophyte makes haploid spores by meiosis
- spores grow by mitosis to form a haploid gametophyte
- gametophyte makes haploid gametes by mitosis
- fertilization of 2 gametes results in a diploid sporophyte
life cycles in fungi/some protists
- only diploid stage is the single-celled zygote
- zygote produces haploid cells by meiosis
- haploid cells grow by mitosis to form multicellular organisms
- haploid adults produce gametes by mitosis that fuse to create diploid zygotes
which types of cells can undergo mitosis/meiosis?
- all cells can divide by mitosis
- only diploid cells can divide by meiosis
significance of fertilization and meiosis in all 3 life cycles
- contributes to genetic variation in offspring
- maintain chromosomes number
divisions and result of meiosis (overview)
- 2 sets of cell divisions result in 4 daughter cells with 1/2 as many chromosomes as the parent
- meiosis I (reductional division): homologs form a tetrad and separate; results in 2 haploid daughter cells with replicated chromosomes attached at centromere
- meiosis II (equational division): sister chromatids separate; results in 4 haploid daughter cells with unreplicated chromosomes
prophase I
- typically >90% of meiosis time; chromosomes condense
- synapsis: homologous chromosomes loosely pair up, align gene by gene to form a tetrad
- crossing over: nonsister chromatids exchange DNA segments
tetrad
pair of homologous chromosomes (4 chromatids) with chiasmata, x-shaped regions where crossing over occurred
metaphase I
- tetrads line up at metaphase plate with 1 chromosome facing each pole
- microtubules from 1 pole attach to kinetochore of 1 chromosome of tetrad; microtubules from other pole attach to kinetochore of other chromosome
anaphase I
- pairs of homologous chromosomes separate
- one chromosome moves towards each pole, guided by spindle apparatus
- sister chromatids remain attached at the centromere and move as one unit towards pole
telophase I and cytokinesis
- each cell half has a haploid set of chromosomes (each w/ 2 sister chromatids)
- cytokinesis occurs simultaneously, forming 2 haploid daughter cells (cleavage furrow or cell plate)
***no chromosome replication occurs between end of meiosis I and beginning of meiosis II
prophase II
- spindle apparatus forms
- chromosomes move towards metaphase plate
metaphase II
- chromosomes at metaphase plate
- **due to crossing over, sister chromatids no longer genetically identical
- kinetochores of sister chromatids attach to microtubules from opposite poles
anaphase II
- sister chromatids separate, move as newly individual chromosomes toward opposite poles
telophase II and cytokineiss
- chromosomes arrive at opposite poles and decondense; nuclei form
- cytokinesis separates cytoplasm
- end result is 4 haploid daughter cells that are genetically distinct from each other and the parent
main differences in mitosis vs meiosis
Mitosis: conserves # of chromosome sets, produces genetically identical cells
Meiosis: reduces # of chromosome sets, producing cells that differ genetically
roles of mitosis and meiosis in organisms
Mitosis: asexual reproduction, development from zygote, growth, repair
Meiosis: reduces # of chromosomes, increases genetic variability
3 events unique to meiosis
- synapsis (chromosomes connect and exchange genetic info)
- tetrads line up at metaphase plate (paired homologous chromosomes instead of individual replicated chromosomes)
- homologous chromosomes separate instead of sister chromatids
importance of mutations for genetic variation
- original source of genetic diversity (even asexual), change in organism’s DNA
- create different versions of genes at the same locus called alleles
- reshuffling of alleles during crossing over produces variation
3 mechanisms contributing to genetic variation
- independent assortment of chromosomes
- crossing over
- random fertilization
independent assortment of chromosomes
- homologous chromosomes orient randomly during metaphase I
- each pair of chromosomes sorts into daughter cells independently
- 2^n combinations possible
what does crossing over produce?
produces recombinant chromosomes, combining DNA inherited from each parent
random fertilization
- any sperm can fuse with any ovum (unfertilized egg)
- natural selection results in accumulation of genetic variation favored by the environment
2 theories for passing of traits
blending hypothesis - genetic material from 2 parents “blends” together, like paint colors
particulate hypothesis - parents pass on “discrete heritable units” (what we now call genes); offspring inherit a unit from each parent
- documented by Mendel
- explains how genes can skip generations
advantages of Mendel’s quantitative approach with pea plants
- many varieties of pea with different traits of distinct heritable characteristics
- mating can be controlled; each flower has sperm-producing stamens and egg-producing carpels
- cross-pollination involves dusting one plant with pollen from another
- he could track characteristics with only 2 distinct forms
characteristic vs trait
characteristics are recognizable features on organisms (e.g. flower color) whereas traits are variants of characteristics (e.g. purple or white flower)
Mendel’s general methodology
- ***used true-breeding varieties (produce off spring of the same variety during self-pollination; a pure lineage)
- **tracked only characteristics with 2 distinctive forms
- mated 2 contrasting, true-breeding varieties (P generation) to produce a F1 generation
- F1 generation either self- or cross-pollinated to produce a F2 generation
results of Mendel’s crosses
- all F1 hybrids purple
- many F2 were purple, but some were white (3:1 ratio)
Mendel’s general learnings from the experiment
- reasoned only purple factor affected F1 color, but the factor for white flowers must not have been destroyed since it reappeared in the F2 generation
- called the purple color a dominant trait and the white color a recessive trait
- same pattern of inheritance observed with 6 other characters
4 concepts of Mendel’s Model
- alternative versions of genes (alleles) result in variations of characteristics
- organism inherits 2 alleles at a locus, one from each parent, which may be identical or differ
- if the 2 alleles differ, the dominant allele determines appearance and the recessive allele had no effect
- law of segregation: the 2 alleles of a single trait segregate during gamete formation and end up in either egg or sperm
* **(corresponds to knowledge today about how chromosomes are sorted into gametes, but he didn’t know about chromosomes!!)
punnet squares and examples of genetic makeups
diagrams for predicting results of a cross between individuals of a known genetic makeup
homozygous: 2 identical alleles at a locus
heterozygous: 2 different alleles at a locus (not true-breeding)
phenotype vs genotype
because dominant and recessive alleles are expressed differently, an organism’s traits don’t always reveal genetic makeup
genotype: genetic makeup (which alleles)
phenotype: physical appearance (which traits expressed)
test crosses and their importance
***allow us to tell if an individual with a dominant phenotype is homozygous or heterozygous!
test cross: mystery individual bred with homozygous recessive individual; if any offspring display recessive phenotype the mystery parent must be heterozygous
law of independent assortment
***Mendel’s 2nd law of inheritance; relates to dihybrid crosses
- each pair of alleles segregates independently of other pairs during gamete formation
- only applies to genes on different, non-homologous chromosomes
- genes located near each other on the same chromosome actually tend to be inherited together (less likely for only one to cross-over)
monohybrid vs dihybrid
- monohybrids produced by true-breeding parents that differed in a single character (F1 heterozygous for 1 character)
- F2 is the result of a monohybrid cross
- dihybrids produced by true-breeding parents differing in 2 characters (F1 heterozygous for 2 characters)
- F2 is the result of a dihybrid cross
usefulness of dihybrid crosses
can be used to determine whether 2 characters are transmitted together (on the same chromosome) or independently
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