Studied by 11 people
genetics
the study of heredity and hereditary variation
heredity
the transmission of traits from one generation to the next
traits
- passed from parent to offspring through genes
- segments of DNA that code for basic units of heredity
inheriting chromosomes
how offsprings acquire genes from parents
asexual reproduction
- single individual
- no fusion of gametes
- clones: offspring are exact copies of parent
- mutations: only source of variation
- mitosis
- ex: bacteria
sexual reproduction
- two parents
- offspring are unique combinations of genes from parents
- genetically varied from parents and siblings
- organisms have both a diploid and haploid number
homologous chromosomes
- pair of chromosomes (same size, length, centromere position) that carry the same genetic information
- one from mom; one from dad
karyotypes
display of chromosome pairs ordered by size and length
somatic/body cells
- diploid/2n
- ex: humans (2n = 46)
gametic/sex cells
- haploid/n
- ex: humans sperm and egg (n = 23)
diploid
two complete sets of each chromosome
haploid
one set of each chromosome
eukaryotes
have DNA packaged in chromosomes
types of chromosomes
- autosomes
- sex
autosomes
chromosomes that do not determine sex (pair 1-22)
sex chromosomes
- X and Y (23rd pair)
- eggs: X
- sperm: X or Y
life cycle
sequence of stages in the reproductive history of an organism from conception to its own reproduction
fertilization and meiosis
alternates in sexual life cycles
fertilization
sperm cell (haploid) fuses with an egg (haploid) to form a zygote (diploid)
meiosis
- process that creates haploid gamete cells in sexually reproducing diploid organisms
- results in daughter cells with half the number of chromosomes as the parent cell
- two rounds of division: meiosis I and II
meiosis example
humans
- diploid: 2n = 46
- meiosis produces sperm and eggs that are haploid: n = 23
mitosis
- occurs in somatic cells
- 1 division
- results in 2 diploid daughter cells
- daughter cells genetically identical
- purpose: growth and repair
meiosis
- forms gametes (sperm/egg)
- 2 divisions
- results in 4 haploid daughter cells
- daughter cells genetically unique
- purpose: reproduction
meiosis key events
- prophase I: synapsis and crossing over
- metaphase I: tetrads (homologous pairs) line up at metaphase plate
- anaphase I: homologous pairs separate
meiosis I
interphase, prophase I, metaphase I, anaphase I, telophase I and cytokinesis
meiosis II
no interphase, prophase II, metaphase II, anaphase II, telophase II and cytokinesis
prophase I
- synapsis
- crossing over
- every chromatid that is produced has a unique combination of DNA
synapsis
homologous chromosomes pair up and physically connect to each other forming a tetrad
crossing over/recombination
- occurs at chiasmata
- DNA is exchanged between homologous pairs
metaphase I
independent orientation/assortment (tetrads line up at the metaphase plate)
anaphase I
- pairs of homologous chromosomes separate
- sister chromatids are still attached
telophase I and cytokinesis
- nuclei and cytoplasm divide
- haploid set of chromosomes in each daughter cell
prophase II
- no crossing over
- spindle forms
metaphase II
- chromosomes line up at the metaphase plate
- chromatids are unique (due to crossing over in meiosis I)
anaphase II
sister chromatids separate and move towards opposite poles
telophase II and cytokinesis
- 4 haploid cells
- nuclei reappear
- daughter cell genetically unique
- makes sperm and egg cells
crossing over
- produces recombinant chromosomes
- exchange genetic material
independent assortment of chromosomes
- chromosomes are randomly oriented along the metaphase plate during metaphase I
- each can orient with either the maternal or paternal chromosomes closer to a given pole
fertilization
- any sperm can fertilize any egg
- females born with egg cells
- male's sperm made throughout life
how meiosis lead to genetic variation
- crossing over
- independent assortment
- random fertilization
DNA and RNA
carry genetic information
genetic code
shared by all living systems
gregor mendel
austrian monk who experimented on pea plants (many varieties, controlled mating, short generation time) and discovered the basic principles of heredity
true breeding
- homozygous
- organisms that produce offspring of the same variety over many generations of self pollination
- ex: purple pea plants will only produce purple offsprings
P generation
true breeding parental generation
F1 generation
- first filial
- hybrid offspring of P generation
F2 generation
- second filial
- offspring of the F1 generation
punnett squares
- diagrams used to predict the allele combinations of offsprings from a cross with know genetic compositions
- capital letters = dominant traits
- lower case letters = recessive traits
homozygous
- organism that has a pair of identical alleles for a character
- ex: dominant = AA
- ex: recessive = aa
heterozygous
- organism has two different alleles for a gene
- aka hybrids
- ex: Aa
genotype
- the genetic makeup (alleles) of an organism
- can't be seen
phenotype
- an organism's appearance
- determined by genotype
principles of heredity
- law of segregation
- law of independent assortment
law of segregation
- the two alleles for the same trait separate during gamete formation and end up in different gametes
- happens during anaphase II
alleles
alternative versions of a gene
law of independent assortment
- genes for one trait are not inherited with genes of another trait
- only applies to genes that are located on different chromosomes (not homologous) or genes that are very far apart on the same chromosome
multiplication rule (laws of probability)
the probability that two or more independent events will occur together in some specific combination
addition rule (laws of probability)
the probability that two or more mutually exclusive events will occur
pedigrees
family trees that give a visual of inheritance patterns of particular traits
dominant trait (pedigree)
- one parent must have the trait
- does not skip a generation
X-linked trait (pedigree)
- linked to the X chromosome
- males are more commonly affected than females
generation
- each row in the pedigree represents a generation
- labeled with roman numerals
affected individual
- person who has the trait being studied
- shaded symbol
unaffected individual
- person who does not have the trait
- unshaded symbol
carrier
- individual who carries one copy of a recessive allele but does not express the trait
- half-shaded symbol
autosomal dominant
- pattern of inheritance where only one copy of the dominant allele is necessary for the trait to be expressed
- appears in every generation
autosomal recessive
- pattern where two copies of the recessive allele are necessary for the trait to be expressed
- may skip generations
X-linked dominant
- mode of inheritance where the trait is carried on the X chromosome
- only one dominant allele is needed for the trait to be expressed
X-linked recessive
- pattern where the trait is carried on the X chromosome
- two recessive alleles are needed for females to express the trait
- males only need one to express the trait
Y-linked (holandric)
- trait carried on the Y chromosome
- affects only males
- very rare
non-mendelian genetics
- varying degrees of dominance
- many traits are produced through multiple genes acting together
- some traits are determined by genes on sex chromosomes
- some genes are adjacent or close to one another on the same chromosome and will segregate as a unit
- some traits are the result of non-nuclear inheritance (ex: chloroplast and mitochondrial DNA)
incomplete dominance
- neither allele is fully dominant (blend)
- ex: red flowers crossed with white flowers will produce pink offspring
codominance
- two alleles that affect phenotype are both expressed
- ex: roan cow fur (red and white hair both expressed)
multiple alleles
- genes that exist in forms with more than two alleles
- ex: human blood group
epistasis
- phenotypic expression of a gene at one locus affects a gene at another locus
- ex: coat color in mice (one gene codes for pigment and a second gene determines whether or not that pigment will be deposited in the hair)
polygenic inheritance
- effect of two or more genes acting on a single phenotype
- ex: height, skin color
sex-linked genes
- gene located on either the X or Y chromosome
- Y-linked: found on Y chromosome
- X-linked: found on X chromosome
father X-linked alleles
can be passed to all of their daughters, but none of their sons
mother X-linked alleles
can be passed o both daughters and sons
X-linked trait due to recessive allele
- females only express trait if homozygous
- males will express the trait if they inherit it from their mother because they only have one X chromosome (hemizygous)
X-linked disorders
- duchenne muscular dystrophy: progressive weakening of muscles
- hemophilia: inability to properly clot blood
- color blindness
X-inactivation
- during development, most of the X chromosome in each cell becomes inactive
- inactive X in each cell of a female condenses into a barr body (helps regulate gene dosage)
genetic recombination
production of offspring with a new combination of genes from parents
parental types
offspring with the parental phenotyp
recombinants
offspring with phenotypes that are different from the parents
linked genes
genes located near each other on the same chromosome that tend to be inherited together
crossing over (linked genes)
- chromosomes from one paternal chromatid and one maternal chromatid exchange corresponding segments
- explains why some linked genes become separated during meiosis
- further apart two genes are on the same chromosome = higher the probability that a crossing over event will occur between them and the higher the recombination frequency
linkage map
genetic map that is based on recombination frequencies
map units
- distance between genes
- one map unit = 1% recombination frequency
- 50% recombination means that the genes are far apart on the same chromosome or on two different chromosomes
non-nuclear DNA
- some traits are located on DNA that is found in the mitochondria or chloroplast
- both chloroplasts and mitochondria are randomly assorted to gametes and daughter cells
animals
- mitochondria are transmitted by the egg
- all mitochondrial DNA is maternally inherited
plants
- mitochondria and chloroplasts are transmitted in the ovule
- both mitochondrial and chloroplast determined traits are maternally inherited
chi square
- form of statistical analysis used to compare the actual results (observed) with the expected results
- determines whether the data obtained experimentally provides a "good fit" to the expected date
- determines if any deviations from the expected results are due to random chance alone or to other circumstances (ex: data collection error)
- designed to analyze categorical data
observed (actual) values
- the numbers that you get in your data
- usually no calculations
expected values
- based on probability
- need to do calculations
degrees of freedom
# of categories - 1
x² > critical value
- there is a statistically significant difference between the observed and expected population
- reject null hypothesis
x² < critical value
- there is not a statistically significant difference between the actual and expected values
- fail to reject null hypothesis
environmental factors
- can influence gene expression and lead to phenotypic plasticity
- individuals with the same genotype exhibit different phenotypes in different environments
- ex: soil pH can affect flower color
mutated alleles
- tay-sachs disease: autosomal recessive disease, mutated HEXA gene (body fails to produce an enzyme that breaks down a particular lipid), affects central nervous system and results in blindness
- sickle cell anemia: autosomal recessive disease, mutated HBB gene (sickled cells contain abnormal hemoglobin molecules)
