bio unit 5: all topics

unit 5: heredity

genetics

the study of heredity and hereditary variation

heredity

the transmission of traits from one generation to the next

genes

segments of DNA that code for basic units of heredity. traits passed through genes from parents to offspring, and the offspring acquire genes from parents by inheriting chromosomes

traits —> genes —> chromosomes

asexual reproduction

1. single individual

2. no fusion of gametes

3. clones: offsprings are exact copies of parent

4. mutations are the ONLY source of variation

5. can produce asexually through mitosis

sexual reproduction

1. two parents (male/female)

2. offspring are unique combinations of genes from parents

3. genetically varied from parents and siblings

homologous chromosomes

a pair of chromosomes (same size, length, centromere position) that carry the same genetic information

*one is inherited from mom and one is inherited from dad

karyotypes

a display of chromosome pairs ordered by size and length

somatic (body) cells

diploid, or 2n (two complete sets of each chromosome)

humans: 2n=46

gametic (sex) cells

haploid, or n (one set of each chromosome)

humans (sperm and eggs): n =23

types of chromosomes

1. autosomes

2. sex chromosomes

DNA is packaged in chromosomes in eukaryotes

autosomes

chromosomes that do not determine sex (humans have 22 pairs)

sex chromosomes

X and Y

• eggs: X (humans: 22+X)

• sperm: X or Y (humans: 22 + X or 22 + Y)

**all sexually reproducing organisms have both a diploid and a haploid number

life cycle

sequence of stages in the reproductive history of an organism from conception to its own reproduction

*fertilization and meiosis alternate in sexual life cycles

fertilization

when a sperm cell (haploid) fuses with an egg (haploid) to form a zygote (diploid)

meiosis

a 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

**involved TWO rounds of division: meiosis I and meiosis II

example of meiosis

humans: diploid, 2n=46

meiosis produces sperm and eggs that are HAPLOID: n=23

mitosis vs. meiosis

both processes are similar but have some key differences

mitosis (overview)

1. occurs in somatic cells

2. 1 division

3. results in 2 diploid daughter cells

4. daughter cells are genetically identical

meiosis (overview)

1. forms gametes (sperm/egg)

2. 2 divisions

3. results in 4 haploid daughter cells

4. each daughter cell is genetically unique

key events in meiosis

1. prophase I: synapsis and crossing over

2. metaphase I: tetrads (homologous pairs) line up at the metaphase plate

3. anaphase I: homologous pairs separate

meiosis I: interphase

cell goes through G1, S (DNA is copied), and G2

meiosis I: prophase I

1. synapsis: homologous chromosomes pair up and physically connect to each other forming a tetrad

2. crossing over (recombination) occurs at the chiasmata and DNA is exchanged between the homologous pairs

   a. each chromatid that is produced has a unique combination of DNA

3. nuclear envelope breaks down. the centriole pairs start to move to opposite poles and the chromosomes condense

meiosis I: metaphase I

1. the spindle has formed completely

2. independent orientation: tetrads line up at the metaphase plate (line up in the middle of the cell)

meiosis I: anaphase I

1. spindle fibers begin to contract

2. pairs of homologous chromosomes separate and pulled by centromeres to the poles

   a. sister chromatids are still attached

meiosis I: telophase I

1. nuclear envelopes start to reform around the groups of chromosomes

2. chromosomes start to decondense back into chromatin

meiosis I: cytokinesis

1. the cytoplasm and nuclei physically divide

2. there is now a haploid set of chromosomes in each daughter cell

meiosis II: prophase II

1. nuclear envelope breaks down

2. centriole pairs start to move to opposite poles and the chromosomes condense

3. NO crossing over, spindle starts to form

meiosis II: metaphase II

1. chromosomes line up in the middle of the cell

   a. because of crossing over in meiosis I, the chromatids are unique

2. spindle has formed completely

meiosis II: anaphase II

1. spindle fibers begin to contract

2. sister chromatids get separated as they are pulled by their centromere to the poles

meiosis II: telophase II

1. nuclear envelopes start to reform around the groups of chromosomes

2. chromosomes start to decondense back into chromatin

meiosis II: cytokinesis

1. cytoplasm physically divides

2. 4 haploid cells are formed —> each daughter cell is genetically unique

3. cells are now in interphase

meiosis review

1. early meiosis I: parent cell —> 2n = 4

2. end of meiosis II: each daughter cell —> n=2

how does meiosis lead to genetic variation?

1. crossing over

2. independent assortment of chromosomes

3. random fertilization

crossing over

produces recombinant chromosomes: they 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

random fertilization

any sperm can fertilize any egg

meiosis + fertilization

meiosis followed by fertilization ensures genetic diversity in sexually reproducing organisms and provides genetic variation that plays a role in natural selection

mendelian genetics

Gregor Mendel studied inheritance and created two laws that can be applied to the study of genetics

common ancestry

1. DNA and RNA carry genetic information

2. the genetic code is shared by all living systems

Gregor Mendel

Austrian monk who experimented on pea plants and discovered the basic principles of heredity

why pea plants for experimentation?

1. many varieties

2. controlled mating

3. relatively short generation time

pea plant traits

mendel tracked characteristics that came in two forms:

1. color

2. seed shape

true breeding

organisms that produce offspring of the same variety over many generations of self pollination

example: true breeding purple pea plants will ONLY produce purple offspring with self pollination

generations

1. P generation: true-breeding parental generation

2. F1 generation (first filial): hybrid offspring of P generation

3. F2 generation (second filial): offspring of the F1 generation

punnett squares

diagrams used to predict the allele combinations of offspring from a cross with known genetic compositions

1. capital letters = dominant traits

2. lower case letters = recessive traits

homozygous

an organism that has a pair of identical alleles for a character

1. homozygous dominant: AA

2. homozygous recessive: aa

heterozygous

an organism has two different alleles for a gene

ex: Aa

genotype

the genetic makeup (alleles) of an organism

ex: AA, Aa, aa, etc.

phenotype

an organism’s appearance, which is determined by the genotype

ex: purple, white, smooth, wrinkled, etc.

testcross

help to determine if the dominant trait is homozygous dominant or heterozygous

principles of heredity

1. law of segregation

2. law of independent assortment

law of segregation

the two alleles for the same trait separate during gamete formation and end up in different gametes

1. example: each gamete for P generation will contain one allele for flower color (in this case they are true breeding so every gamete has the same allele)

law of independent assortment

genes for one trait are NOT inherited with genes of another trait:

1. instead of following one trait in his crosses, this time Mendel followed TWO traits (ex: pea pod color AND pea pod shape)

2. this law only applies to: genes that are located on different chromosomes (NOT homologous) OR genes that are very far apart on the same chromosome

mendel’s discoveries

mendel notices that the cross between purple and white true breeding pea plants produced ONLY purple F1 offspring

**white characteristic did NOT disappear, it came back in the F2 generation

dominant vs. recessive

1. mendel hypothesized that the purple flower must be a dominant trait to the white flower, which is a recessive trait

   a. performed same crosses for each of the 7 characteristics of pea plants and found the same results

2. found that F2 generation was ALWAYS a 3:1 ratio

mendel’s model

explains the 3:1 generation in the F2 generation:

1. alternative versions of genes (alleles) account for variations in inherited characteristics

2. for each character, an organism inherits TWO copies of a gene (TWO alleles), one from each parent

3. if two alleles at a locus differ, then the dominant allele determines the appearance and the recessive allele has no noticeable effect

alleles

alternative versions of a gene

(somatic cells are diploid, they contain TWO copies of each chromosome)

monohybrid crosses

a cross between the F1 hybrids (ex: Bb x Bb)

1. the law of segregation was determined by doing crosses between true-breeding plants which produced F1 hybrids, known an monohybrids (ex: BB x bb produce F1 that are all Bb)

dihybrid crosses

a cross between F1 dihybrids (ex: YyRr x YyRr) —> produces a 9:3:3:1 phenotypic ratio

1. the law of independent assortment was determined by doing crosses between plants that were true breeding for two traits, which produced F1 hybrids known as dihybrids (ex: YYRR x yyrr, all F1 dihybrids would be YyRr)

how to solve genetic problems

1. write down symbols for the alleles (sometimes they are given)

2. write down genotypes given

   a. if phenotypes are given, then write down the possible genotypes

3. determine what the problem is asking, and write out the cross as: [genotype] x [genotype]

4. set up the punnett square

laws of probability

the laws of segregation and independent assortment reflect rules of probability:

1. multiplication rule

2. addition rule

multiplication rule

the probability that two or more independent events will occur together in some specific combination

1. ex: if you flip a coin twice, what is the probability that it will land heads up both times? ½ x ½ = ¼

addition rule

addition rule: the probability that two or more mutually exclusive events will occur

1. ex: what is the chance of rolling a dice and it lands on a 1 OR 6? 1/6 + 1/6 = 1/3

pedigrees

family trees that give a visual of inheritance patterns of particular traits

reading pedigrees

1. if a trait is dominant, one parent must have the trait (**dominant traits do not skip a generation)

2. if a trait is X-linked, then males are more commonly affected than females

why do some traits not follow the ratios predicted by Mendel’s laws?

1. varying degrees of dominance

2. many traits are produced through multiple genes

3. some traits are determined by genes on the sex chromosomes

4. some genes are adjacent or close to one another one the same chromosome, causing them to segregate as a unit

5. some traits are the result of non-nuclear inheritance (ex: chloroplast and mitochondrial DNA)

complete dominance

homozygous dominant and heterozygous individuals are phenotypically the same; the kinds of traits Mendel worked with in his experiments

degrees of dominance

1. incomplete dominance

2. codominance

3. multiple alleles

incomplete dominance

neither allele is fully dominant; F1 generation has a phenotype that is a mix of those of the parental generation

ex: red flowers x white flowers = pink offspring

codominance

two alleles that affect phenotype are both expressed

ex: human blood group, type AB blood (A and B are both expressed)

multiple alleles

genes that exist in forms with more than two alleles

ex: human blood group, alleles- IA, IB, i

multiple genes

in many cases, two or more genes are responsible in determining phenotypes:

1. epistasis

2. polygenic inheritance

epistasis

the 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 codes for whether or not that pigment will actually show up

polygenetic inheritance

the effect of two or more genes acting on a single phenotype

ex: height, human skin color

sex-linked genes

a gene located on either the X or the Y chromosome:

1. Y-linked genes: genes found on Y chromosome

   * very few Y-linked genes, so very few disorders

2. X-linked genes: genes found on X chromosome

inheritance of X-linked genes

1. fathers can pass X-linked alleles to all of their daughters, but NONE to their sons

2. mothers can pass X-linked alleles to BOTH daughters and sons

recessive X-linked traits

if an X-linked trait is due to an recessive allele:

1. females will only express the trait if the are homozygous

2. because males only have one X chromosome, they will express the trait if they inherit it from their mother

   a. called hemizygous

   b. due to this males are much more likely to have an X-linked disorder

X-linked disorders

1. duchenne muscular dystrophy

   a. progressive weakening of muscles

2. hemophilia

   a. inability to properly clot blood

3. color blindness

   a. inability to correctly see colors

X-inactivation

females inherit two X chromosomes (double than males); during development, most of the X chromosome in each cell becomes inactive

the inactive X in each cell of a female condenses into a Barr body (helps to regulate gene dosage in females)

genetic recombination

production of offspring with a new combination of genes from parents

1. parental types: offspring with parental phenotype

2. recombinants: offspring with different phenotypes than parents

linked genes

genes are located near each other on the same chromosome that tend to be inherited together

causes of genetic variation

meiosis and random fertilization generate genetic variation in offspring due to:

1. independent assortment of chromosomes

2. crossing over in meiosis I

3. any sperm can fertilize any egg

linked genes: crossing over

during crossing over, chromosomes from one paternal chromatid and one maternal chromatid exchange corresponding segments; crossing over helps to explain why genes become separated during meiosis:

1. the further apart two genes are on the same chromosome, the higher the probability that a crossing over event will occur and the higher recombination frequency

mapping distance

scientists can map genes and their locations on chromosomes

linkage map

genetic map that is based on recombination frequencies

1. map units: distance between genes

2. ONE map is equivalent to a 1% recombination frequency; it expresses the relative distances along chromosomes

3. 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 found in the mitochondria or chloroplasts; both chloroplasts and mitochondria are randomly assorted to gametes and daughter cells

non-nuclear DNA: animals

in animals, mitochondria are transmitted by the egg, NOT the sperm; therefore, ALL mitochondrial DNA is maternally inherited

non-nuclear DNA: plants

in plants, mitochondria and chloroplasts are transmitted in the ovule, NOT the pollen; therefore, both mitochondrial and chloroplast determined traits are maternally inherited

statistical analysis: chi square

a form of statistical analysis used to compare the actual results (observed) with the expected results

helps to:

1. determine whether the data obtained experimentally provides a “good fit” to the expected data

2. determine if any deviations from the expected results are due to random chance alone or other circumstances

3. designed to analyze categorical data

chi square (x²) formula

use the equation to test the “null” hypothesis —> the prediction that the data from the experiment will match the expected results

(reminder): null hypothesis

the hypothesis that says there is no significant different between specified populations and considers any observed difference being due to sampling or experimental error

environmental effects on phenotype

various environmental factors can influence gene expression

1. phenotypic plasticity

phenotypic plasticity

individuals with the same genotype exhibit different phenotypes in different environments

ex: soil pH can affect flower color, temperature can change coat color in rabbits, UV exposure can increase melanin production in the skin

genetic disorders

some genetic disorders can be linked to:

1. mutated alleles

   a. Tay-Sachs disease

   b. sickle cell anemia

2. chromosomal changes

   a. nondisjuction

Tay-Sachs disease

autosomal recessive disease with a mutated HEXA gene:

1. body fails to produce an enzyme that breaks down a particular liquid

2. affects central nervous system and results in blindness

sickle cell anemia

autosomal recessive disease with a mutated HBB gene:

1. sickled cells contain abnormal hemoglobin molecules

nondisjunction

chromosomes fail to separate properly in meiosis I or meiosis II:

1. karyotyping can detect nondisjunction (ex: down syndrome, 3 copies of chromosome 21)