Bio Final

Learning Objective, lecture 13:

•Recognize chromosomes as the physical units of inheritance.
•Explain how genes determine traits.
•Differentiate between mitosis and meiosis as two forms of cell division. Recall the key sequence of events for each.
•Relate the alignment of homologous chromosomes in meiosis to the generation of genetic variation in gametes.

How are an organism's traits determined? 13

"Units of heredity" - physically transmitted as nucleic acid sequences (DNA, RNA)

Types of Cell Division 13

binary fission (prokaryotic cells),
• 1 parent cell →divides into 2 daughter cells with identical genetic information
• For growth (= reproduction in single-celled organisms)

mitosis (eukaryotic cells),
• 1 parent cell →divides into 2 daughter cells with identical genetic information
• For growth (and maintenance), sometimes reproduction in single-celled organisms

meiosis (eukaryotic cells)
• 1 parent cell →eventually divides into 4 daughter cells, each with ½ genetic information of parent
• For reproduction (eventually)

Chromosome Terminology 13 (insert image from leg 13)

Set up Punnett squares of monohybrid crosses and describe the expected ratios of offspring in the F1 and F2 generations. (14)

A monohybrid cross is a genetic mix between two individuals who have homozygous genotypes (GG X gg)

expected ratio of F1 generation: all heterozygous dominant

expected ratio of F2 generation (if F1 allowed to self-pollinate):
1:2:1

Mitosis 13

Means "division of the nucleus" -this is why only eukaryotes undergo mitosis

Objective: to partition genetic material in a precise way to ensure that each new cell gets one (and only one) full set of genetic information
• In a diploid cell (2n), this means dividing 4n correctly into 2 cells with 2n each

Phases of Mitosis : List them 13

interphase, prophase, metaphase, anaphase, telophase

Phases of Mitosis: (before) 13

•Before mitosis begins, the cell is in interphase
- Includes gap phases & S-phase
- In this state, DNA dispersed throughout nucleus
•In preparation for mitosis, chromatin must condense

Phases of Mitosis: 13

(Interphase)
• Chromosomes have been copied
• Sister chromatids connected at centromere

(Prophase)
• Chromosomes condense
• Nucleus may dissociate
• Microtubules attach to centromeres

(metaphase)
• Chromosomes align at the metaphase plate

(anaphase)
• Microtubules pull sister chromatids apart

(Telophase)
• Chromosomes start to decondense
• Nucleus may reform

(Cytokinesis)
• Cells separate

Result of Mitosis 13

2 identical daughter cells

Meiosis 13

•Objective: to ensure that each cell gets a complete set of chromosomes -one from each homologous pair

• Parent cell is diploid (2n), daughter cells are haploid (n) →this means dividing 4n correctly into 4 cells with n each

•Two rounds of division:
• Meiosis I
• Meiosis II

•Pre-meiosis, starting in G2:
• DNA replicated in S-phase prior to G2

Humans have 23 pairs of chromosomes. How many different (unique) combinations of chromosomes can you get in haploid cells? 13

2^23 = 8.6 millions

meiosis I

(prophase I / metaphase I / anaphase I / telophase I / cytokinesis I) 13

(Prophase)
• Homologous pairs find each other
• (Each one is two sister chromatids held together at centromere →2 blue sister chromatids, 2 red sister chromatids per homologous pair)

(Metaphase I)
• Homologous pairs line up at metaphase plate

(Anaphase I)
• Homologous chromosomes separate

(Telophase I)
• Each to-be daughter cell now has one of each chromosome (chr. 1, 2, 3, etc.)
• (each chromosome still consists of 2 sister chromatids)

(cytokinesis I)
• Daughter cells separate

meiosis II 13

the second phase of meiosis consisting of chromatids separating, along with the two diploid cells splitting in two

Result of Meiosis 13

•1 parent cell →4 daughter cells (2 rounds of division)

•Half the genetic material of the parent cell (one full set of chromosomes -haploid, n)

Genetic Recombination 13

The regrouping of genes in an offspring that results in a genetic makeup that is different from that of the parents.

mitosis vs meiosis differences 13

• Homologous chromosomes don't need to line up in mitosis, but they do in meiosis (meiosis I)
• Crossing over occurs in meiosis (meiosis I), but not in mitosis

Karyotype 13

the full set of chromosomes in a cell

diploid human cell with 22 pairs of homologous chromosomes + 1 pair of sex chromosomes (1 X, 1 Y). In total 23 for human

Homologous pairs 13

one from egg, one from sperm

Genes and alleles 13

•Gene -a stretch of DNA that encodes an RNA or protein product (once transcribed and translated)

•Alleles -variant forms of genes
• One gene can have many different variant forms -each of these variant forms is an allele of that gene

Genes in blood type, reference look at lec 13

Genes in blood type, reference look at lec 13

Cell Cycle 13

insert the image from leg

Describe Mendel's work on inheritance of traits, including his experimental set-up and the conclusions of particulate inheritance. (14)

Relate gametes (& meiosis) to the law of segregation. (14)

Law of segregation:
The two alleles for each trait segregate during gamete formation, and then unite at random (one from each parent) at fertilization. NOT DONE YET^^^

What are genes? (14)

units of heredity (stretches of DNA that encode a specific RNA or protein product)

What are alleles? (14)

variant forms of a gene

What is a genotype? (14)

set of alleles at a gene

What is a phenotype? (14)

observable trait (relate to genotype)

What does heterozygous mean? (14)

different alleles for a gene (each homologous chromosome carries a different allele at the gene)

Autosomes vs sex chromosomes- what are they? (14)

Autosomes - Any chromosome that isn't a sex chromosome, 22 are autosomes.
Sex Chromosomes - 23rd pair is the sex chromosome and it differs between male and female - Female is XX and male is XY

What is true breeding? (14)

true breeding is a kind of breeding wherein the parents would produce offspring that would carry the same phenotype. This means that the parents are homozygous for every trait.

What is a self-cross? (14)

Crossing two individuals with the same genotype

What is a test cross? (14)

cross with known homozygous recessive individual

What is a monohybrid cross? (14)

Monohybrid:
individual that is hybrid for one trait (heterozygous for trait)

Monohybrid cross: cross between two individuals monohybrid at the same trait (e.g. all F1 self-pollinations and F1 x F1 crosses from lecture 14 part 1)

What is a dihybrid cross? (14)

Dihybrid: individual that is hybrid for two traits (heterozygous for both)

Dihybrid cross: cross between two individuals that are dihybrid at the same two traits

What is a reciprocal cross? (14)

perform first cross where female = trait form 1, male = trait form 2, then a second cross where female = trait form 2, male = trait form 1 (if trait is sex-linked, results should differ between crosses)

What is a carrier? (14)

individual who carries an allele that is not seen (means allele must be recessive because it is there but "hidden")

What does homozygous mean? (14)

two of the same allele for a gene (both homologous chromosomes carry the same allele)

What is the process of Meiosis? (13)

Meiosis I
Prophase I - Spindles form, Chromosomes condense and line up with homologous pairs, crossing over occurs here
Metaphase I - chromosomes line up at equator with homologous pairs
Anaphase I - Chromosomes are pulled away from their pair
Telophase I - two new nuclei are formed
Cytokinesis - splits the cell into two slightly different cells

Meiosis II
Prophase II - Spindles form, chromosomes are already condensed with a sister chromatid
Metaphase II - Chromosomes line up
Anaphase II - Chromatids are pulled away from each other
Telophase II - New nuclei developing
Cytokinesis - splits the cell into 4 unique daughter cells

What is one way to determine possible combinations of a multihybrid cross. (14)

Use probability

Product rule:
probability of two independent events occurring together is the product of their individual probabilities

P(1 and 2) = P(1) x P(2)

What is the law of independent assortment? (14)

During gamete formation, different pairs of alleles segregate independently of each other.

What is the process of Mitosis? (13)

\- Prophase - Spindles form, chromosomes condense and appear as two sister chromatids joined at centromere
- Metaphase - chromosomes line up at equator of cell and spindle fibers attach to centromere
- Anaphase - chromatids are pulled apart poleward
- Telophase - two new nuclei are forming on the poles of the cell
- Cytokinesis - The cell is divided into 2 identical cells

Describe the four patterns of Mendelian inheritance: autosomal dominant, autosomal recessive, sex-linked (X-linked) dominant, and sex-lined (X-linked) recessive. (14)

autosomal dominant: non-sex chromosomes have one or two dominant alleles (Bb/BB)

autosomal recessive: non-sex chromosomes only have recessive alleles (bb)

sex-linked dominant: sex chromosome has one or two dominant alleles
X^B X^B
X^B X^b
X^B Y

sex-linked recessive: sex chromosome has only recessive alleles
X^b X^b
X^b Y

Explain the physical basis of linkage between traits and how this shows up in offspring phenotypes. (14)

Set up Punnett squares of dihybrid crosses and describe the expected ratios of offspring in the F1 and F2 generations. (14)

Explain the physical basis of linkage between traits and how this shows up in offspring phenotypes. (14)

Describe how we can estimate the frequency of alleles in a population. (16)

Explain the Hardy-Weinberg principle and how it relates to changes in allele frequency in a population. (16)

Allelic relationships: Three types 15

•Dominance
•Incomplete dominance
•Codominance

Incomplete dominance 15

•Phenotype of heterozygote is an intermediate between parental phenotypes

E.g. Andalusian chicken →heterozygote neither black nor white, but blue

Allele frequencies change due to... (16)

•Genetic Drift -nonselective & random, based on sampling error
•Gene Flow -nonselective & random, based on migration
•Natural Selection -selective, based on various selective pressures

Recap vocabulary: Crosses

Monohybrid cross
Dihybrid cross. 15

• Monohybrid cross -looks at one trait that differs between parents being crossed (alleles of one gene)

• Dihybrid cross -looks at two traits that differ between parents being crossed (alleles of two genes)

What is genetic drift? (16)

A change in allele frequency from generation to generation due to chance
• E.g.: Just by chance, the population (stick figure example) mated in a way that didn't exactly produce the same allele frequencies in the next generation

Human Inheritance:Pedigrees
Learning Objective 15

•Use pedigrees to identify the mode of inheritance of a trait.

What types of genetic drift are there? (16)

Founder effect:
Immigrants establish new population; new population is likely to have different allele frequencies than source population by chance
decreases genetic variety

Bottleneck effect:
High mortality strikes due to event, leftover population is likely to have different allele frequencies than original population by chance,
decreases genetic variety

What is the Hardy-Weinberg principle? (16)

What are the assumptions of Hardy-Weinberg principle? (16)

Random mating

•No natural selection
The gene in question does not affect fitness under present conditions and is not under selection

•No genetic drift
No changes to frequency of alleles by randomness

•No gene flow
No individuals entering/leaving the population
•No mutation
Alleles are not changing

Reading Pedigrees 15

• Pedigrees represent families (here, genetically linked individuals)
• Symbols represent people, and the lines between them represent how they are linked
• Generations are marked with Roman numerals
• Individuals within generations are marked with numbers
• Traits of interest or diseases are shown by filling in the symbol for an individual →indicates "affected" individual

Patterns of inheritance 15

•Autosomal dominant
•Autosomal recessive
•X-linked dominant
•X-linked recessive

Inherited diseases 15

• Disease-causing mutations arise in cells
• Chemicals that cause mutations →mutagens (if cancer-causing, then "carcinogens")
• Irradiation
• Errors in DNA replication and repair
• Only mutations carried in germline cells can be passed down

• Somatic cells: all diploid cells in the body
• Germline cells: egg- and sperm- forming cells, haploid-

• Congenital: born with, but not inherited from parents (e.g. mutation that doesn't exist in parents, that arose in embryo or during development of offspring)

Patterns of Inheritance: X-linked inheritance vs Y-linked 15

(X-linked)
• If recessive: affected mothers will always have affected sons; usually more males affected than females; never passed from father to son
• If dominant: affected fathers will always have affected daughters but not affected sons

(Y-linked)
• Cannot affect females; affected fathers always have affected sons

Patterns of Inheritance: Vertical vs Horizontal pattern 15

(Vertical)
• affected individual always has an affected parent
• usually dominant trait

(Horizontal)
• unaffected parents may have affected child
• usually recessive trait

Pedigrees 15

•A format of family tree used to record relationships between individuals in a family →also record phenotypes
• Traits of interest, diseases

What is gene flow?

Recap vocabulary: Extensions to Mendelian genetics

Linkage, Incomplete dominance, Codominance, Gene interactions, Quantitative traits, Pleiotropy. 15

• Extensions to Mendelian genetics

• Linkage -traits travel together, do not display independent assortment

• Incomplete dominance -intermediate phenotype between the two alleles' phenotypes

• Codominance -both alleles' phenotypes simultaneously expressed (not intermediate)

• Gene interactions -multiple genes influence one trait

• Quantitative traits -multiple genes contribute to a trait to create a continuum of phenotypes

• Pleiotropy -one gene affects multiple traits

What are the four types of natural selection? (16)

1\. Directional Selection:
favors one extreme of a certain phenotype, causing the average phenotype to change in one direction
- genetic variation is reduced

2. Stabilizing Selection:
favors the intermediate of a certain phenotype, maintaining average phenotype
- genetic variation is reduced

3. Disruptive Selection:
favors the two extremes of a phenotype
- genetic variation is increased

4. Balancing Selection:
no single phenotype is favored in all populations of a species at all times
- genetic variation is maintained

Mendel's study system: single-gene traits (with only two alleles per trait with a clear dominance relationship) for his crosses.
BUT
MOST traits are not so simply determined: Why? 15

Gene interactions →multiple genes influence one trait

Relate Hardy-Weinberg equilibrium to evolution. (16)

When a Hardy-Weinberg equilibrium has been reached, evolution is not occuring

Recognize the consequence of different types of selection on genetic variation. (16)

Beyond Mendelian Inheritance: Learning Objectives 15

•Define allelic relationships beyond simple dominance -include codominance, incomplete dominance.
•Define monogenic vs. polygenic traits.
•Explain how gene interactions involve multiple genes to determine a single trait.
•Explain how quantitative traits result in a continuum of a phenotype.
•Recognize mutation as the source of new alleles of a gene.

Extensions to Mendel's rules: 15

•Not all traits are passed down the way Mendel saw the "antagonistic pairs" passed down in pea plants!
•... Linkage -important exception to independent assortment
•... Allelic relationships -not just dominant and recessive
•... Single-gene traits -most traits are not determined by just one gene