From Evolution to Inheritance
- Darwin’s evolution theory: population growth is limited, advantageous traits are selected, natural selection weeds out un-advantageous traits - Occurs over many generation, generally gradual and slow process - Selection operates on existing variation
o Darwin didn’t understand how inheritance works or where variation comes from
Gregor Mendel on inheritance
- The unit of inheritance is the gene
- Gene: DNA segment that codes a functional product
- Allele: Variant of a gene
- Inheritance occurs by:
o Alleles segregating equally
o Different genes assorting independently
- Combining ideas of natural selection and inheritance
- Darwin: changes occur because…
- Mendel: changes are passed down by… We also discuss several other topics like What is the difference between fundamental attribution error and actor observer bias?
o Genetic change from generation to generation by natural selection
- Today: Where does variation come from? How are changes passed down? – cell, chromosomes, cell cycle
Where is the DNA?
- DNA in humans: Nucleus and mitochondrion
o Reside in cells
Gametes (reproductive cells)
- Structure containing DNA
- Only found in nucleus!
- In humans: 46 chromosome
o This is the diploid number
Diploid= full set of chromosomes (2n)
- Number varies by species
o E.g. Dogs- 78; Tobacco- 48
- Sex chromosome (X, Y)
- In humans: 22 autosome pairs and 1 pair of sex chromosomes - Matching pairs are called homologous chromosomes Don't forget about the age old question of What is the meaning of the need to belong in psychology?
o Same gene but different alleles
o E.g. Human chromosomes 1-22
Sex chromosomes in females (XX)
Chromosomes in the cell
- Somatic cells have diploid number of chromosomes We also discuss several other topics like When do antibiotics discover?
- Gametes (unfertilized): Haploid
o Half set of chromosomes (n)
o Once fertilized, becomes diploid
Importance of chromosomes to inheritance
- Chromosomes from each parent is inherited
o Thus traits are passed down to the next generation
- New variation can be introduced
Cell Cycle: Cell Division
- To produce new cells
- Necessary for inheritance process If you want to learn more check out Who are the yeomen farmers?
o Produce gametes
o Mitosis and cytokinesis
- 90% of the entire cell cycle
- Cell growth (want to have enough cell growth to survive after split) - Chromosome duplication, DNA replication
- Four major phases: prophase, metaphase, anaphase, and telophase - Results in identical somatic cells
- Sister chromatids condense
o Sister chromatids= identical copy of chromosome
- Chromatids align
- Chromatids spate to oppose poles
- Membranes reform
- Two separate nuclei form Don't forget about the age old question of The rda (recommended daily allowances) for nutrients are generally what?
- Two nuclei into separate daughter cells
At the end of mitosis
- Daughter cells are identical somatic cells
- Daughter cells are diploid or haploid
Meiosis: inheriting change
- Produce gametes
- Gametes are haploid or diploid
- Separation of homologous chromosomes
- Daughter cells are not identical
o Daughter cells are diploid (2n)
o Identical somatic cells
o Sister chromatids separate
o Daughter cells are haploid (n)
o Homologous chromosomes separate
The point about cell cycle
- Cell growth, reproduction => inheritance
- New variation from chromosomes:
o Introduced during meiosis
o 3 examples:
Translocation We also discuss several other topics like Where are most adolescents living today?
1.Crossing Over (AKA Recombination)
- Exchanging genetic material between homologous chromosomes - Results in recombination
- Exchanging genetic material between nonhomologous chromosomes - Rare occasion: often disadvantageous
- Incorrect number of chromosomes due to failure to separate o Trisomy: gain in number
E.g. Down’s syndrome
o Monosomy: loss in number
E.g. Turner’s syndrome
Loss of one X chromosome in females (X instead of XX) Cell cycle and chromosomes
- Crossing-over always occurs!
o New combination of genes
- But limited to existing variation
o New variation comes from changes to DNA
DNA: Deoxyribonucleic acid
- String of nucleotides
o Nucleotide: Pentose (sugar), phosphate group, nitrogenous base - Double-stranded helix structure
- Adenine (A)
- Cytosine (C)
- Guanine (G)
- Thymine (T)
- Complementary bases: A-T, C-G
How does DNA change?
- Mutation: changes DNA sequence
- Occurs often during DNA replication
o Which phase of the cell cycle?
- How do you go from DNA to advantageous trait?
o Change to DNA=> protein=> traits
DNA Replication: a short summary
- During interphase, in the nucleus
- Separation of double strands
- Each strand replicates itself
- Forms a new strand
o One new and one original strand
After DNA replication: transcription
- From DNA to RNA
- Double- stranded DNA separates
- DNA strand serves as a template
o mRNA synthesized
mRNA= messenger ribonucleic acid
Complementary to DNA, but uses uracil instead of thymine Ex: If DNA strand is ACGTCC
RNA strand: UGCAGG
- mRNA transports out of nucleus
o into cytoplasm
- Attaches to ribosome
- From RNA to protein
- tRNA (transfer RNA) binds to mRNA
- tRNA carries an anticodon, matches with mRNA codon
o (Anti)codon: group of 3 nucleotides
o Each codon: associated with amino acid
Codons and amino acids
- 20 amino acids
- More than 20 codons
o Multiple codons can translate a single amino acid
Genetics and Inheritance
Feb. 2, 2017
- Daughter cells are haploid after mitosis or meiosis?
- Three examples of chromosomal variation
- DNA=> RNA=> protein=> trait=> inherited by mitosis and meiosis
o Translation: from mRNA=> tRNA=> amino acid
- Today: From mutations to traits, principles of inheritance At the end of translation
- “Stop” codon: amino acid chain breaks off
- Amino acid chain makes a polypeptide
- Chain(s) of polypeptide makes a protein
DNA to trait
- Population with abundant variation, advantageous traits already exist - Variation comes from changes to DNA (mutation)
- DNA=> (amino acid) => protein=> trait
- Do all mutations affect traits?
Mutation: affecting traits or not
- To change trait: need to change the protein
o But not all mutations change the protein
- Silent or Synonymous Mutation (Same)
o Does not change amino acid
- Non-Synonymous Mutation (Not the same)
o Changes amino acid
- *Depends on where mutation occurs in the codon
Codon and amino acids
- Changes to a codon:
- Most 3rd position changes are synonymous
- All 2nd position changes are non-synonymous
- Most 1st position changes are non-synonymous
Mutations: Type of earwax
- Wet VS. dry earwax
- ABCC11 gene
o GGG=> AGG: changes amino acid
o Non-synonymous mutation
- Non-synonymous mutation changes the type of earwax Mutations and inheritance
- Mutations on DNA=> may or may not change the trait
- But all mutations are inherited=> then how exactly are they inherited? - Mendel’s plant breeding experiments that he investigated the mechanics of inheritance
o Focused on what is inherited?
o How is it inherited?
- Advantage of using pea plants:
o Short generation time (maybe a matter of a few months) o Clear cut traits (Yellow or green, tall or short, etc.)
o Large numbers possible (100s and 100s of samples)
Mendel, genetics, inheritance
- What is inherited?
o Genes and alleles
- How is it inherited?
o According to the laws of inheritance
1.Law of segregation
- Two alleles for each trait segregate equally
o Source of allele has no influence
o Doesn’t matter whether allele comes from father or mother 2. Law of independent assortment
- Different genes assort independently of each other
o E.g. seed shape and flower color do not influence each other’s inheritance
Understanding Mendelian Genetics
- Using the example of garden pea shape (R or r)
o Genes and allele; phenotype (shape=> smooth or wrinkled) and genotype (combination of alleles=> R or r)
Each letter = allele
o Homozygous and heterozygous
Genotype has same alleles or different alleles
o Diploid and haploid
o Dominant and recessive
Uses capital and small letter
Refers to phenotype
- Each square represents:
o Law of segregation
- Using pea shape (R, r)
o One parent is heterozygous
o One parent is homozygous recessive
o Homozygous or heterozygous offspring?
Principles of Mendelian Genetics
- If both parents are homozygous recessive, offspring phenotype? o Recessive
- If one parent is homozygous dominant, the other is homozygous recessive, offspring genotype?
o Offspring phenotype: Dominant
Mendelian Genetics and Inheritance
- Based on traits determined by a single gene
- Traits are discrete: clear dominant, recessive
- Follows the laws of inheritance:
o Alleles segregate equally
o Genes assort independently
- Does Mendel’s law of inheritance always apply?
- Law of independent assortment says:
- But: when two genes are located close to each other o Transmitted together (do NOT assort independently) 2. Incomplete dominance
- Petunia flower color (C):
o Possible diploid genotypes?
o Phenotypes based on Mendelian principles?
- But heterozygous is an intermediate color (Red and white can make pink)
o Trait is not discrete (not a clear dominant, recessive)
- Mendel: heterozygous genotype would be what phenotype o But both traits can be equally expressed
o Would show both the red and white
Example: ABO Blood type
- What are the phenotypes?
o A, B, AB, O
- Genotypes for A blood type?
o AA, AO
- Between A and O, which is dominant and recessive?
o Dominant- A, Recessive- O
Codominance in Abo Blood Type
- AB blood type: dominant and recessive?
o Both traits are expressed, example of codominance
Week 2: Genetics, Mendelian Inheritance
1. What is the evolutionary synthesis? Explain the contribution of Darwin and Mendel. Based on the evolutionary synthesis, how is evolution defined?
Evolutionary synthesis is the combining of ideas of natural selection and inheritance. Darwin’s evolutionary theory stated that population growth is limited, advantageous traits were selected and natural selection weed out unadvantageous traits. While Gregor Mendel said inheritance occurred by alleles segregating equally and different genes assorting independently. Evolution is defined as genetic change from generation to generation by natural selection.
2. What are chromosomes and where are they located? What is the difference between diploid and haploid number of chromosomes?
A chromosome is a structure containing DNA that is located in the nucleus. Somatic cells have a diploid number, a full set of chromosomes(2n). Gametes have a haploid number, a half set of chromosomes(n).
3. Explain homologous chromosomes, autosomes, and sex chromosomes. Apply the types of chromosomes to humans.
Homologous Matching pairs; same gene but different alleles.
Autosomes Any chromosome that is not a sex chromosome (Chromosomes 122 in the human)
Sex chromosomes A chromosome involved with determining the sex of an organism (X, Y)
4. Which cells are diploid and which are haploid?
Somatic cells have a diploid number and Gametes have a haploid number.
5. What is the point of the cell cycle? What occurs during interphase?
The cell cycle produces new cells which is necessary for the inheritance process. Cell growth, chromosome duplication, and DNA replication occur during interphase.
6. Briefly explain the four phases of mitosis. What is the difference between mitosis and meiosis? How are the daughter cells different?
Prophase Sister chromatids condense
Metaphase Chromatids align
Anaphase Chromatids separate to opposite poles
Telophase Membranes reform and two separate nuclei form
Mitosis results in identical somatic cells,diploid, while meiosis is produces gametes, haploid. In mitosis the daughter cells are identical but in meiosis they are not identical.
7. What is the difference between homologous chromosomes and sister chromatids? Explain whether they have the same or different genes and alleles.
Homologous chromosomes are matching pairs that have the same gene but different alleles. Sister chromatids are identical copies of chromosomes with the same genes and alleles.
8. Explain the difference between crossingover (recombination), translocation, and nondisjunction. Highlight how variation is introduced in each example.
Crossing over Exchanging genetic material between homologous chromosomes; Results in recombination
Translocation Exchanging genetic material between nonhomologous chromosomes; Rare occasion, often disadvantageous.
Nondisjunction Incorrect number of chromosomes due to failure to separate
9. What are the four nitrogenous bases of DNA and which are complementary to each other? What are the nitrogenous bases for RNA?
DNA Adenine, Cytosine, Guanine, and Thymine; Complementary AT, CG RNA Adenine, Cytosine, Guanine, and Uracil; Complementary AU, CG
10. Briefly explain the process of DNA replication, transcription and translation. Make note of where each process occurs in the cell and the end result of each step.
Replication Occurs during interphase in the nucleus. It is the separation of double strands, each strand will replicate itself.
Transcription DNA transcribes into RNA. The DNA serves as a template. (In the nucleus still)
Translation RNA turns into protein. tRNA binds to mRNA (In cytoplasm?)
11. Describe the relationship between mRNA, tRNA, codons, amino acids, and protein. tRNA binds to mRNA. tRNA carries an anticodon and matches with mRNA codon creating a protien. Multiple codons can translate to a single amino acid.
12. Do all mutations affect traits? Describe synonymous and nonsynonymous mutations and how they relate to codons, codon position, and amino acids.
No because to change a trait you need to change the protein and not all mutations change the protein. Synonymous mutations do not change the amino acid. Nonsynonymous mutations change the amino acid. Synonymous and nonsynonymous mutations depend on where the mutations occurs in the codon. Most 3rd position changes are synonymous. All 2nd position changes are nonsynonymous. Most 1st position changes are non synonymous.
13. What was the advantage of using pea plants by Mendel?
The pea plants had a short generation time with clear cut traits and large numbers were possible.
14. What is the unit of inheritance? Explain Mendel’s law of segregation and law of independent assortment.
Genes and alleles are inherited. The law of segregation says that two alleles for each trait segregate equally (meaning it doesn’t matter whether the allele comes from the mother or father). The law of independent assortment says different genes assort independently of each other (Ex: seed shape and flower color do not influence each other’s inheritance)
15. Know and apply the definitions homozygous, heterozygous, genotype, phenotype, diploid, haploid (e.g. gamete genotype), dominant, and recessive to Mendelian genetics. The phenotype decides what trait might be inherited while the genotype is the combination of alleles which decide whether the trait is dominant or recessive. If a genotype has the same alleles it is homozygous and if it has different alleles it is heterozygous. All chromosomes are diploid except for sex chromosomes that are haploid.
16. Know and understand the possible offspring genotype from parents that may be any scenario of homozygous and heterozygous genotypes.
17. What is “Mendelian genetics”?
Based on traits determined by a single gene, the traits are discrete (clear dominant and recessive traits), and follow the laws of inheritance.
18. What is linkage and how does it violate Mendel’s law?
Linkage is when two genes are located transmitted together. This violates Mendel’s law because he stated that genes assort independently.
19. What is incomplete dominance and how does it violate Mendel’s principles? Use the petunia flower color as an example.
Incomplete dominance can have a trait that is not discrete when heterozygous. This violates Mendel’s law because he stated traits must be discrete. An example of this is when a red and white flower create a pink flower.
20. What is codominance and how is it different from incomplete dominance? Explain using the example of ABO blood type.
Codominance occurs when both traits can be equally expressed. This is different from incomplete dominance because in incomplete the traits are not clear (they mix together to make pink) whereas codominance clearly expresses both (see a red and white on the flower). An example of this is ABO blood type. In AB blood types both A and B are equally dominant traits, known as codominance.