Genetics and Heredity Notes

Unit 7 Genetics and Heredity

Overview

• Unit focuses on genetics and heredity.
• Includes lessons on meiosis, Mendel and heredity, traits and probability, mutations and genetic diversity, and genetic engineering.
• Explores the heredity of Huntington’s disease as the unit project.

Driving Questions

• How are traits passed from parents to offspring?
• Why do offspring from the same parents look different?
• How does trait diversity arise over generations?
• How can we determine the probability of a trait being passed on?
• Can scientists alter genetic material, and how can humans use this ability?

7.1 Meiosis

Chromosomes and Meiosis

• DNA is the genetic material that codes for proteins.
• In complex organisms, DNA is packaged into chromosomes within the cell nucleus.
• A karyotype shows the 23 pairs of chromosomes in a human cell.
• Homologous chromosomes are pairs with the same length and general appearance, one from each parent; they contain copies of the same genes (alleles may differ).
• Autosomes (chromosome pairs 1-22) contain genes for characteristics not directly related to sex.
• Sex chromosomes (X and Y) directly control the development of sexual characteristics.
  • XX = female, XY = male
• Body (somatic) cells are diploid (2n), containing two copies of each chromosome (46 in humans).
• Germ cells in reproductive organs produce gametes (sex cells).
• Gametes (sperm and eggs) are haploid (n), containing one copy of each chromosome (23 in humans); only DNA in gametes is passed to offspring.
• Sexual reproduction involves the fusion of two gametes (fertilization) to form a single nucleus.

The Process of Meiosis

• Meiosis: nuclear division that divides one diploid cell into four haploid cells reducing the chromosome number by half.
  • Involves two rounds of cell division: meiosis I and meiosis II.
• Homologous chromosomes: two separate chromosomes, one from each parent, same length and genes, but not identical.
• Sister chromatids: duplicated chromosomes attached by the centromere.

Meiosis I

• DNA copied during S phase before meiosis.
• Meiosis I separates homologous chromosomes, producing two haploid cells with duplicated chromosomes.
  1. Prophase I: Nuclear membrane breaks down, centrosomes move, duplicated chromosomes condense and pair up gene for gene.
  2. Metaphase I: Homologous chromosome pairs randomly line up along the middle of the cell attached to spindle fibers.
  3. Anaphase I: Paired homologous chromosomes separate and move to opposite sides of the cell; sister chromatids remain together.
  4. Telophase I: The cell undergoes cytokinesis; the nuclear membrane reforms in some species; spindle fibers dissassemble.

Meiosis II

• Meiosis II separates sister chromatids, resulting in chromosomes that are not doubled.
• DNA is not copied between meiosis I and meiosis II.
  5. Prophase II: Nuclear membrane breaks down, centrosomes move, spindle fibers assemble.
  6. Metaphase II: Spindle fibers align the 23 chromosomes at the cell equator; each chromosome still has two sister chromatids.
  7. Anaphase II: Sister chromatids are pulled apart and move to opposite sides of the cell.
  8. Telophase II: Nuclear membranes form, spindle fibers break apart, and the cell undergoes cytokinesis.
• Mitosis produces two genetically identical cells, whereas meiosis produces four haploid cells.

Gametogenesis

• Haploid cells from meiosis undergo additional changes to produce mature gametes.
• Egg formation (female gamete): Starts before birth, not finished until fertilization; only one of the four cells produced by meiosis becomes an egg, while the others become polar bodies.
  • Nearly all of the zygote’s cytoplasm and organelles come from the egg; embryo mitochondrial DNA is identical to the mother’s.
• Sperm formation (male gamete): Starts with a round cell, ends up as a streamlined cell.
  • The sperm cell’s main contribution to an embryo is DNA, as well as the ability to move.
  • DNA is tightly packed and most of the cytoplasm is lost; sperm cell develops a flagellum.

Comparing Mitosis and Meiosis

• Mitosis occurs in body cells for growth and development.
• Meiosis occurs in germ cells.

Mechanisms of Genetic Variation

• Meiosis and sexual reproduction increase genetic diversity within a population.
• Gametes have different combinations of genes due to crossing over and independent assortment.

Independent Assortment

• Homologous chromosomes line up randomly during metaphase I.
• The arrangement of any homologous pair doesn't depend on any other pair.
• Number of possible chromosome combinations: Combinations = 2^n (where n = number of different chromosomes).

Crossing Over

• Exchange of chromosome segments between homologous chromosomes during prophase I.
• Results in new combinations of genes (genetic recombination).

Fertilization

• Fusion of two gametes, producing a zygote with a unique combination of genes.
• Mixing and matching of genetic material during meiosis and fertilization result in genetic variation.

Gene Duplication

• Unequal crossing over can result in one chromosome having two copies of a gene (gene duplication) and the other having none (gene deletion).
• Gene duplication provides opportunities for one copy to maintain the original function while the other evolves new functions.

7.2 Mendel and Heredity

Mendel’s Groundwork for Genetics

• Traits are distinguishing characteristics that are inherited.
• Gregor Mendel’s experiments with pea plants established the basis of our understanding of heredity.
• Mendel chose pea plants for their fast reproduction and easily controlled pollination.
• He began with purebred plants and controlled pollination to cross plants with specific traits.
• Mendel used mathematics to analyze data.
• Traits Mendel studied: seed shape, seed color, pod shape, pod color, flower color, flower position, and stem length.
• Genetic cross: mating of two organisms.
• P (parental) generation: original plants in a cross.
• F1 (first filial) generation: offspring of P generation.
• F2 generation: offspring of F1 generation allowed to self-fertilize.
• Mendel concluded that traits were inherited as discrete “factors” (genes) that pass from parent to offspring.
• Law of Segregation: During gamete formation, alleles separate, so each gamete receives only one allele for each gene.

Traits, Genes, and Alleles

• Gene: piece of DNA that provides instructions for making a protein.
• Locus: specific location of a gene on a pair of homologous chromosomes.
• Allele: alternative forms of a gene at a specific locus.
• Homozygous: having two of the same alleles at a specific locus.
• Heterozygous: having two different alleles at a specific locus.
• Genotype: actual genetic makeup (combination of alleles).
• Phenotype: physical characteristics or traits expressed.
• Dominant allele: expressed when two different alleles are present.
• Recessive allele: only expressed when two recessive copies are present.
• Uppercase letters represent dominant alleles, lowercase letters represent recessive alleles.

Extending Mendelian Genetics

• Many traits don't follow simple dominant and recessive patterns.

Incomplete Dominance and Codominance

• Incomplete dominance: heterozygous phenotype is intermediate between two homozygous phenotypes (blended result).
• Codominance: both alleles are equally expressed and appear separately in the phenotype.

Multiple Alleles

• More than two alleles are possible in a population (e.g., human blood type: IA, IB, i).
• IA and IB are codominant; i is recessive.

Sex-Linked Traits

• Genes on X or Y chromosomes (sex-linked genes).
• Recessive genes on the single X chromosome in males are expressed.
• X inactivation: one X chromosome in females is randomly inactivated to balance gene expression.

Polygenic Traits

• Multiple genes contribute to the overall phenotype (e.g., height).
• Show a continuous range of phenotypes; often show a bell-shaped curve when graphed.

Epistasis

• One gene (epistatic gene) can interfere with the expression of other genes (e.g., albinism).

Genes and the Environment

• Environmental factors (temperature, diet, light, pH) influence gene expression.

Genomics

• Genomics analyzes the DNA sequence of specific organisms and compares it to other organisms to gain information about a gene’s particular function.

7.3 Traits and Probability

Predicting Generations

• Punnett square: model that tracks alleles each parent can donate to predict the outcome of crosses.
• Genotype: alleles the organism carries for a certain characteristic.
• Gamete: contains one allele for each trait in an organism’s DNA.
• Completed Punnett square shows possible genotypes for coat type: homozygous dominant (WW), heterozygous (Ww), or homozygous recessive (ww).

Calculating Probabilities

• Probability = number of ways a specific event can occur / number of total possible outcomes
• Calculate the probability of two independent events occurring together by multiplying the probability of the individual events.
• The probability of an event that can occur in more than one way is equal to the probability of the individual events added together.

Determining Types of Crosses

• Monohybrid cross: examines one trait.
• Homozygous-homozygous cross: a homozygous dominant parent crosses with a homozygous recessive parent.
• Heterozygous-heterozygous cross: a heterozygous traits cross with another heterozygous traits.
  • Genotypic ratio of 1:2:1.
  • Phenotypic ratio is 3:1 of dominant:recessive phenotypes
• Heterozygous-homozygous Cross: results in two offspring with heterozygous genotype and two offspring with the homozygous recessive genotype

Analyzing the Inheritance of Two Traits

• dihybrid cross examines the inheritance of two traits.
• Heterozygous-heterozygous dihybrid cross results in a phenotypic ratio of 9:3:3:1.

Sex-Linked Inheritance

• Inheritance of genes located on sex chromosomes.
• Female donates an X chromosome and male can either donate an X (female) or Y chromosome (male).
• Sex-linked inheritance results due to sex chromosomes, not solely sexual characteristics.
• Dominant: Recessive phenotypes can also be used to determine inheritance.

Pedigrees

• Pedigree is a family tree that tracks a trait through multiple generations.

7.4 Mutations and Genetic Diversity

Gene Mutations

• Changes in DNA sequence (mutations) may result in diseases.
• Gene mutations: changes in the DNA sequence of a single gene; typically, happens during DNA replication.
• Mutagens: agents that change DNA or increase the frequency of mutation (e.g., UV rays, chemicals, viruses).
• Thymine dimer: UV light can cause neighboring thymine nucleotides to bond together, forming a thymine dimer; the dimer causes the DNA to kink, which interferes with replication.
• Enzymes repair thymine dimers.

Point Mutations

• One nucleotide is substituted for another.
• Silent mutation: changes a codon, but not the amino acid.
• Missense mutation: substitution of a base results in a change in a codon which changes an amino acid.
• Nonsense mutation: mutation results in a “stop” codon being formed, the protein will not be completed.
• Sickle cell anemia: point mutation alters the gene coding for hemoglobin; glutamic acid is substituted by valine.

Frameshift Mutations

• Insertion or deletion of one or more nucleotides.
• Changes the reading frame, disrupting the amino acid sequence.

Trinucleotide Repeat Expansions

• Sections of DNA that consist of repeating nucleotides are called trinucleotide repeats.
• During replication, DNA polymerase may “slip” and make duplicate copies of the repeated sequence.

Chromosomal Mutations

• Changes in chromosome segments or whole chromosomes.
• Change the amount of genetic material or change the structure of a chromosome.

Gene Duplication

• Homologous chromosomes don't align with each other and a segment of one chromosome may break off and attach itself to the other chromosome, resulting in one chromosome with two copies of a gene or genes.
• Multiple gene copies are present, allows gene mutation.
• Mutated genes may encode proteins with new structures and functions.
• Polyploidy: the entire genome is duplicated.

Gene Translocation

• A segment of one chromosome moves to a nonhomologous chromosome.

Nondisjunction Mutations

• Homologous chromosomes do not separate during anaphase of meiosis.
• Gametes have an abnormal number of chromosomes.

Effects of Mutations

• Mutations only affect the offspring if in germ cells.

Impacts On Phenotype

• Chromosomal mutations affect many genes and major impact on the organism.
• Smaller size mutations (gene mutations) can also have a big effects on the organism.
• Mutations can also affect noncoding regions.

Impacts On Genetic Diversity

• Mutations in germ cells are source of genetic diversity in an organism’s genome.
• Mutation provides a mechanism for a population to adapt.
• Environmental factors helps a population to select better phenotypes.

7.5 Genetic Engineering

Isolating Genes

• Genetic testing: analyzes a person’s DNA to determine the risk of having or passing on a genetic disorder.
• DNA microarrays: tools that allow scientists to study many genes; small chip with dotted genes.
• Polymerase chain reaction (PCR): amplifies target sequences from collected patients to get needed material for testing.

PCR Steps

1.  Separating: thermocycler heats DNA until  complementary strands of DNA separate; separation happens around 95 celcius
2.  Binding: thermocycler cools to around 55 celcius,  and the primers bind to the separated DNA strands
3.  Copying: the thermocycler is heated to 72 celcius, and DNA Polymerase attaches to the primer segments and begins adding complementary nucleotides

Cloning and Engineering

Cloning Organisms

• Asexual reproduction results in genetically identical offspring.
• Binary fission (bacteria): asexual reproduction where chromosome is replicated, and the cell splits into two daughter cells.
  Mammalian Cloning: Cloning occurs upon replacing the nucleus of an unfertilized egg with the nucleus of an animal’s cell.

Cloning Ethics

• As cloning advances, it's ethically important to determine whether they should be treated.

Engineering Genes

• genetically engineering it's now capable to change traits or introduce a new one

Recombinant DNA

• Organisms can be from the same/different species
• Add foreign DNA to Plasmid

Editing Genes with CRISPR

• It gives ability to cut DNA at a specific point
• A good solution for problems, as it has become time saving.

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