Genetics: Heredity, DNA, and Chromosomes

Genetics: Genetic Characteristics and Inheritance

• The nucleus is the largest organelle in cells
• The nucleus contains chromosomes, which contain genes
• Genes contain coded instructions for the cell's creation and function
• The nucleus directs all cellular activity
• Genes are hereditary, passed down through generations

Review of DNA

• DNA (Deoxyribonucleic acid) consists of four nitrogenous bases
  • Adenine pairs with Thymine
  • Cytosine pairs with Guanine
• The order of nucleic acids tells cells how to make different molecules
• Each DNA strand pairs with a complimentary strand
• The two DNA strands create a double-helix structure

Core Concepts of Genetics

• Genetics studies inheritance and variation of inherited traits controlled by chromosomes
• Chromosomes house the blueprints for cell organization and activity in the form of genes
• Genes are sections of DNA that instruct the cell to make specific molecules

Chromosome Structure

• Chromosomes are condensed coils of DNA
• A DNA molecule runs the chromosome's length
  • About 50% of the chromosome comprises proteins for support, protection, repair, and copying DNA sections
• Chromosomes possess give distinct attributes:
  • The amount will not vary across the same species
  • Each chromosome has characteristic shapes
  • They come in pairs known as homologous pairs
  • They hold hereditary traits
  • They can make identical copies of themselves

Fixed Chromosome Numbers

• The number of chromosomes varies between species
• Some species share the same chromosome number
• All members of the same species have the same number of chromosomes
  • Humans have 46 chromosomes
  • Sunflowers have 34 chromosomes
  • Rats have 40 chromosomes
  • Fruit flies have 4 chromosomes

Shape of Chromosomes

• Chromosomes have a specific shape consisting of a long, thin structure
• Chromosomes consist of two chromatids joined by a centromere
• Each has a centromere position and a specific length which is how chromosomes are told apart.

Homologous Chromosome Pairs

• Homologous chromosomes are the same in structure
• One chromosome from each pair comes from each parent
• Karyotypes are pictures of chromosome pairs
• Karyotypes are useful for identifying chromosomal abnormalities

Chromosomes and Hereditary Factors

• A particular gene is always at the same location (locus) on the chromosome, loci is the plural
• There are different versions of the genes, referred to as alleles
• A genome is the total of all genes within an organism
• Homozygous genes are when two versions of a gene are the same
• Heterozygous genes are when the versions are different

Chromosome Replication

• Chromosomes create copies through replication
• A chromosome creates a copy by duplicating its chromatid
  • Forming two sister chromatids joined by a centromere
• During cell division, the centromere divides and one chromatid goes to each daughter cell

Meiosis and Its Role

• Meiosis is the process organisms use to create gametes, or sex cells
• Meiosis is similar to mitosis, but it
  • creates four cells instead of two
  • end cells are haploid, instead of diploid
• Haploid cells have one set of chromosomes (23 in humans)
• Diploid cells have two sets, or pairs, of chromosomes (46 in humans)

Meiosis and Variation

• Meiosis introduces variation into the species
• Homologous chromosomes split into seperate gametes during meiosis
• During Prophase I, crossing over may occur
• The exchange of DNA sections between chromatids on homologous pairs is Crossing Over.

Meiosis II

• Sister chromatids, different due to crossing over, separate
• Four new gametes are created during Meiosis II
  • For humans, this includes egg or spermatozoa
• A diploid embryo will form when a fertilization event occurs between two haploid gametes

Genetic Recombination

• Equal lengths are exchanged between homologous chromatids during crossing over
• The equal sharing ensures no deletion or addition of gene loci
• Only one sister chromatid changes at a time, so each is now different from each other

Gregor Mendel's Contributions

• Gregor Mendel is the father of modern genetics theory
• Mendel discovered the principles of heredity
  • by studying the inheritance of 7 characteristics within common pea plants
• Mendel concluded that
  • hereditary factors determine characteristics
  • these factors occur in pairs
  • The two copies separate during gamete formation
• Knowledge of chromosomes and genes did not yet exist at this point in scientific history

Mendel's Success Factors

• Experiments were carefully planned using large samples
• Accurate data recording was used
• Simple, contrasting characteristics not controlled by polygenes were used
• Studied characteristics appeared on different chromosomes
• The use of mathematics to interpret experimental results was prevalent

Investigating Single Gene Traits

• Monohybrid crosses examine single genetic traits controlled by one gene
• Mendel chose pea plants with opposing characteristics for his work

The Experiment Overview

• Two pure-bred parent plants of opposing traits(P generation)
• When cross-bred, offspring only show one trait (F1 generation)
• When two of the F1 offspring are cross-bred, both traits reemerge (F2 generation)
• Specific ratios appear in the F2 generation's offspring

Genotypes and Phenotypes

• A genotype is the genetic makeup of an organism
  • "What are the genes?"
• A phenotype is the expressed physical features of an organism
  • "What does it look like?"
• Mendel saw that pea plants could have one characteristic, yet produce offspring with differing characteristics

Dominance in Alleles

• Dominant Allele: A gene version that is always expressed, represented by a capital letter. (ex: P for purple)
• Recessive Allele: A gene version only expressed when the genotype has two copies. Represented by a lower-case letter (ex: p for white)
• White flowers did not show up in the F1 generation but showed up later in F2
• An organism has 2 copies of genes, one from each parent
• Meaning the F1 generation still carried the gene but purple was expressed because of it's dominance

Explaining Inheritance Patterns

• P Generation (Parent):
  • Parent 1: Phenotype: Purple, Genotype: PP
  • Parent 2: Phenotype: White, Genotype: pp
• During gamete production, each parent offers one gene copy
  • As the gene copies are identical, Parent 1 can only give P, while Parent 2 can only give p
• When the gene copies are the same, the organism is homozygous

F1 Genotype and Phenotype

• F1 hybrids are created when the Parent 1 contributes only a Purple gene (P) and parent 2 contributes only a (p) gene
• F1 Generation:
  • Genotype: Pp
  • Phenotype: Purple
• Heterozygous offspring have one of each allele from a homozygous dominant and homozygous recessive parent

Offspring and Alle Alleles

• Combinations made when two heterozygous Pp parents have offspring:
  • PP – Homozygous Dominant (Purple)
  • Pp - Heterozygous (Purple)
  • pp - Homozygous Recessive (White)

Mendel's Conclusions

• Experiments were repeated many times, then data was collected
• Conclusion within organisms where breeding factors that control traits
  • There are two inheritance factors in each cell
  • One factor is donated from each parent
  • During reproduction the factors split and pass to the next generation
  • The genes have forms like white or purple
  • One is dominate over the other factor

Punnett Squares

• To calculate the likelihood of specified offspring a tool called a punnet squares is used
  • ¼ of the offspring will be PP
  • ½ of the offspring will be Pp
  • ¼ of the offspring will be pp
  • ¾ of the offspring will be Purple
  • ¼ of the offspring will be White

Mendel’s Inheritance Principles

• Law of Segregation: Alleles separate during meiosis such that each gamete has a single allele copy
• Law of Independent Assortment: Traits separate independently when forming gametes
• Law of Dominance: An organism with at least one dominant allele will express that allele in their phenotype

Independent Assortment Explained

• Traits are inherited separately of each other
• Scientists use a dihybrid cross to find the probabilities of offspring when looking at two traits at once
• Phenotypic ratios:
  • 3:1
  • 9:3:3:1

Beyond Mendel's Laws

• Incomplete Dominance
• Codominance
• Sex-linked Characteristics
• Polygenic Traits
• Epigenetics

Incomplete Dominance

• Phenotype happens when heterozygous individuals show somewhere in between the two homozygous phenotypes
• In these cases, one allele is not fully dominant over the other.

Codominance

• More than one allele is dominant for a given trait
• Blood types are a good example of codominance
  • A, B, and O are blood type alleles
• Humans have three alleles for blood type:
  • IA - Type A
  • IB – Type B
  • i – Type O
• Allows for six different genotypes and four phenotypes
• The A and B alleles can be dominant meaning they show both traits

Sex-Linked Genes

• Traits are found on sex chromosomes, which for humans, are X and Y
• Women have two X chromosomes, while men only have one
• Since the X has far more genetic code, men need a sole copy of the allele for the appearing trait.

Cat Hair Color Genes

• Two types are present on X chromosomes of cat hair - black and orange
• The Y chromosome does not contain a cat hair color gene
• Male cats only have a single color, while two alleles allow for patchwork (calico) results in females

Polygenic Traits Overview

• Genes looked at so far are monogenetic, and controlled by a singular gene
• In reality, polygenetic traits are controlled by multiple genes
• Interactions between Height, Skin and Eye color and many other genes control human traits

Polygenic Traits and Bell Curves

• Polygenic traits shown in graphs have bell curves
• Most people are found at the middle range of characteristics, whereas small amounts are on either extreme

Understanding Epigenetics

• Epigenetics or environmentally impacted factors can heavily impact how genes are expressed
• Identical twins can vary as they genetically age and phenotypically express themselves
• One's behavior, and appearance can be changed over time with things like diet, drugs, and life choices.