Genetics
Genetics is the study of heredity and variation of living organisms and how genetic information is passed from one generation to another.
1. Cell Division and the Cell Cycle
Types of Cell Division
There are 3 types of cell division, depending on the type of cell
* Binary Fission (prokaryotes)
* Mitosis (eukaryotes)
* Meiosis (eukaryotes)
*Prokaryotic cells don’t have a nucleus whereas eukaryotic cells do
Asexual vs Sexual Reproduction
Aseuxal Reproduction - Reproduction where an organism mades an identical replica to itself
Sexual Reproduction - Reproduction of an organism by combining genetic material from two individual different sexes
Eukaryotic
In eukaryotic cells, reproduction and cell divisions are different
Cell Division = Making more cells within an individual
Reproduction = Combining dna to create offspring (Sexual)
Why do cells divide?
Purpose of Cell Division in Eukaryotes:
* Repair
* Replace
* Reproduce
* Growth
Cell Cycle
Cells have their own life spans called cycles because they grow, reproduce, and have programmed cell death. The cell cycle is a very delicate process controlled by genes.
* Malfunctions in the cell cycle can lead to cancer.
Cancer
Cells have a very specific instructions when to divide and when to grow
These instructions are controlled by genes. If the genes are mutated in some way, they will not work properly and cell division may become out of control, leading to cancer.
Cell Growth and Division
A cell will grow until it reaches a certain point and then divide into two cells.
Cells divide because as the cell grows, the nucleus doesn’t grow with it. This means the cell becomes too large to function.
If cells continue to grow larger…
* The cell becomes less efficient in moving wastes out and nutrients in
* The demands of the cell’s DNA increases
* The bigger the cell, the more energy you need to function
What do cells need?
Cells require oxygen, water, and nutrients, and expel wastes; all transported access the cell membrane
Larger cells need more nutrients and make more waste; therefore, smaller cells are more efficient
Surface Area to Volume ratio
Cell division
Cell division is the process in which cellular and genetic material from one
Pre-Cell Division
Before the cell divides, the following must occur
1. DNA must be replicated (copied) so that each cell gets its own copy of DNA
2. New organelles and proteins must be created so that each cell is equipped for survival
Chromosomes
Chromosomes are located in the nucleus of the cell
* Chromosomes - made of DNA (many genes) + proteins (such as histones)
* Gene - a segment of DNA that codes for a particular protein, located on the chromosomes
Histones are proteins that help keep the tightly coiled
What is a chromosome?
* Chromosomes can vary in form
* Chromatin is the uncoiled, decondensed form of chromosomes
* Chromatin only take shape during cell division
1. Terminology - Genetics
Somatic Cells vs Gametes
Somatic cells: body cells, or every cell in your body except egg cells or sperm cells
* Examples of Somatic : heart cell, liver cell, bone cell
* Somatic cells go through mitosis
Gametes: Sex cells (sperm or egg)
* Gametes undergo meiosis
Diploid Cells
* A cell or an organism consisting of two sets of chromosomes
* One set from the mother and another set from the father that form chromosome pairs
* Somatic cells (body cells) are diploid
* In humans, the diploid number is 46 in each body cell (23 maternal; 23 paternal)
* Symbolized as 2n
* n is the number of chromosomes
Haploid Cells
* A cell or organism consisting of one set of chromosomes (half)
* Gametes are haploid
* In humans, the haploid number is 23 (egg - 23, sperm - 23)
* Symbolized as n
Meiosis
* Cell division for gametes
* Halves the number of chromosomes
* Occurs in all sexually reproducing organisms
* Two stages
* Meiosis 1
* Meiosis 2
* Begins with a germ cell that contains 46 chromosomes
* Ends with 4 gametes each containing 23 chromosomes
2. DNA and Base pairing
What’s in the nucleus?
DNA - Deoxyribonucleic Acid
* A molecule that determines the inherited characteristics of an organism
* Instructions for producing new cells with the same characteristics as the original cell
* Double stranded helix structure (Ladder)
* It’s this shape because of the chemical bonds of some of the components of DNA
Genes - a segment of DNA on a chromosome that encodes for a particular protein
* The recipe for making proteins
* Each protein has a special sequence
* Proteins that make you, you
* These are called traits
Chromosomes - Long strands of DNA wrapped tightly around histone patches and coiled to form chromosomes
* A double-stranded threadlike structure that carries genetic material
* Chromatid is ½ of the chromosome
Alleles - Possible Versions of a gene (I.e. Hair colour)
* B = brown hair
* b = blonde hair
* An allele is the specific code for a gene found
* Max two alleles for one gene
DNA from a single human cell extends two meters long
DNA consists of a series of units called nucleotides
* Each nucleotide is made up of three parts
* A sugar called deoxyribose
* A phosphate group
* And one of four nitrogen-containing bases
Nitrogen Bases
The nitrogen bases are:
* Adenine (A)
* Guanine (G)
* Thymine (T)
* Cytosine (C)
Each nitrogen base is attached to a sugar
Base-Pair Rule
DNA = twisted ladder
“Steps” of the ladder = pairs of nucleotides
The order of these steps = the instructions for your body
Adenine - Thymine
Guanine - Cytosine
Only these pairs can bond together
A T A T C A T G C G G G
T A T A C T A C G C C C
With some mutations, you can have an addition of a nucleotide or a deletion
Sometimes there’s no effect, but sometimes there's a significant effect
3. Meiosis
* Cell division for gametes
* Halves the number of chromosomes
* Occurs in all sexual reproducing organisms
* There are two stages of meiosis (two divisions or reduction division); meiosis I and meiosis II
* Begins with a germ cell that contains 46 chromosomes
* Germ cell with divide through meiosis to become a sex cell which has half the number of chromosomes
* Ends with 4 gametes each containing 23 chromosomes
Meiosis & Chromosome #
Every cell has a nucleus; every nucleus has chromosomes. The number of chromosomes depends on the species
Humans - 46
Chickens - 78
Diploid (2N) vs Haploid (N)
Cells in your body have 46 pairs - they are called diploid
Gametes (sperms and eggs) only have half (23) - haploid
Why bother with Meiosis?
Homologous Chromosomes
Chromosomes come in matching sets called homologous chromosomes
* Chromosomes that are the same in size, have the same centromere position, and code for the same genes match up
* One from each parent
Crossing over
* A process in which homologous chromosomes exchange genes
* The points of which they cross over are called chiasma
* Important for genetic variation, without it, gametes would be identical
* Occurs during Prophase 1
Oogenesis - Meiosis in females; makes eggs
During oogenesis, only one cell turns into an egg
The other three are polar bodies which die off
Spermatogenesis - Meiosis in males; makes sperm
During spermatogenesis, all four cells develop into spermatozoa
4. Errors in Meiosis
Terminology
Independent assortment - Homologous chromosomes pair up and separate randomly, it is independent from the other pairs
Twins
* Born during the same birth
* Monozygotic (Identical twins
* Dizygotic (Fraternal)
Dizygotic
* Genetically similar as normal siblings
Monozygotic
* Share nearly identical DNA
* Same sex
Identical twins
Why?
* Arises from a single sperm and egg
* Zygote is created but during early stages of development
* Early stages of an embryo it gets split into two
* Share the same genetic information because they're from the same egg and sperm
* They can be either in the same sac or different sac
Fraternal Twins
* Placenta is never shared
* Never in the same sac
* When two separate eggs are fertilized by two separate sperm cells at the same time
* Just as genetically similar as any siblings
Errors in Meiosis
How do we get genetic variation
* During prophase I our genetic variation happens during crossover
* In metaphase I - shuffling parental DNA into various combinations (independent assortment)
Karyotype
Can show:
* Chromosomal sex (XX and XY)
* The male determines the sex of the offspring
* Missing chromosomes
* Additional chromosomes
* Missing / additional parts of chromosomes
It CANNOT show
* Errors in the genetic code
Errors in the Number of Chromosomes
Trisomy = Additional chromosomes
* Ex. Down syndrome
* Trisomy 21
Monosomy = Missing chromosome
* Ex. Turner syndrome
* Monosomy 23
Errors occur during Anaphase 1 or anaphase 2
NonDisjunction - Failure to separate
Nondisjunction in Meiosis I
* Failure of homologous chromosomes to separate during anaphase I
* RESULT:
* 2 gametes with an additional chromosome
* 2 gametes with missing chromosome
Nondisjunction in Meiosis II
* Failure of sister chromatids to separate during Anaphase II
* RESULT
* 2 gametes with the correct number of chromosome
* 1 gamete with an additional chromosome
* 1 gamete with a missing chromosome
Errors in Single Chromosomes
* Caused by an error in crossing over in Prophase I
Deletion
* A section of the genetic code is removed
Duplication
* A section of the genetic code is repeated 2 or more times
Inversion
* A section of the genetic code is flipped
Translocation
* A section of one chromosome is added to a non homologous chromosome
5. Reproductive Strategies and Technologies
Selective breeding
* Process of breeding plants and animals for desirable traits
* Ex. Two kinds of dogs with a particular trait you want, you’ll breed them together
* Ex. Breeding the largest flowers to produce larger flowers
E.g. You want to breed large cows for meat
1. Select largest female and male
2. Sexual reproduction produces variation but you're more likely to get larger cows
3. Continue to do this for generations until all offspring are large
Artificial Insemination
* Process by which sperm are collected and placed into the reproductive tract of a female
* Benefit makes sperm from high quality males more widely available through online resources
* Allows farmers and pet owners to choose desirable traits for the male parent
Embryo Transfer
* Process involves fertilizing an egg artificially and then transferring it into a recipient female
* Embryos can be shipped very easily, which eliminates the need to physically ship an animal from one place to another
Assisted Reproductive Technologies In Vitro Fertilization (IVF)
* The technique used to fertilize egg cells outside the female's body
* Test tube babies
Preimplantation Genetic Diagnosis (PGD)
Parents who have a history of genetic disorders in their family may choose to use a process that allows for the diagnosis of genetic disorders soon after fertilization
Arguments
* Eugenics
* Selecting for gender
* Specific Traits
6. Understanding Inheritance
Terminology
Genotype vs Phenotype
Genotype: The combination of alleles for any given trait
* Genetic makeup
* E.g. I^A I^A
* Genotype = Genes
Phenotype: The physical appearance of an organism with respect to a trait
* How an organism looks like
* E.g. Blood Type A
* Phenotype = Physical
Dominant vs Recessive
These terms are used when describing an allele
* Dominant Allele: The form of a trait that always appears when an individual has a gene for it
* Denoted as CAPITAL LETTER
* Recessive Allele: The form of a trait that only appears when an individual has two genes for it
* Denoted as small letter
Homogeneous vs Heterogeneous
Homogeneous: Both alleles for a trait are the same in an individual
* Homozygous Dominant: BB
* Homozygous Recessive: bb
Heterogeneous: Different alleles for a trait in an individual
* Bb
Punnett Square
7. Dihybrid Crosses
Studying the inheritance of two traits simultaneously
Law of Independent Assortment
Two traits are inherited independently
* One trait doesn’t affect the other
* E.g. Color of the pea plant doesn’t affect the height
The colour of the seed doesn't affect the size of the seed
Dominant genes are expressed in the first generation
* First generation of pea plants were all tall because tall is the dominant gene
* But the first generation is still the carrier of the recessive gene
* The second generation has more potential to be short and green
8. Complex Patterns of Inheritance
Autosomal Inheritance: the inheritance of traits whose genes are found on
Other Complex Pattern
1. Incomplete Dominance
2. Comdominance
3. Multiple Alleles
4. Sex linked
5. Polygenic Inheritance
1. Incomplete dominance
* The blending of 2 traits: neither one is dominant or recessive
* E.g. A purely red and purely white flower who cross and the result is a pink flower
2. Codominance
* In codominance, both alleles for a trait are equally expressed in a heterozygote; both alleles are dominant
* E.g. Roan
* Red cows crossed with White cows
* Roan refers to cows that have red and white splotches
3. Multiple Alleles
A gene that has more than two alleles
Multiple Alleles: Human Blood Types
* A is dominant to O
* B is dominant to O
* A and B are co - dominant
4. Sex Linkage
1. In addition to their role in determining sex (female=XX, male = XY), the sex chromosomes have genes for many characteristics
2. Genes located on a sex chromosome are called sex linked genes
3. In humans we’re usually referring to them as X-Linked characters: genes located on an X chromosome
4. It’s more commonly known as a X Linked instead of Y Linked because there’s a higher chance of something going wrong in the X chromosome since everyone has it
5. The Y chromosome is also smaller, which means it carries less traits
Autosomal traits - Hair colour
X Linked traits - male pattern baldness
Y Linked Traits - Webbed toes
Recessive alleles in Humans
1. If a sex linked trait is due to a recessive allele
* A female will express the phenotype only if she is homozygous recessive
* Since males only have one X chromosome, they're more likely to be for example, colourblind
5. Polygenic Inheritance
10. Pedigrees
Analyzing patterns of inheritance
Humans breeding studies cannot be conducted in the same way as Medel’s plants
Why?
* Cannot specifically designate mates
* Cannot accumulate large enough numbers for statistical purposes
Pedigrees are flow charts of a family which follows a specific trait
Each row represents one generation of a family
They’re identified by a roman numeral
Individuals are identified by arabic numbers
Pedigree Patterns
Pedigrees are helpful to determine possible modes of inheritance for a trait
1. Autosomal Dominant
2. Autosomal Recessive
3. Sex- Linked
1. Autosomal Dominant
Only one allele is needed for the trait to be expressed
If an individual expresses the trait, ONE of their parents MUST also express the trait
It affects males and females equally
* Affected individuals are usually observed in every generation
2. Autosomal Recessive
Two alleles are needed for the trait to be expressed
If an individual has the trait, BOTH their parents must be carriers
It affects men and women equally
* It can skip a generation
3. Sex Linked
Affected individuals tend to be disproportionately one sex over another