SBI3U Genetics

Genetics

The branch of biology dealing with heredity and the variation of inherited characteristics

Heredity

The passing of traits from parents to offspring

Traits (Alleles)

• Characteristics or features of an organism

• E.g. nose shape, height

Hapsburg’s Jaw

• Likely a result of inbreeding in the case of the royal family

• Increased homozygosity of the trait in the family over generations

Cell Division

• The continuity of life is based on the reproduction of cells, or cell division

• Single-celled organism

  • E.g. binary fission

• Multi-celled organism

  • E.g. cells in bone marrow make new red blood cells

• Most cell division involves the distribution of identical genetic material to two daughter cells, essentially cloning

• The molecule deoxyribonucleic (DNA) contains our genetic information

• A cell’s DNA must be copied before it divides, ensures the daughter cells each end up with a complete genome

• This occurs during the S phase of Interphase

Genome

The complete complement of an organism’s or a virus’s genetic material

Gene

A segment of a DNA molecule that codes for a particular trait (allele)

Locus

The location of a gene in a chromosome

Chromatin

• A mass of thread-like strands of DNA

• When the cell is not dividing, DNA is in the form of a long, thin chromatin fibre

Chromosomes

• Cellular structures containing a DNA molecule and associated proteins

• After replication (S phase), DNA is condensed and neatly packaged for distribution

• Each duplicated chromosome has two sister chromatids

• Sister chromatids: joined copies of the original chromosome

• Centromere: region on chromatid

• Contains specific DNA sequences

• Attachment site for sister chromatids

• Formed by proteins

• Creates “waist” on duplicated chromosome

• Chromatid arms on either side

• Chromatid: a specific term for a replicated chromosome

The 3 Stages of the Cell Cycle

1. Interphase

   1. Cell growth

   2. Normal functions

   3. Chromosome replication

2. Mitosis

   1. Nucleus division

   2. Equal genetic material distribution

3. Cytokinesis

   1. Cell division

   2. Identical daughter cells

Interphase

• Between dividing stages

• G1: initial cell growth

• S: DNA duplication

• DNA is in the form of chromatin

• G2: cell undergoes its function

• Purpose

  • Replenishes dead/dying cells

  • Organism growth and development

  • Produces somatic cells (e.g. organs, skin, bones, blood, etc.)

G0 Phase

• Resting phase

  • The cell is not dividing nor planning to divide

• Also called the state of quiescence

Cell Death - 2 Types

• Necrosis

  • The cell may die due to external factors

  • E.g. toxins, infections, trauma

• Apoptosis

  • Programmed and targeted

  • E.g. white blood cells

Mitosis Vocabulary

• Diploid (2n) - contains 2 complete sets of chromosomes - 46 in humans

  • Body cells (somatic cells) are diploid

• Haploid (n) - contains a single set of unpaired chromosomes - 23 in humans

  • Sex cells (gametes - sperm and egg) are haploid

• A chromatid is one strand of a chromosome

• Two chromatids that are the same are called sister chromatids

Prophase (preparation phase)

• Chromatin condenses into chromosomes

• Nuclear membrane dissolves and disappears

• Spindle fibres begin to form

• Nucleolus dissappears

Metaphase (organizational phase)

• Chromosomes align at metaphase plate

• Spindle fibres assist

• Attachment at centromere of sister chromatids

Anaphase (separation phase)

• Chromosomes separate in identical sets

• Centromeres divide

• Sister chromatids pulled by spindle fibres

• Move to opposite cell poles

Telophase

• Chromosomes reach opposite cell poles and unwind

• Spindle fibres dissolve

• Nuclear membrane forms around chromosomes

Cytokinesis

• Cytoplasm and organelle splitting

• Results in two daughter cells

• Different process in plant and animal cells

Meiosis

• Hereditary material is exchanged (mixed, transferred)

• Species-specific chromosome numbers

• Humans: 36 chromosomes (23 pairs)

• Diploid (2n): 46 chromosomes

• Haploid (n): 23 chromosomes

Meiosis Vocabulary

• Diploid (2n): contains two complete sets of chromosomes, one from each parent

• Haploid (n): cells have half the usual number of chromosomes

• Gametes (n): also called a germ/sex cell, sperm or egg

• Somatic/Body cells (2n): all cells that are not gametes

Early Interphase

• Meiosis is preceded by interphase

• Chromosomes have not yet condensed

Late Interphase

• Chromosomes have replicated

• Cell goes from 2n to 4n

Meiosis Prophase I

• Chromatin condenses into chromosomes

• Homologous chromosomes pair up (synapse), form tetrads

• Crossing over in late prophase I

  • Increases genetic diversity

Crossing Over/Genetic Recombination

• AKA synapsis

• Homologous chromosomes exchange DNA

• Offspring genetically different to parents and each other

Meiosis Metaphase I

• Chromosomes line up at cell equator

• Centromeres attach to spindle fibres

• Random assortment of homologous chromosomes

Meiosis Anaphase I

• Homologous chromosomes separate

• Move along spindle fibres

• Head towards poles/ends of cells

• Chromosomes separate independently

• Independent assortment occurs

• Increases diversity

Meiosis Telophase I & Interkinesis

• Cell begins to divide into 2 daughter cells

• Any combination of maternal/paternal chromosomes possible

• 2 daughter cells formed

Meiosis Prophase II

• Nuclear membrane dissolves

• Spindle fibres begin to form

Meiosis Metaphase II

• Chromosomes line up on spindle fibres

• Sister chromatids attached at centromeres

• Random assortment of sister chromatids

Meiosis Anaphase II

• Centromeres divide

• Sister chromatids separate and move to opposite poles

• Chromatids become chromosomes

• Chromosomes separate independently

• Independent assortment increases diversity

Meiosis Telophase II & Cytokinesis

• Meiosis ends with the formation of 4 cells

• Cells formed are prospective gametes

The Numbers

• Number of Parent Cells

  • Mitosis: 1

  • Meiosis: 1

• Number of Divisions

  • Mitosis: 1

  • Meiosis: 2

• Number of Daughter Cells

  • Mitosis: 2

  • Meiosis: 4

• Number of Chromosomes in Parent Cells

  • Mitosis: 46 chromosomes

  • Meiosis: 46 chromosomes

• Number of Chromosomes in Daughter Cells

  • Mitosis: 46 Diploid

  • Meiosis: 23 Haploid

Location and Function

• Location

  • Mitosis: In somatic (body) cells

  • Meiosis: In germ/sex cells

• Function

  • Mitosis: Growth, repair, replace

  • Meiosis: Gamete creation, reproduction

• Asexual or Sexual

  • Mitosis: Asexual

  • Meiosis: Sexual

Advantages and Disadvantages

• Mitosis

  • Advantages

    • Creates genetically identical cells for uniform organs (e.g. skin, liver)

  • Disadvantages

    • All cells share susceptibility to same diseases

• Meiosis

  • Advantages

    • Mixes genetic material for variation

  • Disadvantages

    • Organism cannot reproduce independently, requires time and energy

Mistakes in Separation

• Aneuploidy: abnormal chromosome count

• Improper chromosome separation in meiosis

• Caused by nondisjunction

• Meiosis I: Homologous chromosomes fail to separate

• Meiosis II: Sister chromatids fail to separate

Aneuploidy Examples

• Monosomy: Karyotype missing one chromosome

• Polysomy (Trisomy): Extra chromosomes present

• Polyploidy: 3 sets of chromosomes (3n) due to nondisjunction

• Somy: Individual chromosome sets

• Ploidy: All chromosomes in nucleus

Karyotypes

• Karyotype: organized picture of chromosomes

• Examines chromosomes for testing

• Identifies genetic problems by:

  • Counting chromosome

  • Checking for structural changes

• Tests performed on various tissues:

  • Amniotic fluid

  • Blood

  • Bone marrow

  • Placenta tissue

DNA (Background Information)

• Stands for Deoxyribonucleic acid

• Stores and transmits genetic information from parent to offspring

DNA Structure

• Wrapped tightly around histones (proteins) and tightly coiled to form chromosomes

• Double Helix (Twisted ladder)

A History of DNA

• Friedrich Miescher

  • Investigated compound in nucleus

  • Named it nuclein (1869)

• Rosalind Franklin

  • Captured x-ray photo of DNA (1952)

• Watson and Crick

  • Interpreted Franklin’s x-ray

  • Described DNA double helix (1953)

DNA consists of…

1. A 5-carbon/pentose sugar called deoxyribose

2. A PO4 group or phosphate

3. A nitrogen containing base

There are 4 different bases in DNA

• Purines

  • Adenine (A)

  • Guanine (G)

• Pyrimidines

  • Thymine (T)

  • Cytosine (C)

DNA Base Pairs

• Adenine always bonds with Thymine

• Cytosine always bonds with Guanine

Genetic Diversity

Different arrangements of nucleotides in a nucleic acid (DNA) provides the key to diversity among living organisms

Types of Mutations

• Chromosomal Mutations: Affecting whole or a part of a chromosome

• Gene Mutation: Changes to the bases of the DNA in one gene

Mutations

• Permanent DNA sequence alteration

• Occurs randomly: DNA replication errors

• Spontaneous mutations: Mutagens (radiation, chemicals)

• Types: Chromosomal, Single-gene

How is the DNA affected?

* Point mutation
* Single base pair substitution, insertion, or deletion
* Redundancy: Possibly not affected
* New amino acid: Similar properties, potential improvement
* Possible outcomes: Enhanced success, detrimental effects

Sex Determination

* 22 pairs are autosomes, 1 pair are the sex chromosomes
* Female gives X and X, Male gives X and Y, Male determines sex

Dominant/Recessive Inheritance

• Dominant Inheritance

  • Only one copy of the gene (from either parent) needs to have a mutation for the trait to be expressed

• Recessive Inheritance

  • Two copies of the gene (from each parent) must have a mutation for the trait to be expressed

Earlobe Variation

• Single gene determines earlobe attachment

• Gene has two alleles

• One allele inherited from each parent

• Dominant allele leads to detached earlobes (indicates at least one dominant allele (RR or Rr))

• Recessive allele leads to attached earlobes (indicates two recessive alleles (rr))

Human Variation

• Some traits occur as a few discrete types

  • Attached or detached earlobes

  • Most genetic disorders

• Other traits are controlled by multiple genes

  • Height

  • Weight

  • Eye colour

Examples of Inherited Autosomal Recessive Disorders

• Sickle Cell Disease

• Tay-Sachs Disease

• Cystic Fibrosis

An Example of an Autosomal Dominant Disorder

* Huntington’s Disease

X-linked/Sex Linked Disorders

• X-linked disorders are caused by genes on the X-chromosome

• Many more x-linked traits than y-linked traits because the X-chromosome is much larger than the Y-chromosome

• X-linked disorders are generally seen in males

  • Males have only one X-chromosome, therefore, no dominant gene to cancel out the recessive gene

X-linked Recessive Disorders

• Affect males more than females

• Females need two copies of the gene to be affected

• Female Carrier Risks

  • 50% chance daughter become carriers

• Affected Male Offspring

  • Daughters: always carriers

  • Sons: not carriers, inherit Y-chromosome

• Examples

  • Duchenne Muscular Dystrophy

  • Hemophilia (AKA Royal Disease)

X-linked Dominant Disorders

• Dominant gene for disorder is carried on the X-chromosome

• One copy of the gene will cause the disease

• Less common than the X-linked recessive disorders

• Example

  • Fragile X Syndrome

Gregor Mendel

• Worked in a monastery growing pea plants

• All of the alleles (traits) that Mendel worked with exhibited complete dominance in their inheritance

Dominant Alleles

Traits that are physically expressed in phenotype (physically), if one or both of the genes code for the dominant allele

Recessive Alleles

Traits that are physically expressed in phenotype (physically), if both of the genes code for the recessive allele

Phenotype

• The physical expression of a gene

• Example: The pea plant has purple flowers

Genotype

• The genetic expression of a gene

• Dominant alleles are represented by capital letters

• Recessive alleles are represented by lowercase letters

• Each individual gene uses one letter to represent it

Homozygous or Heterozygous

• Homozygous (Purebred): Two copies of the same allele

• Heterozygous (Hybrid): Two copies of different alleles

Mendel’s Law of Independent Assortment

• Chromatids align independently at the metaphase plate

• Occurs during Metaphase I and Metaphase II

• Alleles separate into gametes independently

• The allele for one gene in a gamete does not influence the allele received for a different gene

Mendel’s Law of Segregation

• Gametes receive one copy of each chromosome and gene

• Chromosomes are randomly sorted during metaphase, leading to a mix of maternal/paternal DNA in gametes

• Each gamete is unique

• Gametes separate during Anaphase I and II

True Breeding/Purebred

• Mendel developed true breeding plants

  • Homozygous Recessive

  • Homozygous Dominant

• Male Sex Organs: removed from all flowers

• Record of phenotypes of originating plants was kept

• Fertilization: pairing female flowers with male pollen of the same traits

• Goal: achieve true breeding plants

Monohybrid Crosses

Looks at only one gene, on one chromosome

Dihybrid Crosses

• Looks at two genes, each on different chromosomes

• Phenotypic Ratio → 9:3:3:1

Cloning

• Involves making exact copies of the original organism genetically

• The aim is to replicate exceptional individuals

  • E.g. cows that produce extra milk

• Dolly the sheep was the first successful mammal clone born in 1996

  • She lived for a few years before being euthanized due to several complications

Genetic Screening

• Identifies high-risk individuals that pass on inherited disorders through various procedures

• Provides choice to not have children or screen embryos for genetic disorders

Genetic Counselling

• Advises prospective parents about genetic disorder risk to future children

• Gathers background information to make recommendations and allow families to control environmental factors

Prenatal Diagnosis

• Tests the fetus for genetic problems using amniocentesis or chorionic villus sampling

• Karyotype is made to show all chromosomes in an individual

Recombinant DNA

* A plasmid in a bacteria is used to produce missing proteins to treat people with defective genes

Gene Therapy

• Inserts a working gene into cells to correct some hereditary defects

• Stem cells are used to divide and differentiate production

The Human Genome Project

* Identifies junk DNA and the human genome’s approximately 30,000-35,000 genes with 3164.7 x 10^12 base pairs

Incomplete Dominance

• A third (new) phenotype appears as heterozygous in a blend of the dominant and recessive phenotypes

• Neither allele is completely dominant over the other allele

• Result: a heterozygous phenotype

• Example: white and red together makes pink

Co-dominance

• Two equally dominant alleles are expressed at the same time

• Heterozygous phenotype will have both phenotypes visible

• Capital letters that are different

• Result: new variation (heterozygous)

• Example: black and white together makes black and white

Multiple Alleles

• More than 2 alleles for the trait

• Many possible genotypes and phenotypes

• Multiple alleles exist in a population

• Individuals possess only 2 alleles of a gene

Blood Types

• Blood phenotypes are controlled by a combination of 2 or 3 different alleles

• A combination of co-dominant and dominant genetic traits

• 3 blood types: IA, IB, and IO

• The alleles for blood types A and B are co-dominant

• Blood type O is homozygous recessive

• The letter “I” represents immunoglobin

Blood Groups

• Red blood cells contain antigens

• Antigens determine blood type

• The body produces antibodies to guard against foreign cells or organisms

• Antibodies are found in blood plasma, all throughout the body

• Each antibody targets a specific type of antigen

Rh Factor

• Antigen/protein on red blood cell surfaces

• Blood can have the protein or not

• Rh(+) is the most common blood type

• Follows dominant inheritance

• Rh(+): ++ or +- genes

• Rh(-): -- genes

Relevance of Rh Factor

• Rh(-) mother and Rh (+) baby: potential for mother to develop antibodies against Rh(+) fetus

• Antibodies developed: if mother’s blood comes into contact with baby’s blood

• Future pregnancies risk: blood crossing placenta, damaging baby’s red blood cells, and causing anemia

• Solution to prevent Rh antibodies: Rh immune globulin injection