SBI3U Unite 2: Genetics

Genetics

How information in organisms is passed from parent’s to offspring. This includes the study of genes, heredity, and the variation of traits.

How is heridtary structured

Chromosome —> Genes —> Alleles

Gene locus

specific position of a gene on a chromosome that can determine traits

What are genes?

Heritable factors that control specific characteristics

Alleles

Different forms of a specific gene that can vary between individuals

Chromosomes

Contained in the nucleus of all eukarayotic cells and they vary widely in shape, number, size. In multicellular orgnaims, chromosomes exsit in sets

Haploid Nuclei

Gametes/sex cells. There is one of each chromosome type. In humans, haploid cells contain 23 (half a full set) chromosomes. haploid cells fuse together during sexual reproduction (egg and sperm).

Diploid Nuclei

Two of each chromosome type (and 2 copies of each gene). In humans haploid cells contain 46 chromosomes. These are body/somatic cells. After haploid egg and sperm egg fuse in ertilization to produce a diploid zygot, that zygote divideds by mitosis to form an embryo.

What is the exception to the 2 copies of each gene in diploid nuclei?

Males only have one copy of each gene on the x and y sex chromosome.

What are the different types of chromosomes?

Humans have 23 different types that differ in length and position of the centromere. 22 pairs of Autosomes (1-22) and 1 pair of Sex chromosomes (23). Each chromosome carries a different set of genes in a specific order

What does chromosome number define?

A species.

Does chromosome number change?

Rarely. Typically, it is conserved across millions of years of evolution but sometimes chromosomes can fuse together or split. For example, during human evolution, two ancestral ape chromsomes fused to form chromosome 2 (humans have 23, apes have 24)

How are genes distrubuted?

Amongst an organism’s chromosomes. Location of a gene on a chromosome is called the locus of the geme. all members of a species have the same chromsome number with the same genes in the same loci along their chromosomes.

Genome

the whole genetic information of an organism

What is the genome made from in humans, plants, and prokaryotes?

humans: 46 chromosomes in nucleus + mitochondial DNA

Plants: chromsomes in nucleus + DNA in mitochondria and chloroplasts

Prokaryotes: smaller genome- DNA in a circular chromsome and any pasmids

The Human Genome Project

Began in 1990 and 2003 discovered the code. Determined that 99.9% of genome is identical among humans. Rapid research/development happened due to public AND private competition. ~3.2 billion base letter sequences of human DNA and scientists are still in the process of decoding these sequences to ID genes. They also discovered that a lot of the genome is junk and doesn’t have a function (~70%). This is good because the important code is less likely to have mutations

How many genes to humans have?

~23,000

Asexual REproduction (Mitosis)

• New individual is produced from single parent by cell division

• two daughter cells receives one copy of each chromosome

• two daughter cells are genetically identical to one another and the parent cell

Sexual Reproduction

• New individual is produced from the fusion of two sex cells

• sex cells typically come from two different organisms

• Offspring are NOT genetically identical

Cloning

• mode of asexual reporduction

• process of forming identical offspring from a single cell or tissue

• form of biotechnolofy that has a lot of ethical concerns

Plant Cloning

Used commercially especially on valuable plants to ensure desirable traits - ie roses, produce like apples, etc.

Animal Cloning

• First done in 1996 on a mammal (sheep named Dolly)

• cloned from adult body cells, 277 attempts

• More have been cloned but it is unethical because they die early due to health concerns

• Since Dolly was cloned from a 6 year old lamb, all her tissues were 6 years old at her “birth” and therefore she died 6 years early

• Cloning can be done if there’s a positive like restoration

Mass Production of Livestock and Crops (application/importance of cloning)

• Parent organisms of high quality are chosen

• Produces offspring of high quality as well

• Very expensive

• Consumers tend to avoid purchasing cloned animal products due to ethics

• Can pass on detrimental health effects as well

• Reduces genetic diversity

Cloning Genetically Modified Organisms (application/importance of cloning)

• Carry genes from other organisms

• Exhibit characteristics of the other species

• Commercial insulin produced this way (bacteria and yeast are engineered to contain copies of the human insulin gene)

  • Human insulin has been inserted into a plant for the first time and has been successful

  • Very cost effective

Cloning Endangered Species (application/importance of cloning)

• Viable option if DNA is available

  • For extinct species this can be a problem

Genetically Modified Organisms

• most food is gm’d

• ie supplemented to meet nutritional needs

• Any food that has to be transported more than 30km is not organic

• omega 3’s/fatty acids are typically added to foods because they are hard to find in nature

GM bananas

• genes for the production of a hepatitis vaccine. In developing countries, the bananas they receive typically have hepatitis vaccine in them which makes vaccinations readily available for people without access to healthcare

GM Pig

• gene involved in production of omega-3 fatty acids so that pigs produce bacon and other pork products contained “healthy fats”

Fertilization

the union of gametes, doubles chromosome number – so to prevent chromosome number from doubling each generation, sex cells must be haploid.

Meiosis

enables the sexual life cycle of eukaryotes by producing such haploid cells. a process that divides one diploid eukaryotic nucleus to form four haploid nuclei. The original diploid cell is divided twice in this process – Meiosis I and Meiosis II. Since the chromosome number is halved, Meiosis is called a reduction division

Gametes

• Male - sperm.

  • Produced in the male gonads called the testes

• Female - oum (ova plural)

  • produced in female gonads called the ovaries

Fertilization formula

sperm + ovum (egg) = zygote

Homologous Chromosomes

•Chromosomes of the same size and structure, with the same sequence of genes (i.e. same genes at the same gene loci)

•Despite having the same genes, one may have different alleles for the gene!

•In humans, one of each chromosome type comes from the mother and one of each type comes from the father to make 22-23 homologous pairs*

*Since males have one X and one Y chromosome as their 23^rd pair, it is technically not homologous…

X vs Y Chromosome size

The X chromosome is very large and codes for ~2,00 out of the 23,00 genes in the human genome and is necessary for human development. The Y chromosome only codes for about 100 genes.

Karyotype

The results from genetic screening. It gives sex info, tells in an individual has the correct chromosome number, but can’t tell allels from this.

Does chromosome size determine importance?

No. chromosomes 13-18 are the most improtant for development and errors on these chromosomes lead to the majority of miscarriages.

Spermatogenesis

Te process by which sperm cells are produced. the primary spermatocyte splits into 2 secondary spermatocyes which splits into 4 sperm cells.

Oogenesis

The process by which egg cells (ovum) are produced in females. The primary oocyte splits into a secondary ocite and a polar body. The seconday oocyte produces an egg and onther polar body. and the first polar body produces 2 more polar bodies. the polar bodies die and only one ovum is produced from each primary oocyte.

Meiosis Interphase

S pahase, DNA replication begins. Chromosomes are replicated and these sister chromatids are attached to eachother at the centromere. Then further growth and preparation takes place. Essentially a homolgous pair of chromosomes replicates.

Prophase I

• nuclear membrane dissolves

• centrioles migrate to separate poles of the cell.

• Synapsis: homologous chromosome associate with each other to form bivalents.

• Crossing over: non-sister chromatids tails cross over and recombines the alleles. The point where they cross over is called the chiasma.

• Process of crossing over leads to more variation in the gametes through the recombination of linked alleles

Metaphase I

• random orientation occurs and leads to independent assortment

• Each bivalent aligns independtely and hence each daughter cell gets a different mix of choromosomes.

• Significant source of variation becausee around 8,388,068 different combinations can be made in humans!

Mendel’s Law of Independent Assortment

The presence of an allele of one of the genes in a gamete had no influence over which allele of another gene is present. Basically, if you are blonde, you must have blue eyes is a false statement. There is no coinheritance.

What is true at the end of metaphase I

The goal of genetic variation has been completed BUT there are still too many chromosomes

Anaphase I

• spindle fibres contract

• homologous pairs are separated and pulled to opposite poles BUT sister chormatids remain attatched

• reduction division

Telophase I

• new nuclei form

• cytoplasms divides through cytokineses

• nuclei are not diploid — contain one pair of sister chromatids that are not exact copies

Prophase II

• cell doens’t go back into interphase because we don’t want to replicate the DNA in S phase

• nuclei breaks down, but there’s no crossing over

Metaphase II

• non-sister chromtids/homologous chromosomes randomly migrate to the middle

• random orientation happes again to increase genetic variationbut not as extreme as in MEtaphas I

• spindle fibers form and attach at centromeres

Anaphase II

• spindle fibres contract

• centromeres break

• non-sister chromatids are pulled to opposing poles

• non-disjunction will lead to two gametes containing the wrong chromosome number

Telophase II

• new haploid nuclei are formed

• cytokineses splits the cell

• four haploid gamete cells

• fertilization of these haploid gametes will produce a diploid zygote

Genome

The complete complement or entirety of an organism's DNA.

Chromosome

A cellular structure composed of DNA and associated proteins in the nucleus that carries genetic information.

Gene

A basic unit of heredity made of a DNA sequence located at a specific chromosomal locus that determines a trait.

Locus (plural: loci)

The fixed physical coordinate or site on a chromosome where a specific heritable sequence is positioned.

Genotype

The specific combination of alleles or genetic makeup present in an organism.

Phenotype

The observable physical traits and characteristics displayed by an organism.

Allele

One of two or more alternative versions of a specific genetic sequence (often designated by letters like $P$ or $p$).

Homozygous

Describing an organism or cell having two identical versions of a given gene (such as $PP$ or $pp$).

Heterozygous

Describing an organism or cell having two different versions of a given gene (such as $Pp$).

Trait

A heritable physical or physiological feature of an organism.

Variety

A distinct phenotypic variant or form of an inherited character.

True-bred organism

An individual that consistently produces identical offspring across generations when self-pollinated or bred with its own kind.

Hybrid

An offspring resulting from the mating or crossing of two genetically distinct true-breeding parental lines.

P generation

The initial parental set of individuals crossed in a genetic breeding experiment.

$F_1$ generation

The first filial group of offspring resulting directly from crossing parental individuals.

$F_2$ generation

The second filial group of offspring produced by self-pollination or intercrossing of first-generation ($F_1$) individuals.

Gregor Mendel

The 19th-century Austrian monk who discovered fundamental rules of heredity through systematic breeding experiments with garden pea plants.

Monohybrid cross

A mating experiment designed to track and analyze the inheritance pattern of a single genetic character.

Dominant trait

A characteristic that is visibly expressed whenever its determining factor is present, masking any contrasting alternative.

Recessive trait

A characteristic that remains masked when paired with an opposing factor and is only expressed when two matching copies are present.

Law of Dominance

The principle stating that when parents with contrasting pure traits are crossed, only one phenotypic form is expressed in the hybrid generation while the other remains hidden.

Law of Segregation

The principle stating that an organism's two inherited factors for a character separate during gamete formation and recombine at fertilization.

Law of Independent Assortment

The principle stating that hereditary factors for separate characters distribute into gametes completely independently of each other.

Punnett square

A grid diagram used to visualize genetic crosses and predict the probabilities of possible offspring genotypes and phenotypes.

Genotypic ratio

$1\,PP : 2\,Pp : 1\,pp$ (a $1:2:1$ ratio).

Phenotypic ratio

$3\,\text{dominant (purple)} : 1\,\text{recessive (white)}$ (a $3:1$ ratio).

Test cross

A breeding strategy used to determine whether an organism expressing a dominant phenotype is homozygous or heterozygous by mating it with a homozygous recessive individual.

Test-cross outcome interpretation

An all-dominant offspring pool indicates a homozygous dominant parent, whereas a $1:1$ ratio of dominant to recessive offspring indicates a heterozygous parent.