Chapter 17: Genetics

Selective Breeding

Choosing and breeding specific plants and animals for particular features or behaviours. Began in 9000 BC in the Middle East with the selection of grain strains. Also used in the breeding of dogs.

Pangenesis

A theory proposed by Aristotle says that sperm and egg consist of particles, called pangenes, from all parts of the body. Upon fertilization, pangenes develop into the parts of the body from which they came.

Anthony Van Leeuwenhoek

1677, discovered living sperm in Semen under a microscope. Believed the head of the sperm was a homunculus that came from the father, but developed in the mother.

Regnier de Graaf

Proposed that the egg, not the sperm, contained the entire person, sperm only stimulated the egg to develop.

Traits

A characteristic that an organism can pass on to its offspring through its genes.

Genes

DNA segments that serve as the key functional units in hereditary transmission.

Alleles

Different forms of a gene.

Gregor Mendel

A monk and teacher who took care of pea plants and observed many different traits, or characteristics. For each trait, offspring either displayed a maternal or paternal phenotype rather than a blend.

True Breeding Plants

Plants that exhibit the same characteristics generation after generation. Used by Mendel and acquired by self-pollinating plants.

Monohybrid Cross

A genetic cross that involves a single pair of genes that is responsible for one trait.

Complete Dominance

When an individual with one recessive and one dominant form has the same observable physical characteristic as an individual with two dominant forms.

Mendel's First Law

Also known as the law of segregation, all individuals have two copies of each factor. These copies segregate randomly during gamete formation, and each gamete receives one copy of every factor.

Wilhelm Ludwig Johannsen

A botanist and geneticist who coined the term for mendel's factors (alleles).

Genotype

The combination of alleles for any given trait.

Phenotype

The outward expression of a trait, the physical form you observed.

Homozygous

An individual with two identical alleles for a trait.

Heterozygous

An individual with two different alleles for a trait.

Punnet Square

A chart that shows all the possible combinations of alleles that can result from a genetic cross.

Test Cross

A cross between the organism of unknown genotype and a homozygous recessive organism. The unmown genotype could be either homozygous or heterozygous.

Dihybrid Cross

A genetic cross that involves individuals with two independent traits that are present in alternate forms.

Mendel's Second Law

The law of independent assortment states that alleles of one gene sort into gametes independently of the alleles of another gene.

Incomplete Dominance

Describes a condition in which neither of the two alleles for the same gene can conceal the presence of the other. This results in a third heterozygous phenotype which is an intermediate between the two homozygous alternatives. Uppercase and lowercase letters are not used to identify the alleles when representing incomplete dominance.

Sickle Cell Anemia

A disorder that follows an incomplete dominance mode of inheritance. Caused by a specific form of the gene that directs the synthesis of hemoglobin, the molecule that carries O2 in the blood. Less O2 in the blood leads to distorted blood cells, causing blockages. Normal hemoglobin = HbA, sickle cell hemoglobin = HbS.

Heterzygote Advantage

Heterozygous individuals have an advantage over homozygous dominant or homozygous recessive individuals. Eg. People with HbA and HbS have the sickle cell trait and are resistant to malaria.

Familial Hypercholesterolemia

A genetic condition that prevents the tissues from removing low-density lipoproteins (bad cholesterol) from the blood. People who are homozygotes have six times as much cholesterol as the normal amount (a heart attack by age 2). People who are heterozygotes have two times as much cholesterol (a heart attack by age 30).

Co-dominance

A situation in which both alleles are fully expressed, resulting in a third heterozygous phenotype expressing one allele in some cells and the alternative allele in others, such as a roan horse with a mixed coat colour.

Chromosome Theory of Inheritance

A basic principle in biology stating that genes are located on chromosomes and that the behaviour of chromosomes during meiosis accounts for inheritance patterns.

Mode of Inheritance

The manner in which a genetic trait is passed from one generation to the next.

Autosomal Dominant

A trait is dominant and inherited on an autosome as opposed to a sex chromosome.

Autosomal Recessive

The trait is recessive and inherited on an autosome as opposed to sex chromosomes.

Thomas Hunt Morgan

A biologist who was initially skeptical of Sutton's gene chromosome theory but later proved it through experimentation on fruit flies. He observed that there was a sex linked inheritance of the eye colour gene.

Sex Linked Traits

Genes that are located on X or Y chromosomes display different inheritance patterns than genes located on autosomes.

Colour Blindness

An abnormal condition caused by an X-linked recessive allele and characterized by the inability to distinguish different colours of the visible light spectrum clearly.

Linked Genes

Genes found on the same chromosome (can be inherited together during meiosis). Result in less than 50 recombinants. E.g. because genes for hair and eye colour are close, they will undergo crossing over less often, causing them to produce fewer recombinant gametes.

Crossing Over

A random event that occurs with equal probability at nearly any point on the sister chromatids except near the centromere. Likely to occur between genes that are far apart instead of close together.

Alfred Sturtevant

A student of Morgan who observed that linked genes are separated with consistent, yet different frequencies across various gene pairs. He proposed that these frequencies could be explained by assigning each gene a specific linear position on the chromosome. He refined the chromosome theory.

Chromosome Mapping

Also known as gene mapping, based on the number of recombinant gametes for each set of genes, determining their distance relative to each other on a chromosome.

Map Unit

Defined as the distance between points on a chromosome where crossover is likely to occur in 1% of all meiotic events.

Map Distance

The distance between genes on a single chromosome.

Recombinant Types

Offspring with allele combinations different from the parental generation (produced by crossing over and exchange of DNA between chromatids).

Parental Types

Offspring that retain original allele combination (from the same chromatid).

Non-Linked Genes

Results in 50% parental gametes and 50% recombinants.

Recombination Frequency

The % of times crossover occurred as gametes were formed. Recombination frequency = # of recombinant types/total # of offspring x 100%.

Barr Body

An inactive X chromosome that is condensed tightly. Which one forms a Barr Body is random.

Tortoiseshell Cat

- Calico or tortoiseshell coat colour in cats is a visible consequence of the inactivation of one X chromosome in females.

- The gene determining orange or black fur is located on the X chromosome.

- Tortoiseshell cats are heterozygous for the coat colour gene, possessing one X chromosome with the black allele and another with the orange allele.

- During early embryonic development in female cats, one of the two X chromosomes in each cell is randomly deactivated.

- The deactivated X chromosome remains the same in the daughter cells of that original cell.

- The resulting coat pattern of orange and black patches reflects clusters of cells with different inactivated X chromosomes (one expressing black, the other expressing orange).

Multiple Alleles

A gene with more than two alleles. E.g. An individual has only 2/3 genes but for the population there are 3 possibilities total.

Blood Types

- The ABO blood type system is determined by three alleles: IA, IB, and i.

- The IA allele leads to the presence of the A antigen on red blood cells.

- The IB allele leads to the presence of the B antigen.

- The i allele results in no antigens being present on the red blood cells.

- The i allele is recessive to both IA and IB.

- The IA and IB alleles are codominant, meaning that if both are present, both the A and B antigens will be expressed.

- Specific genotypes (combinations of these alleles) result in different blood phenotypes (A, B, AB, or O) and the corresponding antigens on the red blood cells.

Order of Dominance

An ordering of alleles from most dominant to least.

Continuous Traits

Traits for which phenotypes vary gradually from one extreme to another. Eg. height in humans, ear length in corn, kernel colour in wheat, and weight of beans. Controlled by more than one gene (up to five or more for a trait).

Polygenetic Traits

Traits that are controlled by many genes.

Polygene

A group of genes that contribute to the same gene. Only dominant alleles.

Pedigree

A type of flowchart that uses symbols to show the patterns of relationships and traits in a family over many generations.

Polydactyly

An autosomal dominant condition that occurs in extra fingers or toes.

Huntington's Disease

An autosomal dominant condition where a lethal disorder in causes brain deterioration after 15 years. Symptoms appear after 35 (irritability, memory loss, involuntary muscle movements, dementia, and loss of muscle control).

Marfan Syndrome

An autosomal dominant condition that affects the connective tissues. Can affect all body systems, leads to long bones, abnormal curvature of spine, eye problems, glaucoma, increased risk of retinal detachment, faulty heart valves, and respiratory problems.

PKU

Phenylketonuria, an autosomal recessive disorder which affects the development of the nervous system. Caused by the absence/defect of tyrosine, an enzyme. This causes phenylalanine to convert to phenylpyruvic acid which is toxic. Babies with PKU look healthy but if they are not treated they can develop mental handicaps. Newborns are routinely tested for PKU and then put on a temporary diet.

Cystic Fibrosis

An autosomal recessive disorder that causes a buildup of thick mucus in the lungs and digestive system. These people are at risk for pneumonia, respiratory failure, and difficulty digesting food. Treat with physical therapy, nutritional supplements and antibiotics. This is the most common lethal genetic condition.

Hemophilia

A condition that affect the bodies ability to produce proteins associated with blood clotting. These people can experience serious blood loss from cuts and bruises. Males are at a greater risk. This is an x-linked recessive trait.

Duchenne Muscular Dystrophy

An x-linked recessive trait where one cannot make the muscle protein dystrophin. This causes their muscles to weaken and degenerate over time. Symptoms appear at 3-5 years and life expectancy is 20.

Sir Archibald Garrow

An english physician who coined the phrase "inborn errors in metabolism" for alkaptonuria, albinism, cystinuria, and pentosuria.

Retinoblastoma

Tumour development on the retina of young children, which is usually fatal if not treated.

Prader-Willi Syndrome

A developmental disorder, causing decreased muscle tone, short stature, and an insatiable appetite; can lead to life threatening obesity.

Alpha Thalassemia

A condition in which defective hemoglobin binds to oxygen poorly.

Nieman-Pick Disease

Brain and nervous system impairment due to accumulation of lysosomes filled with cholesterol.

Maple Syrup Disease

Inability to break down 3 amino acids causing an accumulation of by-products and nerve degeneration; usually fatal if untreated.

SCID

Severe combined immunodeficiency disease, a deficiency in one enzyme resulting in minimal immune response and susceptibility to all diseases; bone marrow transplant used to replace immune system.

Genetic Screening

Identifying people who are at risk of developing particular genetic conditions or of passing these conditions to their children.

Alkaptonuria

Causes affected individuals to produce black urine.

Albinism

The lack of the skin pigment melanin.

Genetic Counsellors

Can estimate the risk of inheriting a particular genetic condition while also explaining symptoms of genetic conditions and available treatments.

Epigenetics

The study of changes in organisms due to gene expression (rather than alteration of the genetic code itself). Having a particular allele does not necessarily mean that it will be expressed. Genes can be turned "on" or "off" by methylation of the DNA. Environmental factors can trigger methylation. For example, people with BRCA1 or BRCA2 gene alleles are more likely to develop breast cancer, but only if these genes are turned "on".

Pleiotropic Genes

Genes that have more than one effect; genes that have different effects at different times in the life cycle.