ch 14 ; mendel and the gene idea

14.1 Mendel used the scientific approach to identify two laws of inheritance.

Mendel used the scientific approach to study the inheritance patterns of pea plants.

He chose pea plants because:

1. They had many varieties

2. They had a short generation time and produced a large number of offspring

3. The mating could be strictly controlled, so Mendel would be completely sure of

He started his experiments with true-breeding plants, hybridizing them and seeing the resulting offspring.

One of his hypothesis (the blending model of inheritance) suggested that a cross of purple and white true breeding parents would result in all F1 offspring being pale purple (a mix of purple and white). However, all of the offspring were purple. Then when the F1 self-pollinated, the results show a 3:1 ratio of purple to white.

He had developed a model to explain this.

1. Alternative versions of genes account for variations in inherited characters.

   These alternative versions are alleles. Each gene is a sequence of nucleotides at a locus along a particular chromosome. The DNA at that locus can vary slightly, which is what affects the inherited character of the organism.

2. For each character, an organism inherits two copies (alleles) of a gene, one from each parent

   Every somatic cell in a diploid organism has two sets of chromosomes, one set inherited from each parent. A genetic locus is represented twice in a diploid cell, once on each homologue of a specific pair of chromosomes.

3. If the two alleles at a locus differ, then the dominant alleles determines the organism’s appearance; the recessive allele has no noticeable effect on the organism’s appearance.

4. The law of segregation states that the two alleles for a heritable character segregate during gamete formation and end up in different gametes.

   An egg or sperm only gets one of the two alleles present in the somatic cells. If an organism has identical alleles for a particular character (is homozygous) then that allele is present in all gametes. If different alleles are present, then 50% get the dominant allele and 50% get the recessive allele.

Punnett squares are diagrams used to predict the offspring of parent’s whose genotype is known.

Test crosses are used to determine an organism’s genotype. You test cross by breeding an organism of unknown genotype with a recessive homozygote. If the offspring are heterozygous/show the dominant trait, the unknown organism was homozygous dominant. However, if there were offspring with the recessive trait, the unknown organism is heterozygous.

Two or more traits can also be followed in a cross. The law of independent assortment states that each pair of allele segregates independently of any other pair of allele during gamete formation.

• this law only applies to unlinked genes (those that are on different chromosomes or far away on the same chromosome)

Thus, a dihybrid cross between (AaBb x AaBb) would result in 4 types of gametes— AB, Ab, aB, ab— which can combine in 16 equally probable ways, resulting in 4 genotypes. This can be calculated with 2^n, n being the amount of different alleles (e.g., 2^4 if there are 4 different alleles like in this case).

14.2 Probability laws govern Mendelian inheritance.

Probability ranges form 0-1.

Multiplication rule states that to determine the probability of 2 independent events occurring together, we multiply the probability of one event by the probability of the other event. E.g., x AND y occurs = p(x) * p(y)

addition rule states that the probability that any one of two or more mutually exclusive events will occur is calculated by adding their individual probabilities. (e.g., x OR y occurs: p(x) + p(y)

14.3 Inheritance patterns are often more complex than predicted by simple Mendelian genetics

Inheritance of character by a single gene may deviate from simple mendelian patterns.

Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical. One allele in a pair shows complete dominance over the other.

Some genes have no completely dominant alleles, and hybrids are a mix of both parental varieties. This is incomplete dominance, which occurs when phenotype of F1 hybrids is in between the phenotypes of the two parental varieties.

Codominance occurs when two dominant alleles affect the phenotype in separate, distinguishable ways. For example, in humans heterozygous for MN blood cells, their blood doesn’t show either/or of M molecules or N molecules, but both.

An allele is called dominant because it’s seen in the phenotype, not because it ‘subdues’ a recessive allele.

Tay-Sachs disease: is an inherited disorder in humans, where the brain cells of a child w/ tay-sachs disease cannot metabolize certain lipids bc a crucial enzyme doesn’t work properly. The lipids accumulate, causing seizures, blindness, and degeneration of motor and mental performance and death within a few years. It’s a recessive autosomal disease. At the molecular level though, the normal allele and Tay-sachs allele are codominant. But the phenotype of someone heterozygous with Tay-sachs is considered not affected because the amount of enzymes they do have is sufficient to prevent lipid build up.

Dominant alleles aren’t necessarily more frequent in a population—for example, polydactyly is dominant, but it occurs only in 1 out of 500 babies in canada.

Most genes exist in more than two allelic forms— e.g., ABO blood groups, which have 3 possible alleles that each add a different (or neither) carbohydrate to molecules on red blood cells.

Pleiotropy is the property of most genes to have multiple phenotypic effects. A single gene can affect a number of characteristics. They’re responsible for multiple symptoms of some hereditary diseases, like cystic fibrosis and sickle-cell disease.

Epistasis is when the phenotypic expression of a gene at one locus alters the phenotypic expression of a gene at a second locus.

• e.g., in Labradors, one gene determines pigment colours (alleles B for black and b for brown), the other determines whether the pigment will be deposited in the hair (E for deposition). Gene E/e is epistatic to B/b and therefore the coat colour is determined by the E/e locus.

Quantitative characters are those that vary in the population along a continuum. Usually indicates polygenic inheritance.

Polygenic inheritance is an additive effect of two or more genes on a single phenotype.

• e.g., skin colour in humans.

Sometimes, the phenotype of a character depends on environment along with genotype (like nutrition etc).

Multifactorial means that many factors, both genetic and environmental, collectively influence phenotype

14.4 Many human traits follow mendelian patterns of inheritance.

pedigree is a family’s history for a particular trait assembled into a family tree throughout generation. Inheritance patterns can be traced and described using pedigrees.

carriers are heterozygotes that may transmit the recessive allele to their offspring.

Cystic fibrosis is the most common lethal genetic disease in Canada, affecting one out of every 2,500 people of European descent. It results from defective or absent chloride transport channels in plasma membranes leading to a buildup of chloride ions outside cell. Symptoms from mucus buildup and abnormal absorption of nutrients in the small intestine.

Sickle-cell disease affects one out of 400 African-Americans. It’s caused by the susbtitution of a single amino acid in hemoglobin protein in red blood cells. In homozygous individuals, all hemoglobin is abnormal (sickle-cell). Symptoms include physical weakness, pain, organ damage, and even paralysis. Heterozygotes are usually healthy but may suffer some symptoms. About 1 in 10 African Americans carries the sickle cell allele— heterozygotes are less susceptible to the malaria parasite, so there’s an advantage to being heterozygous.

Some disorders are caused by dominant alleles. Dominant alleles that cause a lethal disease are rare, and arise b mutation. Achondroplasia is a form of dwarfism that is caused by a rare dominant allele.

Huntington’s disease is caused by a lethal dominant allele that has no obvious phenotypic effect until individual is 35-45.

There are multifactorial diseases that depend on a genetic component plus a significant environmental influence— heart disease, diabetes, cancer, alcoholism, certain mental illnesses, etc. In this case, the hereditary component is polygenic.

Amniocentesis is a technique that can determine whether the developing fetus has Tay-sachs disease. Chorionic villus sampling (CVS) is an alternative technique where the physician inserts a narrow tube through the cervix to the uterus and suctions out a tiny same of tissue from the placenta— these cells are derived from the fetus and have the same genotype, so they can be karyotyped.

What is the difference between an allele and a gene?

• Genes are DNA sections that code for specific proteins or functional RNA, whereas alleles are variations of these genes. E.g., a gene exists that codes for a specific eye colour, and an allele are the different versions of these genes (e.g., allele for blue eyes, allele for brown eyes).

• An individual’s combination of alleles is known as their genotype.

When and why is a testcross is used?

• A test cross is used when the genotype of an organism is unknown.

• To test cross, you mate an unknown organism with a known recessive true-breeding organism. If all offspring are dominant heterozygous, the organism was dominant homozygous. If some offspring are recessive, then the organism was heterozygous.

How can you predict potential gametes based on parental genotypes?

• Organisms inherit two alleles for each gene, one from each parent.

• Each pair of allele segregates independently of any other pair.

• So if a parent cell has AaBb, the gametes could be AB Ab aB ab

• If there are more than one genes being passed down, they can be calculated with 2^n, n being the amount of alleles (etc, AaBb is 2^4 , or 16 different possible genotypic outcomes of both parents gametes combining).

Why might the inheritance by a single gene deviate from simple Mendelian patterns?

• When alleles aren’t completely dominant or recessive

• When a gene has more than two alleles

• When a gene produces multiple phenotypes

What is the difference between complete dominance, incomplete dominance, and codominance?

• Complete dominance: heterozygous phenotype same as homozygous dominant

• Incomplete dominance: heterozygous phenotype intermediate between two homozygous phenotypes

• Codominance: both phenotypes expressed in heterozygotes

How are the alleles resulting in the ABO blood system both completely dominant and codominant?

• They are completely dominant because A and B are dominant to O— the genotype AA or AO both result in blood group O

• They are codominant because when A and B are together they have a separate phenotype— AB.