Mendel & Genes Chapter 14

Mendel’s contributions Used scientific methodology when conducting experiments. Used a quantitative approach Discovered fundamental principals of heredity The Law of Segregation The Law of Independent Assortment

Mendel’s hypothesis 1. Alternative versions of genes (alleles) account for variations in inherited characters. 2. For each character, an organism inherits two alleles, one from each parent. 3. If alleles differ, the dominant one will be expressed and the recessive allele will not affect the organisms appearance. 4. The 2 alleles for each character separate (segregate) during gamete production.

The law of Segregation The 2 alleles for a particular character are packaged into separate gametes. Thus, an ovum and a sperm each get only one of the two alleles that are present in the somatic cells of the organism. Allele = a gene that has more than one variation or different versions of a gene. i.e. purple flowers or white flowers. Yy Yy

The law of Independent Assortment Pairs of alleles separate independently during gamete formation. Inheritance of one trait does not influence the inheritance of another trait. Each character is INDEPENDENTLY INHERITED alleles separate as if it were a monohybrid cross. SsBb SbsBsb SB Parents Gametes Example: Color and Height for Peas

Terminology Character: a heritable feature (a gene codes for a particular character) Trait: variations of a character (a trait is coded by an allele-defined later) Hair color, spot patterns on cats… P generation: parental generation F1 generation: first filial generation (offspring of the P generation) F2 generation: second filial generation (self pollinating or F1 cross offspring)

Terminology continued Homozygous: 2 identical alleles Heterozygous: 2 different alleles Phenotype: organisms visible trait Genotype: organisms genetic make up Testcross: this is important and you should know this! Used to reveal the genotype of an organism that shows a dominant trait. Completed by crossing the organism with the unknown genotype with an individual expressing the recessive trait (a homozygous recessive individual)

Extending Mendel

Genotype & Phenotype Simplest Arrangement: each gene has only two alleles, one of which is completely dominant to the other. Incomplete dominance: not blended;3phenotypes: both parent & heterozygote Example: snapdragon flowers - Red & White =Pink CRCR CRCR CWCW CWCW CRCR C R C R C W CWCWCWCW

Phenotype & Genotype Codominance: Both Alleles reflected 2 alleles, distinguishable affects M and N blood groups MM, NN, MN Molecules on RBC’s MN is not INTERMEDIATE Tay-Sachs disease: lack enzyme to metabolize lipids in brain cells 2 Tay-Sachs alleles= disease Heterozygotes & homozygotes with 2 working alleles= normal but heterozygotes have less enzyme Tay-Sachs alleles and normal alleles produce enzyme

Dominance Frequency and dominance not related Polydactyly dominant Only in 1:400 people Achondroplasia, a form of dwarfism, has an incidence of 1 in 10,000 people. Heterozygous individuals have the dwarf phenotype % of the population are homozygous recessive for this trait & not achondroplastic dwarfs Range from complete dominance to codominant One allele does not subdue the other; they reflect the pathways of expression

Multiple alleles The ABO blood groups in humans are determined by three alleles, I A, I B, and i. Both the I A and I B alleles are dominant to the i allele The I A and I B alleles are codominant to each other. Because each individual carries two alleles, there are six possible genotypes and four possible blood types. I A = oligosaccharside on surface of RBC’s

ABO Blood groups

A quick problem… A man with A blood marries a woman with group B blood. Their child has group O blood. What are the genotypes of these individuals? What genotypes and frequencies would you expect in offspring from this marriage?

Pleiotropic genes Gene affects more than one phenotypic character Sickle-cell anemia Multiple health problems can result from this one genetic

Epistasis a gene at one locus alters the phenotypic expression of a gene at a second locus. For example, in mice and many other mammals, coat color depends on two genes. One, the epistatic gene, determines whether pigment will be deposited in hair or not. Presence (C) is dominant to absence (c). The second determines whether the pigment to be deposited is black (B) or brown (b). The black allele is dominant to the brown allele. An individual that is cc has a white (albino) coat regardless of the genotype of the second gene.

Epistasis A cross between two black mice that are heterozygous (BbCc) will follow the law of independent assortment. However, unlike the 9:3:3:1 offspring ratio of an normal Mendelian experiment, the ratio is nine black, three brown, and four white.

Polygenic inheritance the additive effects of two or more genes on a single phenotypic character. For example, skin color in humans is controlled by at least three different genes. Imagine that each gene has two alleles, one light and one dark, that demonstrate incomplete dominance. An AABBCC individual is dark and aabbcc is light.

Polygenic A cross between two AaBbCc individuals (intermediate skin shade) would produce offspring covering a wide range of shades. Individuals with intermediate skin shades would be the most likely offspring, but very light and very dark individuals are possible as well. The range of phenotypes forms a normal distribution.

Nature vs Nurture Phenotype depends on environment and genes. A single tree has leaves that vary in size, shape, and greenness, depending on exposure to wind and sun. Humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests. Even identical twins, genetic equals, accumulate phenotypic differences as a result of their unique experiences. Very old and hotly contested debate.

Nature vs nurture The product of a genotype is generally not a rigidly defined phenotype, but a range of phenotypic possibilities, the norm of reaction, that are determined by the environment. In some cases the norm of reaction has no breadth (for example, blood type). Norms of reactions are broadest for polygenic characters. For these multifactorial characters, environment contributes to their quantitative nature. Genotype can refer not just to a single genetic locus, but to an organism’s entire genetic makeup. An organism’s phenotype reflects its overall genotype and unique environmental history

Pedigree analysis 3rd generation lacks a widow’s peak, but both her parents have widow’s peaks, then her parents must be heterozygous for that gene If some siblings in the second generation lack a widow’ peak and one of the grandparents (first generation) also lacks one, then we know the other grandparent must be heterozygous and we can determine the genotype of almost all other individuals.

Pedigree analysis We can use the normal Mendelian rules, including multiplication and addition, to predict the probability of specific phenotypes. For example, these rules could be used to predict the probability that a child with WwFf parents will have a widow’s peak and attached earlobes. The chance of having a widow’s peak is 3/4 (1/2 [WW] + 1/4 [Ww]). The chance of having attached earlobes is 1/4 [ff]. This combination has a probability of 3/4 x 1/4 = 3/16

Human disorders From the relatively mild (albinism) to life- threatening (cystic fibrosis). The recessive behavior of the alleles occurs because the allele codes for either a malfunctioning protein or no protein at all. Heterozygotes have a normal phenotype because one “normal” allele produces enough of the required protein Two carriers have a 1/4 chance of having a child with the disorder, 1/2 chance of a carrier, and 1/4 free.

Human disorders Multifactorial basis. These have a genetic component plus a significant environmental influence. Multifactorial disorders include heart disease, diabetes, cancer, alcoholism, and certain mental illnesses, such a schizophrenia and manic- depressive disorder. The genetic component is typically polygenic.

Human disorders Huntington’s disease, a degenerative disease of the nervous system. The dominant lethal allele has no obvious phenotypic effect until an individuals is about 35 to 45 years old. The deterioration of the nervous system is irreversible and inevitably fatal. Tip of chromosome 4

Cystic fibrosis One of every 2,500 whites of European descent. One in 25 whites is a carrier. The normal allele codes for a membrane protein that transports Cl - between cells and the environment. If these channels are defective or absent, there are abnormally high extracellular levels of chloride that causes the mucus coats of certain cells to become thicker and stickier than normal. This mucus build-up in the pancreas, lungs, digestive tract, and elsewhere favors bacterial infections. Without treatment, affected children die before five, but with treatment can live past their late 20’s.

Human Disorders Tay-Sachs disease is lethal recessive Dysfunctional enzyme that fails to break down specific brain lipids. Symptoms: seizures, blindness, and degeneration of motor and mental performance a few months after birth. Inevitably, the child dies after a few years. Among Ashkenazic Jews (those from central Europe) this disease occurs in one of 3,600 births, about 100 times greater than the incidence among non-Jews or Mediterranean (Sephardic) Jews.

Human Disorders: Sickle Cell

Human Disorders Some genetic tests can be detected at birth by simple tests that are now routinely performed in hospitals. One test can detect the presence of a recessively inherited disorder, phenyketonuria (PKU). This disorder occurs in one in 10,000 to 15,000 births. Individuals with this disorder accumulate the amino acid phenylalanine and its derivative phenypyruvate in the blood to toxic levels. This leads to mental retardation. If the disorder is detected, a special diet low in phenyalalanine usually promotes normal development