Mendel and the Gene Idea Chapter 14 Biology – Campbell • Reece

Gregor Mendel 1822-1884 A monk from Austria Taught school Began breeding garden peas in the abbey garden to study patterns of inheritance

Why Pea Plants? They are available in many varieties They grow quickly and produce many offspring Have several “either-or” traits Flower color, flower position, seed color, seed shape, pod shape, pod color, stem length

Mendel’s Experiments Mendel decided to cross plants with opposite traits He always started with varieties that were true-breeding The crossing of two true-breeding varieties is called hybridization

Mendel’s Experiments P generation – parental generation F1 generation – first filial generation F2 generation – second filial generation

Mendel’s Experiments

Mendel’s Conclusions Alternative version of genes (different alleles) account for variations in inherited characters. For each character, an organism inherits two alleles, one from each parent

Mendel’s Conclusions If the two alleles differ, then one, the dominant allele (P), is fully expressed in the organism’s appearance; the other, the recessive allele (p), has no noticeable effect on the organism’s appearance The two alleles for each character segregate (separate) during gamete production (law of segregation)

Genetics Vocabulary Homozygous – two identical alleles for a trait (PP or pp) Heterozygous – one of each allele for a trait (Pp) Phenotype – the physical appearance of the trait (purple flowers) Genotype – the genetic makeup (PP, pp or Pp) Testcross – the breeding of a homozygous recessive with an organism of unknown genotype

Genotype vs. Phenotype

Punnett Squares Used to predict a cross Monohybrid (one trait) Dihybrid (two traits) Law of independent assortment

Rules of Probability The rule of multiplication – to predict the chance of two or more independent events occurring simultaneously, multiply the individual probabilities The rule of addition – the probability of an event that can occur in two or more different ways is the sum of the individual probabilities

Rules of Probability What is the probability of getting at least two recessive traits in the offspring of the following cross: PpYyRr x Ppyyrr? ppyyRr = ¼ x ½ x ½ = 1/16 ppYyrr = ¼ x ½ x ½ = 1/16 Ppyyrr = ½ x ½ x ½ = 2/16 PPyyrr = ¼ x ½ x ½ = 1/16 ppyyrr = ¼ x ½ x ½ = 1/16 Total = 6/16 or 3/8

Incomplete Dominance Where the F1 hybrids have an appearance somewhere between the phenotypes of the two parent varieties Red snapdragons x white snapdragons

What is a dominant allele? Complete dominance – one allele is completely dominant over the other (purple vs. white flowers) Incomplete dominance – a sort of blending of the two traits (red, white, and pink snapdragons Codominance – both alleles “show” (roan coloring in cattle) The dominant trait is not necessarily more common in a population

Multiple Alleles Three or more alleles determine a trait Blood type: 4 types: A, B, AB, or O Based on the presence of a certain carbohydrate on the surface of the cell (A or B) May have one substance (type A or B), both (type AB), or neither (type O) A and B are codominant

Multiple Alleles

Pleiotropy The ability of a gene to affect an organism in many ways For example: sickle-cell disease causes multiple symptoms

Epistasis A gene at one locus (location on the chromosome) alters the phenotypic expression of a gene at a second locus For example: fur color in mice

Polygenic Inheritance The characters vary in the population along a continuum An additive effect of two or more genes on a single phenotypic character For example: skin color & height

Pedigrees An analysis of the results of matings that have already occurred The information is assembled into a family tree

Pedigrees Circle – female Square – male Shaded – has the trait Unshaded – does not have the trait Sometimes you will see a half-shaded symbol which represents a carrier of the trait

Recessive Disorders An allele that causes a genetic disorder codes either for a malfunctional protein or for no protein at all Heterozygotes will be “normal” (they are carriers of the trait) Homozygous recessive individuals will have the disorder

Recessive Disorders Cystic fibrosis – missing protein for chloride ion transport, results in build up of mucus in the pancreas, lungs, digestive tract, and other organs (most will die before their 5th birthday) Tay-Sachs – dysfunctional enzyme that fails to break down lipids in the brain (child dies within a few years)

Recessive Disorders Sickle-cell – a substitution of a single amino acid in the hemoglobin protein When blood oxygen is low, crystals form, causing the blood cells to become sickle shaped Carriers have increased resistance to malaria

Dominant Disorders If a lethal dominant allele kills an offspring before it can reproduce, the allele will not be passed on Achondroplasia – a form of dwarfism Huntington’s disease – a degenerative disease of the nervous system

Multifactorial diseases Diseases that have a genetic component and are influenced by environmental factors. Include: heart disease, diabetes, cancer, alcoholism, and certain mental illnesses such as schizophrenia and manic-depressive disorder Hereditary component is often polygenic

Fetal Testing Tests are available for Tay-Sachs, sickle-cell & cystic fibrosis, to determine if the parents are carriers Amniocentesis – amniotic fluid can be drawn and analyzed to find if there are genetic disorders Chorionic villus sampling (CVS) – fetal tissue is taken from the placenta

Fetal Testing

Fetal Testing Ultrasound – uses sound waves to produce an image of the fetus Fetoscopy – a needle-thin tube containing a viewing scope and fiber optics is inserted into the uterus (provides a 3-dimensional image) In 1% of cases, amniocentesis or fetoscopy causes complications

Newborn Screening Some genetic disorders can be detected at birth Phenylketonuria (PKU) – a recessive disorder in which children cannot break down the amino acid phenylalanine Toxic levels can cause mental retardation A special diet can prevent retardation