Mendel and the Gene Idea

 F 1 produces equal amounts of 4 possible genotypes  F 2 reveals even more genotypic possibilities (9:3:3:1)  Dihybrid cross is equivalent to two monohybrid crosses (12:4 or 3:1)  Illustrates the Law of Independent Assortment

Many genes do not follow a Mendelian inheritance pattern Incomplete Dominance Co-dominance Multiple alleles Pleiotropy Polygenic Inheritance Lethal Dominance Environmental Influence on Gene Expression 3

 The phenotype of the heterozygous genotype is intermediate between the phenotypes of the homozygous genotypes  E.g. snapdragons  Heterozygotes differ from homozygotes  Predictable 1:2:1 ratio  Different than “blending” hypothesis  No testcross necessary

Both alleles are dominant and affect the phenotype in two different but equal ways Andalusian chickens show this pattern of inheritance. If you cross a black (BB) chicken With a white (WW) chicken You get black+white speckled (BW) chicken

 More than two possible alleles controlling one trait  Example3 alleles in blood type – OAB  4 possible phenotypes = O, A, B, AB  6 possible genotypes Note: this is also an example of co-dominance

 Genes that have more than two alleles  More than two alleles exist in a given population  However, any one individual only has two of these alleles  Example - Coat color in rabbits (4 alleles) Himalayan RabbitFull Color RabbitAlbino Rabbit Chinchilla Rabbit

 Gene influences multiple characteristics  A singe gene influences more than one phenotypic trait.  Genes that exert effects on multiple aspects of physiology or anatomy are pleiotropic

 Multiple genes have an additive effect on a single character in the phenotype  Example: Skin Color or height  Usually is described by a bell-shaped curve with majority clustered in the middle

 T/t x T/t =  T/T T/t t/t  1 : 2 : 1 ratio at conception  0 : 2 : 1 ratio at birth

 pH of the soil will change the color of hydrangea flowers from blue to pink

 Temperature will affect color change in fur

 Body temperature affects color in Siamese cats  Height is affected by nutrition

 In humans, pedigree analysis is used to determine individual genotypes and to predict the mode of transmission of single gene traits 15

 Huntington disease is a progressive nerve degeneration, usually beginning about middle age, that results in severe physical and mental disability and ultimately in death  Every affected person has an affected parent  Two unaffected parents will not produce affected children. (aa x aa)  Both males and females are affected with equal frequency.  Pedigrees show no Carriers.

Dwarfism Polydactyly and Syndactyly Hypertension Hereditary Edema Chronic Simple Glaucoma – Drainage system for fluid in the eye does not work and pressure builds up, leading to damage of the optic nerve which can result in blindness. Huntington’s Disease – Nervous system degeneration resulting in certain and early death. Onset in middle age. Neurofibromatosis – Benign tumors in skin or deeper Familial Hypercholesterolemia – High blood cholesterol and propensity for heart disease Progeria – Drastic premature aging, rare, die by age 13. Symptoms include limited growth, alopecia, small face and jaw, wrinkled skin, atherosclerosis, and cardiovascular problems but mental development not affected.

 Albinism = absence of pigment in the skin, hair, and iris of the eyes  Most affected persons have parents who are “normal” (Aa x Aa)  The parents are heterozygous for the recessive allele and are called carriers (Aa)  Approximately 1/4 of the children of carriers are affected (aa)  Close relatives who reproduce are more likely to have affected children.  Both males and females are affected with equal frequency.  Pedigrees show both male and female carriers. 19

Congenital Deafness Diabetes Mellitus Sickle Cell anemia Albinism Phenylketoneuria (PKU) – Inability to break down the amino acid phenylalanine. Requires elimination of this amino acid from the diet or results in serious mental retardation. Galactosemia – enlarged liver, kidney failure, brain and eye damage because can’t digest milk sugar Cystic Fibrosis – affects mucus and sweat glands, thick mucus in lungs and digestive tract that interferes with gas exchange, lethal. Tay Sachs Disease – Nervous system destruction due to lack of enzyme needed to break down lipids necessary for normal brain function. Early onset and common in Ashkenazi Jews; results in blindness, seizures, paralysis, and early death.

MENDEL AND HEREDITY Why we look the way we look...