Mendel and the Gene Idea

 Monk  Pea Plants  many varieties, easy to reproduce and control, tracked traits that were “either-or”, started with true breeding plants

 P generation = true breeding parents Purple x White  F1 generation = hybrids from P generation cross All purple plants  F2 generation = offspring from F1 self- pollination 3:1 ratio of purple to white plants P Generation (true-breeding parents) Purple flowers White flowers  F 1 Generation (hybrids) All plants had purple flowers F 2 Generation

1. Alternate versions of genes ALLELES Found on Homologous Chromosomes Represented with letters Figure 14.4 Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers

2. Each trait is controlled by 2 alleles – one contributed from each parent

3. The Law of Segregation The 2 alleles for a trait segregate during gamete formation (meiosis) and end up in different gametes

 Mendel’s Other Law: The Law of Independent Assortment Genes located on different chromosomes are inherited independently Exceptions: genes far away on same chromosome (many map units apart) allowing crossing over to occur can be inherited independently * Genes located close together and are usually inherited together are called “linked” ex: red hair, freckles

4. Some alleles are dominant while others are recessive Dominant alleles “mask” recessive Dominant alleles = capital letters Recessive alleles = lowercase letters

 Homozygous = 2 of the same alleles Homozygous Dominant = DD  Shows dominant trait Homozygous Recessive = dd  Shows recessive trait  Heterozygous = 1 dominant, 1 recessive allele Heterozygous = Dd  Shows dominant trait and masks recessive  Genotype = allele make up (ex: DD)  Phenotype = physical trait

 Punnett Squares Aka Monohybrid Cross  1 trait  Explains Law of Segregation

 Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype Is the organism DD or Dd?!  Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait

 The testcross Figure 14.7  Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp If PP, then all offspring purple: If Pp, then 1 ⁄ 2 offspring purple and 1 ⁄ 2 offspring white: p p P P Pp pp Pp P p pp APPLICATION An organism that exhibits a dominant trait, such as purple flowers in pea plants, can be either homozygous for the dominant allele or heterozygous. To determine the organism’s genotype, geneticists can perform a testcross. TECHNIQUE In a testcross, the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent. RESULTS

 Shows inheritance of two traits Ex: seed color AND seed shape Typical Outcome for 2 heterozygous individuals: 9:3:3:1

 Complete Dominance Phenotype of Dd is the same as DD  Codominance Shows 2 dominant traits Ex: BB = black, WW = white, BW = black AND white  Incomplete Dominance Shows blended pattern Ex: C R C R = Red, C w C w = White, C R C W = Pink

 Most traits are controlled by 2 alleles (ex: DD)  Some traits have more than 2 alleles ABO Blood Type I A, I B, i are the 3 alelles Table 14.2

 1 gene has multiple phenotypic effects Ex: Sickle Cell Anemia

 Ex: Hair, Skin, Eye Color  Additive Effects  AaBbCc aabbcc Aabbcc AaBbccAaBbCcAABbCc AABBCcAABBCC 20 ⁄ ⁄ 64 6 ⁄ 64 1 ⁄ 64 Fraction of progeny

 A gene at one locus alters the phenotypic expression of a gene at a second locus (masks the other gene)  Ex: Laborador Retrievers 2 genes: (E,e) & (B,b) pigment (E) or no pigment (e) pigment concentration: black (B) to brown (b) Outcome differs from 9:3:3:1

 Controlled by genetics and environment  Ex: Skin color in humans, hydrangea color  Nature vs. Nurture

 Circles = Females  Squares = Males  Shaded = Expresses phenotype  Half-Shaded = Carriers  Parents joined by horizontal lines  Offspring listed below parents in birth order

 Most are recessive (require homozygous recessive genotype) Carriers = heterozygous individuals that do not show trait, but “carry” the gene for it (Ex: Aa) Cystic Fibrosis: defective chloride channel in cell membranes lead to build up of thickened mucus Tay-Sach’s disease: brain cells cannot metabolize lipids Sickle-Cell Anemia: misshaped blood cells

 Huntington’s Disease: Dominant, degenerative disease of nervous system  Achondroplasia: Dominant form of dwarfism (“AA” is lethal)  Many disorders are multifactorial Heart disease Cancer Diabetes

 Fetal Testing Amniocentesis: withdrawl of fluid from amniotic sac to collect cells for viewing and karyotyping Chorionic villus sampling (CVS): small amount of tissue suctioned from placenta Ultrasound: non-invasive, reveals structures  Newborn Screening Blood: test for PKU (Phenylketonuria)  Recessive disorder that leads to mental retardation  Requires a change in diet