Genes and Chromosomes The Chromosome Theory of Heredity Mutations Regulation of Gene Expression
Objectives • State the chromosome theory of heredity • Explain how gene linkage affects inherited traits • Describe the process of crossing-over and explain how it increases genetic variety • Describe gene mapping • Describe the process of sex determination and the patterns of inheritance for sex-linked traits
Chromosome Theory of Heredity • Mendel’s work was incomplete because he never asked an important question: Where in the cell are the factors that control heredity? Where are the genes?
What do you see in this picture? What do you recall about chromosomes?
Chromosome Theory and HeredityKey terms • Chromosome theory of heredity • Linked gene • Linkage group • Recombinant • Sex chromosome • Autosome • X chromosome • Y chromosome • Sex-linked
Genes and Chromosomes • Nucleuscontainschromsomes • chromosome=threadlike structure in a cell that contains the genetic information that is passed on from one generation of cells to the next
Genes and Chromosomes • Walter Sutton • 1902, discovered gene location • Chromosome theory of heredity genes are located on the chromosomes and each gene occupies a specific place on a chromosome • Each gene may exist in several forms or alleles • Each chromosome contains just one of the alleles for each of its genes • Sutton’s development of the chromosome theory is an example of how the work of one scientist builds on the work of another scientist
Gene Linkage • Genes on a chromosome are linked together • They are inherited together • Linked genes do not undergo independent assortment • Linked genes=genes that are inherited in a group
Gene Linkage • Thomas Hunt Morgan studied drosphilia • Effects of gene linkage • Morgan crossed purebred gray bodies and normal wings with purebred black bodies and small wings • Gray (G) black (g) Normal wings (W) small wings (w) • F1 should have been gray with normal wings (GgWw) • When F1 crossed with black small-winged drosphilia (ggww) Morgan did not observe the expected results • Most gray-bodied drosphilia had normal wings and most black-bodied flies had small wings • Gene for body color and gene for wing size were somehow connected, or linked • They could not assort independently
Gene Linkage • Linkage groups • Morgan studied more and more genes • Discovered genes fell into distinct linkage groups of genes that always tended to be inherited together • The linkage groups (chromosomes) assorted independently, but all genes on one group were inherited together • Because homologous chromosomes contain the same genes, there is one linkage group for every homologous pair of chromosomes(drosphilia has four linkage groups, four pairs of chromsomes) • A cobra has 38 chromosomes. How many linkage groups would this make?
Tomorrow! • Crossing over in linked genes • Gene mapping • Sex linkage
Crossing-over • During prophase I of meiosis, homologous chromosomes may exchange sections of their chromatids in a process called crossing-over • Increases genetic variety
Crossing-Over • Linkage groups explains some of the results of the drosphilia crosses but does not provide a complete explanation • 83% have gene combinations like their parents • 17% have new gene combinations • Recombinants=individuals with new combinations of genes
Crossing-Over • If the genes for body color and wing size are linked, why aren’t they linked all the time? • Morgan proposed that linkages could be broken some of the time • If two homologous chromosomes were positioned side by side, sections of the two chromosomes might cross, break, and reattach. • This process would rearrange the genes on the chromosome and produce new linkage groups
Gene Mapping • Further reasoned that crossing-over occurs at random along the linkage groups, and the distance between two genes determines how often crossing-over occurs between them • Close together crossing-over is rare • Far apart crossing-over more common
Gene Mapping • Knowing the frequency with which crossing-over between two genes occurs makes it possible to map the positions of genes on a chromosome • Today we have detailed maps of Drosphilia that pinpoint the locations of more than 1500 different genes
Sex Linkage • 1905 American biologist Nettie Stevens discovered that not every chromosome has a corresponding homologous chromosome • Discovered female mealworm contain 20 large chromsomes and male contain19 large and one small • One of male chromosome pairs is not homologous • The pair has very different shapes • Same thing was found in drosphilia
Sex Linkage • These “mismatched” chromosmes are the sex chromosomes • female sex chromosomes=two matching sex chromosomes (XX) • Male sex chromosomes=two dissimilar sex chromosomes(XY) • Y chromosome=small and hook shaped • The other chromosomes, which are the same in both males and females, are called autosomes
Sex Linkage • Sex determination • The sex chromosomes in the male’s gametes determine the sex of the offspring
Sex Linkage • Sex determination • When female gametes are produced, meiosis separates one of the X chromosomes into each egg cell • In the male, meiosis separates the X and Y chromosomes so that 50% carry X chromosome and 50% carry Y chromosome • When an X sperm fertilizes an egg a female is formed • When a Y sperm fertilizes an egg a male is formed
Sex Linkage • Genes on sex chromosomes • Sex chromosomes also carry genes that affect other traits • Sex-Linked a gene located on one of the sex chromosomes
Non-Disjunction • Occurs during meiosis • Some gametes contain extra chromosomes • Some gametes are missing chromosomes
Klinefelter’s Syndrome • Male XXY sex chromosomes • Sterile • Show female characteristics • Underdeveloped testes • Breast development • Poor beard growth
Turner’s Syndrome • FemaleX_ • Mental retardation • Sterile • Short in stature • Underdeveloped ovaries • Increased chance of thyroid problems
Tomorrow! • Gene Mapping Lab!
Board work 21 • How are genes related to chromosomes? • How does crossing-over make genetic mapping possible? • What are sex chromosomes? Autosomes? • Why are the effects of recessive sex-linked alleles seen more often in males than in females?
Mutations • Mistakes in duplicating genetic info and transmitting it to the next generation are rare, but they do occur • Mutations=change in the genetic material of the cell • Not all are harmful • No effect • Slight effect • Harmless • beneficial
Mutations • Mutations in reproductive cells (germ cells) • Germ mutations • inheritable • Mutations that affect other cells of the body • Somatic mutations • Cancer • 2 levels • Chromosomal mutations • Gene mutations
Chromosomal mutations • Involve • Segments of chromosomes • Whole chromosomes • Entire sets of chromosomes • Results in change in number or structure • 4 types • deletions • Duplications • Inversions • translocations
Chromosomal mutations • Deletions • The loss of part of a chromosome
Chromosomal mutations • Duplications • Opposite of deletion, segment of chromosome is repeated
Chromosomal mutations • Inversions • Part of a chromosome becomes reversed
Chromosomal mutations • Translocations • Part of one chromosome breaks off and attaches to another, nonhomologous chromosome
Chromosomal mutations • Nondisjunctions • Involve whole chromosomes or complete sets of chromosomes • Failure of homologous chromosomes to separate normally during meiosis • Not coming apart • 1 chromosome involveextra copy in one cell and loss from another • More than 1dramatic increase in number, producing triploid (3N) or tetraploid (4N) organisms • Extra sets of chromosomespolyploidy • Almost always fatal in animals • Plants are often larger and hardier
Gene mutations • Involve • Individual genes • Cause • Chemical change that affects DNA
Gene mutations • Point mutationsaffect no more than a single nucleotide
Gene mutations • Insertion or deletion of nucleotide • Frameshiftmutationscompletely change the polypeptide product produced by a gene
Board Work 22 • Compare a chromosomal mutation and a gene mutation. • What is a somatic mutation? How does it differ from a germ mutation? • How does nondisjunction result in chromosomal mutations?
Regulation of Gene Expression • Gene interactions • Incomplete dominance • Codominance • Polygenic inheritance • Gene expression in prokaryotes • Operon • Repressor • Gene activation • Gene expression in eukaryotes
Regulation of Gene Expression • As biologists have intensified their studies of gene activity, it has become clear that interactions between different genes and between genes and their environment are critically important
Gene Interactions • Dominance • How genes interact with each other • Remember…. • A gene is a section of DNAcodes for a polypeptide • Dominant allele codes codes for a specific polypeptide that works, recessive for one that does not work
Gene Interactions • Incomplete dominance • Inheritance in which an active allele does not entirely compensate for an inactive allele
White carnation R=red r=white Red carnation Pink carnation F1 generation All pink F2 generation 1 red 2 pink 1 white Pink carnation
Gene Interactions • Codominance • Condition in which both alleles of a gene are expressed • Written as capital letters with subscripts or superscripts • Ex: B1 and B2 or R and R’ • Seen in many organisms • cattle=red hair is codominant with white hair (HRHW) • Look roan or pinkish white • Chickens=black feather are codominant with white feathers (FBFW) • Erminant chickens (speckled black and white)
Gene Interactions • Polygenic Inheritance • A trait that is controlled by two or more genes • Many traits are produced by the interaction of many genes…polygenic • Shape of your nose • Color and markings on an animal’s coat