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 Heredity Key terms Chromosome theory of heredity Linked gene Linkage group Recombinant Sex chromosome Autosome X chromosome Y chromosome Sex-linked

Genes and Chromosomes Nucleus  contains chromsomes – 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 XY XXXXY XXXXY

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 Female  X_ – Mental retardation – Sterile – Short in stature – Underdeveloped ovaries – Increased chance of thyroid problems

Tomorrow! Gene Mapping Lab!

Board work 21 1.How are genes related to chromosomes? 2.How does crossing-over make genetic mapping possible? 3.What are sex chromosomes? Autosomes? 4.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 involve  extra copy in one cell and loss from another – More than 1  dramatic increase in number, producing triploid (3N) or tetraploid (4N) organisms Extra sets of chromosomes  polyploidy – 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 mutations  affect no more than a single nucleotide

Gene mutations Insertion or deletion of nucleotide – Frameshift mutations  completely change the polypeptide product produced by a gene

Board Work 22 1.Compare a chromosomal mutation and a gene mutation. 2.What is a somatic mutation? How does it differ from a germ mutation? 3.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 DNA  codes 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

rr RRr R Rr RRRRr r rr R=red r=white F1 generation All pink Red carnation White carnation Pink carnation F2 generation 1 red 2 pink 1 white

Gene Interactions Codominance – Condition in which both alleles of a gene are expressed Written as capital letters with subscripts or superscripts – Ex: B 1 and B 2 or R and R’ Seen in many organisms – cattle=red hair is codominant with white hair (H R H W ) » Look roan or pinkish white – Chickens=black feather are codominant with white feathers (F B F W ) » 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

Gene Expression in Prokaryotes The genes of a single organism cannot be activated ate the same time – Make many molecules it did not need Waste energy – Must be able to produce the product of a gene quickly and in adequate amounts

Gene Expression in Prokaryotes When the product of a gene (a specific protein) is being actively produced by a cell, we say that the gene is being expressed – Within a single organism, some gene are rarely expressed, some are constantly expressed, and some are expressed for a time and then turned off How does a cell “know” when to make a protein and when not to – Turn off and turn on?

Gene Expression in Prokaryotes The Operon – Genes and regions of DNA that operate together; consists of a gene cluster and regions involved in the regulation and expression of that cluster – Consists of Operator=region of chromosome near the cluster of genes in an operon to which the repressor binds when the operon is “turned off” Promoter=region of chromosome next to the operator in an operon to which RNA polymerase binds at the beginning of transcription

Gene Expression in Prokaryotes The Operon – The gene cluster in the operon studied by Jacob and Monod produces enzymes that break down lactose Bacteria does not produce enzymes in large amounts unless lactose is present – Lactose induces production of enzymes to break down lactose for use as food – This operon system  inducer because it induces the production of enzymes » Enzymes not produced in the absence of lactose

Gene Expression in Prokaryotes The Repressor – When repressor nears the operator it attaches itself to the operator so that it sits between the promoter and the genes Position blocks the access of RNA polymerase

Gene Expression in Eukaryotes Inducers induce the activation of genes – Bind directly to DNA and either start or increase transcription of particular genes – mRNA produced during transcription may be altered before it is used to make protein during translation The presence of DNA sequences that do not code for protein – Exons=sequences that are complementary code for protein “expressed” – Introns=segments that are not complementary and do not code for protein “intervening”

Board work 23 1.How do gene interactions affect gene expression? 2.Compare incomplete dominance and codominance. 3.What is a polygenic trait? 4.“Mutations in introns are less likely to affect phenotype than mutations in exons.? Defend or refute this statement.