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

• Nucleuscontains 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

X YX XX XYX XX XY

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

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

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– Frameshift mutationscompletely 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 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

r rR Rr RrR Rr Rr

R rR RR Rrr Rr rr

R=redr=white

F1 generationAll pink

Red carnation

White carnation

Pink carnation

Pink carnation

F2 generation1 red2 pink1 white

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

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 systeminducer 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.