12.1

Thomas Sutton in 1902 proposed that genes are located on chromosomes

Called the Chromosome Theory of Inheritance For most of the life of the cell, chromosomes are

too elongated to be seen under a microscope & are  called chromatin

Before a cell gets ready to divide, each chromosome is duplicated & condenses into short structures

Each chromosome is composed of a single, tightly coiled DNA molecule 

The two DNA strands are homologous (duplicates) and are held together by the centromere

While they are still attached, the duplicated chromosomes are called sister chromatids

Fertilization restores the diploid chromosome number and paired condition for alleles in the zygote

Chromosomes can be categorized as two types --- autosomes & sex chromosomes

Autosomes are non-sex chromosomes that are the same number and kind between sexes

Sex chromosomes determine if the individual is male or female

Sex chromosomes in the human female are XX and those of the male are XY

Males produce X- and Y-containing gametes; so males determine the sex of offspring 

All animals have a characteristic number of chromosomes in their somatic or body cells called the diploid (or 2n) number.

The gametes or sex cells (egg & sperm) contain half the number of chromosomes as a

body cell; known as the haploid number (n) of chromosomes

Diploid (2n) numbers of OrganismsDiploid (2n) numbers of Organisms

ManMan 4646

DogDog 7878

FruitflyFruitfly 88

CrayfishCrayfish 200200

CornCorn 2020

Thomas Hunt Morgan worked with fruit flies & confirmed that  genes were on chromosomesa. Fruit flies are cheaply raised in common laboratory glasswareb. Females only mate once and lay hundreds of eggsc. Fruit fly generation time is short, allowing rapid experiments

Experiments involved fruit flies with XY system similar to human system

Besides genes that determine sex, sex chromosomes carry many genes for traits unrelated to sex

X-linked gene is any gene located on the X chromosome that are missing on the Y chromosome

X-linked alleles are designated as superscripts to X chromosome

Newly discovered mutant male fruit fly had white eyes

Mutant White-eyed  & Wild, Red-eyed 

Cross of white-eyed male with dominant red-eyed female yield expected 3:1 red-to-white ratio; however, all white-eyed flies were males

An allele for eye color on the X but not Y chromosome supports the results of the cross

Heterozygous females are carriers that do not show the trait but can pass it on

Males are never carriers but express the one allele on the X chromosome

Red-green color-blindness is X-linked recessive

What are the results of crossing a colorblind male with a female carrier for colorblindness?

Trait:     Red-Green Trait:     Red-Green ColorblindnessColorblindness

Alleles:     XAlleles:     XCC    normal     normal visionvision

                 X                 Xcc        colorblindnesscolorblindnessXCXc       x    Xc Y

female male

XXcc YY

XXCC XXCCXXcc XXCCYY

XXcc XXccXXcc XXccYY

Genotypes:   XCXC ,XCY, XCXc, XcY Genotypic Ratio: 1:1:1:1 Phenotypes:

normal vision female, normal vision male, colorblind female, colorblind male

Each chromosome has 1000's of genes All genes on a chromosome form a

linkage group that stays together except during crossing-over

Some genes located on the same chromosome tend to be inherited together 

Linked genes were discovered by Thomas Hunt Morgan while studying fruit flies

Linked alleles do not obey Mendel's laws because they tend to go into the gametes together

Crosses involving linked genes do not give same results as unlinked genes

Recombinants result from chromosome crossing over during prophase I of meiosis

Geneticists can use recombination data to map a chromosome's genetic loci (position on a chromosome)

A genetic map lists a sequence of genetic loci along a particular chromosome

Alfred Sturtevant, a student of Morgan, reasoned that different recombination frequencies reflect different distances between genes on a chromosome

The farther apart genes are, the greater likelihood of crossing-over

The closer together two genes are, the less likely of crossing-over occurring

A map unit equals 1% recombination frequency If 1% of crossing-over equals one map unit, then 6%

recombinants reveal 6 map units between genes To determine the frequency of recombinants, the

following formula is used:

Number of recombinants x 100%

Recombination Frequency=  ---------------------------------------------

     Total Number of Offspring

Humans have few offspring and a long generation time so biochemical methods are used to map human chromosomes (Human Genome Project)

Mutations change in the DNA of an organism They increase the number of variations that occur

Germ-cell mutations – changes in gametes don’t affect organism but may be passed onto offspring

Somatic mutations – occur in body cells and affect the organism E.g. cancer

Lethal mutations – cause death before birth Chromosomal mutations include changes in

chromosome number and/or structure

Deletions occur when the end of a chromosome breaks off & is lost

E.g. Cri du chat syndrome– due to a deletion of a portion of chromosome 5

results in retardation & a cat-like cry

Inversion occurs when a piece of a chromosome breaks off & reattaches to the same place but in the reverse order

Translocation occurs when a

chromosome segment breaks off & attaches to a different chromosome

Nondisjunction – failure of chromosome to separate from its homologue during meiosis

Change in genes caused by change in structure of the DNA

DNA bases may be substituted, added, or removed to cause gene mutation

frame shift mutation – when genes are added or removed

12.2

Also called a family tree Squares represent males

and circles represent females

Horizontal lines connecting a male and female represent mating

Vertical lines extending downward from a couple represent their children

A shaded symbol means the individual possess the trait

Half-shaded symbols are carriers

In humans, another well-known X-linked traits is hemophilia (free bleeders that lack clotting factors in their blood)

One of the most famous genetic cases involving hemophilia goes back to Queen Victoria who was a carrier for the disorder and married Prince Albert who was normal

Their children married other royalty, and spread the gene throughout the royal families of Europe

Huntington’s disease – caused by dominant allele on autosome Symptoms – forgetfulness and irritability in

30’s Loss of muscle control, severe mental illness,

death

Patterns of Human TraitsSingle Allele Dominant

Huntington’s diseaseAcondroplasia (dwarfism)

CataractsPolydactyly (extra fingers/toes)

Single Allele RecessiveAlbinism

Systic fibrosisPhenylketonuria (PKU)

Hereditary deafness

X-LinkedColorblindness

HemophiliaMuscular dystrophy

Icthyosis simplex (scaly skin)

PolygenicSkin, hair, eye color

Foot sizeNose length

Height

Multiple AllelesABO blood groups

Single-allele traits – controlled by a single allele of a gene

Multiple-allele traits – controlled by 3+ alleles that code for a single trait

Polygenic – controlled by 2+ genes

Monosomy occurs when an individual has only one of a particular type of chromosome (45 chromosomes instead of 46) E.g. Turner syndrome (X0)

Trisomy occurs when an individual has three of a particular type of chromosome (47 chromosomes instead of 46) E.g. Klinefelter's Syndrome (XXY) Down Syndrome or Trisomy 21 (individual has

three 21st chromosomes)

Duplications – occur when a section of a chromosome is doubled

Fragile X Syndrome caused by an abnormal number of repeats (CCG) results in retardation & long, narrow face becomes more pronounced with age

Environmental factors including radiation, chemicals, and viruses, can cause chromosomes to break causing a change in chromosomal structure

Tay-Sachs – a disorder where the nervous system deteriorates a fatal gene mutation in Jewish people of Central

European Descent

Phenylketonuria or PKU – gene is unable to synthesize a single enzyme necessary for the normal metabolism of phenylalanine results in death