CHAPTER 9

Patterns of Inheritance

Genetics• study of science of heredity• began w/the use of wild type

traits – traits most commonly found in nature

•desirable traits were then bred

Mendel’s Methods• used true breeding plant – made by

self-fertilization• created hybrids by cross-fertilization

(crossing 2 different true breeding plants)

- P generation is parent generation

- F1 generation (1st filial) is offspring of P generation

- F2 generation (2nd filial) is offspring made by F1 x F1

Sect 9.2

Mendel’s Principles

(Principle of Segregation)

1. alternative forms for genes called alleles

2. an organism has 2 genes (alleles): 1 inherited from each parent

- sperm & egg each carry only 1 allele for each inherited characteristic

Sect 9.3-9.4

3. when the alleles of the pair are different, 1 is fully expressed, the other is masked

- dominant allele is expressed

- recessive allele is masked4. Law of Segregation states that the allele pairs separate during gamete formation (meiosis) & restored during fertilization

Punnett Square – diagram used to predict results of a genetic cross

Homozygous – identical alleles for a trait ex: G = green GG g = yellow gg

Heterozygous – 2 different alleles for a trait Gg

Phenotype – the expressed trait (physical appearance green or yellow)

Genotype – organism’s genetic makeup GG, Gg, gg

Mendel’s Principles(Independent Assortment)

• Monohybrid Cross – parents differ only in a single trait

Pod Color G = green g = yellow

Genotype: 50% Gg & 50% gg

Phenotype: 50% green & 50% yellow

Sect 9.5

•Dihybrid Cross – parents differ in 2 different traits

- it follows the law of independent assortment

- each allele pair separates independently during gamete formationP generation: RRYY x rryy

•Testcross – a breeding of the recessive homozygote w/an organism of unknown genotype

Sect 9.6

Practice a testcross

Complications of Genotypes to Phenotypes

Incomplete Dominance – when 1 allele is not dominant over the other (snapdragon)Multiple Alleles – some genes exist in more than 2 allele forms: blood types - A, B, AB, O (phenotypes)

- A & B are codominant

Sect 9.12-9.13

Sect 9.14 Pleiotropy – when a gene has multiple effects

- affects phenotypic characteristics

Ex: sickle-cell anemia (single recessive allele on both homologues) causes formation of abnormal hemoglobin which in turn causes: breakdown of red blood cells, clumping of cells & clogging of small blood vessels, accumulation of sickle cells in spleen

p. 168

NOTE: each of these causes additional effects on an individual

- individuals who are heterozygous are called carriers because they “carry” the disease-causing allele & may transmit it to their offspring

Polygenic Inheritance – an additive of 2 or more genes on a single phenotypic characteristic (skin color controlled by at least 3 genes)

Sect 9.15 p. 169

Chromosomal Theory of Inheritance

•Mendelian genes are located on chromosomes

•Chromosomes undergo segregation & independent assortment

Sect 9.18

Linked Genes

•Discovered in 1908 by William Bateson & Reginald Punnett

•Found on same chromosome•The principle of independent

assortment does not apply because the genes are part of a single chromosome

Sect. 9.19

Chromosomal Basis of Recombination

•Genetic Recombination – production of offspring that combine the traits of 2 parents

•In unlinked genes independent assortment will take place

- parental types – offspring w/same phenotype as one or the other of the parents

Sect 9.20

- Recombinants – offspring having different combinations than either parent

Linked Genes – independent assortment does not take place

- crossing over can occur so new combinations are passed on

- recombination does occur

Mapping ChromosomesCross Over Data Relative

Distance Between Genes

• Use recombination data to assign a position to genes

• A map unit is equal to 1% recombination frequency

• Determined by crossover frequency

• The greater the distance between genes, the greater the chance for crossing over to occur

Sect 9.21

Chromosomal Basis of Sex Determination

• Humans & other mammals have XX & XY• Most insects have XX (female) & XO

(male)• Birds, fish, butterflies, moths have a ZW

system: ZW (female and determines sex) & ZZ (male)

• Most bees & ants are haplo-diploid: female from fertilized eggs (diploid), male from unfertilized eggs (haploid) –

parthenogenesis – virgin birth

Sect 9.22

NOTE: not all organisms have separate sexes

-plants are monoecious (one house), ex: corn

- animals are hermaphroditic – all individuals of a species have the same compliment of chromosomes ex: earthworms, garden snails

Morgan: Sex Linkage• Worked w/fruit flies –

Drosophlia - found that the gene for eye

color is on the X chromosome: R = red r = white

- mated white eyed male w/red eyed female (wild)

* all F1 have red eyes, then mated F1 x F1

Sect 9.23

Which of the following represents the human genome project:

a. The main character in Travelocity commercials

b. Yard art

c. Aimed at sequencing all the DNA on the human chromosomes

Genome• One complete haploid set of

chromosomes of an organism• in humans, 23 chromosomes

w/approximately 3 billion nucleotide pairs of DNA that carry between 50,000 & 100,000 genes

•If genome’s chromosomes were uncoiled and laid end to end, they would make a very thin thread that would be approximately 3 meters long

Karyotype• A photographic overview of a

person’s genome• cells from a person are fixed in

metaphase, stained, & photographed to display all of a cell’s chromosomes

• Individual chromosomes are cut out, paired w/their homologue, & arranged from largest to smallest pairs for the 22 autosomes w/the sex chromosomes placed last

Sect 8.19 p. 144

• the karyotype is used to screen for abnormal numbers of chromosomes or defective chromosomes

Major Chromosomal Alterations & Their Effects

Chromosome Numbers

• nondisjunction - when chromosomes fail to separate during Meiosis I and II

• can cause aneuploidy - abnormal chromosome numbers:

* monosomy (1 less chromosome)

* trisomy (1 extra chromosome)

Sect 8.20-8.22 p.145-147

Human Disorders(nondisjunction/aneuploidy)

1. Down Syndrome - trisomy on chromosome #21

*occurs in 1 of every 700 births

*rounded facial features, varying degrees of mental retardation

2. Patau Syndrome - trisomy on chromosome #13 *occurs in 1 of every 5000 births *causes cleft palate, harelip, brain defects

#13

3. Edwards Syndrome - trisomy on chromosome #18 *occurs in 1 of every 10,000 births *affects almost every organ system

4. Klinefelter Syndrome - trisomy in male

*occurs in 1 of every 2000 births

*has male sex organs but are sterile

(XXY)

5. Metafemale - trisomy in female (XXX)

*occurs in 1 of every 1000 births

*limited fertility but otherwise appear

normal

6. Turner Syndrome - monosomy in female (XO) *occurs in 1 of every 5000 births *no mature sex organs, sterile

Chromosome Structure• Breakage of a chromosome can

cause a variety of rearrangements• fragments are usually lost when a

cell divides in 1 of 4 ways:

1-DELETION = a fragment of the chromosome breaks off and is lost (only dealing with one homologue)

For example, in this picture gene 3 has broken off and been lost.

becomes

(Where did gene 3 run off to?)

2-DUPLICATION = chromosome fragment attaches to a homologue now one homologue has 2 sets of (same) info. and the other is missing info.

(Homologue 1 is leftwithout genes 1 & 2. Homologue 2 ends up with both copies of genes #1 & 2.)

(New Homologue 2)

(Old Homologue 2) OLD{12}334567812345678NEW 34567812{12}345678

3-INVERSION = chromosome breaks off and reattaches in reverse order (only dealing with one homologue)

{234}Becomes

{432}

4-TRANSLOCATION = a fragment breaks off and attaches to a non- homologue (Example – chromosome 1 has a piece break off and attach to chromosome number 2 which is a non-homologue)

Chromo.#1

Chromo. #2

New #2

(What will the new chromosome #1 look like?) 345678

Example of deletion:

Williams Syndrome – deletion of about 15 genes on 1 of the homologous chromosomes in chromosome #7

*occurs in 1 of every 20,000 births

*mild retardation, problems in grasping spatial relationships; possess extraordinary musical talent

*thought to be elves/pixies in medieval folklore

Inherited Disorders Due to Gene Mutations

Human Pedigree - a pedigree shows the occurrence of a trait, seen in a family tree type of style

Recessively Inherited Disorders -

carrier - a heterozygote (Xx) that is phenotypically normal but transmits the recessive allele to the offspring

1. Deafness - severely or totally deaf

Dd = carrier (normal)

DD = normal

dd = deaf

2. Cystic Fibrosis - excessive mucus secretions clog airways of lungs & passages of the liver and pancreas

3. Albinism - lack of (skin) pigmentation

4. Tay-Sachs - an incurable disorder in which the brain deteriorates due to lipid build-up5. Sickle Cell Anemia - red blood cells are defective so they don’t transport O2 tissues properly (caused by point mutation)

Dominantly Inherited Disorders

1. Dwarfism (Achondroplasia)- homozygous dominant results in spontaneous abortion

2. Alzheimer’s Disease-causes mental deterioration (normally no obvious effect until late in life and effects are irreversible and lethal)

3. Huntington’s Disease - degenerative disorder of the brain cells

*no obvious effect until after age 30

*effects are irreversible and lethalWhy are Alzheimer’s and Huntington’s becoming so common?

Sex Linked Traits - fathers pass X linked traits on to all of their daughters and mothers can pass sex linked traits on to both sons and daughtersExamples:

Hemophilia - blood disorder passed from generation to generation

Color Blindness - inability to see certain colors due to malfunctioning light-sensitive cells in the eyes

Duchenne Muscular Dystrophy - progressive weakening and loss of muscle tissue

Risk Assessment and Therapy for Genetic Disorders

•Fetal TestingAmniocentesis - needle obtains

small sample of amniotic fluid *culture cells are taken from

sloughed off cell floating in amniotic fluid

*done around 14-16 weeks of pregnancy*karyotype performed*results in several weeks (risk to pregnancy - 1%)

Chorionic Villus Sampling (CVS) - small tube suctions off a small amount of tissue from the villi of the embryonic membrane (this tissue forms part of the placenta)

*cells are rapidly undergoing mitosis

*done around 8-10 weeks of pregnancy

*perform a karyotype

*results in 1 day (risk to pregnancy 2%)

Ultrasound Imaging - high frequency sound waves (sonar beyond the range of hearing)

*produces a color- enhanced image of fetus - age 18 weeks on*results are immediate (noninvasive and no known risk)*used during amniocentesis and CVS to determine position of fetus and needle or tube

Fetoscopy - needle thin tube w/viewing lens & light source

*produces direct view of fetus

*results are immediate (risk to pregnancy - 10%)

- risks to pregnancy can be complications that can result in maternal bleeding, miscarriage, or premature birth

• Carrier Recognition Counseling

Problem: parents are concerned they are carrier of a recessive genetic disorder; they do not wish to pass the disorder onto their prospective childrenSolution: physicians and genetic counselors now have a growing list of relatively simple biochemical tests that can check a couple’s genotype for genetic disorders

• Identification of Defective Genes and Gene Therapy

- work by Dr. Nancy Wexler on Huntington’s Disease as well as ongoing research making progress in locating defective genes

- her work in Venezuela produced a pedigree linking almost 10,000 people

- this allowed her to find a genetic marker (a DNA strand signaling the presence of a specific allele) and a test to identify for HD in 1983

- she located the HD allele in 1993 and identified the allele’s operation

- set up gene therapy

Problems w/gene therapy:

Technical - new gene must work at the right time and throughout life, and gene therapy works only with cells that currently multiply (nerve cells do not)Ethical - who will have access to it, treat only serious diseases, enhance athletic ability/physical appearance, and treatment of germ cells (makes gametes)

www.biology.ewu.edu/.../ GeneTherapy-Targeted.jpg

Human Genome Project

•Purpose: map all 3 billion nucleotides (international, multi-billion, multi-decade long successful effort)

•Potential: insight & understanding into embryonic development & evolution, aid in diagnosis, treatment, prevention of many diseases

Yeast & Fly Genomes• Reproduces by

budding and doubles every 90 minutes

• sequenced in 1996• 12 million base

pairs of DNA• 6000 genes, at

least 31% have human equivalents

• Lifespan 2-3 months, new generation every 10 days

• sequenced in March 2000

• 165 million base pairs of DNA

• 13,600 genes, 50% have human equivalents

Mouse & Human Genome• Lifespan 2 years, new

generation every 9 weeks

• sequenced in 2001• 3 billion base pairs of

DNA• 40,000 genes• equivalents to human

and some blocks proved impossible to tell apart from human

• Lifespan in U.S. 60-70 years, new generation every 20-25 years

• preliminary draft in June 2000

• Close to final draft in 2004

• 3 billion base pairs of DNA

• 50,000 genes

Ethical Issues

•Who has access to your genome?

•How far, if at all, do we go to re-engineer someone?

•How do we prevent genetic discrimination in the workplace, insurance companies, social settings, etc.?