Honors Biology Chapter 12: Human Genetics p. 246-263

Concept 12.1: The Nucleus Contains an Information- Rich Genome

(DNA contains 6 billion base pairs / body cell nucleus = lots of info in a tiny space!)

I. DNA Packing in a Single Cell

A. Size of DNA
1. 1 DNA molecule/chromosome – each 1000 X longer than the nucleus

a. 46 DNA’s laid end to end = 2 meters

B. Genome = complete set of genetic material in an organism, as defined by the order of bases in the

DNA

1. “Packed” into the nucleus by:

a. See Fig. 12.1, p. 248

b. Histone = small proteins DNA is wrapped around (thread around spool idea)

c. Wrap DNA into helical fiber, then coiled into supercoil, then looped and folded into

chromosome

II. The Human Genome Project

A. What was it – the complete sequencing of every nucleotide on every chromosome in human DNA

1. By 1999 – DNA tech. advances made it possible

2. Rough draft completed in 2000 – govt. funded, with private co, adding data, too

3. Huge task – read p. 249 for scope of project

B. Why Important?

1. Can compare to other species – insight into embryo devel. + evol. Rlshps.

2. ID’ing genes can help diagnose, treat and possibly prevent many common ailments

a. Ex. Allergies, diabetes, cancer

b. 100’s of gene related diseases already id’d

Concept 12.2: Accidents affecting Chromosomes can Cause Disorders

(Meiosis to create sperm or egg can sometimes occur with “accidents” in dividing the chromosomes evenly.)

I. Down Syndrome

A. Karyotype – can show abnormal numbers of chromosomes, or problems with chromosomes parts

B. Trisomy 21 = Down Syndrome

1. Karyotype shows 3 chromosomes of chrom. 21, instead of the 2 there should be.

a. See Fig. 12.3, p. 250

2. Error in meiosis, ususlly in Meiosis I causes it

3. Abnormal #’s of chrom. – usually causes miscarriage, but not with Trisomy 21

4. 1 / 700 births in U.S. is affected by Down Syndrome (named for John Langdon Down, who

described the syndrome in 1866)

a. Symptoms: facial shape, almond eye, below avg. height, sometimes heart defects,

impaired immune system, and varying degrees of mental disability, shorter life

span (middle age)

C. Nonseparation of Chromosomes

1. Nondisjunction = homologous chromosomes or sister chromatids failing to separate during

meiosis

a. Can occur in Anaphase I or II, so gametes have abnormal chrom. # (See Fig. 12.4,

p.251)

b. When fertilization occurs, the abnormal gamete + gamete with normal # will end up

with either 47 or only 45 chrom.

1.) Ex. Down Syndrome (Trisomy 21)

a.) sperm with 23 chorm. may fertt. egg with an extra 21st chrem. (24 total)

23 + 24 = 47 chrom.

b.) when mitosis occurs in the zygote, all the cells end up with the 47

chrom.

2. Causes:

a. Not fully understood

1.) aging women have higher incidence of Trisomy 21 children (See Fig. 12.5)

a.) suggest a rlshp. w/egg cell devel. (Read p. 251)

2.) may not always be mother’s egg, just for Trisomy 21

3. Other nondisjunction disorders (not in text)

a. Klinefelter’s Syndrome – XXY

b. Turner’s Syndrome – XO

II. Damaged Chromosomes

A. Chromosomes may break, leading to gene rearrangements

1. See Fig. 12,6, p. 252

2. Types:

a. Duplication = part of chorm. repeated

1.) not usually fatal, but cause developmental abnormalities

b. Deletion = fragment of a chrom. lost

1.) if lg, major genes may be incomplete or missing

2.) can result in serious effects on body

c. Inversion = reversing a fragment of the original chrom.

1.) less likely than duplication or deletion to produce harmful effects Why?

d. Translocation = fragment of one chromosome attaches to a nonhomologous

chromosome

1.) may involve 2 chrom. with parts translocated, not just one

III. Jumping Genes = Transposons

A. Single Genes may move around

1. Margaret McClintock – 1940’s – corn variety study – See Fig. 12.7, p. 253

2. Genes may move to diff. location in same chromosome, or even to another chrom

a. NOT a translocation

3. Even can land in middle of another gene and disrupt it

a. Ex. Jump to middle of pigment gene, causing spotted kernels

4. Now believe all orgs, even humans, have jumping genes

5. Nobel Prize in 1983 for her work in Genetics

B. See Fig. 12.8, p. 253, and read how can effect Ex. on p. 253

Concept 12.3: Mendel’s Principles apply to Humans

(We can explain human appearance to alleles responsible for disease with Mendel’s principles.)

I. Working with Human Pedigrees

A. Human studies

1. Cannot conduct breeding expts. – immoral

2. Analyze patterns of inheritance in existing families

3. Pedigree = family tree that records and traces the occurrence of a trait in a family

a. See Fig. 12.9, p. 255 – Ex. Traces Earlobe attachment in 3 generations

b. Symbols used:

1.)□ = Male, ○ = Female

2.) colored shapes represent those that show the trait

3.) Parents connectied w/ horizontal line, children beneath them

c. examine pattern in pedigree to determine how inherited

1.) How can you find a recessive trait? Think about it!

2.) Read p. 256, and see Fig. 12.10

II. Disorders Inherited as Recessive Traits (must have 2 alleles to have the disorder)

A. Over 1000 human genetic disoders have Mendelian inheritance patterns =caused by dom or rec.

alleles

1. some are disabling, others lethal

2. Most Human genetic disorders are recessive

B. Some Examples of a range of severity

1. Albinisim

2. Tay-Sachs Disease

3. Cystic Fibrosis (See Fig. 12.12, p. 257)

4. One type of deafness

(5. See powerpoint chart for more)

C. Most people with recessive disorders born to parents who are heterozygous and do not show the

disorder

1. Carriers = individual who has one copy of the allele for a recessive disorder and does not

exhibit symptoms

D. Predict probability of 2 carriers having a child with a recessive disorder (See Fig. 12.11, p. 256)

1. This is what genetic counselors do!

2. Remember, this is a probability, and each child is a separate event with the same chance.

E. Read Cystic Fibrosis symptoms on p. 256

1. 1 in 25 people of European ancestry in U. S. is a carrier!

III. Disorders Inherited as Dominant Alleles (only need 1 allele to have the disorder)

A. Examples

1. Polydactylism = extra fingers and toes

2. Achondroplasia – 1 serouis form of dwarfism (1/25,000 people)

a. all are heterozygous

b. Homozygous dominant = lethal

c. 99.99% of human pop. are homo. rec. for the normal allele – show dom. traits not

always most common (a common mistake people make!)

B. Dom. lethal alleles much less common than rec. lethal alleles - why?

1. for most dominant disorders, the affected person dies before producing any offspring that

could inherit the allele

2. Natural mutations can return the dom. allele to the pop. thru sperm or egg cell

3. Recessive alleles often go undetected, since most have little effect on the carriers of them

C. Dom. lethals with late onset can affect generations of a family before causing death.

1. Ex. Huntington’s disease – Read. P. 257 –does not show symptoms until middle age, after

person has children , See Fig. 12.13

a. Now have test for it, so if runs in family, can be tested prior to middle age, and prior to

having children (Would you want to know if you had the allele? Think about it!)

IV. Sex-Linked Disorders

A. Recessive alleles carried on the sex chromosomes (usually the X)

1. Review how inherited, and why more often shows in males (Read p. 258)

B. Ex. Red-Green Colorblindness (See Fig. 12.14, p. 258)

1. Half-shaded symbols = Carriers

2. ? = cannot tell genotype from pedigree data

C. Ex. Hemophilia

V. Predicting and Treating Genetic Disorders

A. Genetic Counselor = is trained to collect and analyze data about inheritance patterns and to explain

the results and their significance

1. should consult if know a genetic disorder runs in your family, prior to having children

B. Genetic Tests – some can be done prior t birth, to see if embryo has a genetic disorder

1. Ex. Karyotyping – to detect chromosomal disorders (See Fig. 12.3)

2. Fluid analysis – Amniocentesis (ex.)

3. Newborn tests – Ex. PKU test – controlled diet if positive for disease to prevent its

development

Concept 12.4: Genetic Changes Contribute to Cancer

(Now know usual cause of cancer is an accumulation of mutations in DNA, from mutagen exposure or inheritance.)

I. Cancer Genes

A. 2 Classes of Genes direct Cell division

1. Growth Factors = produced by genes that initiate cell division

2. Tumor – Suppressor Genes = produce proteins that stop cell division in particular situations

a. Ex. if cell has damaged DNA , or if cells exceed specific amt. of space

B. Oncogene = cancer causing gene

1. Ex. path to cancer often starts with a mutation to a gene that produces growth factors.

a. Cells with oncogene become overstimulated to divide more often

2. Ex. If tumor suppressor gene also mutated, increase risk of tumor devel.

3. Read p. 260, See Fig. 12.16, and p. 261, Fig. 12.17

II. “Inherited Cancer”

A. Most mutations occur in organ where cancer starts

B. Cancer only “inherited” if occurs in cells that produce gametes; perm or egg cell

1. Ex. mutated version of a tumor-suppressor gene called BRCA1

a.) Increases risk for breast cancer