Biology Chapter 14 The Human Genome
14.1 Human Heredity
I. Human Chromosomes
A. Looking at chromosomes
1. During mitosis, scientists look at chromosomes; photo; cut; paste in pairs
a. called karyotype (Fig 14-1; pg 341)
b. 23 pairs; 46 total
2. 2 Chromosomes are known as sex chromosomes
a. they determine an individual’s sex
b. females = XX (larger)
c. males = XY (large, small)
3. Other 44 chromosomes are called autosomes
4. To quickly summarize total # of chromosomes in a human cell, scientists
write 46XX (female)or 46XY (male)
5. Punnett Square explains why there is a 50:50 ratio of males:females
(Fig 14-2; pg 342)
II. Human Traits
A. Human traits are not always controlled by a single gene; sometimes the
environment may “create” a trait
1. Pedigree (Fig 14-3; pg 242)
a. chart that shows the relationships within a family
2. Example shows how a trait is transmitted through three generations
a. females who have one allele for the trait are only carriers; males
who have only one express the trait
b. female carrier marries male who doesn’t carry the trait
c. they have four children:
i. female carrier, female non-carrier, male w/trait, male w/o
d. female carrier marries male w/o trait; have 3 children
i. male w/trait; male w/o; female non-carrier
3. Polygenic
a. traits that are controlled by many genes
4. Nature v. Nurture
a. genotype is only partly responsible for phenotype
i. environment/nutrition/exercise
b. nutrition allowed average heights to increase 10 cm since 1800s
III. Human Genes
A. Blood Group Genes (Fig 14-4; pg 344)
1. Human blood comes in a variety of genetically determined blood types
a. a number of genes are responsible for blood type
i. best known are ABO blood groups and Rh blood groups
b. Rh is determined by a single gene with 2 alleles
i. positive and negative
ii. Rh = rhesus monkey; animal where it was discovered
iii. positive is dominant
c. ABO is more complicated; three alleles for this gene
i. IA, IB, and i; IA and IB are codominant
ii. IA and IB produce antigens that can be recognized by the
immune system on the surface of RBCs
iii. people with IA and IB, produce both antigens; type AB
iv. i is recessive; IA IA, or IAi produce only A antigen; type A
v. IB IB, or IBi produce only B antigen; type B
vi. ii produce no antigen; type O
d. blood type is usually referred to as Rh and ABO types together
B. Recessive Alleles (Fig 14-6; pg 345)
1. The study of human genes has been helped by looking at genetic
disorders
a. PKU (phenylketonuria)
i. lack enzyme to break down phenylalanine
ii. AA found in milk and others
iii. may build up in tissues and cause mental retardation
iv. can be tested for; placed on low PKU diet
v. recessive allele on chromosome 12
b. Tay-Sachs disease
i. mainly affects Jewish families of central/eastern European
ancestry
ii. nervous system breakdown/death in first few years
iii. no treatment; there’s a test for the allele
iv. parents can know the risk before having children
C. Dominant Alleles (Fig 14-6; pg 345)
1. Examples
a. achondroplasia
i. form of dwarfism
b. Huntington’s disease
i. progressive loss of muscle control/mental function/death
ii. no symptom until 30s or 40s
D. Codominant Alleles
1. Sickle cell disease
IV. From Gene to Molecule
A. Cystic Fibrosis and Sickle Cell Disease are caused by a small change in the
DNA of a single gene
1. Cystic Fibrosis (CF)
a. fatal genetic disease; ½ survive in 20s
b. affects ancestry from N. Europe
c. recessive allele on chromosome 7
d. serious digestive problems
e. thick, heavy mucus that clogs lungs/breathing passageways
f. most caused by deletion of 3 bases in the middle of a sequence
for a protein
i. removes 1 AA; causes protein to fold improperly
ii. cell’s don’t transport it to the cell membrane; its destroyed
iii. cannot transport chloride ions; tissues malfunction
2. Sickle Cell Disease
a. found in 1 of 500 African Americans
b. bend/twisted shape of RBCs
c. more rigid than normal cells; get stuck in capillaries
i. damages cells and tissues beyond blockage
d. physical weakness/damage to brain, heart and spleen
i. fatal in some cases
e. 1 DNA base is changed from normal for hemoglobin (protein
that carries oxygen)
i. less soluble than normal; cause hemoglobin to stick
together
f. maybe caused by malaria mosquitos in west central Africa
14.2 Human Chromosomes
I. Human Genes and Chromosomes
A. Chromosomes 21 and 22 are the smallest autosomes
1. 22 has 43 M bases; 21 has 32 M bases
a. these were the first 2 whose sequences was determined
2. 22 has as many as 545 genes; some important for health
a. genetic disorders include an allele for an allele that causes a
form of leukemia
b. another for neurofibromatosis
i. tumor causing disease of nervous system
c. also has long stretches that don’t code for proteins
i. repetitive; unstable; rearrangements occur
3. 21 has 225 genes
a. amyotrophic lateral sclerosis (ALS); Lou Gehrig’s Disease
4.Chromosome exploration continues today; helps understand
arrangements of genes on chromosomes and how they affect
gene expression and development
II. Sex-Linked Genes
A. There is a special pattern of inheritance on X and Y chromosomes
1. Genes located on these are called sex-linked genes
a. X gene – 100+ disorders mapped here
b. Y gene – smaller; only a few genes
B. Colorblindness
1. 3 genes associated with color vision are located on the X chromosome
a. in males, defect in any produces color blindness
i. inability to distinguish certain colors
b. red-green most common
i. 1 in 10 males; 1 in 100 females
2. Males have only 1 X; all X-linked alleles are expressed in males
a. females have to be homozygous recessive to have traits
3. Sex-linked disorders more common in males
a. very common to pass it on
C. Hemophilia
1. 2 important genes carried on the X chromosome help blood clotting
a. recessive allele in either may produce hemophilia
b. protein for normal blood clotting is missing
c. 1 in 10,000 males is born with it
i. can bleed to death from minor cuts
ii. internal bleeding from bumps and bruises
d. can be treated with injections of normal clotting proteins
D. Duchenne Muscular Dystrophy
1. Progressive weakening and loss of skeletal muscle
a. rarely live past early adulthood
b. 1 of 3000 males born with it
c. caused by defective version of the gene that codes for a muscle
protein
d. looking for a cure; helping by inserting a normal allele into
muscle cells of patients
III. X-Chromosome Inactivation
A. In females, one X-chromosome is randomly turned off
1. Turned off chromosome forms a dense region in the nucleus known as a
Barr body
a. generally not found in males
2. Cat’s fur (Fig 14-14; pg 352)
a. different X being turned off in different parts of the body can
cause spotted fur
b. if a cat’s fur has 3 colors, almost certainly female
IV. Chromosomal Disorders
A. Problems with meiosis
1. Most common problem is when homologous chromosomes don’t
separate (Fig 14-15; pg 352)
a. nondisjunction
b. abnormal numbers of chromosomes get into gametes
B. Down’s Syndrome (Fig 14-16; pg 353)
1. Also known as trisomy (“three bodies”) 21
a. 1 out of 800 born with it
2. Mild to severe mental retardation
a. increased susceptibility to many diseases
b. high frequency of some birth defects
3. Scientists are still trying to figure out sequence for chromosome 21
a. trying to find genes that cause problems with 3 copies
C. Sex Chromosome Disorders
1. Turner’s Syndrome
a. in females, caused by non-disjunction
b. inherits only 1 X chromosome (genotype XO)
c. women are sterile; sex organs don’t develop at puberty
2. Klinefelter’s Syndrome
a. in males; caused by non-disjunction
b. extra X (genotype XXY)
c. interferes with meiosis; prevents reproduction
d. some have been XXXY or XXXXY
3. Babies never born without X; too vital for normal development