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Chromosomes and
Human Inheritance
CHAPTER 12
Disorders caused by individual genes - dominant
 Individuals with the disorder only need 1 allele to
have the disorder

Huntington’s Disease
Caused by a repeat of the sequence CAG
 Mental retardation and uncontrollable movements that usually
don’t appear until middle age


Achondroplasia (mom and son)


A form of dwarfism
Polydactyly

6 fingers or toes
Disorders caused by individual genes – some say
recessive – really codominant
 To have the disorder you need 2
copies of the gene

Sickle cell disease
Defect in hemoglobin (oxygen carrying
molecule in red blood cells) causes red blood
cells to “sickle”
 Carried by 1/12 African Americans
 Carriers are healthy but are highly resistant
to malaria.

12.1 Human Chromosomes
 In humans, two sex chromosomes are the basis of
sex – human males have XY sex chromosomes,
females have XX
 All other human chromosomes are autosomes –
chromosomes that are the same in males and
females
Sex Determination in Humans
 Sex of a child is determined by the father
 Eggs have an X chromosome; sperm have X or Y
Sex Determination in Humans
 The SRY gene on the Y chromosome is the master
gene for male sex determination




Triggers formation of testes, which produce the male sex
hormone (testosterone)
Without testosterone, ovaries develop and produce female sex
hormones (estrogens)
What makes a boy a boy and a girl a girl? It’s not that
simple……….
SRY gene
Karyotyping
 Karyotype
 A micrograph of all metaphase chromosomes in a cell,
arranged in pairs by size, shape, and length
 Detects abnormal chromosome numbers and some structural
abnormalities
 Construction of a karyotype
 Colchicine stops dividing cells at metaphase
 Chromosomes are separated, stained, photographed, and
digitally rearranged
Fig. 12-3a, p. 187
Fig. 12-3b, p. 187
Down Syndrome
 Trisomy 21
Turner Syndrome
 1/ 2,000 live female births
 Short stature, loss of ovarian function, lack or
incomplete development at puberty
 Heart defect, infertility, webbed neck
Klinefelter’s Syndrome
 1/500-1/1000 males
 Small testes and less testosterone than normal (less
facial hair, infertility, breast development)
 Tall
Nondisjunction
 Chromosomes don’t separate in
meiosis
Chromosomal Mutations
 Deletion – involves the loss of part of a chromosome
 Duplication – involves an extra copy of part or all of
the chromosome
 Inversion – reverses the direction of part of the
chromosome
 Translocation – when part of one chromosome breaks
off and attaches to another
Changes in Chromosome Structure
 Cri-du-chat – “Cat’s cry”
 Deletion in Chromosome 5
 Mental impairment, abnormally shaped larynx that causes
infants to make distinct cat-like meow , low set ears
Disorders caused by individual genes – sex-linked
 Genes in sex-linked disorders are
found on the X or Y chromosome
(most are on X)

Color blindness (X chromosome)
Humans have 3 genes for color vision – all
on the X
 Red-green colorblindness = 1/12 males &
1/200 females
 Males have 1 X chromosomes – females
have 2 (the allele for colorblindness can be
masked in females)

Colorblindness
Hemophilia – Remember the Romanovs?
 X-linked recessive
 interferes with blood clotting – bruise easily, internal
bleeding causes joint problems
Duchenne Muscular Dystrophy (DMD)
 1/3500 people – x-linked recessive
 Causes abnormal dystrophin (a protein that supports
muscle fibers) that causes muscle cells to die
 Progressive disorder – usually diagnosed between 3
and 7, wheelchair by 12 and early death from heart or
respiratory failure.
Sex linked problems
 1) A female is a carrier for hemophilia and the dad is
normal


What percent of sons will have hemophilia?
What percent of daughters will have hemophilia?
2) Show how a daughter can be a carrier for colorblindness from
either her mom or dad.
Pedigree Analysis
How is this trait inherited?
How is this trait inherited?
What is the genotype of individual III 1? I 2?
How is this trait inherited?
Well…..it’s not that simple……
 Epigenetics – “above the genome”
 Factors that control gene expression that do not change the
DNA or RNA
 Nova – Ghost in Your Genes!!!!