Genetics and Heredity
Lab Exercise 40
BI 233
Genome
 All of the genes in all of the
chromosomes together are
called the genome
 The study of all the genes
in the human species is
called genomics
Definitions
 Heredity = the inheritance of traits
 Genetics = the study of mechanisms of heredity.
 Genes: A portion of a DNA strand that functions as a
hereditary unit, is located at a particular site on a specific
chromosome, and codes for a specific protein or polypeptide
Gene Expression
 A person has two genes for
every inherited
characteristic.
 If one gene is always
expressed, geneticists call
that gene dominant.
 A gene that is not
expressed when its mate is
different is termed a
recessive gene
Gene expression
 Codominance is the
condition in which an
individual heterozygous for a
gene exhibits the phenotypes
of both homozygote's
(example AB blood)
 Incomplete dominance is
when neither of two alleles is
completely
dominant.(example: sickle
cell anemia at organism level)
Gene Expression
 Polygenic inheritance
occurs when two or more
genes whose additive
effects determine a
particular trait.(skin color)
 Multi allelic
inheritance: refers to
different versions of one
trait on the same gene
(Blood types)
Inherited characteristics
 The characteristic actually
expressed in an individual is
called a phenotype.
 The genetic constitution of an
individual, along with
environmental influences is
called the genotype.
 If both genes for a trait are
the same the individual is
homozygous.
 If genes are not the same they
are heterozygous.
Genes and Chromosomes
 The position of a gene on a
chromosome is called a
locus.
 Genetic maps show the
relative locations of genes
along the chromosome.
 There can be alternate
versions of genes called
alleles which produce
different phenotypes
Centromere
 Each chromosome often has a
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constriction along its length called a
centromere.
The shorter arm is placed on top in
a karyotype and is called the p
arm, the longer arm is on the
bottom and is called the q arm.
Numbered regions and subregions
are then assigned from the
centromere outward
The breast cancer gene BRCA1 is
located at 17q21 meaning that it is
on the long arm of chromosome 17
in the region of 21
Used for gene mapping
Chromosomes
 We inherit 23 DNA
molecules from our
mothers and 23 DNA
molecules from our fathers
for a total of 46.
 22 pairs of these
chromosomes are called
autosomes
 The remaining pair are
called sex
chromosomes.
Taste differences
 Sodium benzoate test –
The ability to taste
something sweet, salty, or
bitter in the paper is
dominant.
 PTC
(phenylthiocarbamide) test
The ability to sense a bitter
taste is dominant.
 Thiourea test – the
ability to taste something
bitter is dominant
Anatomical characteristics of hand
 Bent little finger – If distal
phalanx of the little finger
bends toward the fourth
finger, you have dominant
trait.
 Middigital hair – hair on
middle phalanges is
dominant.
 Hitchhiker’s thumb- If
you can hyperextend the
distal joint of the thumb
noticeable, you have the
recessive trait.
Facial Features
 Pigmented anterior of the
iris – If you have pigment
on the anterior and
posterior of the iris, your
eyes are green, brown,
black and hazel.
 If you lack pigment on the
anterior aspect of the iris,
yours eyes are blue or grey.
 Pigmentation is dominant
Phenotypes of facial features
 Attached earlobes – Lobe
of the ear is attached rather
than free, you have the
recessive trait.
 Widow’s peak – hairline
straight across forehead is
recessive trait
 Tongue roll – curling
tongue is dominant
 Freckles – if your face is
free of freckles you have the
recessive form of this
characteristic
ABO blood types
 There are two dominant
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genes for ABO blood types
Type A (IA):signifies the A
antigen
Type B (IB): signifies the B
antigen.
Type O (i): is recessive to
both Type A and Type B
A person with Type A blood
can have IAIA or IAi
Type B blood can have IBIB or
IB i
Rh Blood Type
 The presence of the Rh
antigen is dominant
Karyotypes
 Standard chromosome chart
 Major clinical genetic tool
 Chromosomes are presented
by size and physical
landmarks that appear during
mitotic metaphase when
DNA is coiled tightly
 Karyotypes can be used to
diagnose many genetic
conditions as well as reveal
effects of environmental
toxins.
Non-disjunction problems
 When the homologous
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chromosomes or sister chromatids
fail to separate at anaphase
An extra chromosome is called
trisomy and a missing chromosome
is called monosomy
Trisomy 21 (Down’s
syndrome)
Signs include impaired physical
development; short stature;
relatively flat face with a flat nasal
bridge and mental retardation
Trisomy 13 & 18 are other
chromosomal trisomies but fetuses
do not usually survive until birth.
Turner’s Syndrome
 XO
 Have only one sex
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chromosome
Designated as 45,X indicating
they have 45 chromosomes
and one X chromosome.
Not noticeable until puberty
when they fail to develop
secondary sex characteristics
and are usually infertile.
Webbed neck
About 97% of fetuses with
Turner syndrome die before
birth.
Klinefelter’s Syndrome
 If an XX egg is fertilized by
a Y-bearing sperm the
result is an XXY
combination.
 Sterile males with usually
average intelligence with
undeveloped testes, sparse
body hair, usually long
arms and legs and enlarged
breasts.
 Often goes undetected
until puberty.
Chromosome Duplications
 Fragile X syndrome
 Caused by repeats on the X
chromosomes
 Most people have about 29
repeats but persons with
Fragile X have over 700
repeats due to duplication.
 A major cause of mental
retardation and is found in
more males because of the
single X chromosomes
Translocation defect
 Different chromosomes
exchange or combine parts
 Exposure to certain viruses,
drugs and radiation can cause
translocations, but they often
arise for no apparent reason.
 No gain or loss of genetic
material is involved
 Can be a cause of Down’s
syndrome when a Robertson
translocation produces 3
copies of the long arm of
chromosome 21
Pedigree
 The study of a family tree and
the careful compilation of
phenotypic records of both
families over a number of
generations.
 By studying human families
for affected and unaffected
members over several
generations we can usually
tell whether a trait is
autosomal recessive or is
carried on a sex
chromosome.
Autosomal Recessive Traits
 Characteristics:
 Most affected individuals have
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unaffected parents
All children of two affected
individuals are affected
The risk of an affected child
from a mating of two
heterozygotes is 25%
Expressed in both males and
females
Examples: albinism, cystic
fibrosis, PKU, etc.
Autosomal Dominant
 Characteristics:
 Every affected individual
should have at least one
affected parent
 Affects males and females
equally
 Two affected individuals may
have an unaffected child
because most affected
individuals are heterozygous.
 Examples: Huntington
disease, Marfan syndrome
X-linked recessive
 Characteristics:
 Phenotypic expression is
much more common in males
than females
 Affect males get the mutant
allele from their mothers and
transmit it to all their
daughters but not to any
sons.
 Examples: colorblindness,
hemophilia, muscular
dystrophy (some forms)
Hemophilia in Royal Family
Is this trait autosomal recessive, autosomal dominant or sex linked?
How can you tell?
Probability
 Is the likelihood of a the
offspring of a particular set
of parents will have a
certain inherited condition.
 Genetic counselors
work with prospective
parents to determine their
possible genotypes for a
variety of traits and then
predict the probability of
their children having those
traits.
Activities
 Follow the instructions in your lab manual to determine your
phenotype for the listed traits.
 After you have determined your phenotype then determine
your possible genotypes.
 Answer the questions in your lab manual using the Punnett
squares and pedigrees
 Be able to answer questions concerning both karyotypes and
pedigrees