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Genetics and Heredity
Lab Exercise 40
BI 233
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
Inherited characteristics
 The genetic constitution of an
individual, along with environmental
influences is called the genotype.
 The characteristic actually expressed
in an individual is called a
phenotype.
 If both genes for a trait are the same
the individual is homozygous.
 If genes are not the same they are
heterozygous.
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.
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
Gene Expression
 A person has two genes for every
inherited characteristic.
 If one allele is always expressed,
geneticists call that gene
dominant.
 A allele that is not expressed when
its mate is different is termed a
recessive gene
Example: Genotype ‘Aa’ and ‘A’ is the
phenotype, then ‘A’ is dominant,‘a’ is
recessive.
Gene Expression
 Incomplete dominance: the
phenotype of two heterozygous
alleles is intermediate between
the phenotype. (sickle cell trait):
Aa – bit of each allele
 Codominant inheritance:
Both alleles are expressed in the
phenotype:
AB – both alleles - making up
AB blood type
Gene Expression
 Most human traits are the result of
polygenic inheritance, which
occurs when multiple genes interact
to produce a phenotype.
 Eye color was believed to be a strict
dominant-recessive trait but we now
know that it is the result of
interactions of many genespolygenic. (other examples
include height and skin color)
Sex Linked Traits
Traits expressed by genes on the X or Y
Chromosome (Sex Chromosomes).
Not as many genes on the Y chromosome
so X-linked traits much more common.
 X-linked recessive trait is always
expressed in a male, because he has only
one X chromosome. Females are called
carriers if she has one allele and must
have 2 recessive alleles to express trait.
Examples: color blindness and hemophilia
 X-linked dominant traits are rare
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 genes for
ABO blood types = Codominance
 Type A (IA):signifies the A antigen.
A person with Type A blood can
have IAIA or Iai
 Type B (IB): signifies the B antigen.
Type B blood can have IBIB or Ibi
 Type O (i): is recessive to both Type
A and Type B. Type O blood must
be ii
Rh Blood Type
 The presence of the Rh
antigen is dominant – 85%
 Rh negative does not have
Rh antibodies without
exposure.
https://www.youtube.com/watch?v=ksC7JxLOrv4
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.
 Use Punnett squares to answer
questions in your packet.
Karyotype
Graphical representation of
pairs of homologous
chromosomes as they appear in
metaphase of the cell cycle.
 They are arranged from largest to
smallest.
 The number and structures are
then evaluated.
 Many chromosomal abnormalities
can be discovered this way
Male
Female
Abnormal Chromosome Numbers =
Aneuploidy
 Monosomy: missing a
chromosome (Turner’s syndrome –
45, missing one X)
 Trisomy: extra chromosomes (46).
 Most autosomal aneuploids are
spontaneously aborted. Mental
retardation is common if individual
survives. Some that do survive are
Trisomy 13 (<5%), Trisomy 18 (<5%)
and Trisomy 21(85%)
Sex Chromosome Aneuploids
Sex chromosome aneuploidy
is less severe than autosomal
aneuploidy.
Examples include:
 Turner syndrome (X,O)
 Triple X females (XXX)
 Klinefelter syndrome
(XXY)
 XYY syndrome in males
Pedigrees
 Geneticists use charts called
pedigrees to display family
relationships and to follow
which relatives have specific
phenotypes and, sometimes
genotypes.
 They help keep tract of
relationships and traits.
 Important for helping families
identify the risk of transmitting
an inherited illness
Queen Victoria’s Pedigree
Pedigree Key
 Dominant or Recessive
 Autosomal or X-linked
Pedigree Examples
Albinism: autosomal recessive
Try to figure out the genotypes
Autosomal dominance
Try to figure out the genotypes
Red-Green Color Blindness
X linked recessive
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