GENETICS
Lab 8
Objetives
Be able to define the following terms
a- genes
j- locus
b- alleles
k-chromosome
c- homozygous
l- sex chrom.
d- heterozygous
m- carrier
e-recessive
n- genotype
f- dominant
o- phenotype
g- lack of dominance (incompl) p- probability
h- sex-linked genes
q- monohybrid
i- reciprocal cross
r- dihybrid
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•
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Be able to interpret genetic data used in
a monohybrid cross, and use this data to
predict the phenotypes of the next
generation.
Be able to interpret genetic data used in
a dihybrid cross, and use this data to
predict the phenotypes of the next
generation.
Be able to solve genetic problems that
involve recessive, dominant, lack of
dominance, and linked traits.
• The inherited characteristics of a diploid
organism are determined at the moment of
sperm and egg fusion. The zygote (2n)
receives one member of each chromosome
pair from each parent. The genetic
information that determines the hereditary
traits is found in the structure of the DNA
molecules in the chromosomes. A short
segment of DNA that codes for a particular
protein constitutes a gene, a hereditary unit.
Both genes and chromosomes occur in
homologous pairs in diploid organisms (see
diagram below).
• In the simplest situation, an inherited trait,
such as flower color, is determined by a
single pair of genes.
• The members of a gene pair may be
identical (e.g., each codes for red flowers)
or may code for a different variation of the
trait (e.g., one codes for red flowers, and
the other codes for white flowers). Again,
in the simplest case, only two forms of a
gene exist. Alternate forms of a gene are
called alleles
• Geneticist use symbols (usually letters like
R or r) to represent alleles when solving
genetic problems. When both members of
a gene pair consist of the same allele,
such as RR or rr, the individual is
homozygous for the expressed trait. When
the members of the gene pair consist of
unlike alleles, such as Rr, the individual is
heterozygous (hybrid) for the expressed
trait.
• The genetic composition of the gene pair
(e.g., RR, Rr, or rr) is known as the
genotype of the individual. The
observable (expressed) form of a trait
(e.g., red flowers or white flowers) is called
the phenotype.
• An understanding of inheritance patterns
enables the prediction of an expected ratio
for the occurrence of a trait in the progeny
(offspring) of parents of known genotypes.
MENDELS’S PRINCIPLES
Gregor Mendel, an Austrian monk, worked
out the basic patterns of simple
inheritance in 1860, long before
chromosomes or genes were associated
with inheritance.
Mendel’s work correctly identified the
existence of the units of inheritance now
known as genes.
• Mendel proposed two principles concerning the
activity of genes, and these principles form the
basis for the study of inheritance.
• In modern terms, these principles may be stated
as follows:
• The Principle of Segregation states that (1)
genes occur in pairs and exist un changed in the
heterozygous state and (2) members of a gene
pair are segregated (separated) from each other
during gametogenesis, ending up in separate
gametes.
• The Principle of Independent Assortment states
that genes for one trait are assorted (segregated
into ghe gametes) independently from genes for
other traits
DOMINANT-RECESSIVE TRAITS
• When a gene pair consists of two alleles,
and one is expressed and the other is not,
the expressed allele is dominant.
• The unexpressed allele is recessive. Many
traits are inherited in this manner. In the
following examples, dominant traits are
underlined.
CONDOMINANCE
• The alleles of some genes are always
expressed in the phenotype and are never
recessive.
• Such alleles exhibit condominance. The
inheritance of color in snapdragons is a
suitable example. When a homozygous redflowering snapdragon is crossed with a
homozygous white-flowering snapdragon,
the F1 progeny always have pink flowers
since both alleles are expressed.
• Sickle-cell anemia in humans is inherited in this
manner. The structure of the hemoglobin
molecule is controlled by a single gene pair
consisting of two alleles, HbA for normal
hemoglobin and Hbs for sickle-cell hemoglobin.
A person who is homozygous for sickle cell is
afflicted with the disease and usually dies early
in life. The heterozygote has relatively few
abnormal hemoglobin molecules, shows no ill
effects, and has an increased resistance to
malaria.
DIGYBRID CROSS
• In garden peas, yellow seed color is
dominant over green seed color, and
round seed shape is dominant over
wrinkled seed shape. The genes for seed
shape and seed color are located on
separate chromosomes. Consider a cross
of a plant producing yellow round seeds
with a plant producing green-wrinkled
seeds. Both plants are homozygous for
both traits (seed color and seed shape).
What kinds of seeds will be produced by
the progeny?
• Based on what is known, you can establish the
genotypes of the parents and write out the cross:
Yellow-Round
x
Green-Wrinkled
YYRR
x
yyrr
• Recalling that the members of a gene pair are
separated into different gametes and
recognizing that one member of each gene pair
must be in each gamete, determine the
gametes. Since both parents are homozygous
for both traits, each can produce only one type
of gamete. Now, set up and complete a Punnett
square”
• Next, mate two of the offspring from the cross
(above) . Determine the gametes, set up and
complete a Punnett square.
Sex-linked Traits
• In humans, sex is determined by a single pair of
sex chromosomes. Females possess two X
chromosomes (XX), and males possess an and
a Y (XY).
• The Y chromosome is shorter than the X and
lacks some of the genes present on the X
chromosome. Those genes that are present on
the X chromosome but absent on the Y
chromosome are the sex-linked genes that
control inheritance of sex-linked traits
• For the sex-linked genes, a female is
diploid and a male is haploid .Therefore, a
recessive allele on the X chromosome of a
male will be expressed, whereas the
recessive allele must be present on both X
chromosomes of a female to be
expressed. Two common sex-linked traits
in humans are red-green color blindness
and hemophilia.
• Reciprocal crosses are used to determine
the mode (sex-linked or autosomal) of
inheritance. The trait in question in the first
cross is associated with the female and in
the second cross with the male. If the
results of both crosses are the same,
autosomal inheritance is indicate. If the
results are different, sex-linked inheritance
is indicated.