Download Essential knowledge 3.A.3:

The chromosomal basis of inheritance provides an
understanding of the pattern of passage
(transmission) of genes from parent to offspring.
Probability and Genetics
 Rules of probability can be applied to analyze passage
of single gene traits from parent to offspring.
 Two basic rules of probability are helpful in solving
genetics problems: the rule of multiplication (or the
rule of and) and the rule of addition (or the rule of or).

Rule of multiplication is that the probability that
independent events will occur simultaneously is the
product of their individual probabilities. For example:
 Question:
 In a Mendelian cross between pea plants that are
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heterozygous for flower color (Pp), what is the probability
that the offspring will be homozygous recessive?
Answer:
Probability that an egg from the F1 (Pp) will receive a p allele
= 1/2.
Probability that a sperm from the F1 will receive a p allele =
1/2.
The overall probability that two recessive alleles will unite,
one from the egg and one from the sperm, simultaneously,
at fertilization is: 1/2 X 1/2 = 1/4.
Rule of addition is that the probability of an event that can occur
in two or more independent ways is the sum of the separate
probabilities of the different ways. For example:
 Question:
 In a Mendelian cross between pea plants that are heterozygous for
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flower color (Pp), what is the probability of the offspring being a
heterozygote?
Answer:
There are two ways in which a heterozygote may be produced: the
dominant allele (P) may be in the egg and the recessive allele (p)
in the sperm, or the dominant allele may be in the sperm and the
recessive in the egg. Consequently, the probability that the
offspring will be heterozygous is the sum of the probabilities of
those two possible ways:
Probability that the dominant allele will be in the egg with the
recessive in the sperm is 1/2 X 1/2 = 1/4.
Probability that the dominant allele will be in the sperm and the
recessive in the egg is 1/2 X 1/2 = 1/4.
Therefore, the probability that a heterozygous offspring will be
produced is 1/4 + 1/4 = 1/2.
 Segregation and independent assortment of
chromosomes result in genetic variation.
 Segregation and independent assortment can be
applied to genes that are on different chromosomes.
Linked Genes are found on the same
chromosome
 Genes that are adjacent
and close to each other
on the same chromosome
tend to move as a unit
 the probability that they
will segregate as a unit is
a function of the distance
between them(Cross-over
frequencies).
 The pattern of inheritance (monohybrid, dihybrid,
sex-linked, and genes linked on the same homologous
chromosome) can often be predicted from data that
gives the parent genotype/phenotype and/or the
offspring phenotypes/genotypes.
 Be prepared to work any of the above type of problems
on the AP Exam.
Linked
 In D r o s o p h i l a t he genes A B D are linked. You cross a
strain that is homozygous for d with a strain that is
homozygous for ab. Below are the phenotypes and progeny
of a test cross. Outline the cross and determine the crossover frequencies and the map for the genes.
PhenotypeNumber
ABd 389
abD 413
ABD 60
abd 68
aBd 29
AbD 34
Abd 3
aBD 4
Certain human genetic disorders can be attributed to the
inheritance of single gene traits or specific chromosomal
changes, such as nondisjunction.
 Sickle cell anemia
 Tay-Sachs disease
 Huntington’s disease
 X-linked color blindness
 Trisomy 21/Down syndrome
 Klinefelter’s syndrome
Sickle cell anemia
 is an autosomal recessive
genetic blood disorder with
overdominance, characterized
by red blood cells that assume
an abnormal, rigid, sickle
shape.
 Sickling decreases the cells'
flexibility and results in a risk
of various complications.
 The sickling occurs because of a
point mutation (substitution)
in the hemoglobin gene.
Tay-Sachs disease
 Another example of a substitution point mutation.
 Tay–Sachs disease is a rare autosomal recessive
genetic disorder.
 In its most common variant (known as infantile Tay–
Sachs disease), it causes a progressive deterioration of
nerve cells and of mental and physical abilities that
commences around six months of age and usually
results in death by the age of four.
Huntington’s disease
 a neurodegenerative genetic disorder that affects
muscle coordination and leads to cognitive decline and
psychiatric problems.
 The disease is caused by an autosomal dominant
mutation in either of an individual's two copies of a
gene called Huntingtin, which means any child of an
affected person typically has a 50% chance of
inheriting the disease.
X-linked color blindness
Trisomy 21/Down syndrome
Klinefelter’s syndrome
 a genetic disorder in which there is at least one extra X chromosome to
a standard human male karyotype, for a total of 47 chromosomes
rather than the 46 found in genetically normal humans
 As babies and children, XXY males may have weaker muscles and
reduced strength. As they grow older, they tend to become taller than
average. They may have less muscle control and coordination than
other boys their age.
 During puberty, the physical traits of the syndrome become more
evident; because these boys do not produce as much testosterone as
other boys, they have a less muscular body, less facial and body hair,
and broader hips. As teens, XXY males may have larger breasts, weaker
bones, and a lower energy level than other boys.
 By adulthood, XXY males look similar to males without the condition,
although they are often taller.
 d. Many ethical, social and medical issues surround human
genetic disorders.
 To foster student understanding of this concept, instructors
can choose an illustrative example such as:
 • Reproduction issues
 • Civic issues such as ownership of genetic information,
privacy, historical contexts, etc.