2. Objectives
1. Identify the mode of inheritance o a
particular trait given the pedigree.
2. Predict the genotypes of parents.
3. Predict the probability of having an affected
offspring.
3. Autosomal Dominant
- If the phenotype associated with a given
version of a gene is observed when an
individual has only one copy, the allele is said
to be autosomal dominant. The phenotype
will be observed whether the individual has
one copy of the allele (is heterozygous) or has
two copies of the allele (is homozygous).
4. Autosomal Recessive
- If the phenotype associated with a given version of a
gene is observed only when an individual has two copies,
the allele is said to be autosomal recessive. The phenotype
will be observed only when the individual is homozygous
for the allele concerned. An individual with only one copy of
the allele will not show the phenotype, but will be able to
pass the allele on to subsequent generations. As a result,
an individual heterozygous for an autosomal recessive
allele is known as a carrier.
5. Sex-Linked or X-Linked Inheritance
- The X chromosome carries hundreds of genes, and many
of these are not connected with the determination of sex.
The smaller Y chromosome contains a number of genes
responsible for the initiation and maintenance of maleness,
but it lacks copies of most of the genes that are found on
the X chromosome. As a result, the genes located on the X
chromosome display a characteristic pattern of inheritance
referred to assex-linkage or X-linkage.
6. Sex-Linked or X-Linked Inheritance
- Females (XX) have two copies of each gene on the X
chromosome, so they can be heterozygous or homozygous
for a given allele. However, males (XY) will express all the
alleles present on the single X chromosome that they
receive from their mother, and concepts such as 'dominant'
or 'recessive' are irrelevant. A number of medical
conditions in humans are associated with genes on the X
chromosome, including haemophilia, muscular dystrophy
and some forms of colour blindness.
7. Pedigree Chart
- Pedigree analysis is also useful when
studying any population when progeny
data from several generations is limited.
Pedigree analysis is also useful when
studying species with a long generation
time.
- A series of symbols are used to represent
different aspects of a pedigree. To the right
are the principal symbols used when
drawing a pedigree.
8. Pedigree Chart
- For those traits exhibiting dominant gene
action:
• affected individuals have at least one
affected parent.
• the phenotype generally appears every
generation.
• two unaffected parents only have
unaffected offspring.
9. Pedigree Chart
- And for those traits exhibiting recessive
gene action:
• unaffected parents can have affected
offspring.
• affected progeny are both male and
female.
10. Pedigree Chart
- This chart shows four
generations of a family
with four individuals
who are affected by a
form of colorblindness.
11. Pedigree Chart
- In this pedigree, the unaffected founding
mother, I-1, and affected founding father, I-2,
are parents to two affected daughters, II-1 and
II-2. The affected founding daughter II-2 and
the unaffected male II-3 who “marries into the
family” have two offspring, an unaffected
daughter III-1 and affected son, III-2. Finally,
this affected male III-2 and the unaffected
female III-3 who “marries in” have an unaffected
son, IV-1.
12. Pedigree Chart
In pedigree analysis, we use certain rules to determine whether
a trait or disorder is autosomal dominant or autosomal
recessive. Here are some key guidelines:
1. Autosomal Dominant Inheritance:
- Affected individuals have at least one affected parent.
- The trait appears in every generation of the pedigree.
- Two unaffected individuals can have an affected child.
- If an affected individual mates with an unaffected individual,
approximately half of their children will be affected.
13. Pedigree Chart
2. Autosomal Recessive Inheritance:
- Affected individuals may have unaffected parents.
- The trait can skip generations.
- Two unaffected individuals can have an affected child if they both carry a
recessive allele.
- If both parents are carriers (heterozygous), approximately 25% of their
children will be affected.
3. Sex-Linked Dominant Inheritance:
- Affected males will have affected daughters but not affected sons.
- Affected females can have both affected sons and daughters.
- If an affected male mates with an unaffected female, none of their sons
will be affected, but all of their daughters will be affected.
- If an affected female mates with an unaffected male, approximately half
of their sons and half of their daughters will be affected.
14. Pedigree Chart
4. Sex-Linked Recessive Inheritance:
- Affected males will pass the trait to all of their daughters, who will be
carriers, but none of their sons.
- Affected females will pass the trait to approximately half of their sons
and half of their daughters.
- If an affected male mates with an unaffected female, none of their
children will be affected, but all of their daughters will be carriers.
- If an affected female mates with an unaffected male, approximately half
of their sons will be affected, and half of their daughters will be carriers.
15. Pedigree Chart
5. Y-Linked Inheritance:
- Only males are affected by Y-linked traits or
disorders.
- Affected males will pass the trait to all of their
sons.
- There is no transmission of Y-linked traits from
fathers to daughters or from affected males to
their unaffected offspring.
- If a male is unaffected, none of his sons will be
affected.
