SAMENVATTING 1
Genes & Genomes
BMW20605 2019-2020
Marieke Roos
,2
Week 1. Phenotypes in families and natural variation
H5. Patterns of inheritance
Locus: The unique chromosomal location of a gene or DNA sequence
Alleles: Alternative versions of a gene
Genotype: All the alleles present at one loci
Phenotype: Observable properties or characters of an organism
Homozygous: Both alleles at a locus are the same
- If both alleles have the same phenotype
Heterozygous: Two different alleles at a locus
Gene: A character that is inherited
A functional unit of DNA
5.1 Monogenic and multifactorial inheritance
Mendelian: Monogenetic characters, caused by genes at one locus
Multifactorial: Caused by genes at more than one loci
- Oligogenic: a small number of loci
- Polygenic: many loci
Dichotomous: Characters you either have or don’t have
All mendelian characters are dichotomous
Continuous or Characters we all have, but to different degrees
quantitative: on quantitative trait loci, QTL
, 3
5.2 Mendelian pedigree patterns
Mendelian: A character determined by variation at a single locus
If transmitted through family it may show characteristic pedigree
pattern as a result of the way chromosomes segregate during
meiosis.
Autosomal or X-linked: Depending on the chromosomal location of the gene.
Y-linked: The Y chromosome carries very few genes. The effect of
variants causes infertility, the pedigree pattern is maleness.
Dominant or recessive: If the character is visible or not in a heterozygous person.
Dominance or recessiveness are properties of characters,
not genes or alleles.
Hemizygous: Males have a single copy of the DNA at the loci of the X
and Y chromosomes.
Symbols used in pedigrees
, 4
Basic Mendelian pedigree patterns
1. Autosomal dominant inheritance
• An affected child usually has at
least one affected parent
• It affects either sex
• It is transmitted by either sex
• A child with one affected and one
unaffected parent has a 50%
chance of being affected
2. Autosomal recessive inheritance
• Affected children are (usually)
born to unaffected parents
• Parents of affected children are
heterozygous, asymptomatic
carriers
• Increased incidence if parents are
related (consanguineous = “of the
same blood”)
• It affects either sex
• After the birth of an affected child,
each subsequent child has a 25%
chance of being affected
3. X-linked recessive inheritance
• It affects mainly males
• Affected males are born to
unaffected parents, with a
heterozygous mother
• Females may be affected if the
father is affected and if the mother
is a carrier, of as a result of non-
random X-inactivation
• There is no male-to-male
transmission
4. X-linked dominant inheritance
• It affects either sex, females more
than males
• At least one parent is affected
• Females are more mildly and
variably affected
• The child of an affected female,
regardless to its sex, has a 50%
chance of being affected
• For an affected male, all his
daughters but none of his sons are
affected
Genes & Genomes
BMW20605 2019-2020
Marieke Roos
,2
Week 1. Phenotypes in families and natural variation
H5. Patterns of inheritance
Locus: The unique chromosomal location of a gene or DNA sequence
Alleles: Alternative versions of a gene
Genotype: All the alleles present at one loci
Phenotype: Observable properties or characters of an organism
Homozygous: Both alleles at a locus are the same
- If both alleles have the same phenotype
Heterozygous: Two different alleles at a locus
Gene: A character that is inherited
A functional unit of DNA
5.1 Monogenic and multifactorial inheritance
Mendelian: Monogenetic characters, caused by genes at one locus
Multifactorial: Caused by genes at more than one loci
- Oligogenic: a small number of loci
- Polygenic: many loci
Dichotomous: Characters you either have or don’t have
All mendelian characters are dichotomous
Continuous or Characters we all have, but to different degrees
quantitative: on quantitative trait loci, QTL
, 3
5.2 Mendelian pedigree patterns
Mendelian: A character determined by variation at a single locus
If transmitted through family it may show characteristic pedigree
pattern as a result of the way chromosomes segregate during
meiosis.
Autosomal or X-linked: Depending on the chromosomal location of the gene.
Y-linked: The Y chromosome carries very few genes. The effect of
variants causes infertility, the pedigree pattern is maleness.
Dominant or recessive: If the character is visible or not in a heterozygous person.
Dominance or recessiveness are properties of characters,
not genes or alleles.
Hemizygous: Males have a single copy of the DNA at the loci of the X
and Y chromosomes.
Symbols used in pedigrees
, 4
Basic Mendelian pedigree patterns
1. Autosomal dominant inheritance
• An affected child usually has at
least one affected parent
• It affects either sex
• It is transmitted by either sex
• A child with one affected and one
unaffected parent has a 50%
chance of being affected
2. Autosomal recessive inheritance
• Affected children are (usually)
born to unaffected parents
• Parents of affected children are
heterozygous, asymptomatic
carriers
• Increased incidence if parents are
related (consanguineous = “of the
same blood”)
• It affects either sex
• After the birth of an affected child,
each subsequent child has a 25%
chance of being affected
3. X-linked recessive inheritance
• It affects mainly males
• Affected males are born to
unaffected parents, with a
heterozygous mother
• Females may be affected if the
father is affected and if the mother
is a carrier, of as a result of non-
random X-inactivation
• There is no male-to-male
transmission
4. X-linked dominant inheritance
• It affects either sex, females more
than males
• At least one parent is affected
• Females are more mildly and
variably affected
• The child of an affected female,
regardless to its sex, has a 50%
chance of being affected
• For an affected male, all his
daughters but none of his sons are
affected
