Hoofdstuk 2 single-gene inheritance
2.1 single-gene inheritance patterns
LO: in the progeny of controlled crosses, recognize phenotypic ratios diagnostic of single-gene inheritance.
First step in genetic dissection is to obtain variants that differ in the trait under scrutiny.
Mendel’s pioneering experiments
>> investigate the inheritance of seven traits of his chosen pea species
All of his plant were from pure lines: all offspring produced by matings within the members of that line were identical.
F1 = first filial generation
Resultst of the first two reciprocal crosses:
Female from yellow line x male from green line F1 peas all yellow
Female from green line x male from yellow line F1 peas all yellow
F2 = second filial generation
Yellow F1 x yellow F1 F2 ratio is 1 green : 3 yellow
F2 green x F2 green F3 all green
F2 yellow x F2 yellow F3 ¼ pure green, ¾ yellow (1/3
all yellows, 2/3 gave ¼ green and ¾ yellow)
F2 yellow x F2 green ½ yellow and ½ green
Mendel’s law of equal segregation
>> zie aantekeningen van hc, is precies wat in dit stukje
staat
2.2 genes and chromosomes
LO: explain single-gene inheritance ratios in terms of chromosome behavior at meiosis.
Somatic cell division (division of cells of the main body) >> mitosis >> 2n 2n + 2n
Sexual cell division (gametes) >> meiosis >> 2n n + n + n + n
Diploid >> 2n >> two copies of each chromosome
Haploid >> n >> one copy of each chromosome
Single-gene inheritance in diploids
>> mitosis
>> Aa Aa + Aa
>> meiosis
>> Aa A + A + a + a
>> ratio = 1A : 1a
,! the physical separation of chromosome pairs during anaphase I of meiosis is the basis for Mendel’s law of equal
segregation
Single-gene inheritance in haploids
>> mitosis
>> A A + A
>> a a + a
! Mitotic division results in the original chromosome number in each
of the two product cells. Meiotic division results in half the original
chromosome number in each of the four product cells.
2.3 the molecular basis of mendelian inheritance patterns
LO: propose reasonable hypotheses to explain dominance and recessiveness of specific alleles at the molecular level.
Structural differences between alleles at the molecular level
When alleles (like A and a) are examined, they are found to be identical in most of their
sequences and differ only at one or several nucleotides of the hundreds or thousands of nucleotide that make up a gene.
A gene can be changed by mutation in many ways. The mutational damage can
occur at any one of many different sites.
Molecular aspects of gene transmission
Replication of alleles during the S phase
DNA molecule is replicated during the S phase. (figuur 2-11)
Meiosis and mitosis at the molecular level
S phase >> two copies of each allele. They are segregated into separate cells. (figuur 2-12)
Demonstrating chromosome segregation at the molecular level
! the rules of segregation enunciated by Mendel applies not only to genes, but to any stretch of DNA along a chromosome.
Alleles at the molecular level
Primary phenotype of a gene >> the protein it produces.
Most mutations that alter phenotype alter the amino acid sequence of the gene’s protein’s product, resulting in reduced
or absent function.
Null alleles: the proteins encoded by them completely lack function
Leaky mutations: reduce the level of enzyme function
Silent mutations: a change to the sequence of a codon that does not change the encoded amino acid.
Dominance and recessiveness
Haplosufficient: one gene copy has enough function to produce a wild-type phenotype
Haploinsufficient: a null mutant allele will be dominant because the single wild-type allele cannot provide enough product
for normal function.
2.4 some genes discovered by observing segregation ratios
LO: predict phenotypic ratios among descendants from crosses of parents differing at a single gene.
A gene active in the development of flower color
The Punnett square is a graphical representation of parental gametes and shows how they randomly unite to produce
progeny genotypes, from which phenotypic ratios of the progeny can be deduced.
,A gene for wing development
A dominant mutation in the heterozygous state will be expressed. A cross between the heterozygous dominant and wild
type parents will result in a 1:1 phenotypic ratio in the progeny.
A gene for hyphal branching
In research on a new mutation affecting a trait of interest, the demonstration of Mendelian single-gene ratios in crossing
analysis reveals a gene that is important in the developmental pathways for that trait.
Predicting progeny proportions or parental genotypes by applying the principles
of single-gene inheritance
The principles of inheritance can be applied in two directions: (1) inferring genotypes from phenotypic ratios and (2)
predicting phenotypic ratios from parents of known genotypes.
2.3 sex-linked single-gene inheritance patterns
LO: in the progeny of controlled crosses, recognize phenotypic ratios diagnostic of X-linked single-gene inheritance
Sex chromosomes
>> sex is determined by a special pair of sex chromosomes.
Females > 2 X chromosomes (XX) > homogametic sex
Males > an X chromosome and an Y chromosome (XY) > heterogametic sex
Dioecious species: those showing animal-like sexual dimorphism, with female plant bearing flowers containing only
ovaries and male plants bearing flowers containing only anthers.
Sex-linked patterns of inheritance
Differential regions (contain most of the genes) > have not counter parts on the other sex chromosome > hemizygous
genes
Sex linkage: linked to X or Y chromosome. Can show phenotypic ratios that are different in each sex.
X linkage: mutant alleles in the differential region of the X chromosome
Y linkage: mutant alleles in the differential region of the Y chromosome
Pseudoautosomal regions 1 and 2: the human X and Y chromosomes have two short homologous regions, one at each
end. One or both of these regions pairs with the other sex chromosome in meiosis and undergoes crossing over.
X-linked inheritance
Males need only inherit a single X-linked recessive allele in order for it to be
expressed in the phenotype; a female must inherit two
Sex-linked inheritance is recognized by different phenotypic ratios in the two sexes
of progeny, as well as different ratios in reciprocal crosses.
2.5 human pedigree analysis
LO: recognize inheritance patterns diagnostic of autosomal dominant, autosomal
recessive, X-linked dominant, X-linked recessive, and Y-linked conditions in human
pedigrees.
Pedigree analysis: deducing single-gene inheritance of human phenotypes by a
study of the progeny of mating within a family, often stretching back several
generations
Propositus: a member of a family who first comes to the attention of a geneticist.
Two phenotypes: the presence and the absence of the disorder.
Autosomal recessive disorder
The affected phenotype of an autosomal recessive disorder is inherited as a recessive allele.
Patterns in a pedigree that would reveal autosomal recessive inheritance:
1. The disorder appears in the progeny of unaffected parents
, 2. The affected progeny include both males and females.
Autosomal dominant disorders
Patterns in a pedigree that would reveal autosomal disorders
1. Normal allele recessive, defective allele is dominant.
2. The phenotype tends to appear in every generation of the pedigree
3. Affected fathers or mothers transmit the phenotype to both sons and
daughters.
Autosomal polymorphisms
Polymorphisms: the coexistence of two or more reasonable common phenotypes of a
biological property. The alternative phenotypes of a polymorphisms (the morphs) are
often inherited as alleles of a single autosomal gene in the standard Mendelian
manner.
Dimorphisms: two morphs
Contrasting morphs are often inherited as alleles of a single gene.
X-linked recessive disorders
features:
1. Many more males than females show the rare phenotype under study
2. None of the offspring of an affected male show the phenotype, but all his
daughters are “carriers”. In the next generation, half the sons of these
carrier daughters show the phenotype.
3. None of the sons of an affected male show the phenotype under study,
nor will they pass the condition to their descendants
X-linked dominant disorders
Features:
1. Affected males pass the condition to all their daughters but none of their
sons
2. Affected heterozygous females mating with unaffected males pass the
condition to half their sons and daughters
Y-linked inheritance
Only males inherit genes in the differential region of the human Y chromosome, with fathers transmitting the genes to
their sons.
! Inheritance patterns with an unequal representation of phenotypes in males and females can locate the genes
concerned to one of the sex chromosomes.
2.1 single-gene inheritance patterns
LO: in the progeny of controlled crosses, recognize phenotypic ratios diagnostic of single-gene inheritance.
First step in genetic dissection is to obtain variants that differ in the trait under scrutiny.
Mendel’s pioneering experiments
>> investigate the inheritance of seven traits of his chosen pea species
All of his plant were from pure lines: all offspring produced by matings within the members of that line were identical.
F1 = first filial generation
Resultst of the first two reciprocal crosses:
Female from yellow line x male from green line F1 peas all yellow
Female from green line x male from yellow line F1 peas all yellow
F2 = second filial generation
Yellow F1 x yellow F1 F2 ratio is 1 green : 3 yellow
F2 green x F2 green F3 all green
F2 yellow x F2 yellow F3 ¼ pure green, ¾ yellow (1/3
all yellows, 2/3 gave ¼ green and ¾ yellow)
F2 yellow x F2 green ½ yellow and ½ green
Mendel’s law of equal segregation
>> zie aantekeningen van hc, is precies wat in dit stukje
staat
2.2 genes and chromosomes
LO: explain single-gene inheritance ratios in terms of chromosome behavior at meiosis.
Somatic cell division (division of cells of the main body) >> mitosis >> 2n 2n + 2n
Sexual cell division (gametes) >> meiosis >> 2n n + n + n + n
Diploid >> 2n >> two copies of each chromosome
Haploid >> n >> one copy of each chromosome
Single-gene inheritance in diploids
>> mitosis
>> Aa Aa + Aa
>> meiosis
>> Aa A + A + a + a
>> ratio = 1A : 1a
,! the physical separation of chromosome pairs during anaphase I of meiosis is the basis for Mendel’s law of equal
segregation
Single-gene inheritance in haploids
>> mitosis
>> A A + A
>> a a + a
! Mitotic division results in the original chromosome number in each
of the two product cells. Meiotic division results in half the original
chromosome number in each of the four product cells.
2.3 the molecular basis of mendelian inheritance patterns
LO: propose reasonable hypotheses to explain dominance and recessiveness of specific alleles at the molecular level.
Structural differences between alleles at the molecular level
When alleles (like A and a) are examined, they are found to be identical in most of their
sequences and differ only at one or several nucleotides of the hundreds or thousands of nucleotide that make up a gene.
A gene can be changed by mutation in many ways. The mutational damage can
occur at any one of many different sites.
Molecular aspects of gene transmission
Replication of alleles during the S phase
DNA molecule is replicated during the S phase. (figuur 2-11)
Meiosis and mitosis at the molecular level
S phase >> two copies of each allele. They are segregated into separate cells. (figuur 2-12)
Demonstrating chromosome segregation at the molecular level
! the rules of segregation enunciated by Mendel applies not only to genes, but to any stretch of DNA along a chromosome.
Alleles at the molecular level
Primary phenotype of a gene >> the protein it produces.
Most mutations that alter phenotype alter the amino acid sequence of the gene’s protein’s product, resulting in reduced
or absent function.
Null alleles: the proteins encoded by them completely lack function
Leaky mutations: reduce the level of enzyme function
Silent mutations: a change to the sequence of a codon that does not change the encoded amino acid.
Dominance and recessiveness
Haplosufficient: one gene copy has enough function to produce a wild-type phenotype
Haploinsufficient: a null mutant allele will be dominant because the single wild-type allele cannot provide enough product
for normal function.
2.4 some genes discovered by observing segregation ratios
LO: predict phenotypic ratios among descendants from crosses of parents differing at a single gene.
A gene active in the development of flower color
The Punnett square is a graphical representation of parental gametes and shows how they randomly unite to produce
progeny genotypes, from which phenotypic ratios of the progeny can be deduced.
,A gene for wing development
A dominant mutation in the heterozygous state will be expressed. A cross between the heterozygous dominant and wild
type parents will result in a 1:1 phenotypic ratio in the progeny.
A gene for hyphal branching
In research on a new mutation affecting a trait of interest, the demonstration of Mendelian single-gene ratios in crossing
analysis reveals a gene that is important in the developmental pathways for that trait.
Predicting progeny proportions or parental genotypes by applying the principles
of single-gene inheritance
The principles of inheritance can be applied in two directions: (1) inferring genotypes from phenotypic ratios and (2)
predicting phenotypic ratios from parents of known genotypes.
2.3 sex-linked single-gene inheritance patterns
LO: in the progeny of controlled crosses, recognize phenotypic ratios diagnostic of X-linked single-gene inheritance
Sex chromosomes
>> sex is determined by a special pair of sex chromosomes.
Females > 2 X chromosomes (XX) > homogametic sex
Males > an X chromosome and an Y chromosome (XY) > heterogametic sex
Dioecious species: those showing animal-like sexual dimorphism, with female plant bearing flowers containing only
ovaries and male plants bearing flowers containing only anthers.
Sex-linked patterns of inheritance
Differential regions (contain most of the genes) > have not counter parts on the other sex chromosome > hemizygous
genes
Sex linkage: linked to X or Y chromosome. Can show phenotypic ratios that are different in each sex.
X linkage: mutant alleles in the differential region of the X chromosome
Y linkage: mutant alleles in the differential region of the Y chromosome
Pseudoautosomal regions 1 and 2: the human X and Y chromosomes have two short homologous regions, one at each
end. One or both of these regions pairs with the other sex chromosome in meiosis and undergoes crossing over.
X-linked inheritance
Males need only inherit a single X-linked recessive allele in order for it to be
expressed in the phenotype; a female must inherit two
Sex-linked inheritance is recognized by different phenotypic ratios in the two sexes
of progeny, as well as different ratios in reciprocal crosses.
2.5 human pedigree analysis
LO: recognize inheritance patterns diagnostic of autosomal dominant, autosomal
recessive, X-linked dominant, X-linked recessive, and Y-linked conditions in human
pedigrees.
Pedigree analysis: deducing single-gene inheritance of human phenotypes by a
study of the progeny of mating within a family, often stretching back several
generations
Propositus: a member of a family who first comes to the attention of a geneticist.
Two phenotypes: the presence and the absence of the disorder.
Autosomal recessive disorder
The affected phenotype of an autosomal recessive disorder is inherited as a recessive allele.
Patterns in a pedigree that would reveal autosomal recessive inheritance:
1. The disorder appears in the progeny of unaffected parents
, 2. The affected progeny include both males and females.
Autosomal dominant disorders
Patterns in a pedigree that would reveal autosomal disorders
1. Normal allele recessive, defective allele is dominant.
2. The phenotype tends to appear in every generation of the pedigree
3. Affected fathers or mothers transmit the phenotype to both sons and
daughters.
Autosomal polymorphisms
Polymorphisms: the coexistence of two or more reasonable common phenotypes of a
biological property. The alternative phenotypes of a polymorphisms (the morphs) are
often inherited as alleles of a single autosomal gene in the standard Mendelian
manner.
Dimorphisms: two morphs
Contrasting morphs are often inherited as alleles of a single gene.
X-linked recessive disorders
features:
1. Many more males than females show the rare phenotype under study
2. None of the offspring of an affected male show the phenotype, but all his
daughters are “carriers”. In the next generation, half the sons of these
carrier daughters show the phenotype.
3. None of the sons of an affected male show the phenotype under study,
nor will they pass the condition to their descendants
X-linked dominant disorders
Features:
1. Affected males pass the condition to all their daughters but none of their
sons
2. Affected heterozygous females mating with unaffected males pass the
condition to half their sons and daughters
Y-linked inheritance
Only males inherit genes in the differential region of the human Y chromosome, with fathers transmitting the genes to
their sons.
! Inheritance patterns with an unequal representation of phenotypes in males and females can locate the genes
concerned to one of the sex chromosomes.
