GENETICA EN
EVOLUTIE
THEORIE
1
, CHAPTER 2 - SINGLE-GENE INHERITANCE
2.1 SINGLE-GENE INHERITANCE PATTERNS
- Pure lines: all offspring produced by matings within the members of that line are identical for the
phenotype in question.
- P: parental generation
- F1: rst lial generation
- Mendel’s rst law: law of equal segregation → in meiosis, the members of a gene pair separate
equally into the cells that become eggs and sperm = gametes → a single gamete contains one
member of the gene pair.
- A gene is a hereditary factor. They come in pairs.
- A gene can have more forms = alleles.
- At fertilisation, gametes fuse randomly, regardless of which alleles they bear.
- Zygote: fertilised egg
- Homozygote: organism has a pair of identical alleles for a given gene.
- Heterozygote: alleles of a gene differ → aka monohybrid.
- An individual can be classi ed as homozygous dominant (Y/Y), heterozygous (Y/y), homozygous
recessive (y/y).
- Genotypes: allelic combinations underlying a phenotype.
- Monohybrid cross = Y/y x Y/y
- 3 : 1 phenotypic ratio and 1 : 2 : 1 genotypic ratio in F2
2.2 GENES AND CHROMOSOMES
- 2 types of cell divisions in eukaryotes:
1. Somatic cell division: division of the cells in the main body.
2. Sexual cell division: cell division that takes place in the sex organs. Meiocytes divide to produce
sex cells such as sperm and eggs, or sexual spores (fungi and algae).
- Diploid organism: somatic cells have two sets of the genome → 2 sets of chromosomes. The
chromosomes are in pairs.
- Haploid organism: somatic cells have one set of the genome.
- When cells divide, each daughter cell has the same chromosomal set as its progenitor.
- Meiosis only takes place in diploid cells.
- Informatieoverdracht van generatie op generatie tijdens seksuele voorplanting
- Meiose 1: paring en uitsplitsing van homologe chromosomen = reductiedeling van diploïde naar
haploïde set chromosomen
- Meiose 2: uitsplitsing van zusterchromatiden over 4 dochtercellen (gameten
- In haploids, meiosis takes place at one special stage of the life cycle when two haploid cells unite to
form a transient diploid meiocyte.
- Meiocytes form only from the union of cells of different mating types.
- In many haploid organisms (especially fungi) they four cells that are the products of a single meiosis
remain together enclosed in a membranous sac → ascus.
2
fi fi fi fi
, 2.3 THE MOLECULAR BASIS OF MENDELIAN
INHERITANCE PATTERNS
- Alleles are generally identical in most of their sequences and differ only at one or several nucleotides of
the hundreds of thousands of nucleotides that make up the gene. → alleles are different versions of the
same gene.
- The replication of DNA during the S phase produces 2 new copies of each of the alleles, that can now
be segregated into separate cells.
- Exon: protein coding region of a gene.
- By changing one or more amino acids, mutations within axons inactivate some essential part of the
protein encoded by the gene.
- An important functional region of a gene is that encoding an enzyme’s active site → region is very
sensitive to mutation.
- Intron: noncoding regions of the gene in between the exons.
- Mutations within introns often prevent the normal processing of the primary RNA transcript.
- Null alleles: the proteins encoded by them completely lack function.
- Leaky mutations: reduce the level of enzyme function.
- Silent mutations: changes within a gene that have no functional impact.
- Recessiveness is observed in null mutations in genes that are functionally haplosuf cient: one gene
copy has enough function to produce a wild-type phenotype.
- Other genes are haploinsuf cient. → Null mutant allele will be dominant because, in a heterozygote,
the single wild-type allele can’t provide enough product for normal function.
- Sometimes, mutations result in a new function for the gene. Such mutations can be dominant, because
in a heterozygote, the wild-type allele can’t mask this new function.
- As a general rule: a null mutation is recessive in a haplosuf cient gene, it will be dominant in an
haploinsuf cient gene.
- A dominant mutation in the heterozygous state will be expressed. A cross between heterozygous
dominant and wild type parents will result in 1:1 phenotypic ratio in the progeny.
3
fi fi fi fi
, - Testkruising tussen heterozygoot en een homozygoot recessief (tester) geeft nakomelingen met
fenotypische verhouding dominant : recessief = 1 : 12.4 some genes discovered by observing
segregation ratios
- Sometimes, the severity of a mutant phenotype renders the organism sterile.
- Test cross: the cross of an individual of unknown heterozygosity (for one gene or more) with a fully
recessive parent. The recessive individual is called tester.
- The principles of inheritance (such as law of segregation) can be applied in 2 ways:
1. Inferring genotypes from phenotypic ratios
2. Predicting phenotypic ratios from parents of known genotypes
2.5 SEX-LINKED SINGLE-GENE INHERITANCE PATTERNS
- Sex is determined by the sex chromosomes X and Y.
- At meiosis in females, the 2 X chromosomes pair and segregate like autosomes. → the gametes are of
only one type and the female is said to be the homogametic sex.
- At meiosis in males, the X and Y chromosomes pair over a short region → heterogametic sex.
- The inheritance patterns of genes on the sex chromosomes are different from autosomal genes.
- Dioecious species are those showing animal-like sexual dimorphism, female plants bearing owers
containing only ovaries and male plants bearing owers containing only anthers.
- Cytogeneticists divide the X and Y chromosomes into homologous and differential regions. The
differential regions contain most of the genes and have no counterparts on the other sex chromosome.
These genes are said to be hemizygous.
- SRY gene determines maleness.
- Genes in differential regions are said to show inheritance patterns called sex linkage. Mutant alleles in
the differential region of the X chromosome show a single-gene inheritance pattern called X linkage.
Mutant alleles of the few genes in the differential region of the Y chromosome show a single-gene
inheritance pattern called Y linkage.
- A gene that is sex linked can show phenotypic ratios that are different in each sex.
- Sex linked inheritance patterns contrast with the inheritance matters of genes in the autosomes.
- If the genomic location of a gene is unknown, a sex linked inheritance patterns indicates that the gene
lies on a sex chromosome.
- The human X and Y chromosomes have two short homologous regions → pseudoautosomal regions 1
and 2 → one or both of these regions pairs with the other sex chromosome in meiosis and undergoes
crossing over.
- The X and Y chromosomes can act as a pair and segregate into equal numbers of sperm.
- Sex linked inheritance is recognised by different phenotypic ratios in the two sexes of progeny, as well
as different ratios in reciprocal crosses.
4
fl fl
EVOLUTIE
THEORIE
1
, CHAPTER 2 - SINGLE-GENE INHERITANCE
2.1 SINGLE-GENE INHERITANCE PATTERNS
- Pure lines: all offspring produced by matings within the members of that line are identical for the
phenotype in question.
- P: parental generation
- F1: rst lial generation
- Mendel’s rst law: law of equal segregation → in meiosis, the members of a gene pair separate
equally into the cells that become eggs and sperm = gametes → a single gamete contains one
member of the gene pair.
- A gene is a hereditary factor. They come in pairs.
- A gene can have more forms = alleles.
- At fertilisation, gametes fuse randomly, regardless of which alleles they bear.
- Zygote: fertilised egg
- Homozygote: organism has a pair of identical alleles for a given gene.
- Heterozygote: alleles of a gene differ → aka monohybrid.
- An individual can be classi ed as homozygous dominant (Y/Y), heterozygous (Y/y), homozygous
recessive (y/y).
- Genotypes: allelic combinations underlying a phenotype.
- Monohybrid cross = Y/y x Y/y
- 3 : 1 phenotypic ratio and 1 : 2 : 1 genotypic ratio in F2
2.2 GENES AND CHROMOSOMES
- 2 types of cell divisions in eukaryotes:
1. Somatic cell division: division of the cells in the main body.
2. Sexual cell division: cell division that takes place in the sex organs. Meiocytes divide to produce
sex cells such as sperm and eggs, or sexual spores (fungi and algae).
- Diploid organism: somatic cells have two sets of the genome → 2 sets of chromosomes. The
chromosomes are in pairs.
- Haploid organism: somatic cells have one set of the genome.
- When cells divide, each daughter cell has the same chromosomal set as its progenitor.
- Meiosis only takes place in diploid cells.
- Informatieoverdracht van generatie op generatie tijdens seksuele voorplanting
- Meiose 1: paring en uitsplitsing van homologe chromosomen = reductiedeling van diploïde naar
haploïde set chromosomen
- Meiose 2: uitsplitsing van zusterchromatiden over 4 dochtercellen (gameten
- In haploids, meiosis takes place at one special stage of the life cycle when two haploid cells unite to
form a transient diploid meiocyte.
- Meiocytes form only from the union of cells of different mating types.
- In many haploid organisms (especially fungi) they four cells that are the products of a single meiosis
remain together enclosed in a membranous sac → ascus.
2
fi fi fi fi
, 2.3 THE MOLECULAR BASIS OF MENDELIAN
INHERITANCE PATTERNS
- Alleles are generally identical in most of their sequences and differ only at one or several nucleotides of
the hundreds of thousands of nucleotides that make up the gene. → alleles are different versions of the
same gene.
- The replication of DNA during the S phase produces 2 new copies of each of the alleles, that can now
be segregated into separate cells.
- Exon: protein coding region of a gene.
- By changing one or more amino acids, mutations within axons inactivate some essential part of the
protein encoded by the gene.
- An important functional region of a gene is that encoding an enzyme’s active site → region is very
sensitive to mutation.
- Intron: noncoding regions of the gene in between the exons.
- Mutations within introns often prevent the normal processing of the primary RNA transcript.
- Null alleles: the proteins encoded by them completely lack function.
- Leaky mutations: reduce the level of enzyme function.
- Silent mutations: changes within a gene that have no functional impact.
- Recessiveness is observed in null mutations in genes that are functionally haplosuf cient: one gene
copy has enough function to produce a wild-type phenotype.
- Other genes are haploinsuf cient. → Null mutant allele will be dominant because, in a heterozygote,
the single wild-type allele can’t provide enough product for normal function.
- Sometimes, mutations result in a new function for the gene. Such mutations can be dominant, because
in a heterozygote, the wild-type allele can’t mask this new function.
- As a general rule: a null mutation is recessive in a haplosuf cient gene, it will be dominant in an
haploinsuf cient gene.
- A dominant mutation in the heterozygous state will be expressed. A cross between heterozygous
dominant and wild type parents will result in 1:1 phenotypic ratio in the progeny.
3
fi fi fi fi
, - Testkruising tussen heterozygoot en een homozygoot recessief (tester) geeft nakomelingen met
fenotypische verhouding dominant : recessief = 1 : 12.4 some genes discovered by observing
segregation ratios
- Sometimes, the severity of a mutant phenotype renders the organism sterile.
- Test cross: the cross of an individual of unknown heterozygosity (for one gene or more) with a fully
recessive parent. The recessive individual is called tester.
- The principles of inheritance (such as law of segregation) can be applied in 2 ways:
1. Inferring genotypes from phenotypic ratios
2. Predicting phenotypic ratios from parents of known genotypes
2.5 SEX-LINKED SINGLE-GENE INHERITANCE PATTERNS
- Sex is determined by the sex chromosomes X and Y.
- At meiosis in females, the 2 X chromosomes pair and segregate like autosomes. → the gametes are of
only one type and the female is said to be the homogametic sex.
- At meiosis in males, the X and Y chromosomes pair over a short region → heterogametic sex.
- The inheritance patterns of genes on the sex chromosomes are different from autosomal genes.
- Dioecious species are those showing animal-like sexual dimorphism, female plants bearing owers
containing only ovaries and male plants bearing owers containing only anthers.
- Cytogeneticists divide the X and Y chromosomes into homologous and differential regions. The
differential regions contain most of the genes and have no counterparts on the other sex chromosome.
These genes are said to be hemizygous.
- SRY gene determines maleness.
- Genes in differential regions are said to show inheritance patterns called sex linkage. Mutant alleles in
the differential region of the X chromosome show a single-gene inheritance pattern called X linkage.
Mutant alleles of the few genes in the differential region of the Y chromosome show a single-gene
inheritance pattern called Y linkage.
- A gene that is sex linked can show phenotypic ratios that are different in each sex.
- Sex linked inheritance patterns contrast with the inheritance matters of genes in the autosomes.
- If the genomic location of a gene is unknown, a sex linked inheritance patterns indicates that the gene
lies on a sex chromosome.
- The human X and Y chromosomes have two short homologous regions → pseudoautosomal regions 1
and 2 → one or both of these regions pairs with the other sex chromosome in meiosis and undergoes
crossing over.
- The X and Y chromosomes can act as a pair and segregate into equal numbers of sperm.
- Sex linked inheritance is recognised by different phenotypic ratios in the two sexes of progeny, as well
as different ratios in reciprocal crosses.
4
fl fl
