CH2: SINGLE-GENE INHERITANCE
- There are many types of analytical approaches to gene discovery, the most common being
detection of single-gene inheritance patterns.
- Inheritance patterns may be recognized in the progeny of controlled matings or crosses
- In this case, we mate a mutated form of the property with a wild type form and the ratio of wild
to mutant reveals whether a single gene controls that difference in property under study.
- The use of mutants in this way is called genetic dissection
- How do we obtain mutants? We visually screen a large number of individuals, looking for a
chance occurrence of mutants in that population
- Single gene inheritance patterns are useful in human genetics
o Cystic fibrosis and Tay-Sachs disease are inherited as a single mutant gene.
2.1 Genes and Chromosomes - An organism’s unique and complete set of genetic information is its genome
- In eukaryotes, the bulk of the DNA of a genome is found in the nucleus of each cell and this
nuclear DNA is divided into units called chromosomes
- Through using chromosome paints as a probe to stain identical chromosomes the same colour,
Diploid we now know that nuclear genetic material of most animals and plants are diploid - contain
two complete genomes and 2 identical chromosome sets.
Haploid - The number of chromosomes in the basic genomic set is called the haploid number (n)
o Ex. Molds, yeasts, mushrooms, and algae ar familiar haploid organisms.
- Each chromosome contains 1 molecule of DNA wrapped around molecular spools called
Nucleosomes, histones nucleosomescomposed of an octamer of proteins called histones
Chromatin - DNA and associated nucleosomes constitute chromatin
- Chromatin is more compact near the centromere
Heterochromatin o Heterochromatin: dense chromatin
Euchromatin o Euchromatin: less dense chromatin
Parts of a Chromosome:
Nucleolar organize - The nucleolar organizer contains multiple repeats of genes encoding ribosomal RNA
nucleolus - A spherical body containing ribosomal RNA, the nucleolus is often visible attached to the
Telomeres - Telomeres are the tips of chromosomes and have a unique replication mechanism different
from that for the rest of the chromosome
- There are untranscribed regions between genes that are variable and a large part of this
intergenic space has resulted from the accumulation of a mobile type of DNA called
Transposable element transposable elementwhich are known to have an effect on genome rearrangement in the
course of evolution.
Introns - Introns are noncoding regions that interrupt the coding segments of a gene, most coding
regions are 1-3kb but large introns can cause a gene to seem enormous.
2.2 Single-Gene Inheritance
Mendel’s Law of Equal - Mendel investigated the inheritance of 7 properties in peas: pea color, pea shape, pod color, pod
Segregation shape, flower color, plant height and position of the flowering shoot.
o Character and trait are used synonymously with property
- For each character, he studied 2 contrasting phenotypes and pure lines – all offspring produced
by matings within the members of that line were identical
o Ex. All progeny of any mating were yellow seeded in the yellow-seeded line.
- The lines that were part of the parental generation (P) produced the first filial generation or
F1 and the crossed descendants of F1 produced the second filial generation or F2.
- - Mendel concluded the following from his experiments:
1. A gene is a hereditary factor producing pea colour
2. Each plant has a pair of this type of gene
3. The gene comes in 2 forms called alleles
4. A plant can be either Y/Y, y/y or Y/y.
5. The Y allele is dominant thus, the phenotype will be yellow and the y allele is recessive
6. In meiosis, the members of a gene pair separate equally into eggs and sperm, this equal
Mendel’s first law separation is Mendel’s first law or the law of equal segregation
7. Hence, a single gamete contains only one member of the gene pair
8. At fertilization, gametes fuse randomly, regardless of which of the alleles they bear.
Zygote - A zygote is a fertilized egg, that develops into a progeny individual.
Homozygote o Homozygote : same pair of identical alleles
Heterozygote o Heterozygote: different pair of alleles
Monohybrid o Monohybrid: a heterozygote for one gene
Genotypes - Genotypes are allelic combinations underlying phenotypes.
Homozygous dominant o Homozygous dominant: Y/Y
Heterozygous dominant o Heterozygous dominant: Y/y
Heterozygous recessive o Heterozygous recessive: y/y
Monohybrid cross - Monohybrid cross is a cross of the same type of genotype i.e. Y/y x Y/y
2.3 The Chromosomal
Basis of Single Gene
Inheritance Patterns - When somatic (body) cells divide to increase their number, the accompanying nuclear division
mitosis is called mitosis , which can take place in diploid or haploid cells and produces daughter cells
with the same genomic content
- Results in one progenitor cell to become 2. 2n 2n +2n
Meiocytes - Most eukaryotes have a sexual cycle and in these organisms, specialized diploid cells called
meiosis meiocytes are set aside to divide to produce sex cells.
- In meiosis, two cell divisions take place and has two nuclear divisions occurring in only diploid
- Results in 2 divisions with 4 cells produced. 2n n+n+n+n
- The difference between mitosis and meiosis is that in meiosis, homologous pairs of sister
chromatids unite to form a bundle of four homologous chromatids, called synapsis, helped by
the Synaptonemal complex (SC)
- Replicate sister chromosomes are called a dyad (Greek word for 2)
- Bivalent: the unit comprising a pair of synapsed dyads
- Tetrad : the four chromatids that make up a bivalent
- At the tetrad stage, crossing over changes the combinations of alleles of several different genes
but doesn’t directly affect single gene inheritance patterns.
o Ex. Start: one homolog carries A and one carries a
o Replication: one dyad is AA and one is aa
o Pairing: Tetrad is A/A/a/a
o First division products: one cell AA, the other cell aa
o Second division products: four cells, two of type A and two of type a.
Single gene inheritance
in haploids - In fungi, there are 2 simple forms of sexes called mating types : MATa and MATα determined
by the alleles of one gene.
- Normal wild type yeast colonies are white, but occasionally, red mutants arise
- Replication of r+ (wild type) and r would give a tetrad of two meiotic products of genotype r+
and two of r, all contained in a membranous sac called an ascus.
- Haploid genetics is simple because a cross requires only one meiosis. Molecular Basis of single-
gene segregation and
Demonstrating chromosome - Restriction fragment length polymorphism (RFLP) uses restriction enzymes that cut DNA at
segregation at the molecular specific base sequences in the genome, followed by a Southern blot.
level - If at a specific location a target site is missing or there is an extra site, and such a site flanks the
sequence hybridized by a probe, then this reveals a RFLP.
The Nature of Alleles and
their products - PKU phenotype is inherited as a Mendelian recessive and is caused by a defective allele of the
gene that encodes the liver enzyme phenylalanine hydroxylase (PAH)