Summary Week 3 | Fundamentals of Genetics | Wageningen | 2025/26

Samenvatting week 3
Ch 2
Key concepts
 The genetic approach to understanding a biological trait is to discover the genes that
control it. One approach to gene discovery is to isolate mutants and check each one
for single-gene intheritance patterns (specific ratios of wild-type and mutant
expression of the trait in descendants).
2.1 single-gene inheritance patterns
 Mendels discovery:
o Used available mutant phenotypes
o Selfed each to obtain pure (true breeding) parental lines
o Made controlled crosses to obtain F1
o Selfed the F1 to obtain F2: analyzed the progeny ratio’s
o Did test crosses: F1 x pure recessive parent
 At meiosis, the members of a gene pair segregate equally into the product cells (often
sperm or eggs) this is know as Mendel’s first law of equal segregation
 A zygote is a fertilized egg
 Equal segregation of a gene pair during meiosis is observable only in heterozygotes,
making them critical tools for genetic analysis, in a cross between a heterozygote and
a homozygous recessive, the gametic ratio produced by meiosis in the heterozyote is
observed in the phenotypic ratio of the progeny
 All 1:1, 3:1 and 1:2:1 genetic ratios are diagnostic of single-gene inheritance and are
based on equal segregation in a heterozygote
2.2 genes and chromosomes
 Mitosis:
o Somatic cells
o Chromosomes replicate  2 identical sister chromatids connected at the
centromere
o No homologous pairing  sister chromatids separate
o Crossing over rare
o Identical daughter cells
o No ploidy change  n->n;
2n ->2n
o Takes < 24h
o Human soma: ong. 3x1013
cells
 Meiosis:
o Germ cells
o Replication +2 divisions
o Homologous pairing
required
o Crossing over required
o Chromosome re-
assortment

, o 2 divisions: 1st separation of homologous chromosomes; 2nd separation of
chromatids
o Non-identical daughter cells (tetrad)
o Reduction of ploidy 2n->n
o n, 3n, 5n sterile
o takes ong. 14 dagys (in human males) and up to decades (females)
 During somatic cell division, the accompanying nuclear division is mitosis. During
sexual cell division, the accompanying nuclear division is meiosis
 n = the number of chromosomes in the genome, and the number 2 indicates that
there are two genomes (chromosome sets) per adult cell
 The somatic cells of diploid organisms contain two copies of each chromosome the
somatic cells of haploid organisms contain one copy of each chromosome
 The physical separation of chromosome pairs during anaphase I of meiosis is the
basis for Mendel’s law of equal segregation
 Chromatid: one of the two side-by-side replicas produced by chromosome division
 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
 Mendelian inheritance is shown by any segment of DNA on a chromosome: by genes
and their alleles and by molecular markers not necessarily associated with any
biological function
 >4500 ‘Mendelian’ human diseases
 Most mutations that alter phenotype alter the amino acid sequence of the gene’s
protein product, resulting in reduced or absent function
 Null alleles: the proteins encoded by them completely lack function
 Leaky mutations: mutant alleles that reduce the level of enzyme function
 DNA sequencing often detects changes within a gene that have no functional impact
at all, so these alleles, silent mutations, are functionally wild type
 Haplosufficient: describes a gene that, in a diploid cell, can promote wild-type function
in only one copy (dose)
 Haploinsufficient: describes a gene that, in a diploid cell, is insufficient to promote
wild-type function in only one copy (dose)
 As a general rule, a null mutation is recessive in a haplosufficient gene, and a null
mutation is dominant in a haploinsufficient gene
 Most mutants are recessive to the wild-type allele
2.4 some genes discovered by observing segregation ratios
 The Punnett square is a graphical representation of parental gametes and shows how
they randomly unite to produce progeny genotypes, from which phenotypes ratios of
the progeny can be decuced
 A dominant mutation in the heterozygous state will be expressed. A cross between
heterozygous dominant and wild type parents will result in a 1:1 phenotypic ratio in
the progeny
 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