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Summary Genetics
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A detailed summary of all the information you'll need to complete 1st-year Genetics modules.
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BLGY1232 Patterns of InheritanceKey terms;
• Genotype – the sum total of the genetic information contained in an organism; the
genetic constitution of an organism with respect to one or a few gene loci under
consideration
• Phenotype – the observable characteristics of an individual resulting from the
interaction between the genotype and the environment in which development
occurs
• Locus – the place at which a particular mutation or a gene resides in a genetic map;
often used interchangeably with gene
• Allele – one of a number of alternative forms of a gene, each possessing a unique
nucleotide sequence; different alleles of a given gene are usually recognized,
however, by the phenotypes rather than by comparison of their nucleotide
sequences
• Diploid – a cell, tissue or organism having 2 chromosome sets
• Haploid – a cell, tissue, or organism having one chromosome set
• Homozygous – a cell or organism having identical alleles at given locus on
homologous chromosomes
• Heterozygous – a cell or organism having different alleles at given locus on
heterologous chromosomes
• Dominant – a relationship between 2 alleles where is one is dominant it is expressed
in the phenotype of the heterozygote
• Recessive – the allele that is not expressed in the phenotype if there is a dominant
allele present
• Wildtype – the prevailing phenotypes or the prevailing alleles, if any, in a natural
population
• Mutant – an allele different from a pre-existing one as a result of mutation; or an
individual carrying such an allele
• Self fertilization (selfing) – producing offspring using gametes derived from a single
organism
• Cross fertilization (crossing) – producing offspring by fusing gametes derived from
different organisms
• True breeding/ pure breeding strains – strains of organisms that when mated
randomly among themselves always produce identical progeny as they are
homozygous at the loci under study
• Pleiotropgy – an effect whereby a single mutant gene affects 2 or more apparently
otherwise unrelated aspects of the phenotype of an organism
Mendelian Genetics
• Gregor Mendel first discovered and formulated the laws of inheritance to found the
science of genetics
• Mendel’s first laws – the principle of segregation states that the 2 members of a
gene pair segregate from each other in the formation of gametes → diploid
organisms produce haploid gametes
,• Mendel’s second law – the principle of independent assortment states that members
of different gene pairs are transmitted independently of one another during gamete
production
• Carried out experiments using true breeding strains of the garden pea (pisum
sativum) which showed different phenotypes for 7 different traits;
➢ Flower (purple vs white) and seed coat (grey vs white) colour
➢ Seed colour (yellow vs green)
➢ Seed shape (smooth vs wrinkled)
➢ Pod colour (green vs yellow)
➢ Pod shape (inflated vs pinched)
➢ Stem height (tall vs short)
➢ Flower position (axial vs terminal)
• Results;
➢ When 2 plants with a different phenotype were crossed the offspring were
not intermediate in appearance between that of the parents but they all
looked like one of the parents
➢ When pea plants producing smooth seeds were crossed with plants
producing wrinkled seeds all the offspring of the first filial produced smooth
seeds → smooth phenotype was dominant to the wrinkled phenotype
➢ When the first filial (F1) plants were selfed both the original parental
phenotypes reappeared and that the second filial generation contained some
plants that produced smooth seed and some that produced wrinkled seed →
organisms contain 2 copies of a gene and when gametes were formed this
gene pair become separated one into each gamete → fusion of haploid
gametes to generate a diploid zygote brought together the 2 members of a
gene pair again
➢ For seed shape the true breeding smooth-seeded peas contained 2 identical
copies of a determinant “S” whilst the true-breeding wrinkled seeded peas
contained 2 identical copies of a different allelic variant of this determinant
“s”
• Can be applied to any organisms that reproduce sexually
• Important models for animal genetics are fruit flies (Drosophila melanogaster) and
nematode worms (Caenorhabditis elegans – 60% in common with humans and
tissues) --
• The best plant model in Arabidopsis thaliana
• In humans we have to rely on pedigree analysis as we cannot carry out matings
between siblings or between offspring and parents
• Fungi have been studied because they are small, simple and haploid → easy ot
manipulate
,BLGY1232 Monohybrid crosses, test crosses and back crosses
Naming genes (Nomenclature)
• Gene names are always written in italics
• Dominant alleles are always written first
• Dominant alleles are written in capitals
• Recessive alleles are written in lower case
• Wildtype alleles are often denoted by “+” → the use of “+” for a dominant allele is
OK if you are analyzing the segregation of alleles at a single locus →not all wildtype
alleles are dominant
• When considering a single gene just use letters
• Loci can be referred to by 3 letters that are an abbreviation of the gene name
The yellow mouse
• Wild type is recessive to the yellow coat colour
• Homozygous Y/Y mice are never found as yellow is a lethal gene so zygotes with 2
copies of Y do not survive
• Yellow is a pleiotropic gene as has multiply phenotypic consequences
• Aberrant segregation; All yellow mice must be heterozygous as if they were
homozygous they would die before being born so do not appear in the progeny
therefore then 2 yellow mice are crossed there is a 2:1 yellow to wildtype
Analyzing genetic crosses
• Represent a cross with a Punnett square
• If you start with SS and ss all offspring with have dominant phenotype
• For F2 if you let them reproduce; ¼ SS, ½ Ss, ¼ ss
• When you get 2:1 you are looking at one gene with heterozygous alleles
• Represent crosses using branch diagrams
• F1 tells you which phenotype is dominant
• F2 segregation ratio tells you how many loci involved
• By selfing we can determine the genotypes of F2 as if they are homozygous they will
breed true and if they are heterozygous they will give you a 3:1
• Test cross – a cross between an individual of unknown genotype and an individual of
unknown genotype and an individual that is true breeding for the recessive trait
• Back cross – a cross between an individual and one of its parents
• Test crosses enable us to distinguish between phenotypically identical but
genotypically different individuals – It is between a plant with the dominant
phenotype and the homozygous recessive parent; if the plant was homozygous all
progeny will have the dominant phenotype, is it was heterozygous there will be a 1:1
of dominant phenotype to recessive phenotype
• If blending inheritance occurs all F1 are the same but the parental phenotypes re-
emerge in F2 generation where the progeny segregate in the ration 1AA:2Aa:1aa →
co-dominance/incomplete dominance
, BLGY1232 Dihybrid crosses, independent assortment and multiple alleles
Dihybrid cross
• Used for monitoring 2 traits at the same time
• 9:3:3:1 2 independent loci each represented by one dominant and one recessive
allele – independent assortment
• Number of segregating gene pairs (n) → number of phenotypic classes (2^n) →
number of genotypic classes (3^n) → total number of genotypes (4^n)
• Trihybrid cross looks at 3 independent traits at the same time
• Branch diagrams for numerous traits are easier
• Expected ratios in trihybrid cross for independent loci each represented by one
dominant and one recessive allele; 27:9:9:9:3:3:3:1
• Independent assortment – members of different gene pairs are assorted
independently of one another at gamete formation
Multiple genes controlling same trait
• When 2 parents create a different offspring which creates 4 different off spring in
the formation 9:3:3:1 there must be 2 genes segregating → the offspring of F1
phenotype depends on 2 alleles located at 2 independently assorting loci
• Allelism – different genes at the same locus
• Complementation - If loci are allelic you do not get wild type phenotypes when you
cross individuals homozygous for different recessive mutant alleles of the same gene
→ if they are different loci wild type progeny result → if we cross 2 individuals who
mutate in he same gene you cant get a wildtype
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