BL414 Genetics Spring 2006
Lecture 3 Outline
January 25, 2006
3.3 Segregation of Two or More Genes
If you cross two or more genes at a time, given that the genes are on different
chromosomes, or unlinked (which we’ll explain later) – the principle of
segregation still holds.
Looking at second genetic locus for pea color: G – yellow, dominant; g-green,
recessive
For a dihybrid cross:
P1 WWGG x wwgg
F1  all WwGg, all round and yellow phenotype
WwGg x WwGg
Phenotype:
F2  Round, yellow
Round, green
Wrinkled, yellow
Wrinkled, green
No. of offspring
315 peas
108 peas
101 peas
32 peas
Phenotypic ratio: 9:3:3:1
Round: wrinkled ~3:1
Yellow:green ~3:1
Looking at the F2 genotypes:
1/16 WWGG (Round, yellow)
2/16 WWGg (Round, yellow)
1/16 WWgg (Round, green)
2/16 WwGG (Round, yellow)
4/16 WwGg (Round, yellow)
2/16 Wwgg (Round, green)
1/16 wwGG (Wrinkled, yellow)
2/16 wwGg (Wrinkled, yellow)
1/16 wwgg (Wrinkled, green)
Ratio
9/16
3/16
3/16
1/16
Principle of independent assortment of genes* (*genes on different
chromosomes, or unlinked genes) :
Segregation of the members of any pairs of alleles is independent of the
segregation of other pairs in the formation of gametes.
Testcross for dihybrid F1:
WwGg x wwgg
 1:1:1:1 of four different phenotypes, 1:1 for each gene locus
Trihybrid cross – 3 genes – principle of segregation and independent assortment
still holds!
P1 WWGGPP x wwggpp
F1 WwGgPp x WwGgPp
F2 phenotypic ratio is 27:9:9:9:3:3:3:1
From segregation of gametes perspective – 8 different gametes can be formed,
which associated randomly – building an 8x8 matrix would show all the possible
genotypes. Your book creates a 3x3x3 cube to see the possible genotypes, which
also works.
3.4 Human pedigree analysis
In humans and animals there is a lower number of offspring per generation (and
a much longer generation time for humans).
 greater deviation from expected ratio’s for allele segregation
Looking at several generations of mating and offspring can demonstrate
segregation and give clues about whether a particular gene is dominant or
recessive.
Pedigree: a diagram of a family tree showing the phenotypes, sexes and
relationships of individuals in multiple generations.
Penetrance is the degree to which a genotype will express a trait. Complete
penetrance of a gene means that all individuals with the genotype will express
the trait.
In a pedigree for a gene that is a rare dominant allele with complete
penetrance:
1) Females and males are equally likely to be affected.
2) Affected offspring have one affected parent.
3) About half of individuals in a sibship with an affected parent are also
affected.
In a pedigree for a gene that is a recessive allele with complete penetrance:
1) Males and females are equally likely to be affected.
2) Affected individuals usually have unaffected progeny.
3) Most affected individuals have unaffected parents.
4) The parents of affected individuals are often relatives.
5) Among siblings of affected individuals the proportion affected is about
25%.
Carrier: heterozygous for a recessive gene.
Consanguineous: related (“same blood”)
Pedigrees can be created using molecular data on a genotype, using a genetic
marker, which could be a DNA polymorphism such as SSRP simple sequence
repeat polymorphism. Molecular data could come from a Southern blot type of
experiment using a probe complementary to a particular sequence or using DNA
that is amplified by PCR using gene-specific primers.
Codominance: the expression of both alleles in a heterozygote.
Codominant pedigree characteristics:
1) Heterozygous genotypes can be distinguished from homozygous
genotypes.
2) Many individuals in the population are heterozygous; many matings
show segregation.
3) Each segregating genetic marker yields up to four distinguishable
offspring genotypes.
3.5 Probability
The event of fertilization involves the chance combination of two haploid
gametes, and therefore the chance combining of two alleles. Therefore we can
calculate the probability of different outcomes based on our knowledge of the
cross – we need to know the starting genotypes.
When we use probability, we are speaking of the outcome of an event. It
designated what the expected outcome is – for a genotype, say “ww,” that has a
¼ probability in a given cross, we expect that for a large number of offspring we
will get close to a ¼ proportionality of offspring with “ww,” perhaps 100 ww
out of 400 total offspring.
Mutually exclusive:
You can only have one or the other, not both. To get the probability of either
event occurring, add their individual probabilities.
Independent possibilities:
One event does not affect the outcome of the next event. Just because you have
rolled a dice ten times and never got a 4, there is no greater chance that you will
get a 4 on the next roll – the chances are still 1/6 every time.
3.6 Incomplete Dominance: the phenotype of the heterozygous genotype is
intermediate between the phenotypes of the homozygous genotypes
 you see a 1:2:1 ratio of phenotypes
Multiple alleles
Mendel looked at cases where there were two alleles for a gene, but in nature,
there are often many more than two alleles of a given gene in a population. At
the sequence level, there could theoretically be 3 x n different sequences of a
gene, if n is the number bases in the coding sequence. However, many of these
possible sequences have not occurred or have been selected against in nature.
Remember that an individual can have only one or two different alleles of a gene.
Epistasis
An interaction between two different genes that perturbs the normal
Mendelian ratios
The C and P genes in peas are epistatic, because they are both required for
production of the purple pigment in the pea flower. Epistasis makes some of the
resulting offspring have indistinguishable phenotypes and the normal 9:3:3:1
ratio is not seen. For CcPp x CcPp  offspring are 9:7 purple: white
Different kinds of epistasis. E.g. in Labrador retriever, the gene for yellow coat
color is epistatic to the gene for black or chocolate coat color. A cross between
labs who are heterozygous for these two genes are expected have a 9:4:3 ratio of
black:chocolate:yellow coat color.
3.7 Complementation test
Mutant screen: isolation of a set of mutants with a given phenotype, often
resulting from intentional mutagenesis of organisms by irradiation or exposure
to chemical mutagens
To determine if two new recessive mutants are alleles of the same gene or
different genes, a complementation test is done – this is simply done by
breeding the homozygous strains to each other.
If the offspring of a1a1 x yy are also mutant in phenotype, the alleles did not
complement each other  the result is noncomplementation, and the alleles are
for the same gene. yy is actually a2a2 and the offspring’s genotype is a1a2
If the offspring of aa and yy are nonmutant in phenotype, then the genes
complemented each other – complementation occurred indicating that they are
two different genes. Each organism had a dominant allele for the other gene, so
the offspring were heterozygous and dominant in phenotype (AaYy).
The principle of complementation: “If two recessive mutations are alleles of
the same gene, then a cross between n homozygous strains yields F1 progeny
that are mutant (noncomplementation); if they are alleles of different genes,
then the F1 progeny are wildtype (complementation).
Complementation group: a groups of mutations that do not complement each
other, i.e. they are all alleles of the same gene