Sarah Margareta Ibrahim▯ Monday, January 14th 2012
BIOL 202 - Basic Genetics
Lecture 4 - Multiple Gene Inheritance I
Reading: Ch. 3 p. 95-104
Mendelʼs Single Gene Traits
• Weʼve been talking about Mendel and Mendelʼs work and Mendel didnʼt just look at one trait.
• When we studied Mendelʼs ﬁrst law we considered that he looked at many different traits (he
looked a seven in particular).
The repeat ability of the results that he got from the monohybrid cross (the 3:1 ratio in the F2
• Mendel didnʼt stop there though, he looked at pairs of traits and thatʼs what weʼre going to look
• This led to the development of a a second law called the law of independent assortment
• When we talked about segregation (the ﬁrst law), we put it into a context of medical genetics.
This is also true of that law that weʼre talking about today but there are relatively few genetic
diseases that are coded for by two separate or three separate loci that we can follow the
segregation pattern into the next generation. The diseases that weʼll be talking about that
have multi-gene basis are really actually controlled by many genes and at that point it
becomes very difﬁcult to use simple Mendelian segregation - so weʼll have to use the
machinery of quantitative genetics (will talk more about this in upcoming lecture).
• Agricultural genetics however, has used Mendelʼs second law in a big way.
Practical Application: Assembling useful genes in modern crop varieties by breeding
• Breeders, prior to Mendelʼs work, were selecting a crop and selecting from within that crop
to improve it
• After Mendelʼs work (particularly after the discovery of the independent assortment rule), it
became clear to breeders that they could combine traits into a single variety and pick and
choose the ones that they want
• Maybe you have a crop in which you want to breed in a new trait (disease resistance for
• Xa4 - disease resistance, gene codes for resistance to bacterial blight disease in rice
• Snb1 - gene codes for tolerance to ﬂood resistance
• Maybe these two traits exist in different varieties and you want to create a cross in which
you introduce the two traits into a single variety
• One parent may have Xa4/Xa4 (has disease resistance) and the little snb1/snb1 (so it
doesnʼt have ﬂood tolerance). The other parent may have Snb1/Snb1 (so it has ﬂood
tolerance) but it doesnt have disease resitance (xa4/xa4)
• So you do a dihybrid cross and create something that is heterozygous for both of those
traits so youʼll have Xa4/xa4 and Snb1/snb1 as the child (F1) and if itʼs dominant the child
(F1) will demonstrate both disease resistance and ﬂood tolerance.
• The problem with this is that the F2 generation might not show the trait. What will happen is
independent assortment - when it selfs some of the progeny may end up with the traits but
some may not.
• Solution: You could self them and select the ones that have the traits that you want and
eventually youʼll end up with an inbred line that completely homozygous for Xa4/Xa4 and
Snb1/Snb1 and youʼll have a commodity that always expresses the trait.
▯ 1 Sarah Margareta Ibrahim▯ Monday, January 14th 2012
• It obliges the farmer to buy the seed from a company every year because if the farmer just
allows the seed to self theyʼll end up with an F1 line that may not be what they want so they
have to keep going to the seed company
• This practice of combining traits led to the green revolution.
• In the 1950s the breeders began to implement Mendelʼs second law of independent
assortment and create inbred lines. This combing of different traits of different varieties meant
that the yields of crop companies have doubled or tripled.
• As climate changes, some of the characteristics are no longer valuable so we need to
• What happens when you donʼt have these genes?
• Irish Potato Famine: Only a single genotype was being grown in Ireland and it was susceptible
to a certain disease (potato blight) which caused the famine because the potatoes died out
• Southern corn leaf blight was another incident in the state where it died out
• Bananas are currently on their way out (Cavendish Banana blight - the bananas are very
homogenous in their makeup)
Svalbard Seed Vault
• In Norway thereʼs an underground vault of seeds in case the world ends.
• Thereʼs like 50 million seeds stored.
• Itʼs always cold in Norway.
• Just incase a species fails, we have a reservoir
Independent Assortment: More of Medelʼs peas
• In picture:
• Yellow and round (top)
• Yellow and wrinkled (second from bottom on the right)
• Green and smooth (third from top on left)
• Green and wrinkled (fourth from top on left)
• This was in a single pea pod that came from a dihybrid cross
▯ 2 Sarah Margareta Ibrahim▯ Monday, January 14th 2012
• The above image is the cross that led to that pea pod.
• In the ﬁrst generation we have a parents that are true breeding for two traits (green or
yellow and round or wrinkled).
• Obviously, each of these can produce can only each produce a single gamete type and
when we cross the two we get a dihybrid (also called a double heterozygote)
• This is exactly what Mendel predicted for single traits - the expression of one trait doesnʼt
affect the expression of another trait.
• The inheritance pattern of round versus wrinkled doesnʼt seem to affect the inheritance of
yellow versus green.
• Will come up with a 9:3:3:1 ratio
What Mendel concluded was that we have two factors (one from each parent) and half of the
time big R will assort with big Y and half the time it will assort with little y. Also, half of the time,
little r will assort with big Y and half the time it will assort with little y. Like this: