Slide 1
To understand genetics and be able to make predictions about
genetic crosses, it is crucial that you understand meiosis. This
diagram, from Tutorial 2, is an overview of meiosis. Make sure that you
understand this diagram and the diagram of independent assortment from
Tutorial 2 before you start to attempt genetics problems.
Slide 2
The most basic example of a genetic cross is a monohybrid
cross. The prefix mono means one - so in a monohybrid cross we are
tracking the inheritance of a single trait. We have already been
introduced to monohybrid crosses when we looked at examples of Mendel's
classic crosses.
When performing a genetic cross, we have a handy tool that we can use called a Punnett square. To create a Punnett square we first need to determine the different gamete types our parent individuals can make with respect to the gene that we are tracking. For example, we now know that the F1 individuals in Mendel's classic cross involving flower color are all heterozygous. Therefore, each F1 individual can make two different possible gamete types with respect to the gene for flower color - gametes with a P and gametes with a p. We construct our Punnett square by lining up these possible gametes along the axes of our square. So a female pea plant can produce two types of eggs - P and p. A male pea plant can produce two types of sperm - P and p. We write these gametes out along the axes of the square and then fill in the squares accordingly.
This Punnett square allows us to visualize the 3:1 phenotypic ratio in the F2 generation. Note, while the phenotypic ratio is 3:1 the genotypic ratio is 1:2:1. 1 homozygous dominant to 2 heterozygotes to 1 homozygous recessive.
The Punnett square allows us to visualize the cross and make predictions about offspring.
Slide 3
When an organism exhibits a dominant trait, we cannot know its
genotype by simply looking at it. For example, this purple-flowered pea
plant - we can't know whether it is homozygous dominant or heterozygous just
by looking at it. But there is a way we can figure out its genotype.
Slide 4
A test cross allows us to determine the genotype of an organism of
unknown genotype. To perform a test cross, it is typical to cross the
individual of unknown genotype (and dominant phenotype) to an individual that
is homozygous recessive. In this example, we don't know whether the
purple flowered individual is homozygous dominant or heterozygoud. But
we do know that a white-flowered individual has to be homozygous
recessive. When we cross our unknown individual with our known
individual - we can tell the unknown individual's genotype by analyzing the
offspring. If all the offspring are purple, our unknown individual has
the genotype PP. If half of the offspring are white and half purple, our
unknown individual has the genotype Pp. Notice, this type of analysis
only works with organisms that have large numbers of offspring.
Slide 5
We can increase the complexity of our genetic analysis by
increasing the number of traits we are tracking. A dihybrid cross tracks
the inheritance of two traits ("Di" means two). To understand this
dihybrid cross, you must understand Mendel's Law of Independent Assortment.
In this example, we are tracking the inheritance of the alleles for seed color (yellow versus green) and seed shape (round vs. wrinkled). The P generation is comprised of true-breeding round, yellow individuals and true-breeding green, wrinkled individuals. The genes for these characters are found on different chromsomes so the genes assort independently of each other.
The F1 generation is all round, yellow - this tells us that round and yellow are the dominant traits (and green and wrinkled are recessive). When we self individuals from the F1 we see a 9:3;3:1 phenotypic ratio in the F2.
Note, in this example each parent can make 4 different possible gamete types (RY, Ry, rY, and ry). This means our Punnett Square is 4 x 4 or 16 squares large.
Slide 6
To understand how to set up the Punnett square - we need to
understand how meiosis and independent assortment create the 4 different
possible gamete types. This image shows a diagram of meiosis in an F1
individual from our previous cross. All F1 individuals are heterozygous
(RrYy). Gene R is found on a different chromosome than gene Y so how
gene R assorts into gametes is independent of how gene Y assorts. In
this example, let's assume that gene R is on the green chromosome and gene Y
is on the blue. There are two possible ways the chromosomes can align
themselves at metaphase I - either light green with light blue and dark green
with dark blue - resulting in RY and ry gametes OR light green with dark blue
and dark green with light blue resulting in Ry and rY gametes.
Therefore, our heterozygous F1 individuals make 4 different possible gamete
types - RY, ry, Ry, and rY.
Slide 7
These are the four gamete types that we line up along the Punnett
Square in our dihybrid cross. Then we fill in the squares (it is best to
keep the Rs together and the Ys together) and determine the phenotypes.