Download Genetics Notes #4

Genetics Notes #4
Dihybrid Crosses and
Polygenic Traits
A dihybrid cross involved two genes that each have more
than one allele
Dihybrid Crosses and
Polygenic Traits
Mendel performed dihybrid cross experiments to see if his
principle of dominance still worked
In his pea plants when looking at colour and shape, it did
but he could see there was more than one gene involved
Independent Assortment
Through his research, Mendel ended up with two laws:
1. Law of Dominance (which is what we have been talking
about)
2. Law of Independent Assortment
Independent Assortment
The law of independent assortment states that genes that
are located on different chromosomes move
independently
Dihybrid Cross
To create a Punnett square for a dihybrid cross, you need to
include one allele for both of the genes in the possible
gametes:
The F1 generation
in an experiment
performed by
Mendel:
Dihybrid Cross
Mendel then produced an F2 generation by allowing selffertilization. He recorded all of the phenotypes and
calculated the ratio.
He ended up with 9/16 yellow, round seeds, 3/16 green,
round seeds, 3/16 yellow, wrinkled seeds, and 1/16 green,
wrinkled seeds
This is known as the 9:3:3:1 phenotypic ratio that occurs
when you cross 2 heterozygous individuals with two alleles
Probability of Dihybrid Crosses
P = # of ways a given outcome can occur
total # possible outcomes
Probability values can be used to predict the chances of
getting a particular genotype or phenotype in an offspring
but in dihybrid crosses, we are looking at the probability
that two outcomes will occur at the same time
Probability of Dihybrid Crosses
Example:
Look at human ear lobes and hairlines .
Free ear lobes are determined by the dominant allele E
Attached ear lobes are determined by the recessive allele e
Widow’s peak hairline is determined by dominant W
Straight hairline is determined by recessive w
Probability of Dihybrid Crosses
Suppose a man with genotype EeWw and a female with
genotype EeWw are expecting a child.
What is the probability that the child will have a straight
hairline and attached ear lobes?
To achieve this phenotype, they will need the genotype
eeww (completely recessive in both genes)
Probability of Dihybrid Crosses
EeWw x EeWw = ?
Since the genes are on separate chromosomes, they will
assort independently.
The first step is to determine the probability of each of the
outcomes separately by using a separate Punnett Square
for each gene
Probability of Dihybrid Crosses
1/2
E
1/2
1/2
W
e
1/2
w
1/2
1/4
EE
1/4
Ee
1/2
1/4
WW
1/4
Ww
1/2
1/4
Ee
1/4
ee
1/2
1/4
Ww
1/4
ww
E
e
W
w
These squares show that the child has ¼ probability of getting attached ear
lobes and a ¼ probability of getting a straight hairline.
Probability of Dihybrid Crosses
We then take the probabilities from each independent
cross and multiply them so that the probability will signify a
dihybrid cross
¼ x ¼ = 1/16
The child in this scenario would have a 1/16 chance of this
phenotype.
Practice
Your turn.
Work in small groups to complete the following questions:
Page 616
#1 a-c
Selective Breeding
Selective breeding is the process of crossing desired traits
from plants or animals to produce offspring with those
traits
Examples of selective breeding are very common on farms
- Corn - most kernels with specific taste
- Wheat - more seeds
- Cows - stronger, more meat
- Pigs - stronger, more meat
Selective Breeding
Purebred individuals have very closely regulated genotypes.
There is a large amount of inbreeding that occurs in these
individuals due to the lack of individuals with the desired
phenotypes.
This inbreeding leads to a very genetically stagnant
population with very low variability
Selective Breeding
New varieties of plants and animals can be produced
through a process called hybridization
This technique attempts to blend certain characteristics
Hybrids tend to be more vigorous than their parents
Polygenic Traits
These are inherited characteristics that are determined by
more than one gene
Skin colour, eye colour, and height are all examples
Polygenic traits have a lot more variability in populations
The genes can show incomplete dominance, codominance,
and can be affected by the environment
Polygenic Traits
These types of traits can make breeding very difficult
In some cases, two different genotypes interact to produce
a phenotype that they are incapable of creating themselves
In other cases, one of the genes will interfere with the
expression of the other, masking its effects - this is known
as an epistatic gene
Questions
Page 623-625
#1-7, 9, 12, 14, 15, 16, 21, 23, 24, 25, 26