Download Part 3 – Theoretical Genetics

Part 3 – Theoretical Genetics – Introduction and Monohybrid Crosses
Assessment Statement
4.3.1 Define genotype, phenotype, dominant allele, recessive allele, co-dominant
alleles, locus, homozygous, heterozygous, carrier and test cross
10.1.4 State Mendel’s Law of Independent Assortment
10.1.5 Explain the relationship between Mendel’s law of independent assortment
and meiosis
4.3.2 Determine the genotypes and phenotypes of the offspring of a monohybrid
cross using a Punnett grid / square
Based upon how organisms look, we can deduce what genes the zygote developed from.
In theory, each organism should have a set of genes from the male and a set of genes
from the female. We all think this is new, done by scientists with computers and fancy
equipment. Theoretical Genetics was actually done by a monk!! (You know, the
chanting guys.)
Gregor Mendel was the first geneticist. The Austrian monk first looked at bees and
noticed that there was some variation in how they looked. He wondered what controlled
these characteristics, or traits.
He turned his curiosity to garden peas. He bred pea plants for several years until he
found 7 traits that occurred generation after generation. This laid the groundwork for the
Chromosomal Theory of Inheritance.
Some of the 7 Traits were:
 round vs. wrinkled
 yellow vs. green
 long stemmed vs. short stemmed
 red flowers vs. white flowers, etc.
Mendel and Theoretical Genetics
Terms to Know
Genotype –
Phenotype –
Homozygous –
Heterozygous –
Dominant Allele –
Recessive Allele –
Co-dominant Allele –
Carrier –
Test Cross – testing a heterozygous genotype versus a homozygous genotype, by
theoretically mating them, called crossing over.
Chromosomal Theory of Inheritance
Based upon the traits that Mendel saw in his pea plants, he labelled some dominant and
some recessive.
Since he crossed the plants with each other, and saw the traits exhibited, he developed
some laws to explain his findings.
Law of Segregation
The pea plants contain 2 heredity factors for each trait. When the gametes form, the two
factors segregate and pass into separate gametes.
The gametes fuse, and the zygote will receive one of each factor from each parent. The
factors remain distinct and re-segregate unchanged when new gametes are formed.
We now know that these factors are called genes, and each homologous pair has the gene
on the same loci. Since the chromosomes go through synpasis (pairing) and separtation
(disjunction), one of each of the chromosomes ends up in a gamete.
Principle of Independent Assortment
If several genes are involved in a cross, they sort themselves out independent of one
another. When they combine, any one of the pair can combine with one of the other pair.
For example:
Since any combination of chromosomes is possible in Metaphase I, any on of a pair of
characteristics may combine with either one of another pair.
Eg. Pea Plants
gene: shape of pea
gene: colour of pea
alleles:wrinkled, round
alleles:yellow, green
When crossing, which we will go over in a bit, two plants that are heterozygous for both
traits (genes), the offspring will show all combinations: green-round, yellow-round,
yellow-wrinkled, and yellow-round. This shows that the genes for shape and colour
inherit independently.
Examples of each as related to Meiosis – See Worksheets
Test Crosses in Theoretical Genetics
How do we get the above? We need to cross the gametes and see what we get. We will
use Mendel’s Monohybrid Cross for pea plants as an example.
Monohybrid – investigation of the inheritance of single contrasting characteristic
There are several parts to a test cross. As we saw earlier, in our definitions, the terms
will be used now.
1. Constructing a Punnett Grid
 A Punnett Grid or Square, is used to find the ratio of the offspring, given parental
phenotypes.
 It is like the multiplication tables you did way back when.
2. Figuring out the parental phenotypes and genotypes. (P)
 Plants showed two phenotypes, tall and dwarf.
 Mendel found that tall plants were dominant and dwarf plants were recessive,
because when the plants were self-fertilized, they produced all tall plants.
Therefore, the homozygous tall plant was either TT, or Tt (genotypes). The
homozygous dwarf plants had to be genotype (tt). What would be the possible
gametes?
3. First Filial Offspring (F1) from the cross of parental (P)
 Mendel found that the offspring were all tall plants. This lead him to conclude
that tall was a dominant phenotype, but the plants also had to have some of the
characteristics of the dwarf plants also.
 He concluded that the genotypes were Tt, based upon the original parental
genotypes.
 Could Mendel have made an error here?
4. Second Filial Offspring (F2) from the self-pollination of the F1 generation.
 Mendel found that when the offspring were crossed, the majority of the F2
generation was tall, but there were some dwarf plants as well. The ratio was 3:1.
 He was able to conclude that the dominant trait was the tall, and those tall plants
had a genotype of Tt or TT, depending on the gametes that fused. In the case of
the dwarf plants, they cannot have T as a genotype, as the dwarf phenotype is
recessive, or t. Therefore, all dwarf plants must be tt for their genotype.
Mendel’s Test Cross
Mendel’s Conclusion from the Monohybrid Test Cross
1.
2.
3.
4.
5.
6.
Within an organism, there are breeding factors controlling characteristics such as
“tall” and “dwarf”
There are two factors in each cell.
One factor comes from each parent
The factors separate in reproduction and either can be passed on to an offspring
The factor for “tall” is an alternative form of the factor for “dwarf”.
The factor for “tall” is an dominant over the factor for “dwarf”.
Steps to solving Genetics Problems 1 – Monohybrid Cross
1.
2.
3.
4.
Figure out the genotypes of the parents
Figure out the kinds of gametes the parents can produce
Set up a Punnett Grid for your mating
Fill in the babies inside the table by matching the egg allele at the top of the
column with the sperm allele at the head of the row.
5. Figure out the genotypic ratio for your predicted babies
6. Figure out the phenotypic ratio for your predicted babies
7. Answer the questions you have been asked
The Problem
In gerbils, there is a gene for fur colour. One allele produces brown fur, the other
produces black fur. The brown allele is completely dominant to the black. Honey and
Toast, two brown gerbils, are mated. In their first litter, of babies, one was black. If they
were to produce a large number of babies, what percentage would you expect to be
brown?
The Solution
Practice Problems
1. What kinds of offspring result when pea plants with heterozygous green pods are
crossed with pea plants with yellow pods?
1. In humans, having dimples (D) is dominant. If two parents are homozygous for
dimples, what is the probability that their child will also have dimples?
2. In humans, straight hair is recessive (w) to wavy hair. What is the probability that
two straight-haired parents will have a child with straight hair?
3. Using the same two parents as in #3, what is the probability that they will have a
child with wavy hair?
4. A women with attached earlobes (f) mates with a man homozygous for free
earlobes (F). What percent of their children would have free earlobes and what
percentage would have attached earlobes?
5. In humans, long eyelashes are dominant (L) to short. If two parents are
heterozygous for eyelash length, what are the possible phenotypes and genotypes
of all offspring?
6. In squash, white fruit is dominant (W) to yellow fruit. What are the possible
genotypes and phenotypes when a heterozygous white squash is crossed with a
yellow squash?
7. Spotted coat is dominant in rabbits and the solid coat is recessive. What are the
genotypes and phenotypes of offspring from a solid-coat rabbit and a
heterozygous spotted-coat rabbit?