Download monohybrid cross

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Inheritance
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Single genes—monohybrid
crosses

To illustrate the first of Mendel’s Principles of Inheritance we
will consider the simplest case—observing the inheritance
pattern of a single gene with two alleles coding for a
particular trait.

This involves conducting a monohybrid cross (‘mono’
meaning single); that is, a cross between organisms that are
heterozygous at a single genetic locus, for example, eye
colour in blowflies and flower colour in snapdragons.
+ The type of experiment that Mendel carried out,
investigating just a single characteristic, is called a
monohybrid cross

There are two alleles controlling pea shape. This means there
are three possible genotypes that the F2 generation of
plants could inherit, leading to two possible phenotypes.
Genotype
Phenotype
homozygous dominant
SS
smooth
homozygous recessive
ww
wrinkly
heterozygous
Sw
smooth
The likelihood of a trait being produced
during a monohybrid cross can be
mapped out using a Punnett Square.
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After his research, Mendel
proposed two laws of inheritance.

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Mendel’s first law: the law of segregation

Alternate versions of genes (alleles) cause variation in inherited
characteristics.

An organism inherits two alleles for each characteristic – one
from each parent.

Dominant alleles will always mask recessive alleles.

The two alleles for each characteristic separate during gamete
production.
Mendel’s second law: the law of independent assortment
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Genes for different characteristics are sorted independently
during gamete production.
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A chance event
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About 1 in 2500 people suffer from a genetic disorder called
cystic fibrosis. What is the chance of particular person
getting it and why is it important to know?

Since the identification of the defective gene in 1989, the
DNA of parents can be analysed to find out if they are one of
the one in 20 that carry the defective gene.

For parents who are carriers for CF, the chance of each child
suffering from the disorder is at least one in 4.
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Analysing pegigree charts and punnet squares can help in
identifying the chances of a parent passing on such defective
genes.
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White eyes—autosomal dominant
inheritance

Consider the consequences of meiosis for the inheritance of
white eye colour in the Australian sheep blowfly. The
production of gametes for each of the genotypes shown
below.
+  Note that homozygous genotypes produce only one type of
gamete: WW individuals produce only W gametes and ww
individuals produce only w gametes.

Therefore, in a cross between WW (red-eye) individuals, all
of the progeny would be WW (red eye).

As long as WW individuals were crossed together, it would
be a pure-breeding strain. Being homozygous in every
generation, it would produce individuals that were
genetically (WW) and phenotypically (red eye) identical.

Similarly, crosses between ww (white eye) individuals would
yield a pure-breeding ww (white-eye) strain.
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F 1 generation

These two pure-breeding strains can be used to parent a new
generation, the F1 generation.

The F1 generation is the result of a cross between two purebreeding strains—in this example WW and ww— and has the
genotype Ww (heterozygous).

Remember that by observing the phenotype of these
heterozygous individuals, we know that the red-eye
phenotype is dominant over the white-eye phenotype.
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F 2 generation

The heterozygous F 1 individuals can now be crossed
together to produce the F 2 generation.

This is a monohybrid cross. Heterozygotes (Ww) produce W
and w gametes in equal frequency. Given that each parent
produces two types of gametes, and gametes fuse randomly
at fertilisation, there are four possible combinations of
gametes.

The eminent geneticist, R.C. Punnett, devised a simple
method of showing the random combination of gametes and
the genotypes of the resulting offspring: the Punnett square.
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
The diagram shows the gametes produced by
the F 1 (Ww) individuals and the genotypes
and phenotypes of the resulting F 2
generation. A 1 : 2 : 1 ratio of the genotypes
WW : Ww : ww is observed in the F
generation.

The 1 : 2 : 1 ratio occurs because:
 in meiosis, heterozygous (Ww) individuals
(both male and female) produce gametes in
a 1 W : 1 w ratio.
 This is referred to as the Principle of
Segregation. The W and w alleles are
segregated (i.e., separated) into different
gametes.
 Fertilisation occurs at random. E.g., a W
sperm has equal chance of fertilising a W
egg or a w egg given that these eggs are
produced in equal frequency.
 The Punnett square takes both of these
factors into account. In the case of blowfl y
eye colour, the 1 WW : 2 Ww : 1 ww
genotypic ratio yields a 3 : 1 red-eye (wildtype) : white-eye phenotypic ratio due to the
dominance of the red-eye phenotype. (b)
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In summary, a 3 : 1 phenotypic ratio will be observed in the F
2 generation for any trait whenever the following four
conditions apply:
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the variation in the trait (in this example, white versus red eye) is
controlled by a single gene
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the gene is on an autosome
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there are two alleles of the gene (in this example, W and w)
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one phenotype is dominant (in this example, red).
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Pedigree charts

Go through for drawing a pedigree chart on page 232 of your
textbook.
+ How to read a pedigree chart?
Can you see the trait in each
generation of the family in
which it occurs?
No
Yes
Sex
linked
recessive
Do the males mainly
show the trait?
Do daughters who
show the trait have
fathers with it also?
No
Autosomal
recessive
Yes
Do the males mainly
show the trait?
Does the trait only
pass from father to
son?
No
On the
Y
chrom
osome
Yes
Do all of the females and
none of the sons show the
trait when the father shows
the trait and the mother
does not?
No
Autosomal
dominant
Yes
Sex linked
dominant
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Do the males mainly show the
trait?
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Do daughters who show the trait
have fathers with it also?
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Do the males mainly show the
trait?
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Does the trait only pass from father
to son?
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Do all of the females and none of
the sons show the trait when the
father shows the trait and the
mother does not?