Download Monohybrid Cross

Mendel’s Methods:
Monohybrid Cross
 Mendel investigated whether the
white-flowered form disappeared
entirely by breeding the F1 purple
flowers with each other.
 Crossing two purple F1 monohybrid
plants is called a monohybrid cross.
 The resulting offspring were called the
F2 generation and surprisingly didn’t
breed true. Some were purple, while
others were white.
 The reappearance of the white form
 The F1 plants are considered monohybrids.
 Monohybrids: Hybrid offspring of two true-breeding parent
plants differing in phenotypes of only one trait.
 In this case, the trait is flower colour and the different parent
phenotypes were purple and white.
raised the question of how a form
can disappear from parent (P) to
offspring (F1) but then reappear in
the next generation (F2).
 In looking at the large datasets
resulting from his monohybrid crosses,
Mendel found that approximately
three-quarters of the offspring took
one form and approximately onequarter took the other form, giving a
three-to-one ratio (3:1).
 This ratio was consistent in the
monohybrid crosses for each of the
seven traits Mendel examined.
 Mendel's careful record-keeping and patience in repeating the crosses were critically important for his
resulting dataset.
 For example, in the F2 generation of an experiment examining flower colour, there were 705 purple-flowered
and 224 white-flowered offspring (Figure 3). Likewise, in an experiment examining seed color, he found 6,022
plants with yellow seeds and 2,001 plants with green seeds.
 If Mendel had not bred so many plants and examined so many different traits, he might have missed a
central discovery. Due to these large datasets, he could begin to observe patterns in proportion and
variability in pea traits across generations.
Tiffany Lim’s BIOL1020 Week 9-12 Lecture Summaries + Diagrams
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BIOL1020 Week 10 Lecture Note Summary
Lecture 18- Mendel’s 2nd Law-Independent Assortment
08.10.14
Objectives:





The difference between an allele and a gene
The principle of independent assortment of alleles
How Punnett squares can be used to predict outcomes of complex crosses
The concept of linked genes
How recombination breaks linkage (except in special cases)
Important Message:
 Independent Assortment - Alleles of genes on different chromosomes segregate independently of one
another
 Some traits are sex-linked – on X or Y chromosomes
 Mendel’s Conclusion: Genes on the same chromosomes will assort independently to gametes, if they are
far apart along the chromosome.
Difference Between Genes and Alleles
 A gene is a unit of hereditary which
codes for a specific trait.
 All individuals of a species have the same genes (where
variation occurs due to gene mutation), but the variants of
the same genes are the alleles. Even if there were no
mutations, still many combinations would result to produce,
still, many variations.
 A single amino acid difference could create a different
protein and/ or function = different allele. Eg. Purple and
white flowers.
 An allele is a variant or alternative form
of the same gene.
 Variation between individuals results from different
combinations of alleles.
 Assuming there are 30 000 genes with 2 alleles per gene,
30 000
there would be 2
possible unique gametes
10000
10
~ 10
>>> 10 people on Earth
Mendel’s Laws
1. Law of Segregation
2. Law of Independent Assortment
1. From last lecture: Alternative versions of genes account for
variations in inherited characters (different alleles confer
different traits). Individuals inherit 2 alleles for each
character, one from each parent and alleles segregate in
progeny  does not result in a blend
2. This lecture: Alleles of different genes assort
independently of each other (2 or more loci combine – what
happens on 1 locus doesn’t mean it happens at another
gene locus)
Mendel’s Conclusions so far
 He observed that the progeny of ALL
possible phenotypes resulted when he
self-fertilised a plant that was
heterozygous for any two of following
traits (see pic) and that F2 generation
recovered all traits.
He concluded that:
 Each gene controls a particular trait:
Eg, Gene A controls flower colour, Gene B controls seed
colour
 Each gene is inherited by progeny independently of the
other. This was true for all 7 of the genes he studied.
 From these 7, he concluded that ALL genes were inherited
independently of each other – this was the case for many
genes but not all
Tiffany Lim’s BIOL1020 Week 9-12 Lecture Summaries + Diagrams
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Incomplete Dominance
 When the resulting offspring looks like
neither of the parents but is a blend of
the two. Neither allele is completely
dominant over the other.
Co-dominant
 When heterozygous, both alleles will
be expressed equally but would not be
a blend of the two alleles.
Overdominance
 Occurs when a heterozygote expresses
a phenotype that is more extreme than
either parent.
 Plant breeders have long known that a hybrid cross of two
heterozygous strains of crops can sometimes result in
offspring that produce a higher yield than their truebreeding parents.
 Example: In nature, the size and number of fruits a plant
makes in any given time is constrained by the balance
between allocating resources to reproduction (fruits)
versus allocating resources to survivorship (leaves).
 On the left is the wild type, which balanced flower
production and plant growth.
 A mutation in this gene shifted the balance and
heterozygous individuals with one functional/ normal copy
of this gene, shifted away from growing leaves towards
growing fruit and had a higher yield than the wild-type
(homozygous for non-mutant allele).
 However, plants that were homozygous for the mutant allele
overproduced leaves and allocated less to fruit production
than the wild-type plants.
Tiffany Lim’s BIOL1020 Week 9-12 Lecture Summaries + Diagrams
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The following are diagrams which I drew to review on the day of the BIOL1020 Final Exam.
Tiffany Lim’s BIOL1020 Week 9-12 Lecture Summaries + Diagrams
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Tiffany Lim’s BIOL1020 Week 9-12 Lecture Summaries + Diagrams
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