Genetic Inheritance According to Gregor Mendel CHAPTER 9
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Transcript of Genetic Inheritance According to Gregor Mendel CHAPTER 9
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Genetic Inheritance According to Gregor Mendel CHAPTER 9• Overview of Inheritance
• Mendel's Pea Plants
• Monohybrid Crosses and Segregation
• Definitions of Genetic Terms
• Dihybrid Crosses and Independent Assortment
• Test Crosses
• The Role of Probability
• Pedigrees and Genetic Conditions
• Human Single-Gene Disorders
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• Gregor Mendel
– Was the first person to analyze patterns of inheritance.
– Deduced the fundamental principles of genetics.
Heritable Variation and Patterns of Inheritance
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In an Abbey Garden• Mendel studied garden peas
• Pea flowers have both male and female parts
• The ovary of the carpel produces eggs by meiosis
• The anthers of the stamens produce pollen (sperm equivalent) by meiosis
• Pollen grains fly from the anthers of the same flower or from another plant to the carpel and fertilize the eggs in the ovary
• Pea flowers are enclosed within petals such that only insect pollination can effectively transfer pollen from one plant to another
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Figure 9.4
How Mendel Fertilized Pea Flower Eggs and Ensured a Single Pollen Source
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• He also created true-breeding varieties of plants.
• Mendel then crossed two different true-breeding varieties.
• Mendel performed many experiments.
– He tracked several characteristics in pea plants from which he formulated several hypotheses.
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Figure 9.5
True Breeding Varieties Available to Mendel
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Figure 9.6a
A Single-Factor (Monohybrid) Cross
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• Hypotheses From The Monohybrid Cross
– Some alleles mask or hide the presence of other alleles; these are dominant alleles symbolized by a capital letter (eg. P).
– There are two gene forms (alleles) for every characteristic in the plant
– The letters P and p were used to designate flower color: P = purple, p = white
– There are alternative forms of genes called alleles
– Alleles that can be masked are called recessive alleles, symbolized by a lowercase letter (eg. p)
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• Every cell in an organism carries two alleles at a time for every characteristic
– For a pea plant flower color, a plant could have cells with PP, Pp, or pp.
Allelic Combinations: Genotype
• Genotype combinations
– PP is homozygous dominant, yields purple flowers
– pp is homozygous recessive, yields white flowers
– Pp is heterozygous, yields purple flowers
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• Phenotype
– An organism’s physical traits
• Genotype
– An organism’s genetic makeup
• Mendel’s law of segregation
– The two members of an allele pair segregate (separate) from each other during the production of gametes.
Some Definitions
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Following Mendel’s True Breeding Cross
PP pp
P P P P
Meiosis Meiosis
p p p p Gametes:
P P p pOr more simply:
Next, determine all the possible ways the gametes of one parent can combine with the gametes of another:
Fill out a Punnett Square
P
P
p p
Pp Pp
Pp Pp
Genotype of the offspring are all Pp in the ratio:
4 Pp: 0 PP: 0 pp (Genotypic ratio)
Phenotypic ratio of the offspring:
4 purple: 0 white
True breeding varieties are homozygous
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Determining the F2 Offspring
Pp x Pp
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Genetic Alleles and Homologous Chromosomes
• Homologous chromosomes
– Have genes at specific loci.
– Have alleles of a gene at the same locus.
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Mendel’s Law of Independent Assortment• A dihybrid cross
– Is the mating of parental varieties differing in two characteristics.
• Two hypotheses for gene assortment in a dihybrid cross are
– Dependent assortment.
– Independent assortment
• Mendel’s law of independent assortment states that
– Each pair of alleles segregates independently of the other pairs during gamete formation.
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Figure 9.8
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Figure 9.23
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Figure 9.9
Genes for Coat Color and Vision Sort Independently into Gametes
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Using a Testcross to Determine an Unknown Genotype
• A testcross is a mating between
– An individual of unknown genotype and a homozygous recessive individual.
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The Rules of Probability• The rule of
multiplication states that
• The probability of a compound event is the product of the separate probabilities of the independent events.
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Figure 9.12
Human Characteristics That Follow Mendel’s Laws
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• A family pedigree
– Shows the history of a trait in a family.
– Allows geneticists to analyze human traits.
Using a Pedigree to Follow an Autosomal Recessive Gene
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Human Disorders Controlled by a Single Gene• Many human traits
– Show simple inheritance patterns.
– Are controlled by genes on autosomes.
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Recessive Disorders
• Most human genetic disorders are recessive.
– Individuals can be carriers of these diseases.
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Dominant Disorders
• Some human genetic disorders are dominant.
– Achondroplasia is a form of dwarfism.