Patterns of inheritance:

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Patterns of inheritance
Mendel and beyond
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(No Transcript)
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Contrasting characters in peas
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Experiment with garden peas I
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In this case a true-breeding tall plant was
crossed with a true-breeding short plant. All
of the plants in the next generation were tall.
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These are the results Mendel obtained when he
crossed two heterozygotes.
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Paint-pot theory of inheritance
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Mendels finding was contrary to the prediction
of the Paint-pot theory of inheritance
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Mendels 1st law-law of segregation
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Mendels law of segregation -explanation of
monohybrid ratio

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Mendel described patterns of inheritance in the
1860s, but it wasnt until the early 1900s that
inherited traits, genes, were linked to cellular
structures called chromosomes. The number of
chromosomes varies among species, but all
chromosomes contain genes arranged linearly at
specific locations, called loci.
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This is how geneticists today represent the cross
from the previous screen. The original plants are
the P generation and their offspring are the F1
generation. The T and t symbols represent
dominant and recessive alleles of a single gene.
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we breed the offspring to one another in the
cross Tt x Tt, and wish to predict the results
we need to follow the production of gametes
during meiosis. These individuals make gametes
with either T or t in equal numbers.
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The chance of gametes of different types
encountering one another is represented on a
Punnett square. The genotypic ratio of
homozygous dominant heterozygous homozygous
recessive individuals is 121.
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Many genetics problems will fall in the category
of a monohybrid cross. They all can be approached
using the model above
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Testcross. a heterozygote was crossed with a
homozygous recessive individual. A situation in
which an individual of dominant phenotype, but of
unknown genotype, is crossed with one or more
recessive individuals. This can provide
information on the unknown genotype.
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Dihybrid cross
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Dihybrid cross
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Dihybrid cross
The inheritance of two contrasting characters
were considered at the same time
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Law of independent assortment
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Mendels 2nd law
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Mendels 2nd lawexplained byobserving movement
of chromosomes at meiosis
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Cell division
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The cell cycle
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Interphase
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Interphase
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Mitosis division of the nucleus
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Prophasemetaphaseanaphasetelophase
Division of the nucleus
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Division of the cytoplasm
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Produce identical daughter cells
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Mitosis and cell cycle
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Meiosis

• Reduction division
• Generate variability in gametes

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Meiosis-Interphase
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Meiosis-prophase I
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Meiosis-metaphase I
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Meiosis-anaphase I
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Meiosis-telophase I
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Meiosis I -animated
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Meiosis II
prophase II
metaphase II
anaphase II
telophase II
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Meiosis II -animated
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Meiosis-overall
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Independent assortment
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Crossing over
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Anaphase I and IInote new gene combinations as
a result of crossing over
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Comparing Mitosis and Meiosis
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Comparing Mitosis and Meiosis
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Incomplete dominance
In some cases, alleles may not interact in a
dominant/recessive pattern. Heterozygotes have an
intermediate phenotype or express both alleles.
The genotypic ratio will match the phenotypic
ratio. Snapdragons demonstrate incomplete
dominance.
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Incomplete dominance II
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Multiple allele
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Linkage and crossing-over
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Recombinant frequency
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Skin colour_a pair of twin sisters
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Continuous or discontinuous
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Polygenic effects on genetic variation
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Polygenic inheritance_skin colour
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Skin colour is a continuous variation because
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How would U explain the very different skin
colour of the twin sisters?
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X-inactivation
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X-inactivation