Transcript The Basis of Heredity

The Basis of Heredity
Inheritance and Meiosis
Definitions
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Genetics = study of genes, the units on
chromosomes that code for traits
Heredity = study of how traits are passed
on
The Inheritance of Traits:
Mendel’s Discoveries
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Czechoslovakian
monk
First to discover rules
of inheritance
Published work in
1865
Mendel’s
Hybrid Peas
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Pure lines: tall plants
made tall plants
Studied 7 contrasting
characteristics
Mendel’s Crosses
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Mendel looked at one trait at a time by
crossing inbred lines.
Hybrid = offspring produced by breeding
two pure lines.
P1 = purebred parents.
F1 = first generation of offspring.
F2 = second generation of offspring.
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P = parental F = filial or offspring.
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Cross of green and
yellow pods
Mendel Crossed Short X Tall
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P1
tall x short
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F1
all tall
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F2
3/4 tall 1/4 short.
hypothesis: 2 factors controlling height.
Factor for shortness was hidden in F1.
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(expected medium)
Short X Tall Continued
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Tall factor = dominant (masked short)
Short factor = recessive (was hidden)
Mendel tested his hypothesis - crossed
plants with other 6 traits
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All other traits acted in same way
The Law of Segregation
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Mendel noticed 2 things
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Explained results with Law of Segregation
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factors controlling traits were distinct units
traits appeared in constant proportions in F2
individual carries 2 factors for each trait
each parent contributes 1 of the 2 factors
TT and Tt = tall and tt = short
genes = units of heredity
Alleles = 2 separate forms of a gene
Genotype Vs. Phenotype
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Genotype = the alleles of an organism
Phenotype = the appearance of an
organism
Homozygous = having 2 identical alleles
TT or tt
Heterozygous = having unlike alleles - Tt
Using Punnett Squares
Test Cross
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Used to determine if an individual is
homozygous or heterozygous
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Incomplete
Dominance
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heterozygote is a
blend of the two
homozygotes
Law of Independent Assortment
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monohybrid cross: involves one pair of
alleles or one gene
dihybrid cross: crosses involving two genes
Mendel did thousands of dihybrid crosses
which led to law of independent assortment
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the inheritance of one gene does not affect the
inheritance of another gene
a tall plant can have either smooth or wrinkled
seeds
Meiosis
The Cellular Basis of Heredity
Chromosome Number
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somatic cells = body cells
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diploid = 2 sets of chromosomes - 2n
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n = # of different kinds of chromosomes
haploid = 1 set of chromosomes
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most organisms contain two complete sets of
chromosomes
humans n = 23(haploid) and 2n = 46(diploid)
homologous chromosomes = matching pairs
Asexual and Sexual Reproduction
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Asexual reproduction
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offspring genetically
identical to parent
offspring formed by
mitosis
potato production
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Sexual reproduction
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2 haploid cells unite to form
a diploid cell
haploid cells = gametes
(sperm and eggs)
fertilization = union of two
gametes to form zygote
zygote grows by mitosis to
form new organism