Chapter 11 Intro to Genetics
Meiosis
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formation of gametes
sperm and egg (haploid)
fertilization—uniting to form a diploid zygote
“reduction and division”
Haploid vs. Diploid
• Cells having two sets of chromosomes are diploid (2n).
• Haploid cells (1n) have only one set of chromosomes.
Meiosis I—highlights
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Interphase I
• duplication
• Prophase I
• Homologues pair up
• Metaphase I
• Homologous pairs line up across from each other in center of cell
• Anaphase I
• Homologous pairs are pulled in separate directions
• Telophase I
• Homologues pair up
Meiosis II—highlights
STARTS WITH TWO CELLS FORMED DURING MEIOSIS I
• Interphase II
• NO replication!
• Prophase II
• Metaphase II
• Anaphase II
Just Like Mitosis
• Telophase II
Development of Gametes
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Spermatogenesis is the process by which sperm cells are produced.
Oogenesis is the process that produces mature egg cells.
Spermatogenesis and Oogenesis in Humans
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Eggs
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Sperm
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4 polar bodies3 degenerate, 1 matures into an egg
400,000 eggs total
only 300-400 mature
300-400 million sperm per ejaculation
continually replaced
Chromosome Numbers
Sex Chromosomes
• Sex chromosomes are chromosomes that determine the sex of an organism.
• Humans have 2 sex chromosomes.
Autosomes
• All of the other chromosomes in an organism
• Humans have 44 somatic chromosomes.
XX = Female
XY = Male
Crossing Over
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portions of homologous chromosomes
exchange genetic material,
occurs during prophase I and results in
genetic recombination= genetic
variety!!!
because homologues are synapsed (close
together)
physically, they join and exchange
DNA= diversity
Results of Mitosis
2 identical daughter cells with the same amount of genetic info in each cell
Results of Meiosis
4 daughter cells with half the original genetic info
Mendelian Genetics: Complete, Incomplete & Codominance
Terms
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GENETICS: The study of heredity or inheritance of traits.
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TRAIT: A specific characteristic that can be passed from parent to offspring
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HEREDITY: the passing of traits/genes from one generation to the next
Mendelian Genetics
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Examines the physical characteristics
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Principals and laws come from experiments by Mendel
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Predicts inheritance patterns from generation to generation from generation to generation
Peas
Parental or P Generation
(Original pair of plants)
• Self-pollination (Pollen and egg are derived from the same plant)
• Cross-Pollination (Pollen and egg are derived from different plants)
• Purebred vs. Hybrid
• Mendel Studied Seven Traits That Bred True
Monohybrid crosses
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Mendel crossed a tall plant with a short plant test.
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The first group of offspring were called F1 generation for
filial . They should have had blended traits, but didn’t….
Proposals
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Law of Dominance: when two different alleles for a single
gene are present, one is dominant and one is recessive.
– A pea plant contains two discrete hereditary factors, one
from each parent
– The two factors may be identical or different
– When the two factors of a single trait are different
• One is dominant and its effect can be seen
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The other is recessive and is not expressed
Dominance
some traits mask expression of others
• Dominant & Recessive allele traits
– Dominant - trait that does not disappear in the F1
– Recessive - trait that disappears in the F1 generation
each individual possess [only] 2 alleles for a specific trait
RR - homozygous dominant
Rr - heterozygous
rr - homozygous recessive
The Law of Segregation
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Each pair of genes segregate or separate during Meiosis
and the sex cells carry only one copy of each gene
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We know that now because we know about meiosis,
but Mendel “discovered” this through analyzing his
results…
The data suggested a theory of inheritance
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Conclusions
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Probability
Results for all seven traits studied:
• The F1 generation showed only one of the two parental traits
• The F2 generation showed an ~ 3:1 ratio of the two parental traits
These results contradicted a blending mechanism of heredity
Gene: a section of chromosome that codes for a trait
Most organisms have 2 copies of every gene
• one from each parent
Alleles: various forms of a trait or gene
Probability is used to show all possible combinations of gametes and the likelihood that each will
occur
• Like flipping a coin
In reality, you don’t get the exact ratio of results
• Rules are up to chance
PUNNETT SQUARES CAN BE USED TO DETERMINE THE PROBABILITY OF AN ALLELE
OCCURING IN THE OFFSPRING
Punnett Squares
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a grid that enables one to predict the outcome
simple genetic crosses
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by the English geneticist, Reginald Punnett
HOMOZYGOUS
• organisms have 2 identical alleles for a
particular trait and are called true-breeding
• Homo = samo
• HH or hh
• AA or aa
• GG or gg
• QQ or qq
HETEROZYGOUS
• organisms have 2 different alleles for
the same trait and are called hybrids
• He-te-ro = dif-fer-ent
• Hh
• Aa
• Gg
• Qq
Results of Monohybrid Cross
PHENOTYPE
(Physical appearance)
3 round to 1 wrinkled
GENOTYPE
(allele make up)
1(RR) to
2(Rr) to 1(rr)
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Dihybrid Crosses
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A cross with two different traits
Conclusions
• Mendelian factors are now called genes
• Alleles are different versions of the same gene
• An individual with two identical alleles is termed homozygous
• An individual with two different alleles, is termed heterozygous
• Phenotype refers to the outward appearance of an individual
• Genotype refers to the specific allelic composition of an individual
Meiosis backs Mendel
• The segregation of chromosomes in anaphase I of meiosis explains Mendel’s observation
that each parent gives one allele for each trait at random to each offspring, regardless of
whether the allele is expressed.
• The segregation of chromosomes at random during anaphase I also explains how
factors, or genes, for different traits are inherited independently of each other.
Beyond Mendel
Incomplete, Codominance, Multiple Alleles, and Polygenic Traits
Incomplete Dominance
Blended Inheritance
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neither gene is dominant
i.e., both contribute to expression of the trait
there is an intermediate phenotype…
Similar to mixing paint
Codominance
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In codominance, both alleles contribute to the phenotype.
In certain varieties of chicken, the allele for black feathers is codominant with the allele for
white feathers.
Heterozygous chickens are speckled with both black and white feathers. The black and white
colors do not blend to form a new color, but appear separately.
Multiple Alleles
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Genes that are controlled by more than two
are said to have multiple alleles.
An individual can’t have more than two alleles.
However, more than two possible alleles can
a population.
A rabbit's coat color is determined by a single
that has at least four different alleles.
alleles
exist in
gene
Polygenic Traits
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Traits controlled by two or more genes are said
be polygenic traits.
Skin color in humans is a polygenic trait
controlled by more than four different genes.
to