Introduction to Genetics and Heredity

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Introduction to Genetics and Heredity
Although these dogs have similar characteristics
they are each unique!
I. Early Ideas About Heredity

• A. The Theory of Blending Inheritance
• Each parent contributes factors that blend in
  their offspring
• - ex. A short plant crossed with a tall plant
  would produce a medium sized plant.
• B. Gregor Mendel
• Born in 1822 in Czech Republic
• Worked as a teacher and performed research on the
  heredity of pea plants
• C. Genetics
• The scientific study of heredity

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• D. Mendels Experiment
• Procedure
• Cross-pollinate purebred pea plants
• Use pea plants with different characteristics for
  the same trait
• - Traits include seed shape and color, flower
  position, plant height etc.

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• D. Mendels Experiment
• 2. Results
• The F1 (offspring) generation had the traits of
  only oneof the parents
• The P1 (parental) generations traits did not
  blend

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• Mendels Experiment
• Conclusions
• Individual factors, which do not blend, control
  each trait of a living thing. These factors are
  called genes.
• The different forms of a gene are called alleles.
• For example, the gene for plant height occurs in
  talland short form.
• Some alleles are dominant, while others are
  recessive.
• The effects of a dominant allele are seen even if
  a recessive allele is present.
• The effects of a recessive allele are seen only
  if a dominant allele is not present.

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II. Using Genetic Vocabulary

• Defining Terms
• 1. DNA is organized into chromosomes. There
  are 23 pairs of chromosomes (46 total) in all
  human cells (except sex cells).
• 2. Genes are small segments of DNA present on
  chromosomes that code for a particular protein.
  Multiple genes are found on one chromosome.
• - The estimated of genes in the human genome
  is between 30,000 and 35,000
• 3. Genes code for proteins, ultimately
  resulting in the expression of specific traits
  (characteristics).

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II. Using Genetic Vocabulary Cont.

• 4. Genes come in different forms
  called alleles. Alleles are either dominant (A)
  or recessive (a).
• 5. Organisms that have two identical alleles
  for a particular trait are said to be homozygous
  (AA) or (aa).
• 6. Organisms that have two different alleles
  for the same trait are heterozygous (Aa).
• 7. The genotype (genetic make-up) for a
  particular trait determines the phenotype
  (physical characteristic).

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III. A Simple Example
The organisms in our fictional example have one
pair of chromosomes per body cell. A gene on the
chromosome codes for either dark (D) or light (d)
flower color.
P1 (parental) Generation
Female
Male
Genotype
DD
dd
Homozygous recessive
Homozygous dominant
Phenotype
Light flower color
Dark flower color
A cross between two parents with different forms
of a gene is called a hybrid.
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To prepare for mating, special cells called
gametes must form. The process used to make
gametes is called meiosis. Gamete cells are
haploid (containing only one homologous
chromosome per pair).
P1 Generation
Male
Female
d
D
D
d
Ovule (egg)
Pollen (sperm)
d
d
D
D
Note In animals, 3 of the egg cells (polar
bodies) will degenerate!
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When organisms mate, 1 sperm cell (or pollen
grain) from dad will randomly combine with an egg
cell (or ovule) from mom. These combined gametes
will produce a diploid (2N) offspring with 2
complete sets of chromosomes.
In this example, all of the pollen cells contain
the recessive allele (d) for flower color and the
ovule cell contains the dominant allele (D) for
flower color
Male gamete pollen or sperm
Female gamete ovule or egg
d
D
Fertilization
F1 (offspring) generation
All offspring will be Dd with Dark flowers
Dd
Genotype
heterozygous
Phenotype
Dark flower color
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IV. Punnett Squares
D
d
D dark flowers
d light flowers
DD
D
Dd
dd
Dd
d
A Punnett square is used to predict the
probability Of producing offspring with certain
characteristics.
The probability of two heterozygous dark flowered
plants Producing a dark flowered plant is 3/4
while the probability Of producing a light
colored plant is 1/4.
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V. Special Cases of Dominant and Recessive
Incomplete Dominance a genetic cross where one
allele is Not completely dominant over another.
Example - red and White flowers combine to
produce pink flowers
Codominance a genetic cross where both alleles
show up In the phenotype for the organism.
Example - red and White flowers combine to
produce speckled red and white Flowers
Multiple Alleles Characteristics that have
more than one Possible allele. Example - the
gene for human eye color Comes in many different
forms.
Polygenic Traits Traits that are controlled by
two or more Genes. Example - about four
different genes control human Skin color