Mendel

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Mendels Laws of Heredity

• Why we look the way we look...

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What is heredity?

• The passing on of characteristics (traits) from
  parents to offspring
• Genetics is the study of heredity

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Mendel used peas...

• They reproduce sexually
• They have two distinct, male and female, sex
  cells
• Their traits are easy to isolate

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What Did Mendel Find?

• He discovered different laws and rules that
  explain factors affecting heredity.

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Rule of Unit Factors

• Each organism has two alleles for each trait
• Alleles - different forms of the same gene
• Genes - located on chromosomes, they control how
  an organism develops

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Rule of Dominance

• The trait that is observed in the offspring is
  the dominant trait (uppercase)
• The trait that disappears in the offspring is the
  recessive trait (lowercase)

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Heterozygous Homozygous

• Heterozygous - if the two alleles for a trait are
  different (Aa)
• Homozygous - if the two alleles for a trait are
  the same (AA or aa)

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Heredity

• The two alleles for a trait must separate when
  the sex cells are formed
• A parent randomly passes only one allele for each
  trait to each offspring

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Heredity

• The genes for different traits are inherited
  independently of each other.

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Phenotype Genotype

• Phenotype - the way an organism looks
• red hair or brown hair
• genotype - the gene combination of an organism
• AA or Aa or aa

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Incomplete Dominance
Neither allele is dominant so there is a blending
of traits when two different alleles for the same
trait occur together. The offspring have a mix of
their parents.
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Incomplete Dominance
Heterozygous individuals 3rd
phenotype

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Incomplete Dominance

• In Four O Clocks,
• if you cross a red
• (which is always pure)
• with a white (that is
• also always pure),
• you get a pink
• (which is alwaysa hybrid).

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Incomplete Dominance
Cross of two pink flowers
What are the possibilities?
genotype ratio
phenotype ratio
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• Multiple Alleles

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Multiple Alleles

• Some human traits are controlled by a single gene
  that has two or more alleles.
• Three or more forms of a gene that code for a
  single trait.
• In this pattern of inheritance, the genes have
  more than two alleles controlling them.

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Multiple Alleles

• Even though a gene may have
• multiple alleles, a person can
• only carry two of those alleles.
• Chromosomes exist in pairs
• Each chromosome in a pair
• only carries one allele for each
• gene

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Multiple Alleles

• Blood type in humans is an example of this
  inheritance pattern.
• The four different blood groups
• A, B, O, and AB
• Are produced by three different alleles
• A, B, and O

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Multiple Alleles
Examples of Blood type crosses
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Blood Type
Practice Problems A mother is AB and a father is
O. Draw the Punnett square. A
B O O   What is the probability their
offspring will have A type blood? ____ Can they
have an offspring with O blood?
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Polygenic Traits
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Polygenic Traits
Some human traits show a large number of
phenotypes because the traits are controlled by
many genes. The genes act together as a group to
produce a single trait.
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Polygenic Traits

• Traits controlled by two or more genes
• Show a wide range of phenotypes
• The phenotype is produced by the interaction
  of more than 1 pairof genes.

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Polygenic Traits

• In humans,
• eye color,
• skin color, hair color are a few controlled by
  many genes.

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Polygenic Traits

• Height is controlled by four genes working
  together.

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Polygenic Traits

• Skin color is controlled by at least three genes,
  each one containing two different alleles.
  Various combinations of alleles produce the many
  skin colors in humans.

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Genetic Disorders
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Genetic Disorder

• An abnormal condition that a person inherits
  through genes or
• chromosomes.
• Genetic disorders are caused
• by mutations.
• Down syndrome and cystic fibrosis are two
  examples of genetic disorders.

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Genetic Recessive Disorders

• Many human genetic disorders are caused by
  recessive genes.
• Occur when both parents have the recessive allele
  for the disorder.
• Parents may be heterozygous and have no symptoms
  and pass the trait onto any offspring.

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Sex-linked Genetics
Ex. Colorblindness
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Boy or Girl?

• The gender of a baby is determined by genes on
  chromosomes.
• There are 23 pairs of chromosomes in each of our
  cells. One of pair of
• chromosomes are called sex
• chromosomes.

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Boy or Girl?

• The sex chromosomes determine whether a person is
• male or female.
• The sex chromosomes are the only pair of
  chromosomes that do not always match.

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• Females
• Two sex chromosomes match XX
• Since both chromosomes are X, all eggs carry one
  X chromosome.

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• Males
• Two sex chromosomes dont match XY, so sperm
  cells will either carry an X or a Y chromosome.

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Sex-Linked Genes

• Some human traits occur more
• often in one gender than the other.
• Sex-Linked Genes Genes on the X and Y
  chromosomes, whose alleles are passed from parent
  to offspring on sex chromosomes

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Sex Linked Gene

• In females, a dominant allele on one X chromosome
  will mask a recessive allele on the other X
  chromosome.
• In males, there is no matching allele on the X
  and Y chromosome. As a result, any allele on the
  X chromosome will produce the trait in a male who
  inherits it.

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Sex-Linked Genes

• Because males have only one X chromosome, males
  are more likely than females to have a sex-linked
  trait that is controlled by a recessive allele.

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Sex-Linked Punnett Square

• Let C Normal Vision c Colorblind
• Cross Normal Male ( ) x Carrier Female (
  )

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Pedigree
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Pedigree

• Pedigree A chart or family tree that tracks
  which members of a family have a particular trait

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