Genetics

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Genetics

• Chapter 11

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Heredity

• Passing of traits from parent to offspring

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Genetics

• The study of heredity
• Gregor Mendel- Austrian monk known as the father
  of genetics

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Mendels Four Hypotheses

1. For each inherited trait, an individual has a
   copy of that gene from each parent.
2. There are alternate versions of genes, called
   alleles. (ex freckles or no freckles)

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Hypothesis continued

• When 2 different alleles occur together, one of
  them may be completely expressed (dominant),
  while the other may not be observed (recessive).
• 4. Alleles separate independently, so that each
  gamete only carries 1 allele for that trait.

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Monohybrid cross

• Mendel began with 1 trait (monohybrid) crosses
• He bred pea plants to produce several generations
• P- the parent generation
• F1 - the first filial generation
• F2 - second filial generation

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• Mendel concluded that each parent has two
  separate factors for a particular trait
• Factors are now called genes.

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Mendels Laws of Heredity

1. Law of Segregation- two alleles separate when
   gametes are formed
2. Law of Independent Assortment- alleles of
   different genes separate independently of one
   another during gamete formation

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Homozygous dominant parent
Homozygous recessive parent
Mendels Law of Segregation
(chromosomes duplicated before meiosis)
meiosis I
meiosis II
(gametes)
(gametes)
fertilization produces heterozygous offspring
Fig. 11-5, p.172
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Independent Assortment
Nucleus of a diploid (2n) reproductive cell
with two pairs of homologous chromosomes
Possible alignments of the two homologous chromoso
mes during metaphase I of meiosis
The resulting alignments at metaphase II
Allelic combinations possible in gametes
1/4 AB
1/4 ab
1/4 Ab
1/4 aB
Fig. 11-8, p.174
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Some modern genetic terms

• Alleles are represented by letters
• Dominant allele is a capital letter
• Recessive allele is a lower case letter

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• Homozygous- identical alleles for a specific
  trait (BB, FF, rr, tt)
• Heterozygous- alleles are different for a
  specific trait (Bb, Ff, Rr, Tt) (aka hybrid)
• Genotype- the set of alleles an individual
  inherits for a trait (i.e.- Rr, Ww, ff)
• Phenotype- the physical expression of a trait
  (i.e. blue eyes, freckles, dimples)

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Punnett Square

• Predicts possible outcomes of traits
• Shows all possible outcomes of a genetic cross

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• Probability (likelihood that an event will occur)
  of a genetic outcome can be predicted
• Ex cross two heterozygous individuals Aa x Aa
• predict ratios of
• 31 for phenotype and
• 121 for genotype

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Practice

• Curly hair is dominant over straight hair. A man
  with straight hair and a woman who is
  heterozygous for curly hair have a child. What
  is the probability that this child will have
  straight hair?

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• Incomplete Dominance- when an individual shows a
  combination of the inherited alleles.
• Ex red snapdragon x white snapdragon will
  produce a pink snapdragon
• straight hair x curly hair wavy hair

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• Codominance- 2 dominant alleles are expressed at
  the same time
• Ex Roan horses show both red and white hairs
  in equal numbers

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Patterns of heredity can be very complex

• Sex-linked trait- a trait whose alleles is
  located on the X chromosome (most are recessive)
• Polygenic trait- trait where several genes
  influence the outcome (eye color, hair color,
  skin color, height, weight)

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• Multiple Alleles- genes with 3 or more alleles
• - Blood type is an example of multiple alleles
  because we have A, B and O alleles and 4 possible
  blood type outcomes
• (A, B, AB and O)

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ABO Blood Type
Range of genotypes
IAIA
IBIB
or
or
IAi
IAIB
IBi
ii
Blood Types
A
AB
B
O
Fig. 11-10a, p.176
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Diploid

• Number of chromosomes found in the body or
  somatic cells of an organism
• 2n

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Haploid

• Number of chromosomes found in the gametes of an
  organism
• n

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Meiosis

• The type of cell division that produces gametes
• Gametes sex cells
• female gamete egg or ovum (plural ova)
• male gamete sperm

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Meiosis
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Formation of sperm and egg
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